DEPARTMENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SURVEY

GEORGE OTIS SMITH, DIRECTOR

BULLETIN 588

THE CONSTITUTION OF THE 

NATURAL SILICATES

BY

FRANK WIGGLESWORTH CLARKE

WASHINGTON

GOVERNMENT PRINTING OFFICE

1914

CONTENTS.

CHAPTER I. Introduction................................................ 5

CHAPTER II. The silicic acids............................................. 10

CHAPTER III. The silicates of aluminum................................... 19

General relations...................................................... 19

The nephelite type ................................................... 21

The garnet type ....................................................... 24

The feldspars and scapolites .......................................... 34

The zeolites ......................................... r ................ 40

The micas and chlorites ............................................... 51

The aluminous borosilicates .......................................... 65

Miscellaneous species ................................................ 74

CHAPTER IV. Silicates of dyad bases....................................... 87

Orthosilicates ......................................................." 87

Metasilicates ........................................................ 94

Disilicates and trisilicates ............................................ 107

CHAPTER V. Silicates of tetrad bases, titanosilicates, and columbosilicates..... 113

APPENDIX ............................................................... 124

INDEX. .................................................................. 125

3

THE CONSTITUTION OF THE NATURAL SILICATES.

By FRANK WIGGLESWORTH CLARKE.

CHAPTER I. 

INTRODUCTION.

In the solid crust of the earth the silicates are by far the most 

important constituents. They form at least nine-tenths of the entire 

known mass and comprise practically all the rocks except the sandstones, quartzites, and carbonates, and even these exceptions are commonly derivatives of the silicates, which break up under various conditions, yielding new bodies of their Own class, together with frees silica 

and limestone. From the geologist's point of view, therefore, the 

silicates are of fundamental importance, and a study of their inner 

constitution may be reasonably expected to shed light upon many 

serious problems. For example, every primitive rock or eruptive 

mass contains an aggregation of silicates, each one of which is capable 

of undergoing chemical change in accordance with limitations imposed 

by the structure of its molecules. When these changes take place 

secondary compounds, alteration products, are formed, and in time the 

rock becomes transformed into new substances, quite unlike those 

 which originally existed. A knowledge of the processes which thus 

occur should be applicable to the study of the rocks and should ultimately render it possible so to investigate a metamorphosed mass as to 

clearly indicate its origin. These processes are dependent on chemical 

structure, and the study of this with regard to the silicates is the purpose of the present memoir.

From the standpoint of the chemist trie problem under consideration 

is one of great importance but also of great difficulty. Some of the 

difficulty is real, some only apparent. At first sight the natural silicates appear to be compounds of great complexity, but this difficulty 

becomes much less serious after careful examination. Few of the natural silicates exist in even an approximately pure condition; many 

that seem fresh have undergone traces of alteration; isomorphous mixtures are exceedingly common; and much confusion is due to defective 

analyses. By multiplied observations these difficulties can be eliminated from the problem, but others yet remain to be disposed of. The 

 5
6 THE CONSTITUTION; OF THE NATUBAL SILICATES.

organic chemist, to whom most of our knowledge of chemical structure 

is due, deals mainly with bodies of known molecular weight, which can 

be measured by the density of a vapor or by cryoscopic methods. To 

the mineral chemist such knowledge is not available, for the compounds which interest him are neither volatile nor soluble, and their 

molecular weights can only be inferred. The simplest empirical formula of a silicate is not necessarily its true formula; the latter may be 

a multiple or polymer of the former; and here we find a difficulty 

which is at present almost insuperable. Strong evidence can be 

brought to bear upon this side of the question, but it is only partial 

evidence and not finally conclusive. The case, however, is by no 

means hopeless, for even the partial solution of a problem is better 

than no solution at all. An. approximation is some gain, and it is 

possible so to investigate the constitution of the silicates as to bring 

many relations to light, developing formulae which express those relations and indicate profitable lines for future research.

The problem is open to attack along several lines, and various 

methods of investigation can be brought to bear upon it. First, of 

course, the empirical formula of each silicate must be definitely ascertained, which involves the discussion of sufficiently numerous analyses 

and the elimination of possible errors due to impurity, alteration, and 

isomorphous admixtures. In this work the microscope renders important service to the analyst and makes his results much more certain. 

By the aid of the microscope many supposed mineral species have 

been proved to be mixtures, and the problem of the silicates has been 

thereby simplified. Indeed, the final outcome of such investigation 

generally indicates, for any given natural silicate, simplicity of composition, and this is what should be expected. These compounds are, as 

a rule, exceedingly stable salts, whereas complex substances are commonly characterized by instability. The mineral silicates are formed 

in nature under conditions of high temperature or are deposited from 

solutions in which many reactions, are simultaneously possible, and 

these circumstances are strongly opposed to any great complications 

of structure. Furthermore, they are few in number, only a few hundred at most being known; whereas, if comrjlexity were the rule 

among them, slight variations in origin should produce corresponding 

variations in character, and millions of different minerals would be 

generated. That few variations exist is presumptive 'evidence that 

only few are possible, and hence simplicity of constitution is reasonably to be inferred. In fact, we find the same small range of mineral 

species occurring under the same associations in thousands of widely 

separated localities, a few typical forms containing a few of the commonest metals being almost universally distributed. The longer the 

evidence is considered, the stronger the argument in favor of simple 

silicate structures becomes.
INTRODUCTION. 7

The empirical formula of a silicate having been established, its 

physical properties may next be considered, and of these the crystalline form and the specific gravity are the most important. From 

identity of form, or complete isomorphism between two species, we 

infer similarity of chemical structure, and the inferences thus drawn 

are often of the highest value. On the other hand, dissimilarity of 

form and identity of composition indicate isomerism, as for example in 

the cases of andalusite and kyanite, and here again we obtain evidence 

which bears directly upon the study of chemical constitution. From 

the specific gravity the so-called molecular volume of a species may be 

computed, and that datum gives suggestions as to the relative condensation of a molecule in comparison with others of similar empirical 

composition. For instance, leucite and jadeite are empirically of 

similar type, but the latter has by far the greater density, together 

with superior hardness. It is therefore presumably more complex 

than leucite, and this supposition must be taken into account in considering its ultimate formula. ° " . From what may be called the natural history of a mineral still 

another group of data can be drawn, relating to its genesis, its constant associations, and its alterability. In this connection pseudomorphs become of the utmost interest and, when properly studied, 

shed much light upon otherwise obscure problems. An alteration 

product is the record of a chemical change and as such has weighty 

significance. The decomposition of spodumene into eucryptite and 

albite, the transformation of topaz into mica, and many like occurrences in nature are full of meaning with reference to the problem 

now under consideration. Just here,, however, great caution is necessary. Mineralogic literature is full of faulty records regarding 

alterations, and many diagnoses need to be revised. Pseudomorphs 

have been named by guesses, based on their external appearance, and 

often a compact mica has been called steatite or serpentine. Every 

alteration product should be identified with extreme care, both by 

chemical and by microscopical methods; for without such precautions there is serious danger of error. Each supposed fact should be 

scrupulously verified.

Closely allied to the study of natural alterations is their artificial 

production in the laboratory. The transformation of leucite'into analcite, and of analcite back into leucite, is a case in point, and the admirable researches of Lemberg furnish many other examples. Work of 

this character is much less difficult than was formerly supposed, and 

its analogy to the methods of organic chemistry renders its results 

highly significant. Atoms or groups of atoms may be split off from a 

molecule and replaced by others, and the information so gained bears 

directly on the question of chemical structure. With evidence of this 

sort relations appear which could not otherwise be recognized, and
8 THE CONSTITUTION OF THE NATURAL SILICATES.

these relations may be closely correlated with observations of natural 

occurrences.

Evidence of the same or similar character is also furnished by the 

thermal decomposition of silicates, a line of investigation which has 

been successfully followed by several investigators. Thus garnet, 

when fused, yields anorthite and an olivine; talc, on ignition, liberates 

silica; and the prolonged heating of ripidolite produces an insoluble 

residue having the empirical composition of spinel. All such facts 

have relevancy to the problem of chemical constitution, and their 

number could easily be enlarged by experiment. As yet the field 

has been barely scratched on the surface; upon deeper cultivation a 

goodly crop may be secured.

The artificial synthesis of mineral species, with the allied study of 

crystalline slags and furnace products, furnishes still more evidence of 

pertinent utility. But here again caution is needed in the interpretation of results. A compound may be produced in various ways, and it 

does not follow that the first rfiethod which is successful in the laboratory is the method pursued by nature in the depths of the earth. The 

data yielded by synthesis are undoubtedly helpful in the determination 

of chemical constitution, but they furnish only a small part of the 

proof needed for complete demonstration, and their applicability to 

geologic questions is extremely limited. For the latter purpose they 

are only suggestive, not final.

Suppose now that the empirical formula of a silicate has been accurately fixed, and that a mass of data such as I have indicated are available for combination with it. Suppose the physical properties to be 

determined, the natural relations known, the alteration products 

observed, its chemical reactions and the results of fusion ascertained; 

what then? It still remains to combine these varied data into one 

expression which shall symbolize them all, and that expression will 

be a constitutional formula. To develop this, the established principles of chemistry must be intelligently applied, with due regard to 

recognized analogies. The grouping of the atoms must be in accord 

with other chemical knowledge; they must represent known or 

probable silicic acids; and any scheme which fails to take the latter 

consideration into account is inadmissible. Not merely composition, 

but function also is to be represented, and the atomic linking which 

leaves that disregarded may be beautiful to see but is scientifically 

worthless. A good formula indicates the convergence of knowledge; 

if it fulfills that purpose it is useful, even though it may be supplanted 

at some later day by an expression of still greater generality. Every 

formula should be a means toward this end, and the question whether 

it is assuredly final is of minor import. Indeed, there is no formula 

in chemistry to-day of which we can be sure that the last word has 

been spoken.
INTRODUCTION. 9

In the development of constitutional formulae for the silicates it 

sometimes happens that alternatives oiler between which it is difficult 

to decide. Two or more distinct expressions may be possible, with the 

evidence for each so strong that neither can be accepted or abandoned. 

In such cases nothing can be done but to state the facts and await the 

discovery of new data, to which, however, the formulae themselves may 

give clues. This sort of uncertainty is peculiarly common among the 

hydrous silicates, and often rises from the difficulty of discriminating 

between water of crystallization, so called, and constitutional hydroxyl. This difficulty is furthermore enhanced by the common 

occurrence of occluded water or water in so-called "solid solution," 

and also by the adsorption of water when a mineral is pulverized for 

analysis. The serious nature of the latter complication was not recognized until quite recently.

In discriminating between rival formulas one rule is provisionally 

admissible. Other things being equal, a symmetrical formula is more 

probable than one which is unsymmetrical. Symmetry in a molecule 

conduces to stability; most of the silicates are exceedingly stable; and 

hence symmetry is to be expected. This rule has presumptive value 

only, as an aid to judgment, and can not be held rigidly. It expresses 

a probability but gives no proof. In a problem like that of the silicates, however, even a suggestion of this kind may render legitimate 

assistance.

There is an extensive literature relative to the constitution of the 

silicates, which, however, has been well summarized by Doelter, 1 

whose summary need not be duplicated here. When necessary suitable reference will be made to the different authorities.

» Handbuch der Mineralchemie, vol. 2, pp. 61-109,1912.
CHAPTER II. 

THE SILICIC ACIDS.

If all the silicates were salts of a single silicic acid the problem of 

their constitution would be relatively simple, but this is not the case. 

Many silicic acids are theoretically possible, and several of them have 

representatives in the mineral kingdom, although the acids themselves, as such, are not certainly known. Their nature must be 

inferred from their salts, and especially from their esters, and this 

side of the problem is the first to be considered.

As silicon is quadrivalent, its orthoacid is necessarily represented 

by one atom of the element united with four hydroxyl groups, thus 

Si(OH) 4 , or, structurally:

H

O

H O Si O H

A

To this acid, orthosilicic acid, the normal silicic esters and many 

common minerals correspond. Its normal salts, reduced to their 

simplest expressions, may be typically represented as follows:

Types. Examples. 

R^SiO, (C2H5 ) 4Si04 

R"aSi04 Mg2Si04 

R1 " 4 (Si04 ). . Al4 (Si04 ) 3 

ZrSi04

Any silicate in which the oxygen atoms outnumber the silicon atoms 

by more than four to one, as, for example, the compound Al2Si05 , must 

be regarded as a basic salt.

By elimination of water orthosilicic acid may be conceived as yielding, first, metasilicic acid, H2Si03 , and, secondly, the. anhydride,

Si02 , thus:

Si(OH) 4 O=Si=(OH) 2 O=Si=O 

10
THE SILICIC ACIDS. 11

Many salts that correspond to metasilicic acid are known, but no 

esters have yet been certainly obtained. The esters first described by 

Ebelmen were supposed to be metasilicates, but all recent investigations have shown them to be ortho compounds, possibly more or less 

impure. Troost and Hautefeuille, however, have described an ester 

having the formula (C2H5 ) 8Si4012 , which is a polymer of a metasilicate, 

but its true nature has not been determined. The simplest formulae 

for typical metasilicates are as follows :

Types. Examples.

R'aSiOs Na2SiO3

RHSi03 . MgSi03

R-2 (Si03 ) 3 Al2 (Si03 ) 3

Riv (Si03 ) 2 Zr(Si03 ) 2

N

The last two examples, Al2 (Si03 ) 3 and Zr(Si03 ) 2 , are salts not 

actually known, but theoretically possible.

By the coalescence of two molecules of orthosilicic acid and successive elimination, molecule by molecule, of water, a series of disilicic 

acids may be produced, thus :

Si(OH) 4 Si=(OH)3 0=Si OH . 0=Si OH

 H20 0 H2O O H20 0

Si(OH) 4 Si=(OH)3 Si=(OH) 3 0=Si

The first of these new acids, orthodisUicic acid, H0Si2 07 , is a sexbasic acid of which several esters are known. It is therefore well 

established, and a number of minerals appear to be salts of it. The 

second acid is a polymer of metasilicic acid, dimetasilicic, and its 

formula is H4Si2 00 . The third compound, metadisilicic acid, 

H2Si205, is represented by no esters, but among its salts are the 

minerals mordenite, ptilolite,*milarite and petalite. By removing 

the last molecule of water the group Si2 04 would remain, a multiple 

ofSi02 .

By a similar process, that is, by the elimination of water from three. 

or four molecules of orthosilicic acid, a series of trisilicic and quadrisilicic acids .may be theoretically developed. These higher acids 

offer many possibilities for isomerism, just as we know to be the case 

among the hydrocarbons. For the present, however, only the trisilicic series need be considered, for above that series the long chains 

of atoms would presumably be unstable. At all events the higher 

series are at present unnecessary for the interpretation of known 

minerals.
12 THE CONSTITUTION OF THE NATURAL SILICATES.

THe trisilicic acids are important and develop as follows:

Two isomers. Two isomers. 

Si=(OH) 3 0=Si OH Si=(OH)3 0=Si OH 0=Si OH

A ' A I, A

Si=(OH) 2

A

Si=(OH) 3 

H8Si3010

Si=(OH) 2 Si=0

A A

Si=(OH)2 

0

Si=(OH) 8 Si=(OH)3 O=Si OH

H6Si309 H6Si309 H4Si308

Si=0

A

Si=(OH)8 

H4Si308

Still another acid is possible to complete the series, H2Si307 ,. to 

which, however, no known minerals correspond.1 The first acid 

of the series, orthotrisilicic acid, has several representatives in the 

mineral kingdom. The second and third, the- trimetasilicitf acids, 

are polymers of metasilicic acid and make, with the similar acids of 

the previous series, four of the same general formula, 7iH2Si03 . To 

these acids the four known modifications of magnesium metasilicate 

may perhaps correspond. The fourth and fifth acids are most important, for they represent the feldspars and appear also in some micas, 

the scapolites, and several other species. Their isomerism is most 

suggestive and possibly accounts for such pairs of minerals as orthoclase and microcline, or eudidymite and epididymite, although the 

latter case is doubtful. The simple name trisilicic acid may be 

assigned to them, for in abundance their salts outrank all the other 

acids of-the series.

Now, by including the quadrisilicic acids for the moment, ignoring 

isomers, and tabulating the several compounds,2 some interesting

relations appear:

Dehydration derivatives of Qrthosilidc adds.

Orthoacids.

H4Si04 ............

E6Si 207 - .........

H Qi f\

H10Si4013 . .......

First 

anhydride.

H2Si03 ....

H4Si206 ----

Second 

anhydride.

Si02 ......

H2Si205 ....

H4Si308 ....

H6Si4On....

Third 

anhydride.

Si,04 ......

H2Si307 ....

H4 Si40IO -.

Fourth 

anhydride.

Si8 06 ......

H2Si409 ....

Fifth 

anhydride.

Si408 .

1 This acid is assumed by Tschermak to be the acid of albite.

2 This form of tabulation has also been employed by Tschermak, Zeitschr. physikal. Chemie, vol. 53, 

p. 350,1905.
THE SILICIC ACIDS. 13

This table can be extended indefinitely, with the result that in each 

vertical column every member below the first differs from the one 

preceding it by the addition of H2Si03 . Furthermore, the first 

anhydride in each series is either H2Si03 or a multiple thereof. That 

is, we have a number of homologous series, quite similar to those 

with which organic chemistry has made us familiar. The final anhydride in every series will be a multiple of Si02 , and that fact seems to 

shed some light upon the possible differences between quartz glass, 

tridymite or cristobalite, and quartz. The commonest associates of 

quartz are the trisilicate feldspars, to which quartz may be related in 

respect to its molecular magnitude. Tridymite and cristobalite, with 

lower specific gravity, are less condensed than quartz and may belong 

in the disilicic series. The still lighter quartz glass is perhaps the 

simplest molecule of all, Si02 . This is hardly more than pure speculation, but the observed relations are certainly suggestive. The denser 

forms of silica are surely polymers of SiO2 .

In the foregoing discussion the silicic acids have been represented 

by "chain" formulae, analogous to the formulae of the aliphatic hydrocarbons. But "ring" formulas of several types are also possible, and 

some authorities prefer them. For example, one type is as follows:

(OH), (OH), (OH),

Si

V

(OH), (OH),

Such formulas can be extended indefinitely, but no matter how many 

silicon atoms are introduced into the ring the saturated compound 

will be a metasilicic acid, nH2Si03 . The successive anhydrides will 

correspond empirically but not structurally to some of the acids of 

the previous scheme, although none can be equivalent to the higher 

orthoacids. This limitation makes the ring system less general than 

the linear or chain system of expressions. Such acids as H4Si04 , 

H6Si2O7 , and H8Si3O10 are impossible under it.

By the coalescence of two or more rings, such as is common among 

the aromatic hydrocarbons, still more complex acids are conceivable, 

thus:

(OH)2=Si Si Si=(OH)s

VV
14 THE CONSTITUTION" OF THE NATURAL SILICATES.

or H4Si308, isomeric with the important trisilicic acids of the chain 

series. 

Again,

000

(OH) 2=Si Si Si Si=(OH)2

"V VV

or H2Si4010, a polymer of the disilicic acid H2Si205 , and so on indefinitely. Here again the limitation holds that the acids with'a higher 

oxygen ratio than appears in the formula H4Si3 08 are excluded from 

the scheme. With triple linkings of oxygen only one silicic acid is 

immediately possible, namely,

0

H O Si 0 Si 0--H

V

or H2Si205 , another isomer of the disilicic acid in the. chain series. 

Two such rings, however, may. be linked together by an oxygen 

atom, thus:

O O

H-0 Si O Si 0 Si 0 Si O H

V V

or H2Si409 , an acid which corresponds to no known compounds. All 

possible acids which appear in the ring formula are included in the 

chain system, at least so far as their empirical formulse are considered. 

It is evident, therefore, from what has been already demonstrated, 

that the chain system is the most complete and general. It is not 

necessary, in the present state of knowledge, to go beyond it, although 

this conclusion should only be held tentatively. It is possible that 

some of the simpler rings may help to interpret some cases of 

isomerism. 1

So far, then, there are only a moderate number of silicic acids 

whose salts appear to need consideration in interpreting the natural 

silicates. They are:

Orthosilicic acid................................ ,H4SiO4

Metasilicic acid................................. ,H2Si03

Orthodisilicic acid..............................H6Si207

Dimetasilicic acid...............................H4Si2Oa

Metadisilicic acid...............................H2Si205

Orthotrisilicic acid.............................. ,H8Si3010

Trimetasilicic acid..............................H6Si309 (two isomers)

Trisilicic acid...................................H4Si308 (two isomers)

i Ring formulse, like some of those given in the text, are used by Vernadsky, Zeitschr. Kryst. Min., vol. 

34, p. 37,1901.
THE SILICIC ACIDS. 15

Many other acids are theoretically possible, and one of them, 

H8Si4012 , is perhaps represented by Troost and Haufcefeuille's ester, 

(C2H5 ) 8Si4 012 . Salts of such acids may occur in the mineral kingdom, 

but so far as present evidence goes the probability of their existence 

is very small.

If the natural silicates were simple normal salts of a few silicic acids 

the problem of their constitution would not be difficult. But relatively few of the" known species are of this description; the greater 

number are double salts, and even triple replacements are not uncommon. Furthermore, there are acid and basic salts to be interpreted, 

and the latter class offers the most serious difficulties. A basic metasilicate, for example, may have the same empirical composition as an 

orthosilicate, so that its ratios,. studied apart from other evidence, tell 

nothing as to the class in which it belongs. For instance, the formula 

Al2Si05 , which represents the composition of three distinct minerals, 

andalusite, silhmanite, and kyanite, admits of' several different 

structural expressions. As a basic metasilicate it may be written

SiO Al 0 Al

and as an orthosilicate it becomes either 

0=A1 Si04=Al, or

If its molecular weight is a multiple of that indicated by the formula 

Al2SiO5 , then the possibilities of isomeric structure become still more 

complicated. Its composition alone does not give its molecular structure, and other evidence, as shown in the introduction to this memoir, 

must be brought to bear before the problem can be even approximately solved. This evidence is sometimes available, sometimes not, 

as will be seen in the systematic discussion of the individual species 

later.

A similar but less troublesome difficulty arises from the common 

occurrence of mixed salts, which may represent one or more silicic 

acids. For example, a well-crystallized silicate on analysis gave 

the following empirical formula: Na2CaAl4Si8 024 . At first sight this 

appears to be a rather complicated metasilicate, but microscopic 

evidence shows that the mineral is a plagioclase feldspar, and the 

formula then is resolvable into 2NaAlSi3O8 + CaAl2 (Si04 ) 2 , or, in petrographic notation, Ab2Ant . A trisili cate and an orthosilicate have crystallized together in isomorphous mixture and simulated a metasilicate.
16 THE CONSTITUTION OF THE NATURAL SILICATES.

In the interpretation of any silicate,' therefore, it becomes important to determine which acid it represents, and that is not often so 

easy to do as in the case just cited. With some minerals the evidence 

seems to be very clear, with others it may be misleading. It. is only 

by careful study of a mineral in relation to other species, and with 

regard to the 'alterations or chemical reactions of which it is capable, 

that this phase of the constitutional problem can be solved. A 

noteworthy attempt in this direction has been made by Tschermak,1 

who in a series of researches has studied the decomposition of silicates 

by hydrochloric acid and sought to identify the silicic acids so liberated. Tschermak has been followed by Himmelbauer, Baschieri, 

Silvia Hillebrand, and others. The validity of his method is sharply 

criticized by Mugge.2 Some of Tschermak's conclusions are in harmony with the generally accepted views; but others are at least surprising. For example, anorthite and olivine are made to be basic 

metasilicates; albite is a salt of the acid H2Si307 ; and garnet and its 

congeners are derived from trisilicic acid, H4Si308 . Even if it be 

admitted that the acids obtained by Tschermak are definite compounds, which has been seriously questioned by Mugge and Van 

Bemmelen, what evidence is there to show that they represent in any 

proper sense the original minerals? In the decomposition of the 

latter many reactions may occur, and the acids finally studied are 

not necessarily those which were first set free. It is safe to say that 

the validity of Tschermak's method is not well established.

Siliceous jellies, obtained by decomposing alkaline silicates with 

acids or by the hydrolysis of SiCl4 or SiF4 , have been studied by many 

chemists with varying results. Norton, and Roth 3 claim to have 

prepared a definite orthosilicic acid from SiF4, but the compound lost 

water steadily on exposure to air. Their memoir contains numerous 

references to the literature of the subject.

A hasty glance over the entire field of the natural silicates will show, 

first, that many of them are most easily interpreted as orthosilicates, 

and, secondly, that by far the greater number are salts of aluminum. 

As regards both abundance and variety the aluminous silicates outrank all the others, and from the wide range of composition which 

they exhibit we can obtain clues to their constitution. In other 

words, they furnish the most evidence, and some of it is of the highest 

import. Their. relations to one another are oftentimes clear and 

unmistakable, so that the constitution of one salt is the key to that of 

a second, and thus generalization becomes possible.

. » K. Akad. Wiss. Wien Sitzungsber., vol. 112, Abth. 1, p. 355,1903; idem, vol. 115, Abth. 1, p. 217,1906; 

 Zeitschr. physikal. Chemie, vol. 53, p. 349,1905; Centralbl. Mineralogie, 1908, p. 225; Zeitschr. anorg. Chemie, 

vol. 63, p. 230,1909; idem, vol. 66, p. 199,1910.

a Centralbl. Mineralogie, 1908, pp. 129, 325; and Van Bemmelen, Zeitschr. anorg. Chemie, vol. 59, p. 225, 

. 1908. See also Serra, R. accad. Lincei Atti, vol. 19, p. 202,1910.

» Am. Chem. Soc. Jour., vol. 19, p. 832,1897.
THE SILICIC ACIDS. 17

The constitution of these aluminous silicates has been studied from 

various points of view. They may be regarded as ordinary silicates^ 

in which the function of ths aluminum is entirely basic, or as salts of 

complex acids containing aluminum as part of the negative radicle. 

That is, the existence of alumosilicic acids is assumed and the various 

minerals are classed as alumosilicates. This mode of interprstation 

has been much in vogue of recent years but is open to the objection 

that it is purely hypothetical. Aluminum may so combine with 

silicic radicles as to form complex ions, but that it does so is quite 

unproved. Some writers have argued that the aluminum of the 

silicates is unreplaceable by other basic radicles, and that the compounds in question are thus different in character from the silicates 

of dyad bases. When, however, andalusite or topaz alters to muscovite, one-half of the aluminum is replaced by the group KH2 , and 

so the argument breaks down. The term "alumosilicate" may, 

nevertheless, be used as one of .convenience, provided that we remember its limitations. The complex ions may exist, but they should 

not be taken too strenuously for granted. The fact that alumina 

combines with silica is alone certain. That the alumosilicates are 

double salts, with all of the aluminum basis, is just as probable as the 

alternative hypothesis. A good summary of the diverse views relative to the alumosilicates is given by Doelter.1 Some of them will be 

considered later as regards their bearing on individual mineral 

species.

A novel interpretation of the alumosilicates has recently been put 

forth by W. and D. Asch,2 and, as it has received considerable attention, it may be briefly noticed here. The authors have developed 

what they call the "hexite-pentite theory," in which rings of silicon 

hydrates are represented as coalescing with similar aluminous rings. 

These rings, as the terminology indicates, may contain either six or 

five atoms of silicon or aluminum alternating with oxygen atoms, 

as in the ring formulae already considered here, and they have a superficial analogy with the benzene ring of organic chemistry. The 

silicon hexite acid, H12Si6018, is evidently a multiple of H2Si03 ; the 

aluminum ring is H6A16012 , equivalent to diaspore. By the coalescence of four such hexite or pentite rings, either two silicic and two 

aluminous or three of one to one of the other, the authors develop 

formulae for 17 alumosilicic acids, having from 6 to 24 replaceable 

hydrogen atoms, and with molecular weights ranging between 873 and 

1,693. From such complicated acids and their successive anhydrides 

the alumosilicates are derived. The acids themselves with few 

exceptions, have no representatives in nature and are purely hypo1 Eandbuch der Mineralchemie, vol. 2, pp. 61-109,1912.

 Die Silicate in chemischer und technischer Beziehung, Berlin, 1911.

43633° Bull. 588 14 2
18 THE CONSTITUTION OF THE NATURAL SILICATES.

thetical. From them, with so many replacements possible an enormous number of salts can be predicated, and isomorphous mixtures, 

altered or impure minerals, and even bad analyses may easily be 

given place in the system. Whether, however, silicates of corresponding complexity could exist at the temperatures of even the 

coolest magma is most questionable.. A generalization which does 

too much may be worse. than no generalization at all. Even for 

such substances as glass the authors of the hexite-pentite theory 

write structural formulae.
CHAPTER III. 

THE SILICATES OF ALUMINUM.

GENERAL RELATIONS.

A strictly logical investigation of the natural silicates might well 

begin with those of magmatic origin, for from them all others have 

been derived. Such a procedure, however, can not be adopted 

exclusively, for the various compounds, primary and secondary, 

are connected by so many interlocking relations that neither class 

should be considered alone. This point is well illustrated by the 

primary alumosilicates, some of whose derivatives are more suggestive than the original species. In this class the only simple 

silicate which has so far been crystallized from a molten mixture of 

silica and alumina is sillimanite, Al2Si05 . More complex salts are 

easily generated from dry fusions, as, for example, nephelite, leucite, 

and anorthite, and each one is the progenitor of many secondary 

minerals. The problem of their structural correlation is the problem 

now in hand.

If we consider first the orthosilicates of aluminum one general relation is easily made apparent. By a general relation I do not mean a 

universal relation, for exceptions are possible, but only that many of 

the salts are connected by a simple regularity or even a law. To make 

this clear it is necessary to recognize the fact that aluminum is 

now properly regarded as a trivalent metal, its formerly supposed 

quadrivalency being no longer admissible. Formulae in which aluminum appears as a tetrad are not valid, and ferric iron, which replaces 

aluminum, follows the same rule.. This point has been established by 

the vapor density and cryoscopic investigations of recent years, and is 

sustained also by the position of aluminum in the periodic classification of the elements. .

This much admitted, the general relation of which I have spoken is 

as follows: Many, perhaps all, of the orthosilicates of aluminum are 

most simply represented as substitution derivatives of the normal salt 

Al4 (Si04) 3 . To'illustrate this rule for present purposes, the following 

examples will suffice:

Aluminum orthosilicate . .Al4 (Si04 )3 

Eucryptite ..............Al3 (Si04 )3Li3

Nephelite............... Al3 (Si04 )3Na3

Kaliophilite.............. Al3 (Si04 )3K3

Muscovite...............Al3(SiO4 )3HK2

Paragonite............. Al3(Si04 )3NaH2

Kryptotile............. AlgCSiC^gEa

Andalusite............. Al3(Si04 )3(A10)3

Topaz................. Ala(Si04 )3(AlF2)8

19
20 THE CONSTITUTION" OF THE NATURAL SILICATES.

These formulae express not only the composition of the minerals but 

also many facts concerning their relations, such as their association, 

their alteration one into another, and so on. Thus, topaz and andalusite are crystallographically akin; both minerals, as well as others in 

the series, alter easily intomuscovite, and these facts become intelligible 

in the light of the formulae given. In the use of the formulae, however, 

one possible misconception must be avoided. They express a relationship of constitution but do not imply that nature first generated 

the normal salt and then actually developed the other compounds from 

it. To emphasize this point an analogy may be drawn from organic 

chemistry. Alizarin, derived constitutionally from anthracene, was 

originally obtained from a glucoside contained in madder root. But 

nobody supposes that the madder plant took anthracene as a starting 

point from which to produce the dye. The constitutional or structural derivation is one thing; the natural origin is quite another.

Whether aluminum orthosilicate as such occurs in nature is still 

a matter of doubt. At all events its existence has not been definitely 

established. It is theoretically possible, and an artificial hydrate of 

the formula Al4 (Si04) 3 .6H20 has been described by Pukall.1 As 

regards its ultimate constitution or chemical structure there is much 

uncertainty. Its formula can be written structurally in several 

ways; as, for instance, with each aluminum atom linked with all three. 

SiO4 groups, or with only one atom so connected. In a sense this 

problem is analogous to that offered by the benzene ring, prism, or 

nucleus, a conception of which the utility is fully recognized, in spite 

of outstanding uncertainties. For practical purposes, that is, for the 

coordination of known facts, expressions like the following are 

sufficient:

,Si04= 

Al SiO4==Al Al Si04=Al Al Si04~CaH

These expressions indicate the observed repla.ceability of aluminum 

atoms by other atoms or groups and have no ulterior significance. So 

long as their limitations are kept in mind they are useful, but beyond 

this it would be unreasonable to go. With prolonged discussion and 

more evidence we may get a deeper insight into the nature of the 

fundamental molecule; at present, speculation concerning it would be 

premature. The relations expressed are clear, no matter what others 

may be revealed in the future. As a working hypothesis, the conception of substitution from a normal salt may be applied to many nonaluminous silicates, as in the magnesian series, the silicates of quadrivalent metals, and so on. These points will be developed in subsequent

i Deutsche cbem. Gesell. Ber., vol. 43, p. 2098, 1910.
THE SILICATES OF ALUMINUM. 21

chapters. For the present we need only to consider the alumosilicates, group by group.

THE NEPHELITE TYPE.

If, now, we start out from-the normal aluminum orthosilicate the 

first and simplest replacement possible is that of a single aluminum 

atom by three monads, giving a compound of the general formula 

Al3 (Si04 ) 3K/3 . This formula represents several well-known minerals, 

and I propose to designate it the nephelite type. At first sight it 

seems to be reducible to the simpler expression B/AlSi04, but that 

expression, as will be seen later, does not indicate all the knowii relations of the group.

The first three representatives of this type are as follows:

Eucryptite................ .............................Al3 (Si04 )3Li3

Nephelite........................:...................... Al3 (Si04 )3Na3

Kaliophilite............................................Al3 (Si04 ) 3K3

These species are all hexagonal, are nearly equal in density, and all 

gelatinize with hydrochloric acid. The second and typical member 

of the series has been made synthetically, and is then found to have 

the composition indicated by the formula. The natural nephelite, 

however, has a composition which is more exactly represented by the 

complex formula B/3Al3Si9034 , in which a little potassium appears 

among the components of B/, and the silica is in excess pf the amount 

required by theory. The potassium is doubtless due to an isomorphous admixture of kaliophilite, and the excess of silica can be explained 

by the presence of a salt isomeric with albite and having the composition Al3 (Si308) 3Na3 . This replacement of Si04 by Si308 appears to be 

common among the silicates, and its recognition clears up many 

discrepancies. In this case one molecule of the trisilicate commingled 

with fifteen of the ortho salt will produce the divergence from normal 

composition shown in the analyses of natural nephelite.

This view of the constitution of nephelite has been.adopted by 

Schaller, 1 and is also favored by Bowen,2 who has studied the fusion 

diagram of the system silica, alumina, and soda. In that investigation Bowen found that at about 1,550° nephelite is transformed into 

a triclinic isomer, a soda anorthite, to which the name carnegieite has 

been given. Foote and Bradley,3 however, have advanced a slightly 

different interpretation of the anomalous composition of natural 

nephelite, ascribing the excessive silica to "solid solution.' 7 These 

three researches represent the most recent and most conclusive work. 

Morozewicz # has explained the divergent analyses of nephelite by 

assuming a series of different nephelite molecules, derived from a

i Washington Acad. Sci. Jour., Sept. 19,1911. * Am. Jour. Sci., 4th ser., vol. 33, p. 439,1912. 

a Am. Jour. Sci., 4th ser., vol. 33, pp. 49, 551,1912. «Acad. Cracovie Bull., 1907, p. 958.
22 THE CONSTITUTION OF THE NATURAL SILICATES.

number of alumosilicic acids. That interpretation seems to be no 

longer tenable. A normal mineral (Na,K)AlSi04 has been described 

by Zambonini 1 under the name of pseudonephelite. It is evidently 

a mixture of nephelite and kaliophilite. The equivalency of these 

species is also clearly proved by an experiment of Lemberg,2 who 

heated nephelite (elaeolite) with -a solution of potassium silicate and 

obtained a product having the composition of kaliophilite.

Eucryptite and nephelite both alter with great ease into muscovite, 

a potassium salt of which paragonite is the sodium equivalent. Furthermore, C. and G. Friedel,3 by heating finely divided muscovite to 

500° in a solution of alkali, obtained nephelite in crystals. From this 

evidence the formulae of muscovite and paragonite become directly 

related to those of the nephelite series, thus:

Nephelite...........................................Al3 (Si04 ) 3Na3

Muscovite. ........................................ .Al3 (Si04 )3KH2

Paragonite...........................................Al3,(Si04 )3NaH2

Physically, the two micas have no resemblance to nephelite, being 

different in form, slightly denser, and refractory toward acids. The 

relationship is purely one of chemical type, and is established by the 

fact of alteration from one into another.

Kryptotile, according to Uhlig,4 is an end member of the mica group. 

If so, its formula becomes AlgCSiOJgHg. The claylike mineral 

leverrierite has apparently the same composition and may be the 

same compound, and another clay, rectorite, is similar but with one 

additional molecule of water, which is lost at 110°. The compound 

may be regarded as an alumosilicic acid, with three replaceable hydrogen atoms, although such an interpretation of it is not necessary.

Through muscovite a connection is recognizable between the foregoing species and the two minerals andalusite and topaz, whose simplest 

formulae, tripled, may be written as follows:

Topaz.............................................. Al3 (Si04)3 (AlF2 )3

Andalusite......................................... Al3 (Si04) 3 (A10)3

Here we encounter the evidently univalent atomic groups

/F 

 Al=0 and Al<

\F

both of which play an important part in various other minerals. The 

two species, topaz and andalusite, are closely allied crystallographically. They have sensibly identical molecular volumes, and both 

undergo alteration into muscovite mica.5 In topaz, as shown by the

1 Chem. Soc. Jour., vol. 98, pt. 2, p. 1078,1910. Abst. from Accad. Napoli Bend., 19JO.

2 Deutsche geol. Gesell. Zeitschr., vol. 37, p. 966,1885.

s Min. Soc. Bull., vol. 13, p. 183,1890.

< Zeitschr. Kryst. Min., vol. 47, p. 215,1910.

5 For a good example of this alteration see Clarke and Diller, U. S. Geol. Survey Bull. 27, p. 9,1886.
THE SILICATES OF ALUMINUM. 23

investigations of Penfield and of Jannasch, hydroxyl commonly 

replaces a part of the fluorine, hydroxyl and fluorine being clearly 

isomorphous. The formula given is that of normal topaz, entirely 

free from alteration.

Obviously the formula of muscovite is the key to all other formulae in 

this group of silicates. Its minimum molecular weight is represented 

by the expression Al3KH2Si3012 and to that the others must conform. 

The general formula Al3 (SiO4 ) 3B/3 is the lowest possible, and the 

formula Na2Al2Si208 , which is often assigned to nephelite, is too small. 

It may represent the isomeric carnegieite, which being stable at high 

temperatures is perhaps molecularly less, condensed than nephelite. 

But of this there is no clear evidence. The tripled formulae are also 

sustained by an experiment of Silber,1 who heated an artificial nephelite silicate in a sealed tube with a solution of silver nitrate and replaced 

one-third of the sodium by silver; that is, one of the three sodium 

atoms seems to be differently combined from the others. This substitution can be expressed structurally in several ways, but its consideration must be deferred until later. Nephelite yields some 

zeolitic derivatives, especially hydronephelite and natrolite, but their 

discussion belongs to another section of this chapter.

To sum up, we have now eight definite species represented by the 

fundamental type Al3 (Si04 ) 3R/3, the first substitution from the 

hypothetical normal orthosilicate of aluminum, and these compounds 

may be divided into three subtypes as follows:

Nephelite. Muscovite. Topaz. 

,Si04=Na3 ,Si04=KH2 ,Si04=(ATF2 ) 3

Al Si04=Al Al Si04=Al Al Si04=Al 

\SiO4=Al \Si04=Al \Si04=Al

symbols which clearly indicate the known chemical relations between 

the several minerals. In six of the eight examples the simplest possible'formulae have been tripled, for otherwise the relationships which 

exist could not be structurally shown. The correctness of this procedure will appear still more definitely in the consideration of the 

groups which follow.

The species sillimanite is isomeric with andalusite, but the structural 

character of the isomerism is not clear. The two species have nearly 

the same molecular volumes, and presumably the same molecular 

weights, but a third isomer, kyanite, is much denser and therefore not 

so easily correlated with the others. It is commonly regarded as a 

basic metasilicate, although that is not its only conceivable structure. 

It is easy to write constitutional formulae for all these minerals, but 

they would be of little real significance except in so far as they repre-

. * Deutsche chem. Gesell. Ber., vol. 14, p. 941,1881.
24 THE CONSTITUTION OF THE NATURAL SILICATES.

seated possibilities. Sillimanite is the most stable compound of the' 

three, and the only one which has been obtained magmatically. At 

high temperatures Jkyanite and andalusite are transformed into 

sillimanite. 1 Structural formulae for andalusite and kyanite have 

been proposed by Zulkowski,2 but they are based upon the minimum 

molecular weight of Al2Si05 and are therefore inadmissible.

THE GARNET TYPE.

By this title I propose to designate the second series of derivatives 

from the normal salt, Al4 (Si04 ) 3, in which two atoms of aluminum 

have been replaced. The general formula of the type obviously is 

Al2 (Si04 ) 3R'6 , and in this series bivalent elements or radicles frequently appear. In lagoriolite, an artificial soda garnet,3 K/6 = Na6 ; 

in prehnite R/6 = Ca2H2 ; and in normal garnet and epidote R'6 = 3R/'. 

There are, therefore, three subtypes to consider one in which all the 

replacing atoms are univalent, one in which all are bivalent, and one 

intermediate between the other two.

Under the first subtype two species may be definitely placed, 

namely, lagoriolite, Al2 (SiO4 ) 3Na0 , and zunyite, which is more complicated. In zunyite R/6 is composed of the univalent radicles 

 A1=F2 , A1=C12 , and A1=(OH)2 , but the species has been 

found in only one locality, and needs further study. At present, 

if we unite the chlorine in it with the fluorine, it may be provisionally 

represented by the expression

,Si04=(A102H2 ) 2 .AlF2 

Al Si04=(A102H2 ) 2 .AlF2 

\8i04==Al

This formula expresses the facts which are now available but is not 

conclusive. Its isometric character, however, helps to connect 

zunyite with the garnet and sodalite groups, as has been shown by 

Brogger.4

"In the second subtype, when B/6 is partly composed of bivalent and 

partly of univalent atoms, two species may be placed, thus:

Prehnite............................................Al2 (Si04 )3Ca2H2

Biotite.............................................Al2 (Si04 )3Mg2HK

Possibly the tetragonal sarcolite, which has the general formula of a 

garnet with the lime partly replaced by soda, may fall here also, but 

the analyses of this mineral are unsatisfactory, and its relations are

1 Vernadsky, Soc. Min. Bull., vol. 12, p. 447,1889; vol. 13, p. 256,1890.

2 Monatsh. Chemie, vol. 21, p. 1086, 1900.

s See Morozewicz, Min. pet. Mitt., vol. 18, p. 147,1898-99. The formula here given to lagoriolite is that of 

the ideally pure mineral. The actual product contains a notable admixture of the corresponding lime 

compound.

* Zeitschr. Kryst. Min., voh 18, p. 209,1891.
THE SILICATES OF ALUMINUM. 25

still uncertain. Biotite will be more fully considered in the section 

devoted to the mica group.

Under the third subtype of this series we find the garnet group 

itself, together with epidote and several related species. The sodalite 

group is also akin to garnet and to the second subtype and will be 

considered in this connection a little later. The generic term garnet 

covers several species, all isometric and strictly isomorphous, in which 

magnesium, calcium, and ferrous iron replace one another, and 

chromium, aluminum, and ferric iron are also equivalent terms. Thus 

we have:

Grossularite.-.......................................... Al2(Si04 )3Ca3

Pyrope................................................ Al2 (Si04 )3Mg3

Almandite............................................. Al2(Si04 )3Fe"3

Spessartite............................................ Al2(Si04 )3Mn3

Andradite............................................ .Fe2(Si04 )3Ca3

Ouvarovite............................................Cr2(Si04 )3Ca3

To these may be added schorlomite, a garnet in which titanium 

occurs both as part of the acid, that is, with TiO4 replacing Si04 , and 

also as TV" among the triad bases, equivalent to aluminum. The 

monoclinic partschinite, isomeric with spessartite, also falls into this 

group.

The several species of garnet occur in a great variety of isomorphous 

mixtures and some of them contain small quantities of alkalies, due to 

the presence of compounds like lagoriolite.1

In the epidote group several species appear, one, zoisite, being 

orthorhombic, whereas the others are monoclinic. These species are 

characterized by the bivalent group of atoms =A1 OH or =Fe OH, 

thus:

Zoisite..............................Al2(Si04 )3Ca2(A10H)

Epidote a........:................... fAl2(Si04)3Ca2(A10H)

Epidote 6............................ lFe2(Si04 )3Ca2(FeOH)

Piedmontite........................ (Al,Mn)2(Si04 )3Ca2 (A10H)

Allanite........................... .(Al,Ce,Fe)2(Si04 ),(Ca,Fe)2(A10H)

Hancockite.......................... Al2 (Si04 )3 (Ca,Pb,Sr)2(FeOH)

or, in general, as compared with garnet,

Garnet.................... ...........W^SiO^W^

Epidote.............................Rw2(Si04 )3B"2 (R"'OH)

A chromium epidote, "tawmawite," containing 11.16 per cent of 

Cr203 has also been described.2

The facts that garnet alters into epidote and that the two minerals 

are often associated give emphasis to the formulae.

1 For elaborate studies of the garnet group see Brogger and Biickstrom, Zeitschr. Kryst. Min., vol. 18, 

p. 209, 1891; Weinschenk, idem, vol. 25, p. 365, 1S9G; Uhlig, Naturh. Ver. preuss. Rheinl. u. Westfalens, 

Verb.., vol. 67, pt. 2, p. 307,1910; and Seebach, Centralbl. Mineralogie, p. 774,1906.

2 See Bleeck, India Geol. Survey Records, vol. 36, p. 254,1907-8.
26 THE CONSTITUTION OP THE NATURAL SILICATES.

Although garnet as a rule is unattacked by acids, and epidote is 

only in part decomposable, both species are so broken up by strong 

ignition as to be readily acted upon by hydrochloric acid, with separation of gelatinous silica. According to Doelter and Hussak,1 garnet 

yields upon fusion sometimes anorthite and an olivine; or meionite, 

augite, and olivine; or melilite and anorthite; and occasionally spinel. 

Epidote, says Doelter,2 yields lime-augite and anorthite, and prehnite 

behaves like garnet. These facts are interesting, but they give no 

direct information regarding chemical structure. By fusion the 

molecules of a silicate are broken down, and on cooling the melt a 

complete rearrangement of the atoms may take place, although not 

necessarily. When calcium alumosilicates are fused they may, as 

in the case of anorthite, recrystallize unchanged, or they may solidify 

as compounds having little or no structural relations to their progenitor. When, however, silicates are broken down by mere calcination and without fusion the reaction may be highly instructive. 

Examples of this kind will be noted later.

In the four species sodalite, haiiynite, noselite, and lazurite we 

have a group of minerals which Brogger has classified as alkali garnets.3 Like garnet, they are all isometric, and they are characterized 

by the. presence of the bivalent groups =A1 Cl, =A1 SO4 Na, and 

=A1 S S S Na. There are also artificial products, ultramarines, in which the groups =A1 S S Na and =A1 S Na 

appear. By adopting Brogger's formulae, which are preferable to 

those formerly proposed by myself,4 these species may be written as 

follows:

. Sodalite................................... Al2 (S104 )3Na4 (AlCl) :

Haiiynite......................../.........Al2(Si04)3Na2Ca(AlS04Na)

Noselite...................................,Al2(Si04 )3Na4 (AlS04Na)

Lazurite.................................. .Al2 (Si04 )3Na4 (AlS3Na)

They fall therefore properly under the second subtype, but are considered at this point on account of their analogies with garnet.

The forrnulge just assigned to these minerals represent, of course, 

the ideally pure compounds, which rarely, if ever, occur in nature. 

The four species are all evidently derived from nephelite, with which 

sodalite is commonly associated, and their composition varies in the 

same manner as that of the parent mineral. Like nephelite, sodalite 

yields natrolite, hydronephelite, and muscovite by alteration. 

Furthermore, C. and G. Friedel, 5 on heating powdered muscovite with 

soda solution and sodium chloride at a temperature of 500°, obtained

1 Allg. chein. Mineralogie, p. 182,1890.

2 Idem, p. 183.

» Brogger and Backstrom, Zeitschr. Kryst. Min., vol. 18, p. 209,1891.

« U. S. Geol. Survey Bull. 42, p. 38, 1887.

6 Soc. min. Bull., vol. 13, p. 183,1890.
THE SILICATES OF ALUMINUM. 27

sodalite artificially, although nephelite was probably first formed as 

an intermediary, and the two species were commingled in the product.

The two hexagonal species, cancrinite and microsommite, are also, 

like sodalite, undoubtedly derivatives of nephelite, but their formulae 

are rather uncertain. At Litchfield, Maine, cancrinite often occurs 

in intimate mixture with nephelite (elseolite). A cancrinite 

described by Zambonini * corresponds very closely to a mixture of 

nephelite and the compound Al2 (SiO4 ) 3Na3Ca(AlC03 ).

In its purest varieties cancrinite approximates to the formula 

Al2 (Si04 ) 3Na4H(AlC03 ), in which a little soda is replaced by lime, and 

the univalent group A1=C03 may be partly substituted by Al= 

Si03 . Microsommite, according to the published analyses, varies 

widely in composition, invariably containing potassium and having a 

notable proportion of chlorine and S03 among its constituents. 

If, however, we assume in it the univalent radicles A1=C12 and 

 A1=S04 , its composition reduces easily to the form Al2 (Si04 ) 3 

(NaK) 3Ca(Al(S04Cl2 )), like cancrinite, both species having then the 

composition of the general type Al2 (SiO4 ) 3B/6 . The theory as proposed, then, assumes univalent complex radicles for cancrmite and 

microsommite, and bivalent radicles for the sodalite group, thus:

In cancrinite group. In sodalite group.

 A1=C13 =A1 Cl

 A1=S04 =A1 S04 Na

 A1=C03 =A1 S8 Na

 Al=Si03

and the typical structures are as follows:

Cancrinite. Sodalite.

xSi04E=Na2 (AlC03 )

Al Si04=Na2H At SiO4=Na2 

\Si04=Al \Si04=Al

The best analyses of microsommite give very nearly

xSi04==Na2 .AlS04 xSi04=Na2 .AlCl2 

1 Al Si04=NaCa + 2 Al Si04=NaCa 

\Si04=Al

with nearly half the sodium replaced by potassium; the radicle 

A1C03 is also sometimes present.

There are arguments both for and against these formulae and the 

peculiar univalent and bivalent radicles assumed in them. The 

assumption of a group Al=Si03 , equivalent to and replacing

i Append ice alia mineralogia vesuviana, p. 35.
28 THE CONSTITUTION" OF THE NATURAL SILICATES.

 A1=C03 , is clearly suggested by the experiments of Lemberg,1 who 

by the action of sodium silicate solution upon elseolite, obtained a 

compound which he designates as a cancrinite containing Na2Si03 in 

place of Na2C03 . By similar reactions with sodium carbonate he produced a substance having the composition of true cancrinite. Hence, 

whatever the ultimate molecular structure of cancrinite may be, we 

are amply justified in assuming in it the replaceability of C03 by Si03 .

These experiments fairly represent a large number of like kind which 

are due to Lemberg, and which are recorded in his papers. Some of 

these will be cited later, but a reference to the work of his colaborer,2 

Thugutt, is in place at this point. Starting from a hydrated nephelite, 

artificially prepared from kaolin, Thugutt succeeded in producing-a 

large series of compounds analogous to sodalite, in which the original 

silicate had taken up, at moderately high degrees of heat and pressure, 

various other salts of sodium, such as the chlorate, selenate, formate, 

oxalate, and so on. These compounds, however, are all hydrated, 

and so differ from the natural minerals of the sodalite group, and they 

are regarded by Thugutt as formed by molecular union. Following 

Lemberg, he regards sodalite as a molecular compound of nephelite 

with sodium chloride, and taking his series of compounds throughout, 

he looks upon the sodium salts which have been added to the fundamental silicate as equivalent in function to water of crystallization. 

In favor of this view he cites many arguments, some of which are 

entitled to considerable weight. Thus, when sodalite is ignited 

NaCl is driven off, whereas if the chlorine were united with aluminum 

A1C13 should be expelled. Similarly, by the action of water alone, 

sodium chloride can be split off from the sodalite molecule, thus indicating a looser form of union than the proposed structural formulas 

show.

But what is molecular union ? To this question there is no satisfactory answer, and even in the case of water of crystallization the term 

is only a confession of ignorance. Unless we assume the existence of 

two kinds of chemical union, it means merely that the structural linking is unknown, and that the problem is laid on one side, conveniently 

labeled for future reference. The constitutional formulae here adopted 

for sodalite and cancrinite are intended to give a provisional solution 

of the problem in their particular cases and to express the genetic 

relationships with nephelite on the one hand and the crystallographic 

analogy with garnet on the other. The objections to them raised by 

Thugutt are serious but not absolutely conclusive. When sodium 

chloride is split off from sodalite the mechanism of the reaction is

1 Deutsche geol. Gesell. Zeitschr., 1885, p. 962.

2 Mineralchemische Studien, Dorpat, 1891. See also Thugutt on cancrinite, Neues Jahrb., 1911, p. 25. 

Zambonini (Contribute allo studio dei silica ti idrati, Napoli, 1908) regards cancrinite as a mixture of silicates, 

with all the water extraneous that is, not essential to the molecules. On the sodalite group see also Silvia 

Hillebrand, K. Akad. Wiss. Wien Sitzungsber., vol. 119, p. 775,1910.
THE SILICATES OF ALUMINUM. 29

quite unknown, and the relative affinities in the molecule are quite 

unstudied. Until these are understood the objections raised by 

Lemberg, Thugutt, and others are not fatal. Furthermore, the presence of a group =A1 Cl does not imply, as Thugutt supposes, the 

splitting off of A1C13 by heat. To effect such a decomposition three 

molecules of sodalite would have to -be broken up, and there is no 

probability that such a disintegration would occur. At all events the 

formulae proposed fulfill a definite purpose, even though they are not 

finally established. They express known relations but not necessarily 

all the relations which the future may reveal. The facts that the 

sodalite-cancrinite minerals are derivable from nephelite and that 

nephelite is again derivable from them are unquestionable.

The question of the molecular structure of a typical garnet, 

Al2 (Si04 ) 3Ca3 , remains to be considered.1 ' If it is regarded as a 

derivative of the normal salt Al4 (Si04) 3 it may be written in at 

least two ways, thus:

1. 2. 

/Si04= 1 Si04=Ca 

Al^SiO = | °a3 Al\Si04=Ca 

\Si04EEAl A<Si04=Ca

That is, isomerism is possible, and of the two species, partschinite and 

spessartite, one may belong to one type and the other to the other. 

In the first expression there is still a replaceable atom of aluminum, 

but in the second expression none; in the first at least one calcium 

atom must link two Si04 groups, whereas in the other no such linkage 

occurs; and these facts may be connected with others. For example, 

garnet alters into mica, and the mica group, as will be seen later, contains members in which the third aluminum atom is replaced. This 

points at once to the first type of formula as preferable, and the 

alterability of garnet into epidote brings the latter mineral into the 

same category.

Zunyite and sodalite, being isometric, should also follow garnet, 

but derivatives of the second type are theoretically possible and may 

exist. Even under the first type alone isomerism is conceivable, and 

the orthorhombic zoisite may be contrasted with the monoclinic lime 

epidote as follows:

Si04=Ca 

/ >A10 H 

Al Si04=Ca

\Si04=Al

Si04=Al OH 

/ >Ca 

Al Si04=Ca

\Si04=Al

i Tschermak (K. Akad. Wiss. Wien, Sitzungsber., vol. 115, p. 217,1906) regards garnet, epidote, zoisite, and 

prehnite as salts of the acid HiSiaOg, but his formulae do not well show the relations of these minerals to 

other species.
30 THE CONSTITUTION OF THE NATURAL SILICATES.

even though we can not assign either species to either formula definitely. My obj ect here is merely to show that the formulae have properties by virtue of which they are able to express known differences. 

Additional evidence for the formula assigned to garnet is supplied 

by the composition of vesuvianite, which is most simply represented 

by the coalescence of two garnet molecules with partial hydration, 

thus :

Garnet. Vesuvianite.

Si04=l ' Si04=)

Ca Ca3

Al Si04= Al Si04=E

^=Al OH

C

,Si04=Al OH

Al Si04=

The formula agrees well with many analyses of vesuvianite, but 

actually, as with other species, its composition varies. About oneseventh of the calcium is commonly replaced by magnesium, and in 

some varieties boron, presumably in the group =B OH, re'places 

in part the corresponding aluminum radicle. 1 Fluorine is also often 

present in small amount as the equivalent of hydroxyl. In short, a 

variety of isomorphous replacements or comminglings are possible 

without affecting the essential structure .as shown by the formula. 

Such replacements are too well known to need detailed discussion 

here.

The true molecular weights of silicates, however, are unknown, 

and it is therefore conceivable that the formulas of garnet and epidote 

should be doubled. These minerals and vesuvianite have nearly the 

same specific gravities, 3.3 to 3.5, for the purely calcic varieties. 

The specific gravities in the cancrinite-sodalite group, on the other 

hand, are about a unit lower, a fact which favors the simpler, lesscondensed molecular structure. On doubling the formulas of garnet 

and epidote the following comparison with vesuvianite and anorthite 

is interesting:

Garnet. Anorthite. 

ySi04==Ca3==Si04v xSi04==Ca3==Si04v

Al Si04=Ca3=Si04 Al Al Si04E=Al2E=Si04 Al

\3iO4==A]3=SiO/'

1 On boron in vesuvianite see Wherry and Chapin, Am. Chem. Soc. Jour., vol. 30, p. 1684, 1908. Weingarten (Centralbl. Mineralogie, 1902, p. 726) represents the mineral by the formula A10H=Si2 07=Ca2 . In 

the former edition of this memoir, U. S. Geol. Survey Bull. 125, vesuvianite was given the formula 

Al2(Si04 )5R8. A1OH. See also U. S. Geol. Survey Bull. 262, p. 72, 1905, for variations in the present formula.
THE SILICATES OF ALUMINUM. 31

Epidote. ' " Vesuvianite. 

xSi04=Ca3=Si04v ,Si04=Ca3=Si04v

Al-Si04E=Al2E=Si04 Al Al-SiO4=Ca3=Si04 Al 

\Si04 Ca Si04/ \Si04 Ca SiO/

A10H A1OH A10H A10H

In these new formulae the essential character of the former expressions is unchanged, but the presumably greater condensation is 

indicated, with the derivation from two molecules of aluminum 

orthosilicate instead of from one. They are also sustained by the 

facts that garnet, epidote, vesuvianite, scapolites, and in some localities anorthite often occur in limestones as products of contact metamorphism; that vesuvianite alters into garnet, garnet into epidote 

and scapolite, and that all four minerals alter into micas and the 

magnesian varieties into chlorites also. The species are connected 

constitutionally and genetically, the analogies connecting them are 

remarkably suggestive and complete, and the formulae here proposed 

render those analogies intelligible. In the Swedish "manganidocrase" a salt occurs which is doubtless the vesuvianite equivalent 

of spessartite, but the compound in a pure state is unknown.

Kyanite, an isomer of andalusite, but of much' higher specific 

gravity, may perhaps be represented as a basic member of the garnet 

series, although it is morphologically very different. The formula

/SiO4==Al2==Si04v 

Al Si04=Al2=Si04 Al 

\3i04 SiO/ J! (A10), (A10).

expresses this relation and also its comparative instability. Both 

kyanite and andalusite, at very high temperatures, are transformed, 

with disengagement of heat, into a third isomer, sillimanite,1 which 

probably has the simplest formula of the three. This, however, is so 

purely hypothetical that it would be useless to discuss the several 

species further.

Two more species, melilite and gehlenite, which, like vesuvianite, 

are tetragonal, may perhaps be best considered .now. Both species 

are very variable in composition, and neither seems to admit of one 

definite formulation. They appear to be mixed silicates, like the 

intermediate plagibclase feldspars and scapolites, but the end members of each series are difficult to identify. An artificial silicate 

recently obtained by Shepherd and Rankin,2 in the Geophysical

1 See Vernaclsky, Soc. Min. Bull., vol. 12, p. 447,1889; vol. 13, p. 256,1890. 

a Private communication.
32 THE CONSTITUTION OF THE NATURAL SILICATES.

Laboratory of the Carnegie Institution, is probably one end of the 

gehlenite series and has the formula Al2Ca2Si07 , which is that of a 

basic metasilicate. Structurally it can be written

/A102Ca 

Si°3 \A102Ca, .-

in which the basic radicle Al<^ )>Ca is analogous to the more

NX

familiar A1=(OH) 2 . The other end of the series is probably a 

silicate of the vesuvianite type,

,Si04EEECa3= 4v

Al Si04=Ca3=Si04 Al

N\Si04=Ca3EESiO//

in which the three replaceable aluminum atoms of the normal orthosilicate are substituted by calcium. If this supposition is correct we 

have the following complete series :

Al6 (Si04) 6Ca 3, anorthite. 

Al4 (Si04) 6Cae, garnet. 

Al3 (Si04) 8Ca8 , in gehlenite.

In the Mexican gehlenite analyzed by Wright 1 there is an approximation to Shepherd and Rankin's silicate, namely, eight molecules of 

that compound commiDgled with one of the other. The comparison 

is as follows : '

sio2 ............... ......... :.............

Ti02 ......................................

A1203 ................... ..'.I..............

Fe2o3 ............;:....:.:A... ...........

FeO......... .....-...,:.-.;....,..........

MnO...........-.-:. :-.;..,. ....-£:. !.....

MffO ' ' " > '' '" ' ' ' '

CaO. ............. .--..,;. '. >?. Na2o ................ i . : >. ....:.....:. .... . . . :

K20... ........ ':.......:...................

H20......... ..............................

Found.

26.33

.03

27. 82

1.43

.50

.01

9 44

39.55

.21

.10

1.85

100. 27

Reduced.

| 26. 69

1 29.12

44.19

100. 00

Calculated.

26.51

28. 98

44.51

100. 00

The second column is recalculated to 100 per cent after uniting iron 

with alumina and recomputing the other bases to their equivalent in

i Wright, F. E., Am. Jour. ScL, 4th ser., vol. 26, p. 545,1908.
THE SILICATES OF ALUMINUM. 33

lime. Other gehlenite analyses reduce equally well but with much 

larger proportions of the orthosilicate compound. The Orawitza 

gehlenite, for example, is very nearly

2 Al2da2SiO7 +1 Al2(3a9SiflO24 .

This commingling of an orthosilicate with a very basic metasilicate is 

not easy to explain, but it seems to fit the actual evidence. It is 

furthermore sustained by an observation of Cathrein,1 who has 

reported pseudomorphs after gehlenite of fassaite, a metasilicate, 

and grossular garnet. The formula commonly assigned to gehlenite, Ca3Al2Si2010, is inadmissible.

 In gehlenite the oxygen is always in. excess of the orthosilicate 

ratio, but in melilite the reverse is generally true. The Vesuvian 

melilites agree nearly with Groth's formula, R6 " R/" Si5010, but the 

mineral from other localities exhibits quite different ratios. An 

artificial "melilite" obtained by Bodlander 2 from Portland cement 

is very nearly

ySi04==Ca3 =Si04v

Al Si04=Ca3 EESi04 Al

as the following comparison shows:

SiOa ... ............... .......................

A1 203~................ .....................

FeaOs.....................................

MgO...... ................................

CaO. ....... ................................

K20....... ....... .........................

Na20........................... ............

Found.

37.96

9.46

2.93

12.77

34.75

1.53

.64

100. 04

Reduced.

38.63

| 11.51

12.99

) 36. 87

|'

100. 00

Calculated.

39.22

11.11

13.07

36.60

100. 00

Other melilites seem to be mixtures of this type of compound with 

the corresponding trisilicates that is, with Si308 in place of Si04 , 

but {he evidence is not conclusive. Such mixtures are found in the 

feldspar, scapolite, and mica groups and are well known. Just as 

the calcic anorthite crystallizes with the sodic albite so probably in 

melilite two compounds, one calcic or magnesian, the other alkaline, 

replace each other isomorphously. Gehlenite and melilite, howi Mln. pet. Mitt., vol. 8, p. 408,1886-87. 

43633° Bull. 588 14 3

2 Neues Jahrb., 1892, vol. 1, p. 53.
34 THE CONSTITUTION' OP THE NATURAL SILICATES.

ever, seem to have one end compound in common, and that compound belongs in the anorthite-garnet series as already shown. Its 

synthesis in the pure state is yet to be effected.

Whether the formulae here proposed are true or not they are useful 

for purposes of correlation. They are, moreover, emphasized by an 

experiment made by Lemberg,1 who has shown that gehlenite, when 

heated to 200° with a solution of potassium carbonate, gives calcium 

carbonate and a product having the composition of a potash mica, 

whereas similar treatment with sodium carbonate converts the 

mineral into cancririite. Gehlenite, garnet, cancrinite, and muscovite are therefore related to one another, and this fact is expressed 

by the formulae proposed. Furthermore, at Orawitza, in the Banat, 

gehlenite is found in rolled pebbles containing grains of vesuvianite, 

a fact which indicates a common genesis for both minerals.

An alternative interpretation of the relations between, melilite and 

gehlenite is offered by the hitherto generally accepted theory of 

Vogt,2 who regards both species as varying mixtures of two silicates, 

one the " gehlenite silicate," R2 '"K,3 "Si2010, and the other akermanite, R4 "Si3010 . The last .compound is found in slags and has 

recently been identified by Zambonini among the minerals of Vesuvius. In it R4" is principally Ca, but with a notable proportion of 

Mg also. A purely calcic silicate of that type has not yet been 

found. Vogt's theory has been seriously questioned by Bodlander 

and Zambonini. The new interpretation now offered seems to be 

more general.

The little-known mineral arctolite is possibly another member of 

this group, with affinities toward prehnite. Its composition is fairly 

expressed by the formula

Al2 (Si04 ) 3CaMgH2

which is that of prehnite with CaMg in place of Ca2 . The integrity of 

this species, however, is not yet fully established.

THE FELDSPARS AND SCAPOLITES.

Although orthosilicic and trisilicic acids are technically distinct and 

from the chemist's point of view should be studied separately, their salts 

containing aluminum occur in such a variety of mixtures that in several groups of minerals the two acids must be considered as mutually 

equivalent and their compounds discussed together. Two such 

groups, closely allied, are the feldspars and the scapolites.

For each of these groups the theory developed by Tschermak has 

met with general acceptance. In the case of the feldspars, Tschermak

> Deutsche geol. Gesell. Zeitsch., p. 237,1892. 2 Neues Jahrb., 1892, vol. 2, p. 73.
THE SILICATES OF ALUMINUM. 35

was undoubtedly anticipated in great part by Hunt, Waltershausen, 

and others, but to him full recognition is due. More recently it has 

been put upon a thorough quantitative basis by the synthetic experiments of Day and Alien.1 According to this theory the triclinic 

plagioclase feldspars consist of albite, AlNaSi308, and anorthite, 

CaAl2Si208, which, commingled in various proportions, give the intermediate oligoclase, labradorite, andesite, and so on. There are also 

the triclinic microcline and its monoclinic equivalent, orthoclase, both 

represented by the formula AlKSi308 , the monoclinic barbierite, 

isomeric with albite, and the recently described carnegieite, or soda 

anorthite, already mentioned as an isomer of nepheline. The mineral 

celsian, BaAl2Si208, is empirically the barium equivalent of anorthite, 

but it is monoclinic and isomorphous with orthoclase.2 Hyalophane 

and other barium feldspars are mixtures of orthoclase and celsian.

The exact nature of the isomeric equivalencies among the feldspars 

is no.t clear; they -may be due to the structure of the salts independently of the acids which they represent, or to isomerisms among 

the acids themselves. The latter possibility was discussed in the 

section on the silicic acids and seems to be the more probable, at lea^t 

so far as the trisilicates are concerned, but for present purposes the 

problem may be left outstanding. In the discussion later of the species eudidymite and epididymite the question of isomeric trisilicates 

will be considered.

For the scapolite series Tschermak has elaborated a theory which is 

closely parallel to that of the feldspars. These tetragonal minerals 

are shown to be most easily interpretable as mixtures of two end 

compounds, meionite, Al6Ca4Si8025 , and marialite, Al3Na4Si0024Cl. 

Neither end compound has yet been found in nature quite free from 

the other, but the variations in composition, in optical character, etc., 

are all accounted for, and the theory, so far as it goes, is satisfactory. 

I have tentatively examined some possible alternative hypotheses, and 

none of them fulfills all necessary conditions so well as this scheme of 

Tschermak's.

Upon studying the feldspars and scapolites more closely, certain 

analogies appear other than those indicated by the parallelism of the 

two series. Both groups of minerals are easily alterable, and both 

yield kaolin as a final product of the change. Furthermore, both alter 

to muscovite, or to pinite, which is only an impure pseudomorphous 

mica, and kaolin crystallographically has close relations with the mica 

family. Feldspars, scapolites, muscovite, and kaolin are therefore 

presumably connected, and the structural formulae of the minerals 

should render the relationship apparent.

i Am. Jour. Sci., 4th ser., vol. 19, p. 93,1905. 2 Strandmark, Zeitschr. Kryst. Min... vol. 43, p. 89,1907.
36 THE CONSTITUTION OP THE NATURAL SILICATES.

The typical empirical formulas to be considered are now as follows:

Albite.................................................AlNaSia08

Anorthite............................................. Al2CaSi208

Meionite............................................... Al6Ca4Si6025

Marialite.....................:......................... Al3Na4Si9024Cl

Muscovite............................................. Al3KH2Si3Oi2

Kaolin................................................. Al2H4Si209

For muscovite the constitution has already been indicated, and this 

clue, together with the general hypothesis of derivation from normal 

salts, enables us to correlate all six of the formulas given. To do this 

it is necessary to triple the formulas of albite and anorthite, and we 

have the following expressions:

' Albite. Anorthite.

Al Si,Os=Al Al Si04=Al2=Si04 Al

Marialite. Meionite.

Si308<=Na2

/ \U Cl Al Si04=Al2=Si04 Al 

Af-Si308^Na2 \Si04=Ca2±=SiO/ 

\Si308E=Al | I

Ca O Ca

Muscovite. = Kaolinite.

,SiO4==KH2 ,OH

Al Si04=Al Al Si04=H3

\Si04=Al N3i04=Al

On this basis marialite becomes the trisilicate equivalent of sodalite, although the two species are quite unlike in form. . Anorthite 

is the calcium salt corresponding to nephelite, which is also alterable 

into kaolin. Again, garnets are known to alter into feldspars and 

scapolite, and, according to Brauns, in the alteration of diabase, 

prehnite and epidote are sometimes derived from anorthite. These 

species, therefore, are all connected by numerous cross relations, all 

emphasizing one another and pointing to a community of molecular 

.type. So far the formulas are highly suggestive, but as yet they do 

not indicate the mechanism of the reaction by which a trisilicate 

feldspar breaks down into kaolin, and they need development in 

that direction. Tschermak,1 from the composition of the silicic acids 

derivable from the several minerals, infers that albite is a salt of the 

acid H2Si307 , and that anorthite is a basic metasilicate. Hinrmel1 Zeitschr. physikal. Chemie, vol. 53, p. 349, 1905.
THE SILICATES OF ALUMINUM. . 37

bauer,1 by the same method, makes meionite ,a metasilicate and 

marialite a derivative of a new acid, H10Si0023. Such formulae fail to 

express the known relations of the minerals at all clearly.

Closely allied to the feldspars in its petrographic relations is the 

isometric mineral leucite, ALKSi206 . Empirically it seems to be a 

metasilicate and is commonly so regarded, but it may easily be conceived as a mixed salt, containing orthosilicate and trisilicate molecules. By alteration it yields orthoclase, nephelite, muscovite, and 

kaolin, and the pseudoleucite of Magnet Cove has been shown by 

J. F. Williams to consist of orthoclase and elaeolite intimately commingled.2 This case probably represents the typical breaking up of 

leucite, the formation of kaolin or of muscovite in other instances 

being due to secondary reactions. On the other hand, C. and G. 

Friedel 3 have obtained leucite synthetically from muscovite as a 

starting point, orthoclase and nephelite being produced at the same 

time, and Lemberg,4 in his experiments, has transformed leucite 

into sanidine, anorthite, and microsommite and also into andesine. 

In a later paper 5 Lemberg describes the action upon various silicates 

of the salt Na2Si03 .8H20, at 200° under pressure, kaolin, albite, 

elseolite, leucite, and analcite all yielding a silicate-cancrinite containing Si03 in place of C03 . These facts connect the several species 

together, but to their explanation the empirical expression AlKSi2Q6 

gives no clue. A formula for leucite, to be satisfactory, must be a 

multiple of this, and several such multiples fulfill the conditions of 

the problem.

The isometric form of leucite suggests at once a relation with the 

sodalite group, and this can be indicated by the quadrupled formula 

Al4K4Si8024 . We then have, as a distinct possibility, the following 

series of molecules, including for comparison the tetragonal marialite.

Sodalite. Marialite. Leucite.

Si04==Na2 Si3O8=Na2 Si04 =K2

/ >A1 Cl / >A1 Cl / >A1 Si04=Al

Al Si04=Na2 Al Si308=Na2 Al Si308=K2

NsiO4=Al

On this basis leucite is clearly reduced to the uniform type of the 

minerals to which it is apparently related, and also of those into 

which it alters, but the formula proposed can not be regarded as final. 

It is offered here only as a first approximation toward answering the 

questions which are suggested and is therefore subject to modification in the future.

i Sitzungsber. K. Akad. Wiss. Wien., vol. 119, p. 115, 1910.

3 Arkansas Geol. Survey Ann. Kept., 1890, vol. 2, pp. 267 et seq.

3 Soc. min. Bull., vol. 13, p. 134, 1890.

* Deutsche geol. Gesell. Zeitschr., 1876, pp. 611-615.

» Idem, 1885, pp. 961, 962.
38 THE CONSTITUTION OF THE NATUKAL SILICATES.

Closely allied to leucite is another isometric mineral, analcite, which 

empirically has the composition AlNaSi206 .H20. This species alters, 

like leucite, into feldspar, and an alteration into prehnite is also 

recorded. Furthermore, Lemberg has shown, in the papers already 

cited, that leuqite, by the action of soda solutions, may be transformed 

into analcite, and that analcite, by similar treatment with potash, 

yields leucite again. With these facts in view, analcite may be 

written

1 Si04=Al 

Al-Si3(V=Na2 +4H20

\Si308EEEAl

exactly equivalent to leucite. That the water is entirely nonconstitutional has been shown by the experiments of G. Friedel,1 who found 

that it could be expelled continuously without change in the crystal 

nucleus. The dehydrated mineral, moreover, could take up water 

again, or instead of water various vapors and gases. These substances seem to be occluded, much as water is held in a sponge, only 

the ratio between the water and the silicate is definitely molecular. 

This phenomenon seems to be peculiarly characteristic of the zeolites, 

which will be considered in the next section of this bulletin.

The close relationship between leucite and analcite is shown not 

only by the work of Lemberg but also by investigations carried out 

in the laboratory of the Geological Survey by Clarke and Steiger.2 

When these minerals are heated with dry ammonium chloride in 

sealed tubes to 350° C., double decomposition takes place and 

ammonium is substituted for the fixed alkalies. In each case the 

new compound has the empirical formula NH4AlSi206 ; that is, an 

ammonium analcite or leucite precisely equivalent to the original silicates is formed. A volatile base has replaced the fixed bases, and 

the substance so formed splits up on ignition in such a way as to shed 

light on its constitution. If, now, ammonium leucite is a true 

metasilicate, a salt of the acid H2Si03 , it should break up, when 

ignited, in accordance with the following equation:

2 NH4Al(Si03 ) 2 =Al2 (Si03 ) 3 + 2 NH3 + H20 + SiO2

and one-fourth of the silica ought to be set free, measurable by 

extraction with sodium carbonate solution. No such liberation of 

silica occurs, and we may therefore conclude that analcite and leucite 

are not metasilicates, but more probably mixed orthosilicates and 

trisilicates, as shown in the constitutional formulae assigned to them 

here. The evidence against their being metasilicates at least seems 

to be conclusive.

i Soc. min. Bull., vol. 19, pp. 94, 363, 1896. , * U. S. Geol. Survey Bull. 207, 1902.
THE SILICATES OP ALUMINUM. 39

One other isometric mineral, pollucite, may perhaps be considered 

here. Its empirical formula, as established by the ana^ses of Wells 1 

and Foote, 2 is that of a metasilicate, H2Cs4Al4S9027, which, however, 

may also be written as a basic trisilicate, thus:

i308=Cs2H

i308=Cs2H 

\3i308=(A10)3

/ If further investigations should show that the water of pollucite 

is not an essential part of the molecule, its empirical formula would be 

very close to AlCsSi206, and the mineral would become the caesium 

' equivalent of leucite and analcite. Pollucite, however, differs from 

those minerals in one notable respect, namely, on heating with dry 

ammonium chloride only one-third of its caesium is replaced by 

ammonium instead of the entire amount. This observation needs to 

be checked by experiments on pollucite from new localities before any 

safe conclusions can be drawn from it. That pollucite is a true 

metasilicate is very doubtful.

Although kaolin mineralogically is not a member of the feldspar 

group, it is properly discussible here as a derivative. The formula 

assigned to it in the foregoing pages is not unimpeachable, but it suggests its relations to the feldspars and micas and also represents the 

fact that the water in it is wholly constitutional. In fact the mineral 

is stable far above the ordinary temperatures of dehydration, so that 

the water can be regarded only as an essential part of the molecule.

In addition to the formula proposed for kaolin the following expressions are possible without assumption of any higher molecular weight:

(1) Si207 .H4 (A10)2

(2) Si2O7 .H2 (A10H) 2 (Brauns's)

(3) Si205 .(AlH202 ) 2 . (Groth's)

Si03 A1H203

Al=SiO,=H2

If the formula be tripled, then kaolin may be written as a 'basic 

trisilicate, thus:

X)H

(6) Al-Si308=H.(AlH202) 2

_______ ______ \3i308i=(AlH202 ) 3 ______________

i Am. Jour. Sci., 3d ser., vol. 41, p. 213, 1891. ' Idem, 4th ser., vol. 1, p. 457, 1896.
40 THE CONSTITUTION OF THE NATURAL SILICATES.

Of all these symbols only the last and the one originally chosen indicate the relations between kaolin and its parent species. As' for these 

two, the formula

/OH

Al Si04=H3 

\Si04=Al

is the simpler and would seem to represent the greater stability. 

Kaolin under ordinary circumstances is scarcely attacked by the 

strongest hydrochloric acid, a fact which seems to be most in harmony with the orthosilicate expression. That expression, therefore, 

is to be preferred, at least until more positive evidence is attainable. 

It is also sustained by the observation of Cornu 1 that kaolin has a 

faintly acid reaction toward litmus. The three hydrogen atoms in 

union with the Si04 group suggest such an.acidity.

After dehydration at low redness, kaolin is completely decomposable by hydrochloric acid, but the ignited mass contains no silica 

soluble in sodium carbonate solution. These facts, developed by 

experiments made under my direction by Mr. George Steiger, seem to 

indicate the formation of a salt, Al2Si2O7 , as the result of ignition, but 

other interpretations are possible. The data are given here simply 

as data that may become available for a fuller discussion of the 

problem by and by. It will be seen later, when the other clays are 

considered, that their formulae are in harmony with that chosen for 

kaolin.

THE ZEOLITES.

By this title is indicated a well-defined group of hydrous silicates, 

unmistakably related to nephelite and the feldspars. Indeed the 

relationship is so close that the several species can often be studied 

genetically, and it has also been established in certain cases by 

synthetic metjiods. The kinship of analcite, which is commonly 

classed as a zeolite, to nephelite and leucite, has already been pointed 

out.

For example, hydronephelite, natrolite, and analcite all occur as 

alteration products of nephelite; 2 natrolite and analcite are both 

derivable by natural processes from albite,3 and analcite yields feldspathic pseudomorphs. Natrolite and hydronephelite may be generated from sodalite, and by artificial means Doelter 4 has produced 

natrolite and analcite from nephelite. All these relations, with others 

both morphologic and genetic, are covered by the types of formulas 

which have already been developed and which can be extended here.

1 Min. pet. Mitt., vol. 24, p. 417,1905; idem, vol. 25, p. 489,1906.

2 See Brogger, Zeitschr. Kryst. Min., vol. 16, pp. 223 et seq., 1890. 

s See Brauns, Neues Jahrb., 1892, vol. 2, p. 1. 

< Neues Jahrb., 1890, vol. 1, p. 134.
THE SILICATES OF ALUMINUM. 41

In a similar way, but rather less completely, many zeolitic minerals 

may be connected with anorthite, the calcium end of'the plagioclase 

feldspar series. For example, by heating anorthite with freshly 

precipitated silica and carbonic acid water at 200°, Doelter obtained 

heulandite. 1 Furthermore, by various wet reactions, some of them 

unfortunately involving several stages, Lemberg 2 has generated 

analcite from chabazite, gmelinite, laumontite, harmotome, phillipsite, stilbite, and heulandite, for some of these minerals studying 

several varieties of one species. It is clear, therefore, that the 

zeolites are connected not only with the feldspars but also with one 

another by many interlacing relations which their constitutional 

formulae ought to symbolize. These relations have been recognized 

by all modern authorities, but their interpretations have been diverse.

In the systematic treatment of the zeolites the most serious difficulty 

is found in the hydration of the several species. To determine what 

part of the water in any one of these minerals is constitutional and 

what is crystalline is not easy, and no fixed criterion exists upon which 

judgment may be based. At present the weight of evidence goes to 

'show that zeolitic water is extraneous to the silicate molecule, at least 

so far as suitable experiments have been made. But until all zeolites 

have been studied by modern methods it would be unwise to assume 

that the rule is universal. Indeed, in some zeolites it seems probable 

that constitutional water or hydroxyl is actually present, for on no 

other basis are the analyses easily interpretable. The work of Friedel 

on analcite, a mineral in which the water is clearly extraneous, was 

cited in the preceding section of this chapter, and analogous researches 

have been conducted by other investigators .3 Many zeolites lose water, 

which is regained without much change of crystalline character on 

subsequent exposure of the minerals to moist air, and this water at 

least can not be regarded as constitutional. The fundamental fact, 

however, that zeolitic water is held in relatively simple molecular 

ratios must not be overlooked, even though it may not be clearly 

explainable. It is also to be remembered that the hydrated silicates 

differ in crystalline form from the parent anhydrous minerals.

In the former edition of this memoir 4 an attempt was made to 

discriminate between essential and nonessential water in the zeolites, 

on the basis of various researches (Damour, Hersch, and others) relative to their dehydration at successive temperatures. The results 

obtained were instructive and of some significance, but the modern

' Neues Jahrb., 1890, vol. 1, p. 128 et seq.

' Deutsche geol. Gesell. Zeitschr., 1885, p. 959 et seq.

s See Rinne (Neues Jahrb., 1896, vol. 1, p. 139; idem, 1897, vol. 1, p. 41), on heulandite and stilbite, and 

Grandjean (Soc. min. Bull., vol. 33, p. 5,1910) on the replacement of zeolitic water by other gases and vapors. 

Zambonini (Contribute allo studio del silicati idrati, 1908) has also done much to show that zeolitic water 

Is essentially absorptive.

< U. S. Geol. Survey Bull. 125,1895.
42 THE CONSTITUTION OF THE NATURAL SILICATES.

work, as just cited, renders a complete revision of the subject necessary. Now, regarding water as not belonging to the true silicate 

molecules, we may discuss the zeolites with reference to their genetic 

relations, beginning with the obviously related starting points, the 

formulae of nephelite and albite. These minerals, as we have already 

seen, are compounds of the same type, one an orthosilicate, the other 

a trisilicate, Al3 (Si04) 3Na3 and Al3 (Si308 ) 3Na3 . From these species 

hydronephelite, natrolite, scolecite, mesolite, analcite, and faujasite 

appear to be derived, either directly or indirectly, as the formulae to 

be proposed clearly show. In nearly every case the simplest empirical 

formula is discarded as not fairly representing the known relations 

between the minerals, and to only one of the above-named species 

 does the rule not apply. That species is hydronephelite, 1 an obvious 

derivative of its original -type and of its more direct parent, sodalite. 

Its formula is Al3 (Si04) 3Na2H.3H20.

For natrolite alternative formulae have been proposed. One, 

Al2 (Si04)5Na2H4 , regards the water of the mineral as constitutional. 

But when natrolite is heated with dry ammonium chloride in a sealed 

tube it is transformed into the compound Al2 (NH4) 2Si3010 , an orthotri-' 

silicate. The simplest formulaf or natrolite, then, is Al2Na2Si3O10 .2H20, 

which is not obviously related to nephelite or to its near relatives 

among the zeolites. Nephelite can be written as a basic orthotrisilicate, but that involves more difficulties than the one it might 

seek to avoid. Natrolite, then, with other species, is best represented in a less immediately obvious manner by doubling its formula 

and bringing it into line with its congeners, especially with sodalite, 

scolecite, mesolite, and edmgtonite. Mesolite, however, is only a 

crystalline mixture of natrolite and scolecite and needs little consideration. Edmgtonite is rather doubtful, but Lemberg, 2 by the action 

of barium chloride solution upon natrolite, obtained a silicate which 

appears to be that mineral. This species is included-here on account 

of its chemical analogy to scolecite. So much premised, the formulae 

now offered are as follows, beginning with the anhydrous type 

species:

Nephelite. Albite. Sodalite.' 

Si04=Na3 Si3O8=Na3 Si04==Na2

Al Si04=Al Al Si308=Al Al Si04 ^:

1 Thugutt (Neues Jahrb., 1910, vol. 1, p. 25) regards hydronephelite as a mixture of natrolite, gibbsite, 

and diaspore; but the mineral analyzed by me was purified by means of Thoulot solution, was homogeneous 

under the microscope, and apparently hexagonal as judged from its optical behavior.

2 Deutsche geol. Gesell. Zeitschr., vol. 28, p. 553,1876.
THE SILICATES OF ALUMINUM. 43

Hydronephelite.

Si04=Na2H 

Al Si04E^

Analcite. 

Si04 =Na2

Al Si308=Na2

i04=Al

3H20

Natrolite.

Si04==Na2

/ >Al-Si3 08=Al 

Al Si04=Na2

4H0

4H20

Scbledte.

Si0=Ca

i308=Al

Edingtonite. 

SiO4=Ba

i,Ofi=Al

6H20

Faujasite. 

Si308==Na

Al Si04

V/Qo

Al Si0

i04EEAl

6H90 19H2O.

The formula assigned to faujasite is quite unlike that usually given, 

but it best fits Damour's analysis, as the subjoined comparison shows:

Si02 .. .................................................

M n

CaO. ..................................................

Na20. .................................................

H20 ..................................................

Found.

46.12

16.81

4.79

5.09

27.02

99.83

Calculated.

47.46

16.14

4.43

4.91

27.06

100. 00

The isometric character of faujasite relates it to analcite and leucite, 

but its immediate derivation was probably from albite.

A group of monoclinic zeolites, closely related in structure to the 

foregoing species, is that formed by wellsite, phillipsite, harmotome, 

and stilbite, to which Pratt and Foote * assign the following general^ 

ized formulae: . .

Wellsite............................................RAl2Si3010 .3H20

Phillipsite.......'.................................. .RAl2Si4012 .4H20

Harmotome........................................RAl2Si5Ou.5H20

Stilbite..............:............................. .RAl2Sia018 .6H20

Am. Jour. Sci., 4th ser., vol. 3, p. 443,1897.
44 THE CONSTITUTION OF THE NATURAL SILICATES.

These formulae make a'beautifully regular series, but unfortunately 

they do not represent the wide variations in composition exhibited 

by some of the species. Harmotome, a barium salt, and stilbite, a 

calcium compound, are fairly constant, except for variable replacements of the dyad radicle by sodium or potassium. Wellsite rests 

on a single analysis, in which calcium, barium, strontium, potassium, 

and sodium appear. In phillipsite R is principally calcium, but with 

varying replacements by potassium and sodium, and the ratio of 

silicon to oxygen is by no means constant. A phillipsite reported by 

Zambonini 1 is very near R'^U2Si3010.4H2O, with alkalies partly 

replacing lime. Other phillipsites are much richer in silica and 

approach a trisilicate in composition. In fact, all phillipsites appear 

to be mixtures of orthosilicates and triplicates, ranging between 

3Si308 : lSi04, and lSi308 : 3Si04 . Such a range and even a greater 

one is only to be expected when we remember that many zeolites are 

derivatives of plagioclase feldspars. The zeolites vary as the feldspars vary between end products, which may or may not be definitely 

known. Stilbite, for instance, represents a hydrated calcium albite 

or trisilicic anorthite, which in the anhydrous condition is yet to be 

discovered.

In this series of silicates, then, we have the plagioclase variation in 

the ratio Si : 0, whereas the ratios R : Al and the degree of hydration 

for each species are constant or . nearly so. The formulse being 

qualified by recognizing the common replacements of lime or barium 

by alkalies, the four "species" may be assigned the following general 

expressions, that for phillipsite representing a fair average between 

its extreme variations :

Wellsite. Phillipsite.

Si04 =R Si04 =Ca

/ Vl SisO8=Al / >A1 Si308==Al

Al Si04^R Al^Si04 ^Ca

Nsi04E=Al \Si808==Al 

6H2O 8H2O.

Harmotome. Stilbite.

Si308=Ba Si3O8=Ca

/ >A1 Si04=Al / >A1 Si3O8=Al

Al Si308=Ba Al Si308-Ca

10H0 12H0

In phillipsite as much as half the calcium may be replaced by potassium. That replacement is characteristic of the species.

i Contribute allo studio dei silicati idrati, p. 114,1908.
THE SILICATES OF ALUMINUM. 45

The hydration of these silicates increases in a very regular and 

remarkable manner, and proportionally to the number of silicon atoms 

in the molecule. For every silicon atom one molecule of water is 

retained. This rule holds true for the typical formulae, but if individual analyses are studied considerable variations will be found. 

Phillipsite nearly always contains an excess -of water. Moreover, the 

low molecular weight of water is the cause of apparent irregularities 

when formulas are deduced from analytical data. A small error in 

the determination of water is exaggerated in the computed ratios. 

The rule is not universal, but it certainly applies to the wellsite-stilbite 

series. Stilbite, at one end, is entirely trisilicate; at the other end 

there should be a pure orthosilicate R2Al4 (Si04) 4 .4H2 0, but no such 

zeolite is known. The orthorhombic thomsonite approaches the 

required composition but not quite closely enough. Empirically 

either lawsonite or its isomer hibschite would complete the series, but 

their hydration appears to be constitutional, and crystallographically 

they belong elsewhere.

Among the plagioclase zeolites, if such a term is admissible, there 

are two, essentially orthosilicates, which may be regarded as hydrated 

anorthite. These species are thomsonite and gismondite, and they 

may be represented as anorthite plus water, using the tripled formula 

for anorthite as developed in the preceding section of this work. It 

is better perhaps to treat them as less condensed molecularly than 

anorthite, because of the loose crystalline structure which permits 

the retention of zeolitic water. On this basis their formulae fall in 

line with those of the other zeolites, as follows :

Thomsonite. Gismondite. 1

Si04=Ca Si04=Ca

/ \A1 Si04=Al / \A1 Si04EEAl

Al Si04^Ca Al Si04^Ca

5H2 0 8H2O

In each of these minerals variations are common, just as among the 

feldspars from which they are probably derived. Gismondite contains some potassium replacing calcium, or, in other words, an 

admixture from orthoclase or microclihe. Thomsonite may have as 

much as half its lime replaced by soda, due perhaps to original 

carnegieite, and it often carries an excess of silica, either in "solid 

solution" or else representing trisilicate groups. Carnegieite, it 

should be remembered, is an isomer of nephelite, a species that, under 

some conditions, alters into thomsonite.

1 See Zambonini (Neues Jahrb., 1902, vol. 2, p. 79) for the composition of gismondite and also of phillipsite. 

See also Sachs, Centralbl. Mineralogie, 1904, p. 215.
46 THE CONSTITUTION OF THE NATURAL SILICATES.

Tnree more zeolites, like stilbite, are entirely trisilicate, namely, 

heulandite, epistilbite, and brewsterite. Heulandite and epistilbite 

are isomeric, or in crystallographic terminology the compound is 

dimorphous. They differ from stilbite in containing less water, 11 

molecules instead of 12. The formula commonly assigned to them, 

if doubled as is done here, assumes only 10 molecules of water, but 

all trustworthy analyses give a larger proportion. Brewsterite differs 

from them in its dyad bases, having barium and strontium with only 

a little lime. In the formula to follow presently the calcium is united 

with barium. In heulandite strontium is often present, and soda to a 

small extent replaces lime. The two formulas, identical in type with 

those which have preceded them, are as follows:

Heulandite. Brewsterite.

Si308=Ca Si3O8=Ba

/ >A1 Si3O8EEAl / >A1 Si308=Al

Al Si308=Ca . Al Si308-Sr

\3iAF=Al \Si308EEEAl 

11H2O 10H20

Brewsterite evidently is derived from an unknown feldspar containing strontium. Edingtonite' and harmotome may represent 

original celsian or hyalophane. Doelter's synthesis of heulandite 

from anorthite has already been mentioned, but an interesting 

observation by Binne 1 remains to be noticed. On decomposing 

heulandite with sulphuric acid he obtained a crystalline form of 

silica, which appeared to be cristobalite. This fact may shed some 

light on the relative molecular magnitude of the zeolite, as was 

suggested in Chapter II, on the silicic acids.

Baschieri,2 working by Tschermak's method, regards heulandite 

and stilbite as salts of an acid, H10Si6O17 . Natrolite and laumontite 

yielded, him orthosilicic acid, and analcite he formulates as a dimetasilicate.

Erionite, an orthorhoinbic zeolite described by Eakle,3 probably 

belongs as a trisilicate with stilbite and heulandite. Its formula, 

which accurately reflects Eakle's analysis, is as follows:

Si308=NaK 

Al Si,O*M

12H2O

1 Neues Jahrb., 1896, vol. 1, p. 139.

2 Zeitschr. Kryst. Min., vol. 46, p. 479,1909. 

s Am. Jour. Sci., 4th ser., vol. 6, p. 66,1898.
THE SILICATES OF ALUMINUM. 47

Laumontite, chabazite, gmelinite, and levynite are plagioclase 

zeolites in which the ratios are empirically metasilicate or nearly so. 

That is, Si04 and Si308 groups appear in equal or approximately 

equal numbers. Laumontite is essentially calcic, with insignificant 

alkaline replacements. In gmelinite alkalies predominate and lime 

is quite subordinate. Chabazite. varies widely from a calcic variety 

to one which is mainly alkaline, and levynite is a lime zeolite with 

Si04 to Si308 as 3 to 2. The variations in composition are quite 

like those which occur among the feldspars, and the crystalline 

comminglings are of the same order. If regarded as a zeolite, analcite is of similar constitution to these species but of lower hydration 

and less variability.1 Lemberg's syntheses of analcite from three of 

them have already been mentioned.

Now, repeating the formula of analcite to facilitate comparison, 

the several zeolites can be well represented as follows :

Analdte. ' Gmelinite.

Si04 ==Na2 Si04 ==Na2

/ >A1 Si04EEAl / >A1 Si04=Al

Al Si0^Na Al Si308=Na2

4H20 12H20

Laumontite. Calcium chabazite. 

Si04 =Ca Si04 =Ca 

/ \M Si04=Al / >A1 Si04==Al 

Al Si0=Ca 38 38

8H20 12H20

Sodium chabazite is empirically identical with gmelinite but different 

in form. This silicate therefore appears to be dimorphous or isomeric, 

and the possibility of isomerism is easily shown. One acid radicle is 

represented in what may be called the side chain of the molecule as 

Si04 . Let that exchange places with an Si308 group and an isomeric 

arrangement is at once given. There are other possibilities, but the 

one is enough for present purposes. -As for the last of these plagioclase zeolites, levynite, the best analyses represent a mixture of three 

orthosilicate and two trisilicate molecules of the same type as the 

other members of the group, with calcium for the dyad radicle, and 

10 molecules of water. The formula commonly assigned to levynite, 

CaAl2Si3010 .5H20, does not fit the facts. From Hillebrand's analysis 

of the Table Mountain levynite we get the following comparison 

between observation and theory:

1 On variations in the composition of analcite see Foote and Bradley, Am. Jour. Sci., 4th ser., vol. 33, 

p. 433,1912.
48 THE CONSTITUTION OF THE NATURAL SILICATES.

SiO*... ................................................

A1203 . .................................................

CaO...................................................

Na20. .................................................

K20..... ................................. I.............

H20............ ........................................

Found.

46.76

21. 91

11.12

1.34

.21

18.65

99.99

Calculated.

46.55

21.98

1 12. 07

[

19.40

100. 00

So far all the zeolites considered, except hydronephelite, are assigned 

one type of formula with varying hydration. If we unite Si04 and 

Si 308 under the indiscriminate symbol X, the general formula becomes

and this covers all variations of composition accurately. R" may 

stand for calcium, strontium, or barium, and R' for either sodium or 

po'tassium. The derivation of the zeolites from feldspar and lenads, 

however, is not always direct. In many cases it can be observed and 

verified, but in others the zeolites seem to have been formed in 

cooling magmas from feldspathic material rather than from the feldspars themselves. Inclosed bubbles of water, perhaps magmatic 

water, have helped to generate the zeolites, especially in amygdaloid 

rocks. The zeolitic amygdules can hardly be explained otherwise, 

and in such a process the SiO4 and Si3O8 radicles may easily be supposed to change places, forming the silicate nuclei corresponding to 

calcium albite on the one hand and sodium anorthite or carnegieite 

on the other.

Regardless of the validity on non validity of the foregoing suggestions, which, by the way, are not new, the constitutional and genetic 

connection between the normal zeolites and the feldspars seems to be 

perfectly clear, and it ought to be easily confirmed by petrographic 

investigation. Data of this kind, in addition to those already cited, 

are even now available, and many alterations of the most pertinent 

kind have been observed. Thus laumontite, heulandite, stilbite, and 

analcite alter into albite or orthoclase; laumontite and stilbite into 

analcite ; chabazite into natrolite ; and gismondite into phillipsite. So 

also alterations into prehnite are recorded on the part of laumontite, 

scolecite, mesolite, natrolite, and analcite, and the identity of chemical 

type seems to be almost unquestionable. From the formulae here 

developed all these alterations become intelligible, and the theory of 

substitution from normal salts is very emphatically sustained.

Several other zeolitic minerals are known, which, however, do not 

belong in the normal series. The two closely allied species mordenite
THE SILICATES OF ALUMINUM. 49

and ptilolite, for example, are to be classed as metadisilicates, and 

their constitution, which I have fully discussed elsewhere, 1 is easily 

expressed by regarding both minerals as mixtures of the two molecules

Al_Si205 > °a Al-Si20 R'

Si205 +6H2O and Nsi2O5 +6H2O

A H Al Si20 H

in which R'=Na or K. In one occurrence of ptilolite the water is 

lower than is required by these formulae, and it seems probable that 

a trihydrate may exist.

The metasilicate zeolite, laubanite, is the precise equivalent of 

ptilolite and mordenite and is easily interpreted thus:

*

/SiO3

A/Q-n > Ca Al SiO3

\Si03 +6H2O

Al-Si0

Pilinite, a similar mineral, seems to be Al2 (Si03 ) 5Ca2 .H20, a monohydrate corresponding to the hexhydrated laubanite. Unlike laubanite, 

pilinite is undecomposed by hydrochloric acid, but physically all four 

of the species here grouped together resemble one another very closely.

Possibly bavenite,2 Ca3Al2Si6O18 .H20, is to be classed with laubanite as a metasilicate, although different in structure.

Foresite and the manganese zeolite ganophyllite are two more 

species of unusual form. Their formulae can be written thus:

Foresite.

/SiA^Al

Al Si04 ==A1

\SiO4 =Ca

Al-Si04 =A1

\Si3O8=Al

Ganophyllite.

vSi308= |

Al Si04 ^]

+ 10H2O \SiO4 *=

Al SiO4 = ]

\SiA=l

[Mn3

1

Mn + THjQ

 Mns

. i Am. Jour. ScJ., 3d ser., vol. 44, p. 101,1893.

2 See Artini, R. accad. Lincel Atti rondiconti, vol. 10, p. 139,1901.

43633° Bull 588^14 t
50 THE CONSTITUTION OF THE NATURAL SILICATES.

These expressions represent fairly well the actual composition of 

the two minerals, the analysis of foresite by Manasse 1 and that of 

ganophyllite by Hamberg 2 being taken for comparison.

SiOa ...........................

Al A

Fea08 .. .........:..............

CaO...........................

MgO. ..........................

MnO. ............... ...........

Na20. .........................

KjO.................... :.....

PbO?......... ................

H20..........................

Foresite.

Found.

48.93 

27.56

5. 16

1.14

16.66

99.45

Calculated.

49. 91

28. 28

5.18

16.63

100. 00

Ganophyllite.

Found.

39.67 

7.95 

.90 

1.11 

.20 

35.15 

2.18 

2.70 

.20 

9.79

99. 85

Calculated.

42.69 

} 8.60

39. 29

9.96

100. 00

Both species need additional study, especially with reference to 

their possible variations.

A still more unusual type of zeolite is stellerite, recently described 

by Morozewicz.3 To this the empirical formula CaAl2Si7O18 .7H2O is 

assigned, which constitutionally is equivalent to

Si025

Al Si2O +7H2O

\SLOJ.q\_/o fiE=Al O o

Grattarola's pseudonatrolite seems to have nearly the same formula, 

but with only 5H20. The structure proposed for these species is, of 

course, only tentative, and may be set aside at some future time.

A considerable number of other zeolitic minerals have been 

described, but their nature is by no means clear. Offretite may be a 

variety of phillipsite, unusually rich in potash, and gonnardite may 

belong with natrolite and scolecite. References to other imperfectly 

known zeolites may be found in Dana's Mineralogy.

The morphological characteristics of the zeolites probably depend in 

great part upon their mode of hydration, but this point needs to be 

developed. So also does the relation between zeolites and kaolin, into 

which the minerals of this group sometimes alter. Furthermore, 

zeolitic substances of indeterminate nature are believed to exist in 

soils and clays, and it is conceivable that such bodies may be intermediately formed during the transition from feldspar into kaolin. In

1 Zeitschr. Kryst. Min., vol. 35, p. 514,1902.

2 Geol. FOren. Forh., vol. 12, p. 586,1891.

8 Acad. Cracovie Bull., 1909, vol. 2, p. 344.
THE SILICATES OF ALUMINUM. 51

studying the mechanism of that change this possibility ought to be 

considered.

THE MICAS AND CHLOBITES.

On account of their wide distribution, their variety of composition, 

and their genetic relations to other species, the micas and chlorites 

form one of the most instructive and interesting families of minerals. 

Two of the micas, muscovite and biotite, have already been noted 

among the members of the first and second of the preceding groups; 

and we have seen how frequently they are produced by the alteration 

of other silicates, some of which have been synthetically derived from 

micaceous material.

As regards the substitution theory, the minerals of this family are 

peculiarly suggestive, for the reason that they form a series of the most 

complete character. Thus, starting from the normal aluminum orthosilicate, we have

Normal orthosilicate................................. Al4(Si04)3

Muscovite.......................................... Al3 (Si04)3KH2

Normal biotite......................................Al2 (Si04)3Mg2KH

Normal phlogopite.................................. Al(Si04)3Mg3KE2

No further substitution of the same order is possible, for the reason 

that it would remove the linking atom of aluminum, and break up 

the fundamental molecule.

Muscovite, the first species in the foregoing series, occurs in nature 

as an independent mineral, and also as an alteration product of 

nephelite, eucryptite, topaz, andalusite, the feldspars, the scapolites, 

and various other natural silicates. All these alterations become 

intelligible in the light of the formulae adopted in this memoir. In its 

more typical occurrences muscovite agrees sharply with the formula 

given, but it varies in composition within well-defined limits. First, 

it ranges toward its sodium equivalent, paragonite, which has the corresponding formula Al3 (Si04 ) 3NaH2 . Secondly, in fuchsite, the 

chromic mica, a chromium salt partly replaces the aluminum compound, and similar ferric replacements are also known. The chromic 

replacement is generally quite small, and so, too, is that of iron, 

although one sericite (a secondary muscovite), analyzed by Sennhofer,1 is very nearly represented by Al2Fe(Si04 ) 3KH2 . Much larger 

replacements of aluminum by vanadium are found in the mineral 

roscoelite, in which as much as 24 per cent of V203 has been 

determined. An ideal roscoelite should have the formula AIV2 

(Si04) 3KH2 , requiring 33.6 per cent of V203, but the pure compound 

is yet to be discovered. In kryptotile, Al3 (SiO4 ) 3H3, we have probably 

the extreme hydrogen end of the muscovite series, and leverrierite 

may be the same species but of different origin. The presence of

1 Min. pet. Mitt., vol. 5, p. 188,1882-83; Dana, E. S., System of mineralogy, 6th ed., table, p. 618,1892, 

analysis No. 43.
52 THE CONSTITUTION OF THE NATURAL SILICATES.

magnesia or of ferrous iron in a muscovite is attributable to small 

admixtures of biotitic molecules.

The most important variation in muscovite is in the direction of 

increased silica. Normal muscovite contains 45.3 per cent of Si02 , 

but varieties exist in which the percentage rises to nearly 59. Muscovites of this class have been designated by Tschermak as phengites, 

and they are most easily explained upon the supposition of trisilicate 

-admixtures. The molecule Al3 (Si308 ) 3KH2 is identical in type with 

ordinary orthosihcate muscovite, and its presence completely accounts 

for all excesses of silica over the normal amount. In Sandberger's 

lepidomorphite, for instance, the orthosilicate and trisilicate molecules occur in nearly equal proportions. All known muscovite may 

be represented by the general formula Al3 (Si04 ) 3K/3 + Al3(Si308 ) 3R/3, 

in which the latter molecule varies from 0 to 50 per cent, and with' 

ferric iron, chromium, or vanadium sometimes partly replacing aluminum. The authenticity of this trisilicic variation is fully confirmed 

by certain of the lithia micas, in which the ratios are entirely trisilicate. Allied to muscovite and paragonite there is also the basic mica 

euphyllite, in which the univalent group A1=(OH) 2 appears. The 

formula of euphyllite appears to be Al3 (Si04 ) 3KH(A102H2 ), -which 

agrees closely with the best analyses.

With the biotites and phlogopites the variability of composition is 

much greater than in the muscovite series. Typical or normal biotites 

may be represented by the subjoined formulae, the actual minerals, 

however, being commonly mixtures.

xSi04=MgK 

Al Si04=MgH

,Si04==Fe"K 

Al Si04EEFe"H

,Si 

Al Si04==MgH

4 

xSi04=Fe"K

Al SiO=Fe"H

These formulae correspond to the following compositions:

SiOa ..........................

AI n

FeA--- -"--- --- --- --

MgO..........................

FeO...........................

KaO...........................

H20...........................

1

43.06

24.40

19.14

11. 25

2.15

100. 00

2

37.35

21.16

29.88

9.75

1.86

100. 00

3

40.27

11.41

17.89

17.89

10.52

2.02

100. 00

4

35.22

9.98

28.18

9.20

1.76

100.00
THE SILICATES OF ALUMINUM.

The siderophyllite of Lewis agrees very closely with No. 2 of these 

formulae. Haughtonite is near an equimolecular mixture of Nos. 1 

and 2, with some ferric replacement of aluminum. Sodium, as in 

muscovite, often partly replaces potassium.

In the normal phlogopite series four typical compounds may occur, 

but the entirely magnesian variety is the only one which is found even 

approximately pure. These compounds are 

6

,Si04=MgK 

Al SiO4==MgH

x 

Al Si04=MgH

,Si04=Fe"K 

Al Si04=Fe"H

/Si04=MgK . 

Al SiO4==Fe"H 

\Si04=Fe"H

equivalent to the following percentage compositions:

SiO

AI n

MgO....... ....................

FeO..........................

K20... .......................

2

5 '

43.27

12.26

28. 85'

11.29

4.33

100. 00

6

40.18

11.38

17.86

16.07

10.49

4.02

100. 00

7

37.66

10.67

8.37

29.70

9.83

3.77

100. 00

8

35.16

9.96

42.19

9.18

3.51

100. 00

A sodium phlogopite containing no potassium has been described by 

Griinling,1 but his analysis is not altogether satisfactory. Aspidolite 

is another mineral which is probably sodium phlogopite, but it needs 

reexamination. In manganophyll, a manganese mica of variable 

composition, with from 9.7 to 17.1 per cent of MnO, the molecule 

Al(SiO4 ) 3Mn3KH2 seems to occur. It is intimately associated with 

the manganese zeolite^ ganophyllite, which is itself micaceous in 

appearance. The two minerals are closely related.

To the typical biotite and phlogopite molecules few natural micas 

actually correspond, although intermediate mixtures are very common. Many of the analyses, moreover, are difficult to interpret with 

any degree of accuracy and for several reasons. The state of oxidation of the iron is frequently uncertain, because in grinding a mineral 

for analysis ferrous compounds may be partly oxidized to the ferric 

condition. In fine grinding, furthermore, some water is adsorbed

i Zeitschr. Kryst. Min., vol. 33, p. 218,1910.
54 THE CONSTITUTION OF THE NATUKAL SILICATES.

from the atmosphere, and an error by no means small is thereby 

incurred. Titanium is also present in many micas, and its exact 

function in them is quite unknown. It may be present as Ti02 

replacing silica, as Ti2 03 replacing alumina, or, which is probably 

more common, as inclusions of rutile. An unusuaF type of mica is 

Breithaupt's alurgite, which, as analyzed by Penfield/ corresponds to 

a mixture of molecules 

2Al3 (Si308 ) 3KH2 

3Al2 (Si04 ) 3E;H4 

3Al2 (Si04 ) 3Mg2KH

with a slight excess of H over K in the last compound. The second of 

these molecules, an alkaline biotite, is the characteristic feature of 

alurgite. Similar compounds, parallel to phlogopite, seem also to 

exist, having the general formula Al(Si04 ) 3R/9, but all of these bodies 

conform sharply to the general theory of the micas and are substitution derivatives of the normal aluminum salt.

In many of the magnesian micas fluorine is found, and the iron 

micas frequently contain oxygen in excess of the amount necessary to 

convert all the silicon into the radicle Si04 . When this excess is real, 

that is, not ascribable to defective analysis, it may be due either to 

alteration or to the replacement of univalent radicles by such groups 

as A102H2 , and K/'OH. Replacements of this kind indicate a transition toward the chlorites, as will be seen later.

Fluorine in the ferromagnesian micas may represent either a group 

like A1=F2 or R" F, and these appear most conspicuously in 

the lithia micas. An average lepidolite, for example, agrees well with 

the formula

/Si308=(AlF2 ) 3 /a04=H,K,Li 

Al Si3O8=K3 + Al SiO4=Al

That is, the mineral is a mixture of a trisilicate with a muscovite, the 

actual proportions varying on both sides of the ratio 1:1. Lepidolite sometimes forms borders on plates of muscovite, and Baumhauer 2 

has shown that lepidolite often contains inclusions of muscovite 

recognizable only under the microscope. Another lithia mica, 

zinnwaldite, is a similar mixture of the same trisilicate with a ferrous

biotite, thus:

/SAESCAIF,), /Si04=FeK

Al Si3O8=K3 + Al Si04=FeH

1 Am. Jour. Sci., 3d ser., vol. 46, p. 289, 1893. Penfleld's interpretation of alurgite is quite different from 

that adopted here.

2 Zeitschr. Kryst. Min., vol. 51, p. 344, 1913.
THE SILICATES OF ALUMINUM. 55

which represents its composition very closely. The calculated composition of the two micas is as follows:

SiOa .. .................................................

A1203 ...... .............................................

FeO....................................................

K20.- ..................................................

Li20..... ..............................................

HaO.- -' ...........................................

F...................................... '.................

Less 0 ................................................

Lepidolite.

51.43

25.50

13.43

4.29

.64

8.13

103. 42 

3.42

100. 00

Zinnwaldite.

46.54

21.58

10.15

TO 97

3.17

.63

8.04

103. 38 

3.38

100. 00

In both minerals a little potassium is commonly replaced by 

sodium, and a little fluorine by hydroxyl. Cryophyllite is near zinnwaldite but more complicated. It forms borders on plates of the 

iron mica annite, which is a mixed silicate between a biotite and a 

phlogopite, and its derived lithia mica exhibits similar complexity. 

Other known lithia micas are varying mixtures of the typical molecules found among the other members of this group of minerals, and 

in irvingite, which is fully two-thirds trisilicate, an alkaline biotite 

appears. Irvingite is well represented by the formula

8Al(Si308 ) 3 (AlF2 ) 3K3Li3 + 9Al2 (Si3O8 ) 3K2H4 + 1 1 Al2 (Si04 ) 3Li3Na3 .

Polylithionite, which is entirely a trisilicate mica, has a quite different type of formula from those already given. The typical mineral 

as shown by Lorenzen's analysis has the formula

/F ,Si308=Na2K 

5A1 F + 1A1 Si3O8=Na2K

which leads to the subjoined comparison:

SiOa .. ................................................. Ai2o3 . ..........................:......................

FeO...............................................

K20 .... .. .................,......................

Na20. .................................................

Li20. ...............................................

F. ....................................................

Found.

59.25

12.57

.93

5.37

7.63

9.04

7.32

102. 11 

3.08

99.03

Calculated.

59.79

12.74

5.85

7.72

9.34

7.88

103. 32 

3.32

100. 00
56 THE CONSTITUTION OP THE NATURAL SILICATES.

In Flink's analysis of polylithionite from another locality potassium 

is in excess of sodium, but otherwise the ratios are nearly the same. 

" In the clintonite group or so-called "brittle micas" we have a 

series of highly basic compounds commonly free from alkalies. They 

are morphologically like biotite and are characterized by the presence 

of the univalent radicle A102R", R" being either Mg; Ca, Fe", or 

Mn. The most basic mica of the group, the end member of the 

series, is xanthophyllite, which has approximately the formula 

Al(Si04 ) 3 (A102R") 9 , with R9 = Mg6Ca3 . In seybertite three of the 

univalent radicles are replaced by hydrogen, and in chloritoid there is 

still more replacement of a different kind. The ideal formulae are 

as follows:

Seybertite. 

/SiO H3

Al Si04=(A102R)3

Xanthophyllite. Chloritoid.

,Si04-(A102Ca) 3

Al Si04=(A102Mg)3 Al Si04=(A102R)3 Al Si04=(A10H)H 

\Si04=(A102Mg)3 \Si04=(A102R)3 N^Si04=(A102Fe)3

In xanthophyllite there is always some hydration, and in the other 

species there are various small replacements of Al by Fe'", of Fe" 

by Mn, etc., as in all the other micas. Ottrelite, a fourth member 

of the series, is like chloritoid, but contains Si3O8 instead of Si04 ; 

that is, its formula is trisilicate The pure theoretical compounds 

have the following composition:

SiOa ...........................

A1203 ..........................

FeO..........................

MgO.......... .................

CaO...........................

H*0....... ...................

Xanthophyllite.

16.45

46.37

21. 91

15.27

100. 00

Seybertite.

21.53

. 42.70

19.14

13.40

3.23

100. 00

Chloritoid.

23.81

40.48

28. 57

7.14

100. 00

Ottrelite.

48.38

97 49

19.36

4.84

100. 00

Xanthophyllite decomposes on ignition into two portions one 

soluble and the other insoluble in hydrochloric acid. The insoluble 

portion has the composition of spinel, a mineral which generally 

accompanies clintonite micas, and of which the formation is rendered intelligible by the formulae. 1 The actual decomposition may 

perhaps be represented by the subjoined equation:

Xanthophyllite. Spinel. Garnet. Magnesia. 

Ca3Mg6Al10 (Si04 ) 3 018 = 4MgAl204 +A12 (SiO4 ) 3Ca3 + 2MgO.

1 For a different interpretation of these micas see the former edition of this memoir, U. S. Geol. Survey Bull. 125,1895; also for details see Clarke and Schneider, U. S. Geol. Survey Bull. 113, p. 27,1893.
THE SILICATES OF ALUMINUM; 57

The lime-alumina garnet and the free magnesia would constitute 

the soluble portion of the ignited mineral. The equation, of course, 

is purely hypothetical and would be difficult to verify experimentally. 

One other mineral, willcoxite, an alteration product of corundum, 

seems to be best classified with the clintonite micas. It appears to 

be a basic analogue of the mixed biotite-phlogopite kind, and its 

analysis gives quite sharply the subj oined formula:

//Si04=(A102Mg) 2Na 

1 Al Si04=(A102Mg) 2H 

\Si04=(A102Mg) 2H

+

ySi04==(A102Mg)Na2 

3 Al Si04=(A102Mg)H2

A little iron is present in the mineral and the sodium is partly replaced 

by potassium.

From some points of view kaolin may be regarded as a member 

of the mica series, especially when its crystalline form is considered. 

With it the calcium mica, margarite, which is commonly classed as a 

member of the clintonite group, can be conveniently correlated. 

Furthermore, margarite yields an alteration product, dudleyite, 

which falls into line with the other two species, thus:

Kaolin.

Al SiO4=H3

Margarite. Dudleyite.

X)H X)H

Al SiO4=CaH Al Si04=CaH

V\Si04=(A10) 3 \Si04=A10H.AlH202

Cookeite, a micaceous mineral found associated with lepidolite and 

encrusting lithia tourmalines, also seems to belong here. Its formula 

is simply written

Al Si04=Li,H(A102H2 ) + H2 0

which agrees well with Benfield's analysis of the cookeite from Maine 

and Schaller's analysis of the California mineral.

SiO

A12 03 - .....:............-.................

CaO......................................

Na 0

K 6

H20............... ........................

F... ............................. ............

Penfield.

34.00

45.06

.45

.04

4.02

.19

.14

14.96

.46

99.32

Schaller.

35.53

44.23

Trace.

2.73

2.11

.31

14.18

1.46

100. 55

Calculated.

35.08

}

44 73

1 4.40

1

} 15. 79

'

100. 00
58 THE CONSTITUTION OP THE NATURAL SILICATES.

These expressions for kaolin and its analogues are suggestive but 

not altogether conclusive. They represent the known facts fairly i 

well, however, and so serve their purpose for the time being. \

By hydration, and sometimes by oxidation, the micas undergo 

alteration, yielding a great variety of products which are known in 

general as vermiculites. This is especially true as regards the ferromagnesian micas, which lose alkalies and take up water with the 

greatest ease, in accordance with what seems to be a well-defined 

law. Thus we have . ,

Biotite. Hydrobiotite. ' \.

,Si04==MgK xSi 

Al SiO4==MgH Al Si04==MgH + 3H20

Phlogopite. . Hydrophlogopite.

,Si04=MgK ,Si04=MgH

Al Si04=MgH Al Si04==MgH + 3H2

\Si04=MgH .

These micas occur in nature in great variety of admixture, and the 

corresponding vermiculites show a parallel complexity. In the normal 

series, however, the alteration commonly follows the line indicated by 

the formulae, and the vermiculite is simply the mica with H in place 

of K or Na, plus 3 molecules of loosely combined water. Two of these 

molecules are, as a rule, given off at 100°, and regained in moist air, 

suggesting an analogy between the vermiculites and the zeolites.. 

Some vermiculites are only monohydrated, and many of the so-called 

species which have received names are mere mixtures of altered and 

unaltered micas, representing stages of transition between the 

originarmineral and the final product. Maconite, lucasite, and philadelphite are incompletely altered micas of this kind.

Jefferisite is quite near the normal hydrobiotite, kerrite approximates to a hydrophlogopite, and lennilite is a mixture between the 

two, but in all three of these minerals the hydration is somewhat 

irregular, and there are the usual replacements of aluminum and magnesium by ferric and ferrous iron.

In some of the vermiculites basic radicles appear, corresponding 

to the excesses of oxygen over the normal ratios that are found 

among the micas themselves. For example, roseite and proto vermiculite may be written:

Roseite. Protovermiculite. 

' ,Si04E=(MgOH) 2H ,Si04EEH3

Al SiO4E=(AlOH)H + 2H2O Al SiO4=(MgOH) 3 + 3H2O 

\Si04==(A10H)H
THE SILICATES OF ALUMINUM. 59

expressions which fit the actual analyses fairly well. Such formulae, 

however, must be interpreted with much caution. They do not 

necessarily imply that these micas are definite compounds; they 

merely symbolize one kind of alteration to which the minerals of this 

group are subject. They mark a transition between the micas and 

the chlorites, and similar but more complex examples are found in 

hallite, painterite, pyrosclerite, vaalite, and pattersonite. Caswellite is another altered mica, rich in manganese and lime but of 

uncertain formula. Indeed, it is hardly worth while to write formulae 

for these minerals, for none of them seems to be a single definite 

compound.

Between the micas and the more basic chlorites the relations are 

exceedingly close. All the species are foliated, all or nearly all are 

monoclinic, and to each of the ferromagnesian micas one or more 

chlorites, higher in magnesia and water, seem to correspond. The 

exact formulation of the chlorites, however, is not a simple matter. 

Some of the so-called " species" are not homogeneous; others are 

isomorphous mixtures; and in all of them replacements of one dyad 

base by another, or of aluminum by iron, occur. In the chlorites 

the basic univalent and bivalent groups Mg OH, Fe" OH, 

 A1=(OH)2 , =A1 OH, Fe'"=(OH) 2 , and =Fe"' OH appear, 

but their precise identification is complicated by uncertainties in the 

hydration of the minerals. In general, the water shown by the analyses 

is constitutional, but in some chlorites it may be extraneous, like the 

water of the zeolites. .An exact study of the hydration of the chlorites by modern methods is yet to be made.

Some so-called chlorites, containing alkalies, are' obviously mixtures of chlorites and micas, but the true species are all referable to 

the types of molecule represented by biotite and phlogopite except a 

few that fall more nearly into line with margarite and kaolin. Typical 

chlorites, which, however, are rarely if ever found pure, may be represented thus:

Biotite-chlorite. Phlogopite-chlorite.

/SiO^MgOH^H ,Si04=(MgOH)2H

Al Si04=(MgOH)2H Al Si04=(MgOH)2H

^SiO.EEAl \SiO4EE(MgOH)2H

An average pennine contains these two molecules commingled in the 

ratio 1:1, whereas in clinochlore and leuchtenbergite the ratio is 

2 : 3, with the second Al of the biotite formula replaced by 3A1O. 

That is, the formula of a typical clinochlore is represented by

3 Al(Si04 ) 3 (MgOH) 6H3 + 2 Al(Si04 ) 3 (MgOH)4H2 (AlO) 3 .

These expressions give the subjoined compositions for the two 

chlorites.
60 THE CONSTITUTION OF THE NATUBAL SILICATES.

Si02 .. .................................................

A12 03 .. ................................................

MgO. ..................................................

H20........ ............................................

Pennine.

34.35

14.60

38. 17

12.88

100. 00

Clinochlore.

31.58

19.65

36.49

12.28

100. 00

The mixtures may occur in other proportions; the magnesium may 

be partly replaced by iron, and in the varieties kammererite and 

kotschubeite some chromium, equivalent to aluminum, appears. 

A glance at the tables of analyses in the textbooks of Dana and 

Hintze will show how variable in composition these and other chlorites really are.

Three chlorites, rumpfite and the ferrosoferric minerals cronstedtite 

and melanolite, seem to conform very nearly to the biotite type of 

formula, thus 

Rumpfite.

xSiO4E=(MgOH) 2H 

Al Si04=(A102H2 ) 3 

\SiO4=Al

Cronstedtite.

£i04=(Fe''OH) 2 (Fe'''02H2 ) 

Fe SiO4E=(Fe''OH) 2 (Fe'''02H2 ) . 

Nsi(X=Fe-'"

Melanolite.

,Si04=(Fe"OH)H2 

Fe Si04=(Fe"OH)H2

The composition of the three minerals, as given by these formulae, is 

as follows:

Si02 .... ....................................

Al O

Tef>z.... .................................

FpO

MgO......................................

H20............................ ...........

Rumpfite.

30.20

42.78

13.42

13.60

100. 00

Cronstedtite.

20.93

37.21

33.48

8.38

100. 00

Melanolite.

33.70

29. 63

26. 67

10.00

100. 00

In melanolite a little ferric oxide is replaced by alumina.

The two species brunsvigite and delessite may be given either 

biotitic or phlogopitic formulae, according to the character of the
THE SILICATES OF ALUMINUM. 61

hydration. If one molecule of their water is zeolitic in character 

that is, nonessential the minerals are of the biotite type. If all the 

water is constitutional, they are related to phlogopite. Brunsvigite 

is essentially a ferrous chlorite, delessite is ferromagnesian, and the 

two species correspond to the two formulae 

Brunsvigite.

,Si04=(FeOH)2H 

Al Si04=(FeOH)2H 

\Si04=E(A10H)H

Delessite.

Al Si04=(MgOH) 2H 

\Si04==(A10H)H

In delessite the magnesian end compound is sometimes nearly 

approached, but usually about one-fourth of the magnesium is 

replaced by iron, and a little ferric replacement of alumina is als'o 

common. Delessite is a very variable mineral. When computed 

with Mg3Fe1; its average composition is as follows, in comparison 

with brunsvigite: '

Si0a .. ...............:.................................

AI n

FeO. ...................................................

MgO................ ............... ....................

H20............ ........................................

Delessite.

32.97

18.68

13. 19

91 QR

13.18

100. 00

Brunsvigite.

28.04

15.89

44.86

11.21

100. 00

Closely related to the two preceding species are pro chlorite and 

grochauite, to which the following structures may be assigned:

Prochlorite.

>Si04==(MgOH) aH 

Al Si04=(FeOH)2H

\

Si04=E(A10H)(A102H2 )

Grochauite. 

/Si04=(MgOH)3

Al Si04=E(MgOH)3 

\Si04E=(A10H)(A102H2 )

The composition of these two minerals is as follows:

SiO,.. .................................................

AI n

FeO

MgO.........................,...:......-..............

H30. ...... ............... ^.-.... ........... ......

Prochlorite.

28. 28

00 Qfl

23.50

12.66

12.66

100. 00

Grochauite.

27.53

Oq QQ

36. 69

12.39

100. 00
62 THE CONSTITUTION OF THE NATURAL SILICATES.

This prochlorite is fairly typical, but a so-called "prochlorite" 

from Culsagee, North Carolina, 1 is quite different in its ratios and 

contains very little iron. It is allied to metachlorite, as the following 

formulae show:

Culsagee prochlorite.

/Si04=(MgOH)3

Al Si04EE(MgOH) 2H 

The percentage compositions are as follows:

Metachlorite. 

/Si04=(FeOH)3

Al Si04=(FeOH)2H 

\Si04=(A102H2 )H

Si02 .... ...............................................

A1203 . .................................................

FeO

MgO...................................................

H20..................... ...............................

Culsagee 

prochlorite.

28.48

24.21

31.65

15.66

100. 00

Metachlorite.

22. 73

19.32

45.46

12.49

100. 00

Corundophilite is a chlorite with still higher alumina, which probably has the following constitution:

,Si04=(ROH)3 

Al Si04=(ROH)3 

\SiO=(A102H2 ) 3

SiO%.......................................................... 22.68

A1205-....,................................................... 25. 69

tfeO........................................................... 17.88

MgO.......................................................... 20.15

S20........................................................... 13.60

100.00

The calculated composition gives R6 = Mg4Fe2 . The water is a little 

too high, and may be partly vermiculitic. If so, (A1O2H2 ) 3 should be 

replaced by (A10)3 , a replacement which may be desirable in some of 

the other formulae already given. The need of investigating the 

hydration of the chlorites has already been pointed out, but it is 

well to emphasize it here.

Diabantite and thuringite appear to be mixed silicates, intermediate between the vermiculites and the chlorites, thus:

»Analyses 17-20 on p. 654 of Dana's System of mineralogy, 6th ed., 1892.
THE SILICATES OF ALUMINUM. 63

Diabantite.

,Si04=RH xSi04=(ROH)2H 

1A1 Si04=RH + 2 Al Si04=(ROH)2H

Thuringite. 

,Si04=FeH /Si04==(FeOH)3

Al Si04=FeH + Al Si04=(FeOH) 3 

\Si04=(A102H2 ) 3 \Si04=(A102H2 ) 3

In diabantite R12 = Mg7Fe5 , and in thuringite one-third of the 

aluminum is replaced by ferric iron. Hence the following compositions :

Si02 .. .................................................

A1208 .. ................................ ..................

FeaCV............... ..................................

FeO... .................................................

MgO. ..................................................

HaO... .................................................

Diabantite.

34.93

13.19

23.29

18.11

10.48

100. 00

Thuringite.

22.78

19.37

10.13

36.33

11. 39

100. 00

The talc-chlorite of Traversella is another intermediate compound 

simply formulated as shown below. In the actual mineral one-fifth 

of the dyad portion is Fe.

,Si04=(ROH)2H

Al Si04=RH

SiOa ... ........................................................ 39.00

A1203 . ......................................................... 11. 04

FeO........................................................... 12.47

MgO....................................."..................... 27.75

HoO... ........................................................ 9.74

100. 00

Stilpnomelane has a truly chloritic formula, but is distinct from 

the others in being a trisilicate. This is probably true also of the 

very uncertain eknianite, which has a most variable composition. 

The two formulae may be written thus: - .

Stilpnomelane. Ekmanite. 

xSi3 08==(FeOH)2H ,SiO4=(FeOH)3

Al Si308=(FeOH)2H Al Si04=(FeOH)3

\SiO4=(FeOH)3
64 THE CONSTITUTION OF THE NATURAL SILICATES.

The theoretical composition follows:

SiOa .. .................................................

A1203 ----------------------------. ---------------------

FeO. ...................................................

H20............ ........................................

Stilpnomelane.

48/91

4.62

39.13

7.34

100.00

Ekmanite.

40.91

3.86

49.09

6.14

100. 00

Epichlorite appears to be a mixture of stilpnomelane with the equivalent magnesian orthosilicate, Al(Si04 ) 3 (MgOH) 6H3 , in nearly equal 

percentages. The recently described minguetite* is apparently a 

mixture of stilpnomelane and a lepidomelane.

So far all the chlorites conform to the mica type of formula as 

represented by biotite and phlogopite. In general they may be 

regarded as salts of an alumosilicic acid, Al(Si04 ) 3H9 , and its corresponding trisilicate, in which the nine hydrogen atoms are replaceable 

by a variety of basic radicles. The apparent complexity of the 

chlorites vanishes and the relations between them become clear and 

simple.

There is, however, a group of chlorites of distinct character from 

the normal series. Like margarite they model after kaolin in the 

following manner:

Kaolin. Strigovite. Aphrosiderite.

/OH 

Al Si04=H3 Al Si04=(Fe"OH)3

\Si04EEEAl

Al Si04=(Fe"OH)H2 

\3i04=Fe'"

Daphnite. 

/OH

Al Si04=(Fe"OH)3 

\Si04=(A10H)H

Sheridanite.2

/OH

Al Si04=(MgOH) 3 

X\Si04=(A10H)H

The theoretical composition of the four chlorites is therefore as 

follows:

SiOo..........................

A1203 - ........................

Fe203 . ........................

FeO

MgO................ ........".,

H20...........................

Strigovite.

33.61

14.17

22.22

20.00

10. 00

100. 00

Aphrosiderite.

25.32

21.52

45.57

7.59

100. 00

Daphnite.

24.40

20.73

43.90

10.97

100.00

Sheridanite.

30.30

25.76

30.30

13.64

100.00

1 Lacroix, Soc. min. Bull., vol. 33, p. 270,1910.

2 See Wolfi, J. E., Am. Jour. Sci., 4th ser., vol. 34, p. 475,1912.
THE SILICATES OF ALUMINUM. 65

Strigovite is the least satisfactory of these species, for the reason 

that the analyses are discordant. The formula given it in Dana may 

be written A10H(Si04) 2K.(B,OH) 2H, which, however, is of the same 

general type as that employed here.

Several other chloritic minerals have received names, such as 

chamosite, klementite, euralite, pycnochlorite, epiphanite, hullite, 

and chlorophseite, but they are not very well characterized. The 

analyses can all be interpreted in harmony with the other chlorites, 

but it is hardly worth while to discuss them in detail until the several 

minerals shall have been more thoroughly studied. Enough has been 

done to show that the micas, clintonites, vermiculites, and chlorites 

form one systematic group of minerals, and all the valid evidence is 

satisfied. The facts that garnet and vesuvianite alter into chlorites 

and that chloritic pseudomorphs after feldspar are known serve to 

connect still more closely the formulae here adopted with the similar 

formulae of the preceding groups of minerals.

When clinochlore or leuchtenbergite is strongly ignited, it yields, 

like xanthophyllite, a product insoluble in hydrochloric acid, having 

the composition of spinel. This reaction establishes still more definitely the relationship between the chlorites and the clintonite group, 

and it is readily intelligible in the light of the structural expressions. 

The splitting up, under influence of heat, of mixtures containing such 

groups of atoms as MgOH, AlOH, and A102H2 ought to generate 

spinel, and the appearance of a compound of this character is evidence 

in favor of the formulae.

An interpretation of the chlorites proposed by Tschermak has had 

some acceptance, but the scheme is complicated and subject to very 

serious objections. According to Tschermak the "orthochlorites," 

which include pennine, clinochlore, prochlorite, and corundophilite, 

are molecular or crystalline mixtures of serpentine, H4Mg3Si209 , and 

an uncertain substance, amesite, H4Mg2Al2Si09 , with the equivalent 

molecules containing iron. Now serpentine, on strong calcination, 

breaks up into water, olivine, and enstatite, the enstatite being 

insoluble in acids. But the magnesian chlorites, which, if Tschermak's theory were true, should yield about 18 per cent of enstatite on 

ignition, yield none at all.' Spinel is formed instead of enstatite, and 

in quantities proportional to the excesses of oxygen over the orthosilicate ratio. That is, serpentine molecules are not present in the 

chlorites, and the Tschermak hypothesis breaks down.

THE ALUMINOUS BOBOSILICATES.

In this group of minerals, of which tourmaline is the most important, there are five species, namely, tourmaline, axinite, dumortierite, serendibite, and manandonite. There are also several

43633° Bull. 588 14 5
66 THE CONSTITUTION OF THE NATURAL SILICATES.

borosilicates of the rare earths, which may properly be studied with 

them even though they contain no aluminum.

Although tourmaline in its several varieties is apparently quite 

complex, the evidence for its interpretation is abundant and ample. 

Its variations in composition are shown by numerous good analyses, 

its associations are well known, and its alteration products have been 

observed in a sufficient number of examples. From the minerals 

which have been discussed in the preceding chapters it differs essentially in that it contains boron, and the part played by this element 

is a new question to be interpreted.

When tourmaline undergoes alteration, the commonest product is a 

mica, and between the micas and the tourmalines there are very striking analogies. With the lithia micas, lithia tourmalines are generally 

associated; with muscovite and biotite, iron tourmalines occur; and 

magnesian tourmalines accompany phlogopite. In each case the 

composition of the tourmaline seems to bear a relation to that of the 

associated mica. Furthermore, the varieties of tourmaline shade 

one into another through an unbroken series of gradations, and this 

may happen to some extent in one and the same crystal. The genus 

tourmaline, in short, represents a series of compounds, and these are 

parallel to the normal mica series.

Upon the constitution of tourmaline there have been many essays 

written and much controversy. It is not necessary to discuss here 

the literature of the subject in detail, for only two types of formula 

are now seriously considered, and they differ principally in regard to 

the ratio between silicon and oxgyen. Penfield and Foote * represent 

the mineral as a salt of the acid HuAl3B2Si4 021 , which may be written 

structurally thus:

\Si04 =H2 

A1-B0=H

i04 =H2

In this formula the boron appears as the radicle of an acid H4B205 , 

which is a rational compound. So far as its atomic ratios are concerned, the formula fits most of the published tourmaline analyses 

very well, although not all of them, but it does not suggest the wellknown alterability of the mineral into mica. It is also difficult to 

apply in detail, that is, to apportion the several bases replacing 

hydrogen in any symmetrical manner, so as to indicate clearly the

i Am. Jour. Sci., 4th ser., vol. 7, p. 97, 1899. Criticized by Clarke, idem, vol. 8, p. 111,4899. Keply by 

Penfield, idem, vol. 10, p. 19, 1900.
THE SILICATES OF ALUMINUM. 67

end members of the series of compounds which are commingled in 

tourmaline. This difficulty has been avoided in part by later writers, 

notably by Schaller1 and Reiner,2 who triple the Penfield-Foote formula, 

so making the ultimate tourmaline acid H60Si12B6 063 . From this both 

Schaller and Reiner derive a number of distinct compounds, which, 

as mixed crystals, make up tourmaline. The tripled formula, however, is unwieldy, and not easy to represent by a simple and presumably stable series of structures. This objection, is entitled to some 

weight but is not necessarily fatal.

In the Penfield-Foote type of formula the silicon-boron-oxygen 

ratio is represented by Si4B2 021 , which is equivalent to Si6B3 031.5 . In 

the tourmaline formulae proposed in the former edition of this memoir 

the nucleus Si6B3 031 appears, with an oxygen ratio slightly lower than 

that of Penfield and Foote. In general the Penfield-Foote formula 

fits the actual analyses a little better than mine, but the difference is 

very slight and suggests a possible constant error. Heretofore all 

analyses of tourmaline have been made upon finely ground material, 

and fine grinding is accompanied by more or less adsorption of water 

from the air. It also leads to some oxidation of ferrous iron, a change 

which takes place very easily in tourmaline, and these two almost 

certain sources of error tend to raise the oxygen ratio in the minerals 

as analyzed. These sources of error were not known when the published analyses were made and must now be taken into account.

Now, the foregoing considerations being kept in mind, and also the 

alterability of tourmaline into mica, the following formula for the 

fundamental tourmaline acid seems to be probable:

Al Si04=H3 

\Si04=Al B02

Al BO3=H2

I ,Si04=Al BO2

Al Si04=H8

This involves the lowest observed ratio between aluminum and 

silicon, and also the constant ratio between silicon and boron. The 

boron is shown as partly metaborate and partly orthoborate, which 

may be regarded as improbable. As an alternative the boron may 

be represented as the quinquivalent group B307 , the radicle of a possii Zeitschr. Kryst. Min., vol. 51, p. 321, 1913.

* Inaugural Dissertation, Heidelberg, 1913. Reiner-gives a good summary of earlier work on the subject. 

His formujse are those of Wiilflng.
68 THE CONSTITUTION OP THE KA.TUBAL SILICATES.

ble acid H5B307 , a rational compound. But many tourmalines contain fluorine, and some analyses show a deficiency of boron, which is 

not always ascribable to analytical error. Fluorine in such tourmalines may perhaps replace the B02 group, a supposition for which 

there are good arguments, although it may not be absolutely proved. 

To this point reference will be made later.

In the proposed tourmaline acid various replacements of hydrogen 

are possible, by means of which the individual tourmalines, as mixed 

crystals, can be quite accurately formulated. At one end of the 

series, with the group Al5Si6, we find some magnesia and iron tourmalines, at the other end, approaching the.ratio Al9Sie, are the colored 

lithia tourmalines. Of the lithia tourmalines the extreme member 

known is the rubellite from Elba, which approximates in composition 

to -the following mixture of molecules:

/Si04=H3 ,Si04=H3

Al Si04==Al Al Si04=Al Al Si04=Al

N\Si04=Al B02 \Si04=Al B02 \Si04=Al B02

I I I

3A1 B03=Na2 + 4A1 BO3=Na2 + 10 Al BO3=Li2

,Si04=Al B02 ySiO4==Al BO2 xSi04=Al BO2

Al Si0=Al Al Si04=Al Al Si04=Al

\Si04=Al ^i04 Al

This may be compared with Schaller's analysis of the mineral 1 

thus:

Si02 - ..................................................

B2 03 . .................................................

A12 03 . .................................................

Ti O

FeO

MnO..... ..............................................

CaO............. ........................................

Na^O. . ................................................

Li20..... ..............................................

H20.......... ..........................................

F.....................................................

Found.

37.89

10.28

43.85

.04

.11

.11

.07

2.43

1.66

S.47

.10

100. 01

Calculated.

38.08

11.06

43.98

2.69

1.85

2.34

100. 00

The differences here are mainly due to the water and to the small 

neglected impurities. The water found is probably too high, because

i All the analyses of tourmaline cited here were made in the laboratory of the United States Geological 

Survey.
THE SILICATES OF ALUMINUM. 69

of fine grinding. The comparison is also influenced by the fact that 

the mixed molecules are not exactly represented by such simple 

numbers as are assumed in this computation. The latter consideration applies throughout the discussion.

At the other end of the series we have the magnesian tourmaline 

from Pierrepont, New York, as analyzed by Riggs. The mixed 

molecules are nearly as follows:

xSiO4=FeH 

Al Si04=FeH 

\Si(X=Al BO,

xSi04=MgH

Al Si04=MgH Al Si04=MgH 

\Si0^ 44 =Al BO,^^£ 2 ^SiO^Al B0 4 2&

3A1 BO,=NaH + 2'Al BO,=NaH + 5 Al BO,=Ca

/Si04=Al B02 

Al SiO4==FeH

Si04=Al B02 

Al SiO4=MgH 

\Si04=MgH

/Si04=Al B02 

Al Si04=MgH 

N\Si04=MgH

SiOa ................................................... Tin BA................... .................................

A1Al 2O U3 . .................................................

FeA... ...............................................

PeO. ...................................................

MgO. ..................................................

CaO.. .................................................

NaaO..................................................

K20....................................................

H20....................................................

F......................................................:

Found.

35.61

.55

10.15

25.29

.44

8.19

11.07

3.31

1.51

.20

3.34

.27

99.93

Calculated.

35.91

10.48

25.44

8.62

11.17

2.79

1.55

4.04

100. 00

An interesting intermediate tourmaline is the black variety from 

Lost Valley, California, analyzed by Schaller. Its formula is that 

of a mixed crystal, as follows:

xSi04=FeH 

<-& . Al SiO,=FeH

\Si04=Al B02

2 Al B03=Na2

,SiO4=Al B02 

Al SiO4==Al

)4=A1 B02

- 2 Al B03=H2

I xSi04=Al B02

Al SiO4=FeH

xSii

Al Si04==MgH 

\Si04=Al B02

- 1 Al B03=H2

Si04=Al BO2 

Al Si04==MgH
70 THE CONSTITUTION OF THE NATURAL SILICATES.

Si03 .. .................................................

B2 03 ..................:.....:.......:.............:.....

Al O

TiaOa.............. ......................................

FeO. ...................................................

MnO....... ...............:.....................;......

CaO.... ...............................................

MgO.................. .................................

Na^jO. .................................................

H20............. .......................................

Found.

qc afi

10.61

33.28

.36

11.04

.13

.42

3 48

2.16

3.31

100. 75

Calculated.

oc 07

10.42

33 43

11.43

3.18

2.46

3.21

100. 00

It is not necessary to give more examples of tourmaline formulae, 

for all tourmalines are represented by the type of structure proposed 

here. The alkali tourmalines tend toward the more aluminous end 

of the series, the magnesian and iron tourmalines toward the less 

aluminous. All the tourmalines now known are evidently mixed 

crystals, but the complexity of the mixtures is more apparent than 

real. Whether any tourmalines contain ferric iron is still uncertain, 

but the probabilities are adverse to its presence. It appears in many 

analyses, but that is probably due to oxidation, so that the minerals 

analyzed were not quite normal. Ferric tourmalines are theoretically 

possible, but their existence is unproved. The iron of tourmaline is 

at least predominatingly ferrous.

The formulaB for tourmaline adopted here not only express the 

composition of the mineral but also indicate its obvious relation to the 

micas- and its ready alterability into them. A molecule of tourmaline, 

with elimination of boric acid and one atom of aluminum, splits into 

two molecules of the mica type, and the transformation is easily 

understood. Potash is of course taken up. Certain experiments by 

Lemberg, 1 who investigated the action of alkaline solutions upon 

tourmaline, are in accord with these suppositions.

Although otherwise interpreted by Brogger, the minerals cappelinite, melanocerite, karyocerite, and tritomite seem to be chemically 

akin to tourmaline. This view of their nature has already been suggested by Wiik,2 and it is sustained both by chemical and by morphological considerations. Cappelinite is hexagonal, and the other 

species, like tourmaline, are rhombohedral. They are silicates of rare 

earths, which are mostly trivalent, like aluminum; all contain boron, 

and all but cappelinite contain fluorine also. Furthermore, all four 

species, considered together, illustrate the reciprocity between boric 

acid and fluorine, which has been suggested in the discussion of 

tourmaline. Thus, if we compute the atomic ratios from the analyses 

cited by Brogger,3 the following relation appears:

1 Deutsche geol. Gesell. Zeitschr., 1892, p. 239.

2 Zeitschr. Kryst. Min., vol. 23, pp. 421, 422,1894.

a Zeitschr. Kryst. Min., vol. 16, pp. 462-469., 1890.
THE SILICATES OF ALUMINUM. 71

Cappelinite ...........................

Melanocerite. .................;........

Karyocerite. ..........................

Tritomite. .............................

Si.

236

218

216

226

B.

488

92

134

210

F.

304

296

226

B+F.

488

396

430

436

That is, Si : B -f F : : 1 : 2 nearly, variations being due to the fact 

that in the first three minerals the boric acid was determined by 

difference, and also, probably, to the occasional replacement of fluorine 

by hydroxyl. Another source of variation is found in the presence of 

tetrad bases, as will be seen later, but for the moment the relation 

indicated seems to be reasonably clear.

The first number of the group, cappelinite, is a borosilicate of 

yttrium and barium and approximates in composition to

/BO, B02 

Y BO2 +4Y B02

With the earths of uncertain molecular weight, designated as"yttria," 

are a little lanthanum oxide and trifling quantities of Th02 and Ce02 , 

and with the barium are some calcium and alkalies.

The other three members of the group are all more complicated than 

cappelinite, and vary from it in type by containing tetrad oxides, such 

as Ce02 , Th02 , and Zr02 . In eudialyte and catapleiite we have two 

rhombohedral silicates of zirconia, which help to explain these compounds. Catapleiite probably has the constitution (OH) 3Zr.Si308 .R'3 . 

If we regard the tetrad bases in the cappelinite group as forming 

orthosilicates of this same type, the remaincfer of each mineral may 

be written as a mixture of molecules like those already designated, 

but with cerium earths predominating over yttrium, and fluorine 

replacing some boric radicles. Thus, melanocerite is not far from

/OH 

/.OH

< OH + 2R'" BO,=Oa + 7R'" F

Karyocerite may be written similarly, and tritomite becomes 

)H

AV OOH ^ iv \

\SiO4=ECaH xSiO4==H2 .R'' 'F2

These formulae are uncertain and need verification with material from 

other sources. At present they have only a reasonable probability. 

The tetrad silicate in them, however, will be seen to be highly probable when we come to the discussion of the other allied compounds 

in their proper connection later.
72 THE CONSTITUTION OP THE NATURAL SILICATES.

 The second aluminous borosilicate, axinite, is difficult to interpret 

structurally, for the reason that its relations to other species and its 

modes of alteration have not been definitely traced. It is, however, 

easily formulated empirically and is well represented as a varying 

mixture of two molecules:

. Ferroaxinite..................................... HFeCa2Al2BSi4016

Manganaxinite................................... HMnCa2Al2BSi4016

A little magnesia is also shown in the analyses of axinite, which points 

to the probable existence of a magnesium salt exactly equivalent to 

the others. A distinctly magnesian axinite, however, has not yet 

been found.1

The formulae given above seem to make axinite an orthosilicate, 

provided that the boron is regarded as basic and equivalent to 

aluminum. But boron is distinctly an acid-forming element, and 

therefore it is more probable that in axinite it has acid functions. 

On this supposition its structural formula may be best written as a 

mixed orthosilicate and trisilicate, thus:

B03=A1 OH 

Al Si04 =

Si308=

in which R"3 = (Fe, Kn^ Ca2 , the calcium being constant. Other 

structures are possible, but no one seems to have any advantage 

over this. Until further evidence is discovered the proposed expression may be regarded as valid, but it represents only the composition 

of the mineral, and nothing more. It is, however, in conformity with 

the general theory of substitution. 

Dumortierite is much more easily interpreted than axinite. It is

related to andalusite, with which it is often associated, and it alters 

into muscovite. Its composition has been determined by the careful 

analyses of Ford 2 and Schaller, 8 and is simply represented by a significant constitutional formula. By slightly modifying Schaller's 

formula the following comparison is obtained:

Andalusite. Muscovite. Dumortierite. 

,Si04=(A10) 3 ^Si04=KH2 ,Si04==A10.BOH

 Al Si04=Al Al Si04=Al Al Si04=(AlO) 3

\Si04=Al \Si04=(A10) 3

The bivalent group =B OH is evidently equivalent to the corresponding aluminum group. Schaller prefers to write it as two radicles, H and B=0, and his form may perhaps be better.

» For a full discussion of the composition of axinite see Schaller, U. S. Geol. Survey Bull. 490, p. 37, 1911. 

See also the former edition of this memoir, U. S. Geol. Survey Bull. 125,1895. 

8 Am. Jour. Sci., 4th ser., vol 14, p. 426,1902. 

"U. S. Geol. Survey Bull. 262, p. 91,1905.
THE SILICATES OF ALUMINUM. 73

The mineral manandonite, recently described by Lacroix,1 is also 

easily formulated. As written by Lacroix, in close agreement with 

the analysis, its empirical formula is H24Li4Al14B4Si8063, a highly 

basic compound. Regarding the boron as present in the group 

B4O7 , the well-known radicle of borax, manandonite may be given 

the following structure, having some analogy to that of tourmaline:

,Si04=(A102H2 ) 2H 

Al Si04=(AlO2H2 ) 3 

\Si04==Li2

Al B407 H

Ao,=u2

Al-Si04=(A102H2 ) 3 

X-SiO4=(AlO2H2 ) 3

This formula needs to be confirmed by more analyses, and an investigation as to the relations of manandonite to other species.

One other borosilicate, serendibite, has been described by Prior 

and Coomaraswamy, 2 who assign to it the empirical formula 

10R0.5Al203 .B203 .6Si02 , in which RO is partly CaO aind partly MgO, 

with a little FeO. In composition it has some analogy to tourmaline, 

and it also resembles the ultrabasic silicate sapphirme. The latter 

mineral is akin to the brittle micas, in which the univalent group

Mg

appears,. and its formula may be

0 A1=O 

Al 0 Al=0

\Si04=E(A102Mg) 3

although that is not quite certain. With these clues the formula of 

serendibite may be written thus :

Si04EE|

/ Mg3 

Al SiO4= )

\Si04=(A102Ca) 2 

A1-B205=(A10) 3

/Si04=(A102Ca) 2 

Al Si04= | Mg3

i Soc. Min. Bull., vol. 35, p. 223, 1912. 2 Mineralog. Mag., vol. 13, p. 224, 1903.
74 THE CONSTITUTION OF THE NATURAL SILICATES.

which fairly satisfies the known conditions. This formula, however, 

like some of the others, can only be regarded as tentative.

Although it contains no alumina, danburite, CaB2Si208 , is perhaps 

most conveniently considered here. It is sometimes regarded as the 

equivalent of barsowite, CaAl2Si208 , a doubtful isomer of anorthite. 

But barsowite gelatinizes with hydrochloric acid, whereas danburite 

is attacked by the reagent only after ignition. Possibly, the difference may be as follows :

/Si03 

Ca Ca<

XSiO B=0

danburite being a metasilicate. Other formulae are also possible, and 

more data are needed before any conclusion can be reached. The associations of danburite with feldspars, mica, pyroxene, etc., suggest that 

it may be a pseudometasilicate, allied in structure to the aluminous 

constituent of augite.

MISCELLANEOUS SPECIES.

Among the orthosilicates and trisilicates of aluminum, ferric iron, 

and other triad elements, there are a considerable number which do 

not fall conveniently into any of the preceding groups of minerals, or 

which are doubtful as regards their genetic affinities. Some of them 

have obvious relationships to other species, and some are quite 

obscure in character, but all seem to be conformable to the theory of 

substitution.

First in order of importance is the mineral staurolite, a highly basic 

silicate, which is evidently akin to andalusite and siUimanite, and 

which, like them, is orthorhombic. Like andalusite, furthermore, 

staurolite alters to muscovite, an entire crystal becoming transformed throughout into an aggregate of mica scales.

By far the best. evidence as to the composition of staurolite is that 

furnished by the analyses of Penfield and Pratt,1 who adopt Groth's 

formula, HAl5Fe//Si2013 . This, structurally, may be written

/O H

Al Si04=(AlO) 2 

\ >Fe

Si04=(A10) 2

which expresses a partial relation to the micas, andalusite, and so on. 

The theoretical percentage composition calculated from this formula 

agrees well with the results of analysis, except that it gives the silica 

nearly 1 per cent too low, a discrepancy which Penfield and Pratt 

attribute to inclusions of silica in the minerals analyzed.

By means of a slightly different formula the relations of staurolite 

to the other species can be much more- clearly shown, but it assumes

1 Am. Jour. Sci., 3d ser., vol. 47, p. 81, 1894.
THE SILICATES OF ALUMINUM. 75

that the ideal staurolite is not yet known. The expressions proposed 

are as follows:

Andalusite. Staurolite. 

,Si04=(A10) 3 ySi04=(A10) 3 

Al SiO4=Al Al Si04=(A10) 3

\Si04=Fe

. xSi04=Fe 

Al SiO4=(A10) 3 

\3i04=(A10) 3

This formula, in contrast with that of Penfield and Pratt, and with 

their reduced analysis of staurolite from Lisbon, New Hampshire, 

gives the following percentage composition:

SiOa ........................................

AljO...... ............ ......................

Feb........................................ H2o... .....................:..............

Lisbon.

27.44

55.16

15.72

1.68

100. 00

Penfield 

and Pratt.

26.32

55.92

15.79

1.97

100. 00

New 

formula.

27.90

55.35

16.75

100. 00

If, now, we assume that the actual staurolite is slightly altered by 

hydration, some Fe being replaced by H2 and by FeOH + H, the discrepancies between formula and analyses are sufficiently accounted 

for. The new formula is more symmetrical than the old one; it better 

expresses the alterability of staurolite into muscovite; and it seems to 

satisfy the evidence with sufficient completeness. When we remember that staurolite is excessively liable to inclusions and alterations, 

a very sharp agreement between analysis and theory is not to be 

expected.

Still another orthorhombic species, harstigite, has a formula analogous in some ways to that of staurolite. For harstigite there is' but 

one analysis extant, which gives nearly

,Si04=CaH 

Al Si04=CaH

Ca

xSi04= 

Al Si04==CaH
76 THE CONSTITUTION OF THE NATUKAL SILICATES.

This, iii comparison with Flink's analysis, gives the following percentage composition:

SiO,. ..................................................

A1203 .. ................................................

MnO... .......... ........................................

MgO...... ............................................

CaO.. ......................... '............... :........

K20.............. ......................................

Na20.. ................................................

H20... ...... ..........................................

Found.

38.94

10.61

12.81

3.27

29.23

.35

.71

3.97

99.89

Calculated.

39.13

11.09

} 15. 43

30.44

3.91

100. 00

This result is fairly satisfactory. More data relative to harstigite 

are evidently needed.

Two closely related silicates, the calcic lawsonite and the manganous carpholite, may perhaps be analogous in structure tostaurolite 

and harstigite. For both minerals the simplest empirical formula is 

H4R/'Al2Si 2O10 , which, tripled, can assume the following form:

Lawsonite.

/Si04=(A102H2 ) 8 

Al Si04==H3

xSiO4 

Al Si04=H

Carpholite.

,Si04=(A102H2 )s 

Al-Si04=H3 

\Si04=Mn

Ca Mn

ySM

Al-Si04=H3 

\SiO4=Al

Lawsonite is orthorhombic. The isometric hibschite seems to have 

the same composition, but is too incompletely known to be satisfactorily discussed here.

An interesting pair of ultrabasic silicates is furnished by the species 

kornerupine (or prismatine) and grandidierite. Both are orthorhombic; kornerupine alters into kryptotile, and grandidierite yields 

a similar and perhaps. identical derivative. To kornerupine the 

empirical formula Al2MgSi06 has been commonly assigned, which, 

tripled, may be written:

Al Si04=Al
THE SILICATES OF ALUMINUM. 77

in close analogy to andalusite and kryptotile. The recent investigation of prismatine by Uhlig, 1 however, shows that the mineral.is 

more complex and in better agreement with the formula

NaH3Mg6Al12Si7O40 .

Grandidierite,2 described by Lacroix in 1902, has the empirical 

formula R"9 (A1, Fe) 22Si 7056 , with aluminum largely predominant 

over iron. R" is mainly magnesium, but with .somereplacement by 

iron, calcium, and a little sodium. That is, it is a mixed crystalline 

mineral, but of fairly definite type. Both species, with the empirical 

formulas now given them, are expressible by analogous structures, 

thus 

Kornerupine.

//Si04=Al 

Al Si04=(A102Mg)s

Al Si04==Al

Grandidierite.

ySiO^CAlO.E),

Al Si04=(A10) 3 

V04 =(A10)2 

Al Si04=(AlO2R) 3

\Si04 =(A10)2 

Al Si04=(A10) 3

\Si04 =(A102R) 3 

The nacreous or micaceous mineral batavite 3 is perhaps of similar 

type, although Weinschenk has assigned it the relatively simple 

formula, 4H20.4MgO.Al203 .4Si02 . Its composition, however, is 

equally well expressed as follows:

Al-Si04=(A102Mg)

Al Si04=MgH

Al Si04=MgH + 4H20

\Si04=Mg 

Al-Si04=MgH

\Si04==MgH

Batavite, like many other minerals, needs further investigation, 

as the following comparison between analysis and formulae clearly 

shows:

SiOa......................................

AI n

MgO..................... .................

H20.- .....................................

Found.

42.33 

16.35 

28. 17 

13.19

100. 04

Calculated.

Weinschenk.

41.81 

17.77 

27.88 

12.54

100. 00

New 

formula1;

43.30 

15.78 

28.86 

12.06

100.00

i Zeitschr. Kryst. Min., vol. 47, p. 215,1910. a Zeitschr. Kryst. Min., vol. 28, p. 160,1897. 

a Soc. min. Bull., vol. 25, p. 85,1902; vol. 27, p. 259,1904.
78 THE CONSTITUTION OF THE NATURAL SILICATES.

A still more unusual type of silicate is presented by didymolite, a 

mineral recently described by Meister, 1 who assigns it the formula 

2Ca0.3Al203 .9Si02 . Constitutionally this seems to be a basic derivative of a trisilicic anorthite, with the following structure:

,Si308E=A10(A102Ca) 2 

Al Si308=Al

To bityite, a mineral from the tourmaline region of Madagascar, 

described by Lacroix,2 may be assigned the empirical formula 

H7Li2GlCa3Al9Si6034 .

This leads to a structural formula analogous to that of tourmaline, which suggests that bityite may be an intermediate compound 

between tourmaline and cookeite.

Al Si04=LiR// 

\Si04=R"

Al OH

Al Si04==LiR"

\Si04EEE(AlO) 3

The comparison between analysis and formula is as follows:

sio2 ........................ ................. .........;

AI n '

CaO...... .............................................

MgO... ................................................

G1O.......... ...'..... ..................................

Li20...... .............................................

Na20......... .........................................

K20....... .....:.... ...................................

H20........................... .........................

, Found.

31.95

41.75

14.30

.13

2.27

2.73

.40

.16

6.50

100. 19

. Calculated.

32. 58

41.54

1 15. 20

2.26

V 2. 72

I

5.70

100. 00

The water found is probably a little too high, due either to alteration 

or perhaps to fine grinding. Tourmaline, manandonite, serendibite, 

and bityite seem to be closely related species.

The vanadiosilicate or arseniosUicate, ardennite, is a compound of 

quite different type from any so far considered. It is a mixed silicate

Neues Jahrb., 1912, vol. 1, ref. 403. 2 Soc. min. Bull., vol. 31, p. 241,1908.
THE SILICATES OF ALUMINUM. 79

in which sometimes the vanadic radicle predominates and sometimes 

the arsenical group, but the structure of the molecule is the same in 

both varieties. According to Prandtl* its empirical formula is 

H6Mn5Al5 (As,V)Si5 028 , which can be written structurally as follows:

ySiO4=Mn(AlO2H2 ) 

Al SiO4==MnH

Nsi04=Al As04=Mn 

Al Si04=MnH

NxSi04=Mn(AlO2H2 )

The vanadic ardennite is strictly isomorphous with the arsenical 

compound and in it V replaces As.

Among the silicates of aluminum, salts of orthodisilicic acid are 

very rare. The only one which seems to be thoroughly defined is 

iolite, which agrees best with the formula

//Si207 .AlMg 

Al Si207 .AlMg

\Si207 .(A10H)2 

Al Si207 .AlMg

\Si207 .AlFe

/ 

which requires the following percentage composition:

SiOo......................................................... 49.26

A1203 ........................................................ 33. 50

MgO......................................................... 9.85

FeO.......................................................... 5.91

H,0.......................................................... 1.48

100. 00

in close concordance with the best recorded analyses. By alteration 

iolite passes into mica, going through an intermediate stage, however, 

known as chlorophyllite. This substance may be regarded as formed 

by hydration, in which the linking group of Si207 in iolite is split into 

two orthosilicic radicles, yielding two molecules of the type

xSi04==H2 .AlO2H2 

Al Si207=MMg

from which the final transition into a mica is easy. ' If we take E,ammelsberg's analysis of chlorophyllite, recalculate the ferric oxide into

i Zeitschr. Kryst. Min., vol. 40, p. 392, 1905.
80 THE CONSTITUTION OF THE NATURAL SILICATES.

alumina and lime into magnesia, reducing afterward to 100 per cent, 

we get the following comparison between observed fact and the composition of chlorophyllite computed from the foregoing formula:

Si02 . .....................................

AiA-.....'....... ................ . r .......

FeA......... .............................

MgO....... ...............................

CaO................. ........................

H20...... ...................................

Found.

46. 31

25.17

10.99

10.91

.58

6.70

100. 66

\

\

Reduced.

47.99

33.34

11.74

6.93

ioo. oo

Calculated.

48 3Q

32.90

12.90

5.81

100. 00

The agreement is as close as could be reasonably expected. The 

replacement of a little magnesia by a little water in the original alteration product accounts for the discrepancies.

Two other magnesian alumosilicates, of rather uncertain character, 

may possibly be related to iolite; namely, the lasallite of G. Friedel 1 

and the pilolite of Heddle. 2 Both minerals are highly hydrous and 

are fairly represented by the subjoined formulae:

Lasallite.

Al Si207 .H5

\Si207 .Mg2 +7H2O 

Al-Si207 -H5

Pilolite. 

/Si207 .MgH3

Al-Si207 .H5 

^>Si207 .Mg2 +7H20

Al-Si207 .H5 

NsiA-MgH,

The exact hydration is doubtful but not far from that shown in the 

formulae. There are a number of other questionable minerals 

recorded,3 of generally similar composition, but their consideration 

in detail would hardly be profitable. Some of them are probably 

mixtures of magnesian silicates with clays.

Two more aluminous minerals are probably to be classed as orthodisilicates. One barylite, is near

xSi207 .A!Ba 

Al Si207 .AlBa

\Si207 .AlBa

« Soc. min. Bull., vol. 24, p. 12,1901; idem, vol. 30, p. 80,1907.

2 Min. Mag., vol. 2, p. 206,1878.

8 See Dana's System of mineralogy, 6th ed., pp. 705-711,1892.
THE SILICATES OF ALUMINUM. 81

which requires the following percentage composition:

I

j Si02 ......................................................... 35.19

( A1A-------------------------------------------------------- 19.94

( BaO.......,............................:.................... 44.87

100.00

The other silicate, sphenoclase, is approximately Al2 (Si207 ) 3Ca6, which 

may be analogous to barylite in structure or written as a calcium 

salt similar in type to okenite. Both barylite and sphenocLase, however, are uncertain, and their relations are not definitely known.

Well-defined metasilicates of aluminum, or alumometasilicates, 

seem to be few in number. It has already been shown that many of 

the species classed as metasilicates are really mixed salts, trisilicates 

and orthosilicates being commingled.

With beryl, however, Gl3Al2Si6018 , the evidence in favor of a metasilicate structure is fairly good, although the composition can also 

be expressed as that of a basic trisilicate. There are thus two 

alternatives,

I /si3o8X

Gl and Gl< >G1 XSi0

(A10),

But beryl alters into mica, a fact which is favorable to the first of 

these formulae, and all of its commoner alterations seem to take place 

by replacement of glucinum. In the trisilicate formula the alumina 

should be equally replaceable, and so far the evidence is adverse to it. 

Furthermore, Traube 1 has effected the synthesis of beryl by precipitating a mixture of glucinum and aluminum sulphates with a solution 

of sodium metasilicate, and then crystallizing by fusion of the precipitate with boron trioxide. Since the starting point was a metasilicate, 

there is a fair presumption that the product was a metasilicate also. 

Beryl can be written as a pseudometasilicate, but there are no data to 

justify doing so.

The mineral astrolite, described by Reinisch 2 may perhaps be a 

metasilicate. If so its formula is (Na,K)Fe"(AlFe'") (Si03 ) 5 .H2O, but 

the species needs further study. Spodumene and jadeite also have 

apparently metasilicate ratios, but on mineralogic grounds it is best

1 Neues Jahrb., 1884, vol. 1, p. 275. 2 Centralbl. Mineralogie, 1904, p. 108. 

43633° Bull 588 14 6
82 THE CONSTITUTION OF THE NATURAL SILICATES.

to defer their consideration and to take them up in connection with 

the pyroxenes and amphiboles.

The alumosilicates derived from metadisilici'c acid, H2Si2O5, are 

few in number. Two of them, ptilolite and mordenite, have already 

been discussed as zeolites; two others, petalite and milarite, demand 

attention now. The formula of petalite, empirically, is AlLi(Si2 05 ) 2 , 

similar to the metasilicate formula for spodumene, and the two 

species are commonly associated. Petalite, however, has far the 

lower density of the two and is therefore presumably composed of 

smaller molecules. An alteration product of petalite, hydrocastorite, 

approximates roughly

// AW

\SiA-H

which requires the following percentage composition:

Si02 . ........................................................ 60. 60

ALfl3 . ....................................................... 25.75

H,0.....:.. ................................................. 13.65

100. 00

The actual hydrocastorite contains about 4.3 per cent of lime, and 

is doubtless impure. By Doelter * petalite is interpreted somewhat 

differently, being given the empirical formula Al2Li2Si10 024 , but the 

formula here adopted is the one most generally received.

Milarite, HKCa2Al2 (Si2O6 ) 8 , is, like beryl, hexagonal, and its formula 

is analogously to be written

Al-Si205

Ca

/SiA,s!o 

Al Si205 H 

^SiA K

For this species the only evidence is that of its composition. Its, 

genesis and its possible alterations are unknown.

The clays are peculiarly difficult to interpret constitutionally. 

One .member of the group, kaolin, has already been considered, and 

this member is peculiar in being crystalline. The other clays are 

amorphous and of uncertain origin; they often occur in complicated 

mixtures, are difficult to identify with certainty, and still more difficult

i Min. pet. Mitt., 1878, p. 529.
THE SILICATES OF ALUMINUM. 83

to correlate with other species. They undoubtedly represent the 

breaking down of crystalline silicates, to which they are related 

somewhat as kaolin is related to the feldspars, but rarely, if ever, has 

their actual genesis been observed. Furthermore, the integrity of 

some of the clay silicates is in question. According to Stremme 1 

several of them are merely mixtures of colloidal alumina and colloidal 

silica, whereas Thugutt 2 regards them as definite compounds. The 

controversy can probably be settled only by careful thermal investigation as to the character of the hydration, a line of attack which has 

been followed to some extent by Le Chatelier,3 although not yet to a 

finality. The formulae proposed here are therefore to be regarded as 

merely tentative and as a first step toward the better study of the 

several species.

Upon comparing the formula of aluminum orthosilicate. with that 

of kaolin an indication of serial arrangement becomes evident, which 

may be written thus :

Normal salt. ' Westanite or woerthite.

Al Si04=Al Al Si04=Ai

Kaolin. Montmorillonite. Newtonite.

, , . OH

Al Si04=H3 Al Si04=H3 Al OH

Woerthite is an altered sillimanite, and westanite is perhaps a 

similar derivative of andalusite. The newtonite compound has already appeared in the mica series among the components of cookeite 

and rumpfite.

The best that can be said for these formulae is that they are suggestive. In one respect they are highly questionable, for the reason 

that the group Si04=H3 is indicative of loosely combined water, 

whereas in these particular clays the water is quite firmly retained. 

On this point much fuller information is needed, and future evidence 

may prove that the serial relation indicated is apparent only.

The composition of rectorite may be Al3 (Si04 ) 3H3 + 2H20, or that of 

a hydrous kryptotile. Halloysite has the composition of kaolin 

plus one molecule of water, the latter being removable at or about 

100°. Halloysite, however, differs from kaolin in being decomposable 

by hydrochloric acid, and hence it is unlikely that the two species

1 Centralbl. Mineralogie, 1911, p. 205. 3 Zeitschr. physikal. Chemie, vol. 1, p. 396, 1887.

2 Idem, 1911, p. 97; 1912, p. 35.
84 THE CONSTITUTION OF THE NATURAL SILICATES.

have similar structure. Allophane is perhaps (A102H2 ) 2H2Si04 4- 3H20 

or it may be written analogously to andalusite,

Al3 (Si04 ) 3 (A102H2 ) 3 .12H20.

Neither formula is sustained by .any good evidence. Other clays 

are possibly as follows :

Samoite ........................................ . Al4 (Si04)3 .10H20

Cimolite. ........................................ . Al4(Si308 )3 .6H20

Collyrite. ......................................... (A10)4Si04 .6H20

Schrotterite. .................................... .(A102H2 )4Si04 .6H20

Melite... .......... ............................. 1 .(A102H2)4Si04 .4H20

One other member of this group, termierite, has the empirical formula Al2Si6015 .18H2 0. It may be a metadisilicate, but until the 

exact character of its hydration is determined its constitution must 

r.emain in doubt.

None of these formulae can be construed as anything more than a 

temporary suggestion, which may help research. The hydrous ferric 

silicates are, if anything, less satisfactory than the aluminum salts. 

Anthosiderite is representable by the formula Fe4 (Si3 08 ) 3 .2H20, and 

chloropal by the expression Fe2 (SiO4 ) 3H6 . Nontronite, according to 

Weinschenk,1 is the ferric equivalent of kaolin, H4Fe2Si2O9 , but 

Bergeat,2 who has studied the mineral as a derivative of kaolinite, 

assigns it -the more complex formula H8Fe4Sia028 . Mullerite is probably a metasilicate, Fe2 (Si03 ) 3 .2H20. Hisingerite seems to range 

from a ferric kaolin to a ferric halloysite, and further than this it is 

not worth while to go. The remaining iron clays which have received 

specific names are altogether doubtful. The chromium clays, wolchonskoite, alexandrolite, and others, are also of very uncertain 

character.

Several silicates of the rare earths may be properly mentioned 

here on account of their analogy to the alumosilicates.

To cerite, which is of doubtful composition, the provisional 

formula

Ce Si04=CeO.H2

may be assigned. Other earth metals lanthanum, the two didymiums, and others replace a considerable part of the cerium. 

With cerium only the formula requires the following percentage 

composition :

Si02 . ....................-.......:............................ 20. 22

Ce203 ............... v ..................'....................... 73. 71

H90 .................'...................... .......-...'.......... 6. 07

100. 00

Zeitschr. Kryst. Min., vol. 28, p. 150,1897. > Centralbl. Mineralogie, 1909, p. 161.
THE SILICATES OF ALUMINUM. 85

Beckelite is 'perhaps similar to cerite in structure, having the 

probable formula

,Si04=CeO.Ca

Ce Si04=CeO.Ca

\Si04=CeO.Ca

with lanthanum, neodymium, praseodymium, and other elements 

partly replacing cerium.

The yttrium silicate, thalenite, is possibly represented by the simple 

formula Y=Si04 K/, in which K/ is partly H and partly Na or K. 

The ideally pure mineral, however, is yet to be found. Another 

silicate of yttrium and calcium, hellandite, is of quite different type, 

and agrees well with the formula

/SiO4=(YO) 3 

Si0=H

Ca<

\=

Another yttrium mineral,, cei^osite or kainosite, appears to be an 

orthodisilicate containing a carbonic radicle, thus:

/C03 H 

Y Si2O7 .CaH3

The recently described thortveitite * is much simpler, being the 

normal orthodisilicate of scandium,

Sc2Si207

Britholite and erikite are phosphatosilicates, but the published 

analyses' correspond to no simple formulae. All. these species need 

fuller investigation.

Several salts of triad bases may be noted here as having more 

analogy to the compounds of aluminum than to any other silicates. 

Melanotekite, for instance, is a silicate of lead and ferric iron, which, 

according to Warren, 1 has the empirical formula Pb4Fe3Si3015 . Structurally this becomes

^Si04=(FeO)Pb

Fe Si04=(FeO)Pb

\Si04=(FeO)Pb

Kentrolite is probably similar, but with Mn'" replacing Fe"'.

i See Schertelig, Centralbl. Mineralogie, 1911, p. 721. 2 Am. Jour. Sci., 4th sen, vol. 0, p. 116, 1898.
86 THE CONSTITUTION OF THE NATUBAL SILICATES.

Glauconite, T?eK(SiQ<i) 2 .riH.20, is a hydrous silicate which is rarely 

found in even approximate purity; The formula, therefore, merely 

represents the best evidence w,e have as to the constitution of the 

mineral. It may be a pseudometasilicate and equivalent to a hydrated 

potassium acmite, a supposition which would seem to be capable of 

experimental verification. Some iron is commonly replaced by 

aluminum and some potassium by other basic radicles. Glauconite 

is of marine origin, but celadonite, formed by alteration in certain 

volcanic rocks, is probably the same compound.

Pseudobrookite is an orthotitanate of iron, Fe4 (Ti04 ) 3 , and arizonite 

is the metatitanate, Fe2 (Ti03 ) 3 . There are also the bismuth silicates, 

eulytite and agricolite, Bi4 (Si04 ) 3 , which differ in form but are 

identical in empirical composition. They therefore suggest two types 

of chemical structure among the silicates of trivalent bases.
I 

/ CHAPTER IV.

SILICATES OF DYAD BASES. 

ORTHOSILICATES.

Although, the orthosilicates of the dyad metals are presumably simpler than those of aluminum, the problem of their constitution, studied 

in the light of mineralogic evidence, is peculiarly difficult. Starting 

points exist, in the salts of magnesium, iron, manganese, zinc, and 

glucinum, but the derivatives are fewer than in the case of aluminum, 

and the evidence upon which to base argument is correspondingly 

limited.

Expressed in the simplest terms, the normal orthosilicates of this 

 group are represented by the general formula R2Si04 . To this type 

the following minerals correspond:

Porsterite..................................................Mg2Si04

Fayalite................................................... Fe2Si04

Tephroite.................................................Mn2Si04

WiUemite...................................................Zn2Si04

Phenakite................................................. Gl2Si04

Monticellite................................................CaMgSi04

Knebelite.................................................MnFeSi04

Glaucochroite.............................................CaMnSi04

Between these minerals there are many intermediate species or 

varieties, which may be either isomorphous mixtures or double salts 

representing polymers of the fundamental type. Thus, chrysolite or 

olivine* may be a mixture of forsterite and fayalite, or, in the case of 

hyalosiderite, a salt of the formula Mg4Fe2 (Si04 )3 . So also, allied to 

knebelite, we have igelstromite, Fe4Mn2 (Si04) 3 , and in trimerite we 

find the salt Gl3Mn2Ca(Si04 ) 3 .

A close study of the derivatives of these normal salts shows that the 

assumption of polymerization seems to be necessary. If the theory 

of substitution is valid, then the existence of polymers must be taken 

for granted, and it thus becomes possible to develop a system of 

formulse which satisfies all the conditions imposed by the evidence 

now at hand. For some of the species already mentioned the degree 

of polymerization is difficult to determine, and synthetic investigations seem to be needed. In other cases the problem is comparatively 

simple, and the indications as to the true formulae are apparently clear.

»Tschermak regards olivine as a basic metasillcate, but that interpretation is difficult to reconcile with 

an the evidence.

87
88 THE CONSTITUTION OF THE NATURAL SILICATES.

For instance, a good example is furnished by the chondrodite group, 

for which we have the empirical formulae established by Penfield and 

Howe. 1 Structurally written these become

Clinohumite. Humite. Chondrodite.

^o.< .."*-->*

Mg<

 - ' ~ (MgF) 2

(MgF)2

or derivatives respectively of the salts Mg8 (Si04 ) 4 , Mg6 (Si04 ) 3 , and 

Mg4 (Si04 ) 2 , with one atom of magnesium in each case replaced by the 

two univalent Mg F groups. Prolectite is a fourth member of 

the series, described by Sjogren,2 which, on crystallographic grounds, 

is supposed to be simply Mg=SiO4=(MgF) 2 , although it has not been 

analyzed. The fluorine in these minerals is usually replaced in part 

by hydroxyl, but the replacement is rarely complete. The mineral 

leucophcenicite 3 is a similar compound, with a formula analogous to 

that of humite, namely, Mn5 (Si04 ) 3 (MnOH) 2 , but containing slight 

replacements of manganese by calcium and zinc. Gageite is also a 

member of the humite group, and its formula can be written

(ROH)

in which R represents Zn, Mg, Mn, in the ratio 1:3:7. It is closely 

related to leucophcenicite. 4

Clinohedrite occurs in association with forsterite, and the two 

species have nearly the same specific gravity. Hence forsterite may 

be Mg8 (Si04 ) 4 , and, this is the only datum available from which to 

infer its molecular magnitude. The synthetic transformation of 

forsterite into clinohumite, if it could be eflFected, would go far toward 

settling the question.

» Am. Jour. Sci., 3d ser., vol. 47, p. 188,1894.

2 Geol. Inst. Upsala Bull., vol. 1, p. 40,1892-93; idem, vol. 2, p. 99,1894-95.

3 See Penfield and Howe, Am. Jour. Sci., 4th ser., vol. 8, p. 351,1899. 

< Phillips, Am. Jour. Sci., 4th ser., vol. 30, p. 283,1910.
SILICATES OP DYAD BASES. 89

In the case of phenakite the triple formula Gle (Si04 ) 3 is rendered 

probable by the existence of trimerite, Gl3Mn2Ca(Si04 ) 3 . It is also 

emphasized by the species helvite and danalite, which contain 

sulphur, probably combined in the dyad group E, S R . 

Both these species, in all their known occurrences/ agree with the 

general formula

Gl

in which R. may be either Fe", Mn, or Zn. The E is variable, but 

the other constituents are constant. In helvite,' manganese and 

iron occur, and in danalite zinc appears. In the Colorado danalite 

zinc predominates largely over iron, and there is very little manganese. The Rockport .danalite has iron in excess of zinc, and rather 

more manganese. The Cornish danalite is veiy low in zinc, and the 

iron largely exceeds the manganese. The ratio Gl:Si04 ::3:3, however, holds for all.

If phenakite is Gle (Si04 ) 3 , then willemite, which is morphologically 

similar, is probably Zn6 (Si04 ) 3 , with zinc partly replaced by manganese in the variety known as troostite.

Spurrite,1 empirically 2Ca2Si04 .CaC03 , may possibly be assigned 

a structure similar in type to those used in the preceding groups of 

minerals, thus:

Ca

but with no evidence to go upon other than its composition. To the 

associated hillebrandite the formula Ca2Si04 .H2O has been given, 

but the water in it may be constitutional. In that case hillebrandite 

becomes a basic metasilicate, (CaOH) 2Si03 . Molybdophyllite, 

PbMgSi04 .H20, may be similarly constituted.

The three orthorhombic species, bertrandite, calamine, and ilvaite, 

are most conveniently represented as derivatives of the bipolymer 

R,4 (Si04 ) 2 , like chondrodite. Crystallographically ilvaite resembles

i See Wrlght, F. E., Am. Jour. Sci., 4th ser., vol. 26, p. 547,1908.
90 THE CONSTITUTION OP THE NATURAL SILICATES.

humite, but bertrandite and calamine are related to each other. 

The simplest analogous formulae for the three minerals are as follows:

 Bertrandite Calamine. Ilvaite. 

Gl H2 Ca

/v^ Zn< 4\Zn Fe< 4\Fe

/3i04< N3iO/ \SiO/

W \G1-OH I II

(ZnOH) 2 Al OH

Calamine may also be written as a metasilicate, (ZnOH) 2Si03 , with 

half of the formula indicated above, but then the analogy with bertrandite disappears. The structure proposed is therefore preferable, at least until more evidence has been accumulated. Clinohedrite is equivalent to calamine, but with half the zinc replaced by 

calcium. Zambonini 1 interprets calamine as a basic orthodisilicate. 

Baschieri 2 regards ilvaite also as an orthodisilicate.a It is not easy 

to write constitutional formulae for these minerals and to show their 

relation to other species on that basis. Further investigation is 

evidently needed.

To the datolite group a similar constitution is ascribable. The 

species, reduced to their simplest empirical expressions, are these:

Datolite...........:.................................. .HCaBSi05

Homilite..............................................Ca2FeB2Si2010

Euclase.................................................HGlAlSi05

Gadolinite.............................................Gl2FeY2Si2010

By doubling the formulas of datolite and euclase all four of the 

minerals become similar in constitution. Hydrogen here is evidently basic, and boron must play the same part as aluminum and 

yttrium. Assuming these elements to be present in the univalent 

groups BO, A1O, and YO, the subjoined formulas follow:

Datolite. . Euclase.

H2 H2

Ca< 4\Ca ( 

\SiO/

__________(B0)2__________(A10) 2_____,

i Contribute allo studio dei silicati idrati, 1908. a Zeitschr. Kryst. Min., vol. 49, p. 112,1911.
SILICATES OF DYAD BASES. 91

Homilite. 

Fe

/Si04X4

Ca< >Ca

N3iO/

(BO),

Fe

iO

(Y0)

Gadolinite alters with great ease, passing by hydration into a 

brownish-red earthy substance. The analyses of this substance, 

whicn is probably never pure and definite, are not altogether satisfactory, but they indicate in a general way a transformation into the 

compound

Fe'"OH

iO

(Y02H2 ) 2

to which the alteration product very roughly approximates.

It will be observed that all of the foregoing structural formulae in 

this group of compounds are rings or series of rings. From them, 

however, chainlike molecules are derivable, and these seem to exist 

in friedelite, pyrosmalite, and dioptase. The last-named mineral, 

simply written, is CuH2Si04 , but it is morphologically related to the 

two other species, which have a much greater complexity of composition. The following expressions derived from the polymer R8 (Si04 ) 4 

are probably the best to represent existing evidence :

Mn

Friedelite.

i04=H2 (MnCl)

i04=H2 

i04=HMn

Dioptase.

Pyrosmalite is like friedelite, but a large part of the manganese is 

replaced by iron. Possibly karyopilite may be similar, having the 

formula

i04=H2 (MnOH)

^ TTii2

iO4E=HMn
92 THE CONSTITUTION OF THE NATURAL SILICATES.

These formulae are purely tentative and need additional support. 

By synthetic and genetic investigations they may be supported or 

overthrown. That they sustain one another and fit in well with the. 

formulas of the preceding species is all that can be said in their favor. 

Palache * and Zambonini 2 assign more complex formulae to friedelite. 

The chlorine in that mineral seems to be partly replaced by hydroxyl.

Bementite is still another silicate of similar type. In accordance 

with the latest analysis by Steiger, 3 its formula is simply written:

For serpentine, H4Mg3Si2 09 , several formulae are possible, and concerning them there has been much discussion. 'The species commonly 

originates in nature from the alteration of olivine on the one hand and 

from pyroxene or amphibole on the other, and it is therefore conceivable that it may include two or more isomeric compounds. In favor 

of this supposition there is some evidence but nothing conclusive. 

Massive serpentine, chrysotile, antigorite, and other allied varieties 

differ in their physical properties and suggest the existence of isomers, 

but much more investigation is needed in order to decide whether the 

supposition is true or not. . By some authorities serpentine is regarded as an orthosilicate and 

by others as a salt of the acid H6Si2 07 . On the latter supposition it 

becomes Mg=Si2 07=H2 (MgOH) 2 , which may be derived either from

2Mg2Si04 or 2MgSi03 , with loss of magnesium in one case and gain in 

the other. On the orthosilicate basis it is simply derivable from the 

polymer Mg4 (Si04 ) 2 and is related to the intermediate alteration 

product, villarsite, as follows :

Villarsite. Serpentine

Mg Mg H,

*04X 

Mg< >Mg IV

Mg H \g-OH II Mg OH

1 Am. Jour. Sci., 4th ser., vol. 29, p. 177,1910. 3 See Palache, idem, p. 182.

2 Contribute allo studio dei silicati idrati, 1908.
SILICATES OF DYAD BASES. 93

Oh this scheme the. formula for serpentine corresponds with that of 

chrondrodite; and the fact that the latter mineral alters readily into 

serpentine is strong evidence in its favor. In short, that formula 

best indicates the genetic relationships of serpentine, and on such 

grounds is preferable to the alternative diorthosilicate expression. 

The latter is not disproved; it is simply, rendered less advantageous 

as regards existing evidence, which is the evidence now to be interpreted.

In some former investigations, carried on jointly with Dr. Schneider, 1 I sought to obtain experimental data in support of the orthosilicate formula. here assigned to serpentine. By acting on serpentine with dry gaseous hydrochloric acid we found that a part of 

the magnesium could be removed as chloride, whereas olivine and 

the magnesian micas were not attacked. At first it seemed probable 

that the reaction would give a quantitative measure of the magnesium combined as MgOH; but our later experiments and those of 

Lindner 2 have shown that the expectation was not well founded. 

I still believe, however, that the reaction discriminates between those 

magnesium silicates which contain MgOH and those which contain 

Mg and H combined otherwise, for only the members which must 

belong to the first class are acted upon by the reagent. Brauns's 

objections 3 to this supposition, on the ground that the dry hydrochloric acid becomes moist, are not well taken, for the reaction 

always took place at temperatures lower than those at which water 

is given off. His criticisms may apply to the later stages of the 

reaction, after it has once fairly begun, but not to its initiation. The 

magnesian micas which contain several per cent of water are all 

decomposable by aqueous hydrochloric acid, but are scarcely touched 

by the dry gas; whereas, on the other hand, serpentine and the 

chlorites are strongly attacked. After the gaseous acid has acted it 

becomes moist, but very slowly, and most of the moisture is carried 

past the mineral under investigation before it has had time to produce an appreciable effect. It is possible, however, that a slow 

stream of the acid may act differently from a rapid current, and that 

the discordant' results of observation may be due to differences of 

this kind.

When serpentine is ignited water is expelled, and a residue having 

the composition Mg3Si207 is left behind. According to Kammelsberg 4 the water is given off in two portions one-half upon weak

i U. S. Geol. Survey Bull. 78, p. 11,1891; Bull. 90, p. 11,1892; and Bull. 113, pp. 27 and 34,1893. 

' A. Lindner, Inaugural Dissertation, Breslau, 1893.

3 Neues Jahrb., 1894, vol. 1, p. 205.

4 Handbuch der Mineralchemie, 2d ed., p. 506,1875.
94 THE CONSTITUTION OF THE NATURAL SILICATES.

ignition, the other after heating more strongly. - On the orthosilicate theory these stages may be represented thus :

Serpentine. First stage. Second stage.

At the end of the second stage, if the ignition has not been too 

intense, the residue is still decomposable by hydrochloric acid, but 

by prolonged heating it is broken up quantitatively into soluble 

olivine and insoluble enstatite.

Allied -to serpentine is the somewhat doubtful picrosmine, to which 

the formula Mg2H2Si207 is commonly assigned. Although this expression suggests a diorthosilicate, it may also be written

/SiO

M<sio>g

A

which represents picrosmine as a dehydrated serpentine altered subsequently by rehydration, with replacement of one magnesium atom 

by two of hydrogen. This mode of interpretation brings the mineral 

into line with serpentine, and all the known relations of the species 

are adequately expressed.

Several other hydrous silicates seem to belong in this group, but 

their nature is altogether doubtful. Thus we have

Aphrodite.......................................Mg2H4 (Si04)2

Kerolite.........................................Mg(Si04)2H5 (MgOH)

Nepouite, H4Ni3Si2O9 , seems to be the nickel equivalent of serpentine. Garnierite and noumeite, other .hydrous silicates of nickel, 

are too variable in composition to be definitely classed.

METASILICATES.

Although the metasilicates appear at first sight to be extremely 

simple, they are actually rather difficult to interpret. It is easy 

enough to deduce their empirical formulae and to write them afterwards in structural terms, but this is not sufficient. The structural 

formulae must express all known relations for each species, and in 

attempting to satisfy the established conditions the difficulties begin 

to appear. In the first place, metasilicic acid itself is defectively
SILICATES OF DYAD BASES. ' 95

known, and no ester of the form R2Si03 b#s yet been certainly 

obtained. Troost and Hautefeuille's ester (C2H5 ) 8Si4012 suggests the 

possibility that metasilicic acid, like metaphosphoric acid, may polymerize, but an attempt'to draw general conclusions on so important 

a question from one datum only would be most unwise. The possibility of polymeric acids, however, was clearly shown in the general 

discussion of the silicic acids in Chapter II of this bulletin. It is also 

emphasized by the existence of four distinct modifications of magnesium metasilicate, as proved by Alien, Wright, and Clement. 1 

Whether these modifications represent different metasilicic acids or 

not is yet to be discovered.

Again, as we have repeatedly seen, a mineral may be apparently a 

metasilicate and yet really a mixture of orthosilicates and trisilicates. 

Even a basic triplicate can have seemingly metasilicate ratios. All 

these considerations complicate the identification and study of th.e 

true metasilicates to such an extent that only provisional conclusions 

can be drawn from the data now on hand.

A crystallized silicate of sodium, Na2Si03 .8H20, is well known. A 

solution of this salt added to a solution of calcium chloride precipitates a compound which, dried over sulphuric acid, has, according to 

my own observations, the composition Ca2Si2Oe .5H20. This, minus 

the water, is analogous to the mineral wollastonite, from which another 

mineral, pectolite, is derived. If wollastonite, instead of the formula 

Ca2Si206 be given the formula Ca3Si309 , it may be compared structurally with pectolite, as follows:

It must be remembered that the molecular weights of the inorganic 

silicates are not known but only assumed; and the problem suggested 

by the foregoing expressions is to find a set of structural formulae 

which shall represent all the available evidence. Now, wollastonite 

is commonly classed with the pyroxenes, on crystallographic grounds; 

and so too is pectolite. But both species are very easily decomposed 

by even dilute and weak acids, whereas the normal pyroxenes are 

quite refractory, and, furthermore, wollastonite has a lower density 

than any pyroxene which approaches it in composition. Chemically, 

then, these species are dissimilar, and it is very doubtful whether they 

can properly be grouped together. That wollastonite and pectolite 

are true metasilicates, however, is sustained by the fact that when

i Am. Jour. Sci., 4th ser., vol. 22, p. 385,1906.
96 THE CONSTITUTION OF THE NATURAL SILICATES.

pectolite is ignited one-sixth of the silica, proportional to the hydrogen 

ctf the mineral, is split off in the free state and can be determined quantitatively. That is, two molecules of pectolite are decomposed with 

liberation of metasiricic acid, H2Si03 , which, in turn, divides into 

Si02 + H20. In some varieties of pectolite manganese replaces part 

of the calcium, and schizolite is a mineral of similar composition but 

with different ratios, its formula approximating H3Na3Ca3Mn2 (Si03) 8 . 

Schizolite, however, may be an isomorphous mixture of two silicates 

and not a simple compound. It is in no true sense equivalent to 

pectolite.

Empirically the nonaluminous pyroxenes resemble wollastonite in 

their ratios. Thus we have, according to the commonly accepted 

formulae developed by Tschermak, Doelter, and others:

Enstatite, orthorho.mbic...................................Mg2Si206

Diopside, monoclinic......................................CaMgSi206

Hedenbergite, monoclinic................................. CaFeSi208

Rhodonite, triclinic.......................................Mn2Si206

There is also, a great number of other intermediate species or isomorphous mixtures in the pyroxene series, such as bronzite, hypersthene, 

schefferite, sahlite, jeffersonite, and fowlerite, in which we find, 

variously replacing one another, salts of magnesium, calcium, iron, 

manganese, and zinc. All these minerals, however, conform to the 

general formula RSi03 , or R2Si206, which adequately expresses their 

constitution so far as they alone are concerned. This formula can 

be written structurally 

/Si(V 

R< >R \SiO/

which would be satisfactory if the pyroxene series ended here and if 

the amphiboles were unknown.

Going a step further we find in augite a pyroxene containing 

aluminum and having an oxygen ratio greater than in the group 

Si03 . In place of aluminum ferric iron also occurs, and alkalies are 

sometimes present. Leaving these variations out of account, for 

consideration later, we have in augite, as interpreted by Tschermak, 

together with the normal compound R2Si206 , the basic salt RAl2Si06 . 

This substance, however, is not known by itself, unless it is represented by kornerupine, although the latest analysis of that mineral 

tends to negative the supposition. An artificial silicate of similar 

type, K2Al2Si06 has been prepared by Weyberg,1 and may serve to 

strengthen Tschermak's assumption. This salt constitutionally may 

be regarded as a basic orthosilicate 

> Centralbl. Mineralogie, 1911, p. 326.
SILICATES OF DYAD BASES. 97 

,/M 0 K

SiO/

\A1 0 K

and its magnesian equivalent as

.SiO Mg 

\U O/

but neither expression is to be taken as final. The excess of oxygen 

in the pyroxenes may be due, with equal probability, to the equivalent group

 Mg

which has been assumed in preceding sections of this memoir.

Still another series of silicates containing triad bases and also alkalies are classed with the pyroxenes, as follows:

Spodumene.............................................. AlLiSi206

Jadeite.......................................:.......... AlNaSi 206

Acmite.................................................Fe///NaSi200

and their empirical formulae are fairly satisfactory. Structurally 

these expressions become, as metasilicates,

3 R'

and babingtonite, which contains no alkalies, is similar, thus :

iO3

i03 Fe"

Fe'"CSi03

E,'' being =Ca, Fe'', and Mn. The ferric molecule is evidently 

equivalent to two acmite molecules, with Na2 replaced by a linking 

atom of iron.

So far, except partially in the comparison between wollastonite 

and pectolite, the formulae cited for the pyroxenes express composition and composition only. But spodumene, as shown by the 

elaborate research of Brush and Dana, splits up on alteration into a 

43633° Bull. 588 14 7
98 THE CONSTITUTION OF THE NATURAL SILICATES.

mixture of eucryptite, an orthosalt, and albite, a trisilicate. This 

observation suggests two alternatives: Either that spodumene is 

derived from a polymetasilicic acid, or else that it is a pseudometasilicate, a mixed ortho- and tri-salt, like some of the species which 

have already been explained. An analogy with leucite, for example, 

will at once be inferred, and that species, empirically, is strikingly 

like spodumene, thus:

Leucite....................................................AlKSi206

Spodumene................................................ AlLiSi.j00

Like spodumene, leucite alters into a feldspar and a member of the 

nepheline group, but it differs from spodumene in form and in 

density. The specific gravity of the isometric leucite is 2.5, that of 

the monoclinic spodumene is nearly 3.2, and hence we may reasonably 

infer that spodumene has the larger and more condensed molecule. 

In order to explain the relations of leucite, its empirical formula was 

quadrupled, and in that way a relation with the garnet group was 

brought out. For spodumene, regarding it also as a mixed silicate, 

a sixfold multiplication of its formula indicates its greater density, 

and its splitting up into eucryptite and albite, with partial replacement of lithium by sodium, is representable as follows:

Li2 Li

/Si3O8 Al=Si04X

. Al=Si308 Al< >A1 Si04=Al 

\Si3O8=Al SiO/

TLi L Li T 1 2 ' 

From this grouping of atoms the transition into Al3 (Si308 )3Na3 + 

Al3 (Si04 ) 3Li3 is hardly more than simple cleavage, and the relations 

between the three species are intelligibly expressed. Acmite, which 

yields pseudomorphs of analcite, and jadeite, also, probably follow 

the same rule, the formula of one being typical of the others. The 

ferric molecule in babingtonite should be still another instance of the 

same kind, with Fe"3 in place of Li6 , and Fe'" instead of Al. Possibly pyrophyllite, HAlSi206 , is related to spodumene and jadeite, 

much as kaolinite is related to the feldspars.. Pyrophyllite is probably a pseudometasilicate, for silica is not liberated from it upon 

ignition, at least not to any noteworthy extent.

If the formula just developed for spodumene should be sustained, 

it would seem necessary to adjust the other- pyroxenes with it. 

For Tschermak's aluminous constituent of augite this adjustment is 

easily made by taking the formula Al2MgSi06 six times, as was done
SILICATES OF DYAD BASES. 99

for spodumene. The parallelism between the two species is then 

representable as follows:

. Spodumene................................ Al6 (Si308)3(Si04)3Li6

Aluminum-augite.......................... Alfl(Si04)3(Si04)3 (A102Mg) 6

the univalent A102Mg having been recognized among the micas. 

This formula serves to explain the well-known alterability of augite 

into epidote and into mica, and so far at least is useful. I do not, 

however, feel inclined to put very much stress upon it, for as yet it is 

only an expression of analogy, which may or may not prove to be 

valid. It would seem to require the recognition of all the pyroxenes 

as pseudometasilicates, in which case the normal series, containing 

only dyad bases, would become

R R

/SiA R Si04\

R< >R

N3i808 R SiO/

1 1

where four atoms of R are given linking functions, and the other 

four are, so to speak, replaceably combined. On this basis we 

should write

Diopside.......................-.....:...........Mg4Ca4(Si308) 2(Si04)2

Hedenbergite...................................Fe4Ca4(Si308 ) 2(Si04)2

The formula R//4 (A10) 8 (Si04 ) 2 (Si04 ) 2 would be exactly parallel with 

these, and affords another expression for Tschermak's compound, 

Al2RSi06 . Or the ground of simplicity this is preferable to the more 

complex expression based on the formula of spodumene. If to monticellite we assign the quadrupled formula Mg4Ca4 (Si04 ) 4 , and to forsterite the similar formula Mg8 (Si04 ) 4 , diopside becomes equivalent 

to them, in structure, with one-half the orthosilicic radicle replaced 

by the trisilicic Si308 . The pyroxenes and the olivines thus appear to 

be curiously related compounds, although they are unlike morphologically.

This mode of interpreting the pyroxenes is so remote from our 

usual conceptions that I offer it with great diffidence. It unifies the 

group, however; it expresses the observed alterations of the several 

species; and despite its complexity it will be found to be sustained 

and strengthened by evidence brought out in the study of the 

amphiboles.

This last-named group of highly important minerals resembles the 

pyroxenes in composition and is explained by Tschermak in essentially the same way. Their molecular weights, however, are taken
100 THE CONSTITUTION OF THE NATURAL SILICATES.

as double those of the pyroxenes, for the reason that the atomic 

replacements seem to occur by fourths rather than by halves. This 

point is exemplified by a comparison between diopside and tremolite, 

which, reduced to their simplest empirical formulae, become

Diopside. .......... '. .................................. . .CaMgSi206

Tremolite. ............................................... GaMg3Si4Oi 2

The pyroxenes, however, are somewhat heavier than the amphiboles 

and from their greater density we may suppose them to have the 

larger molecules. Hence the formula of diopside should be a multiple 

of that just cited and presumably greater than Ca2Mg2Si4012 . Upon 

this point the phenomenon of uralitization has definite bearing. In 

this process pyroxene is converted into amphibole, with increase of 

volume and little or no change of composition. In other words, a 

complex molecule has been dissociated into simpler molecules a 

phenomenon the direct opposite of polymerization. In the face of 

this evidence it is difficult to see how the current views as to the 

relative molecular magnitudes of pyroxene and amphibole can be 

maintained. The pyroxenes must form the more complex group 

and the amphiboles the simpler.

In the amphibole .group the orthorhombic anthophyllite is the 

equivalent or isomer of enstatite and hypersthene. Then follows a 

monoclinic series, containing tremolite, actinolite, cummingtonite, 

dannemorite, and other minerals, all represented by the general 

empirical formula RSi03 , with calcium, magnesium, iron, or manganese as the bivalent metal. In griinerite the salt FeSi03 exists by 

itself, and in richterite and astochite alkaline silicates appear. If we 

regard the minerals as pseudometasilicates, having molecular weights 

lower than the pyroxenes and with the bases replaceable by fourths, 

the typical amphiboles are most simply represented by formulae like 

the following: - - 

Anthophyllite. Tremolite. 

Mg Mg

xSi04V /SiCU 

Mg< >Mg Mg< >Mg ) NSLO

Mg Ca 

Richterite becomes a mixture of salts,

R R

/oiw4 \ /Si04 \

R< >R + R< >R

XSLO/ NSLO /

R
SILICATES OF DYAD BASES. 101

commingled in ratios near 1:1, and astochite is similar, but with NaH 

in place of Na2 . Potassium may also partly replace sodium. Another 

alkaline amphibole of doubtful character, waldheimite, approximates

R

LO

N

which is the formula of a trisilicate pure and simple, with R = Ca, Fe, 

Mg. The existence of this compound is strong evidence in favor of 

the pseudometasilicate theory, and, as will be seen later, it does not 

stand alone.

Among the amphiboles, as among the pyroxenes, aluminous and 

ferric compounds are common, and with these the minerals approach 

orthosilicate ratios. Tschermak's interpretation of these ratios is 

practically the same as in the pyroxene series, namely, by the assumption of molecules of the form Al2RSi06 or Al4R2Si2012 . An alternative to this view is offered by Scharizer/ who shows that the hornblendes can be explained as mixtures of actinolite, R4Si4012 , with an 

orthosilicate called syntagmatite (R/2R// ) 3Al2 (Si04 ) 3 , whose ratios are 

similar to those of garnet. An amphibole from Jan Mayen Island 

approaches very nearly to syntagmatite in composition, and so also 

does the Canadian hastingsite. If a compound of this type is present 

in the amphiboles it would explain at once their alterability into 

epidote, micas, and chlorites, but so far as the composition of the 

group is concerned neither Tschermak's view nor Scharizer's is absolutely necessary. The Tschermakian molecule, however, can be 

written either as

A102Mg (A10) 2

Al<f \A1 or as Mg<; >Mg 

N3iO/ NsiO/

A102Mg (A10)2

the latter form resembling that of tremolite, and also connecting the 

group still more closely with the olivines. It is also parallel to the 

last formula suggested for the corresponding pyroxene compound, 

being one-half of that formula and identical with it in type.

No amphibole is yet known which corresponds precisely in constitution to acmite and spodumene. In glaucophane we find a species,

i Neues Jahrb., 1884, vol. 2, p. 143.
102 THE CONSTITUTION OF THE NATURAL SILICATES.

which, as a metasilicate, may be written AlNaSi206 + (MgFe) Si03 , 

and in crocidolite we find another similar salt, Fe'"NaSi206 + FeSi03 . 

Crocidolite alters easily, and one of the products of alteration, which 

has been named griqualandite, is very near Fe'"HSi2 06 , the equivalent of acmite in general type. This last compound can be written

H

^Si04X 

Fe<f >F<> NSLO/

and so adjusted as an amphibole-hydrogen-acmite to the remainder of 

the group, but glaucophane and crocidolite are best formulated as 

follows:

Glaucophane. Crocidolite.

(A10) 2 (Fe'"0) a

/S^OgX /Sl3 08\

Mg< >Mg Fe< >Fe 

/- XSi0

which makes them, as trisilicates, precisely equivalent in structure to 

the normal amphiboles. These compounds, and their corresponding 

orthosilicates, commingled with salts like tremolite or actinolite, give 

mixtures which conform in composition to the aluminous hornblendes. 

Rhodusite, which is allied to glaucophane, appears to be a mixed 

silicate of the following constitution:

Na2 (Fe"'0)2

2 Fe< 3 8\Mg + 1 Mg<

XSi3 0/ ^i3

Ma: ' ' Ms

In rhodusite the ratio of Si to 0 is distinctly less than 1 to 3, and 

the same is true of crossite, which may be written as a mixture of the 

two molecules

R (Fe'"0)2

+ 1 

i.0/ xSi,O,

with some Al in place of Fe, and R being = Fe, Mg, Ca.
SILICATES OF DYAD BASES. 103

Arfvedsonite, in which R" is mainly Fe", is represented quite 

closely as a mixture of this order:

Fe Ca Fe

/Si( /SiCU /Si308X

Fe< >Fe + 4 Fe< >Fe + 6 Fe< >Fe

xSiO/ N3iO/ XSi3O/

II ' II 1

(AID), Na2 Na2

Hastingsite, soretite, and phihpstadite may all be formulated in a 

similar way, except that they tend toward the orthosilicate end of 

the series. In philipstadite Si3 08 and Si04 appear in equal proportions; in the two other amphiboles Si04 is largely in excess. Gastaldite and riebeckite are similar species, but the analyses are not 

perfectly conclusive. It is, perhaps, necessary to assume the presence 

in these minerals of acmite-like molecules, riebeckite being empirically 

near 2 Fe'"NaSi2Oe + FeSiO3 . The formula of barkevikite is also 

uncertain.

Several minerals belonging to the group of amphiboles are characterized by the presence in them of noteworthy quantities of titanium. If this is assumed to represent an orthotitanate, the formulse 

all reduce to expressions exactly similar to those given in the preceding 

pages. For example, the type mineral of this group is senigmatite, 

to which the following formula may be assigned :

Fe Fe Fe

/Ti04\ /Si04X /Si308X 

1 Fe< >Fe + 1 Fe< >Fe + 2 Fe< >Fe 

\TiO/

in which R' is f Na and f (AIJJV'OO.

Cossyrite is a similar mixture of molecules, and very near

10 Fe3 (Ti04) 2Na2 

7 Fe3 (Si3O8) 2 (Fe'//0) 2 

10 Fe3 (Si308) 2Na2

with the usual variations due to the replacement of Fe by Ca, Mg, 

etc., of Na by K, etc.
10.4 THE CONSTITUTION OF THE NATUBAL SILICATES.

The other titaniferous amphiboles are also evidently variable 

mixtures, which, in the best analyses, are representable as follows:

Linosite. Kxrsutite. 

1 Ca2Fe(Ti04) 2Na2 2 CaFe2 (Ti04) 2Na2

1 Ca2Mg(Si04) 2 (Fe'"0) 2 2 Ca2Mg(Si04) 2H(A10) ^

2 Mg3 (Si308) 2 (A10) 2 3 Mg3 (Si308) 2 (A10) 2

Anaphorite. . Rhonite. [

1 R3 (Ti04) 2Na2 2 Mg2Ca(Ti04) 2 (RO) 2

3 R3 (Si308) 2Na2 5 Mg2Ca(Si04) 2 (RO) 2

1 R3 (Si308) 2 (Fe'"0) 2 5 Fe2Ca(Si04) 2 (RO) 2

In anaphorite R = Ca, Fe, Mg, in the ratio 1 : 2 : 5; and in rhonite 

RO is about § A10 to J Fe'"0. Rhonite represents the orthosilicate 

end of the series; anaphorite shows the nearest approach to trisilicate 

ratios. It is evident that an indefinite number of similar molecular 

mixtures are possible, but all are likely to be of the same general type. 

A careful study of the best analyses in the pyroxene and amphibole 

groups will strengthen very materially the view here developed that 

the species are not true metasilicates. Although in most of the 

analyses the approximation to metasilicate ratios is very close, there 

are distinct variations toward orthosilicates on one side and toward 

trisilicates on the other, and it is only by assuming that we have 

mixed silicates to deal with that all the anomalies can be made to 

disappear.1 On this theory, if we represent Si308 , Si04 , and Ti04 

groups indiscriminately by the general symbol X, all the amphiboles 

are covered by the following typical symbols, in which R" stands for 

any dyad metal, and R' for K, Na, H, A10, or Fe'"0:

R"4X2

R " "Y" T?' 

3 ^^2 2

R"2X2R'4

or, structurally,

R" R" R'2

/-^\ / -K.\ /^\ /£.' S^\ x

T?'Y J.V \ \R" / * *' R'Y * *' x / " >R" " R"/ \ /*M

\v/ \ir/ \"v/ \A/ 1±- ^J\r

In a similar way all the pyroxenes, except the acmite-spodumene 

group, which has the special formula discussed previously, may be 

represented as formed by mixtures of

R"4X4R"4

i This view of the constitution of the pyroxenes and amphiboles was first advanced by G. F. Becker, 

Am. Jour. Sci., 3d ser., vol. 38, p. 154, 1889. - .
SILICATES OF DYAD BASES. 105

which is in accordance with the' theory developed by Tschermak except 

as to the molecular magnitude of the compounds that is, the 

pyroxenes are essentially bipolymers of the amphiboles, and the 

character of the structure is the same for both groups. The analogy 

between these formulae and those of the olivines has already been 

pointed out, and it is emphasized by still more evidence. Pseudomorphs of pyroxene (fassaite) after monticellite have been found at 

Monzoni and are well known. . Furthermore, Becke x has described 

pseudomorphs of anthophyllite and actinolite after olivine, so that a 

connection between the two groups is clearly indicated. The tracing 

of this connection in a more general way would seem to offer a profitable field for investigation.

Many amphiboles contain water and some contain fluorine. These 

constituents are easily accounted for, being present either as univalent 

radicles like A102H2 and A1F2 , or with H replacing Na. Alien and 

Clement, 2 however, in their study of tremolite found that the mineral 

contained water which could be expelled continuously and therefore 

behaved as if it were not constitutional but in "solid solution." A 

study of their analyses leads to some doubt as to this conclusion. The 

water as determined is needed to completely satisfy the silica, and 

so to give rational formulae to the different samples of tremolite 

which they studied. Additional investigation is plainly needed here.

Another view of the amphiboles has been developed by Penfield 

and Stanley,3 who assume the presence in them of such bivalent 

groups as A12OF2 , A120(OH) 2 , A1203R"; and Ala04R//Na2) and also 

the univalent group MgF. With the aid of such assumptions they 

are able to formulate all the amphiboles as metasilicates. Such an 

interpretation of these minerals is evidently more complicated than 

the formulation adopted here, and it does not provide for the varieties 

in which the ratio Si to 0 is below the metasilicate requirements.

By the hydration of pyroxene or amphibole either serpentine or talc 

may be generated. . Talc has the composition H2Mg3Si4012 , and may 

be written structurally like amphibole either '

Mg<( >Mg or Mg< ;>Mg

Mg

Both expressions are in accord with the fact recorded by Schneider 

and myself,4 that upon the ignition of talc one-fourth of the silica is

1 Min. pet. Mitt., vol. 4, pp. 355, 450, 1881-82.

2 Am. Jour. Sci., 4th ser., vol. 26, p. 101,1908. 

» Am. Jour. Sci., 4th ser., vol. 23, p. 23,1907. 

« TJ. S. Geol. Survey Bull. 78, p. 13,1891.
106 THE CONSTITUTION OF THE NATUBAL SILICATES.

set free quantitatively. This would give the ignited residue the composition shown by the subjoined alternative formulae:

. M

Mg< >Mg ' and Mg\ >Mg 

' \SLO/.

and of these the first would seem to accord the better with the remarkable stability and insolubility of the material. A metasilicate formula, H2Mg3 (Si03) 4 , is also admissible, and accords equally well with 

the evidence concerning talc. The pseudometasilicate expression, 

however, seems to be preferable in view of what is known as to the 

genesis of the species.

The nickel silicate, alipite, H2Ni2 (Si03) 3 , also has metasilicate ratios, 

but there is no further evidence as to its constitution.

Chrysocolla is probably a metasilicate, and perhaps empirically 

CuSi03 .2H20. It can not be well regarded as impure dioptase, for that 

mineral gelatinizes with hydrochloric acid, whereas chrysocolla does 

not. The species, which may be a mixture of compounds, needs 

careful investigation- The same is true of plancheite, to which 

Lacroix 1 assigns the complex formula H2Cu7 (CuOH) 8 (SiO3) 12 . Too 

little is known of this species to admit of any more definite formulation.

The lead silicate alamosite, PbSiO3 , is analogous to wollastonite 

and is therefore, in all probability, a true metasilicate. Agnolite, 

H2Mn3 (Si03 ) 4 .H20, resembles talc in its ratios, and may also be 

classed here.

Leucophanite, NaCaGlFSi2O6', is another definite species which is 

easily figured thus :

/Si03 Gl F /Si03 Ca F

Ca< or Gl<

\Si0 Na \SiO Na 3

but between the two alternatives there is no way of deciding.

The mineral hillebrandite was interpreted by its discoverer, 

Wright, 2 as an orthosilicate of calcium, Ca2Si04.H20. The water, 

however, is not given off at low temperatures, and is probably constitutional. If so, the species is a basic metasilicate,

/Ca OH 

Si03<

X2a OH

1 Compt. Rend., vol. 146, p. 722, 1908; Soc. min. Bull., vol. 31, p. 250, 1908.

2 Am. Jour. Sci., 4th ser., vol. 26, p. 551, 1908.
SILICATES OF DYAD'BASES. 107

This conclusion is borne out by the fact that the mineral gives an 

immediate and deep rose color when its powder is moistened with 

phenolphthalein solution. That reaction is suggestive of the alkaline 

group CaOH, and a new fiuosilicate, custerite, recently discovered by 

Umpleby and Schaller, of the Geological Survey, gives the same 

coloration. Custerite is represented by the formula

/Ca F ' 

SiO/

X!a OH

which is analogous to that proposed for hillebrandite.

Several other species are possibly metasilicates, although the evidence is not sufficient to warrant a definite conclusion. They are 

empirically 

Weinbergerite...........................NaFe"3AlSi4013

Spodiophyllite........................... (Na,K) 2(Mg,Fe)3(Al,Fe)2Si8024

Taramellite..............................Ba4Pe//Fe///4Sii0031

Pholidolite, a hydrous magnesian silicate, containing some potassium 

and aluminum also, seems to admit of no simple and satisfactory 

formula.

DISILICATES AND TRI SILICATES.

Although the existence of the sexbasic acid HeSi207 has been well 

established by the preparation of its esters, its metallic salts are little 

known and uncertain. I have already shown (see discussion of 

serpentine, p. 92) that a mineral may be apparently an orthodisilicate 

and yet be equally well explainable otherwise; and what is true for 

that mineral may be true for others. For the following species the 

orthodisilicate formulae seem to be the best and simplest, even though 

they are not wholly free from objection. They fit existing evidence 

but are not absolutely conclusive.

The typical member of this group of minerals is the hexagonal lead 

silicate, barysilite, Pb3Si207 . The artificial compound from the slags 

of Bonneterre, Missouri, described by Dana and Penfield, is near this 

in composition, and may be.Pb2CaSi207 . Hardystonite, Ca2ZnSi207 , 

is also typical. The pseudodeweylite of Zambonini,1 which has the 

formula Mg3Si2O7 .3H2O, is another compound of this type. The 

formulae assigned to these species are merely their simplest empirical 

expressions. The true molecular weights are not known nor is there 

any available evidence to show whether or no the formulae should be 

doubled or tripled.

Two other lead-calcium silicates appear to belong here. Nasonite, 

described by Penfield and Warren,2 contains chlorine, and ganomalite

i Contribute allo studio del silicati idrati, p. 88,1908. 

a Am. Jour. Sci., 4th ser., vol. 8, p. 346,1899.
108 THE CONSTITUTION OF THE NATUKAL SILICATES.

is regarded by them as the corresponding hydroxyl compound. The 

formulae are simply written as follows:

Nasonite. Ganomalite. 

ySi207 .Ca2 (PbCl) __ /Si207 .Ca2 (PbOH)

>Si207 .Pb2 

Pb< 7

\Si207 .Ca2 (PbCl) \Si207 .Ca2 (PbOH)

The univalent groups Pb Cl and Pb OH are similar to other 

groups which have been assumed elsewhere.

The group of zeolitic calcium silicates, okenite, gyrolite, and apophyllite, are unquestionably related to one another and are best 

represented as salts of H6Si207 . In nature gyrolite may be derived 

from apophyllite and apophyllite also from gyrolite, and Doelter has 

generated appphyllite from okenite by artificial means.1 First, by 

heating okenite with potassium silicate and water to 200°, crystals of 

apophyllite were obtained. Secondly, by heating okenite with 

aluminum chloride, sodium carbonate, and carbonated water at 220°, 

apophyllite, analcite, and chabazite were produced. The most satisfactory general formulae for the three species are these:

Okenite. Gyrolite. Apophyllite.

i207 .H5 /Si207 .CaH3 xSi2O7 .H4 (CaOH)

- Ca< Ca<

i207 .CaH2 ^>Si207 .H4 . )>Si2O7 .H4

Ca<^ Ca<^

Si207 .H5 \Si207 .CaH3 \Si207 .H4 (CaOH)

In apophyllite fluorine may partly replace hydroxyl, and K may 

replace the univalent CaOH. With K : CaOH :: 1 :1 the composition 

of apophyllite becomes

Si02 .....-.-................................................. 52.03

CaO................. ............................:.......... 24.27

K20................ ........................................ 6.79

HoO............ ........................................... 16.91

100. 00

The uncertain mineral plombierite may be a fourth member of this 

group, with the formula

i207 .Ca2 +9H2 0 

i2O7 .CaH3

' Neues Jahrb., 1890, vol. 1, p. 118.
SILICATES OF DYAD BASES. 109

To the calcium-manganese silicate, inesite, various formulas are 

assignable. By Flink it is regarded as 2(CaMn)Si03 .H20. But part 

of the water is stable at temperatures above 300°, and this fact is 

expressed by Schneider's formula (CaMn)Si308 (MnOH) 2 .H20. Both 

formulae agree with the analyses approximately, but the analysis by 

Lundell is better represented by the following mixture:

i207 .GaH3 xSi207 .MnH3

>Si2O7 .Ca2 + 2H20, and Si2 07 .Mn2 +2H3O

Ca< Mn

\Si207 .CaH3 \Si2O7 .MnH3

which requires the following percentage composition:

Si02 . ..................................................

PbO..................................................

MnO...................................................

MgO... ................................................

CaO ...................................................

H20. ...................................................

Found, 

Lundell.

42.92

.73

36.31

.37

8.68

10.48

99.49

Calculated.

42.18

| 37. 44

| 9.84

10.54

100. 00

To inesite from Mexico, which contains less water than is shown 

above, Farrington 1 assigns the formula H2 (Mn, Ca) 6Si6019 .3H20. 

This formula is difficult to represent constitutionally and does not fit 

the analysis as sharply as is to be desired.2

A similar structure probably belongs to the magnesian spadaite, 

as follows:

/Si207 .MgH3 

Mg<(

;>Si207 .MgH2 

Mg<(

\Si207 .MgH3 .

Another magnesian silicate; saponite, is perhaps normally H4 (MgOH) 2 

Si207, although the analyses all show admixtures of some aluminous 

compound.

To cuspidine, a calcium fluosilicate, Zambonini 3 assigns the subjoined formula:

/Ca F 

Ca2Si207<

\Ca-F

1 Field Columbian Mus. Bull., Geol. ser., vol. 1, p. 221, 1900.

2 See also Zambonini, loc. cit.

a Mineralogia vesuviana, p. 273, 1910.
110 THE CONSTITUTION OF THE NATURAL SILICATES.

An analysis of cuspidine from Franklin, New Jersey, by Warren leads 

to the same formula, although Palache * interprets it differently. 

Zambonini, following Groth, regards bertrandite as similar, and 

assigns to it the formula

Gl2Si2O7 .(G10H)2 .

In accordance with Hillebrand's analysis,2 the rare mineral rowlandite should have an orthodisilicate formula as follows:

Y F

which expresses the composition of the species very closely.

Minerals corresponding to metadisilicic acid, H2Si2O5 , are rare. A 

few of them have already been mentioned among the alumosilicates, 

but only two belong here. Bivaite, described by Zambonini,3 is a 

metadisilicate of calcium and sodium and is probably to be figured

>iO5 Na

which suggests a relationship with pectolite.

The rare mineral hyalotekite, which contains boron, agrees very 

closely with the empirical formula R4BFSi6017 , if we regard the water 

in it as hydroxyl replacing fluorine. This can be interpreted either 

as a metadisilicate or as a trisilicate, with the following alternative 

formulas:

R B02

or

i2 05 R F 0=B

Its association with feldspar and schefferite rather favors the trisilicate formula, but the two are empirically identical. If we reduce

i Am. Jour. Sci., 4th ser., vol. 24, p. 185,1907 3 Appendice alia mineralogia vesuviana, p. 16. 

a U. S. Geol. Survey Bull. 113, p. 45,1893.
SILICATES OF DYAD BASES. Ill

Lindstrom's analysis to 100 per cent, after calculating the water 

(ignition) into its equivalent of fluorine, rejecting as impurities the 

traces of A1203 and Fe2 03 , and consolidating like bases, we get the 

following comparison with theory:

Si02 .................. ....................

B203 ... ....................................

PbO... ....................................

CuO................. .....................

MnO. .....................................

G10.......................................

Na20:.................. ..................

CaO......................................

BaO......................................

MgO...... ..................................

K20.......................................

Al O

Fe208 ............................... ........

F.?..3....... ...............................

Cl.......... ...............................

Ignition. ..................................

Less 0 .....................................

Found.

39.47

3.73

25.11

.09

.29

.75

.17

7.82

20.08

.09

.89

.18

.06

.99

.06

.59

100. 37

Reduced.

38. 60

3.63

1 25. 71

|

}

1

J 9.43

1 21. 32

I

2.26

100. 95 

.95

100. 00

Calculated.

38.10

3.71

26.22

9.22

21.59

2.01

100. 85

.85

100. 00

In computing, R" has been regarded^ as Ca : Ba : Pb : : 7 : 6 : 5; 

that is, hyalotekite is a mixture of isomorphous calcium, barium, and 

lead salts in the indicated ratio. The agreement between analysis 

and theory is as close as could be reasonably expected.

Salts of the octobasic orthotrisilicic acid H8Si3O10 seem to be few in 

number, at least so far as present evidence goes. Two of them are 

silicates of nickel, namely,

Connarite. . .......................................

Genthite. .......................................... Mg2Ni2Si3010 .6H20

Deweylite, according to Zambonini,1 is equivalent to genthite, its 

formula being MgSi3 010 .5-6H2O. He shows that the so-called deweylite really represents two species, one having the formula just given, 

the other, pseudodeweylite, being an orthodisilicate. Pseudodeweylite has already been considered.

The complex fluosilicate, meliphanite, is also probably an orthotrisilicate, although other formulae have been proposed for it. Zeophyli Contribute allo studio dei silicati idrati, p. 88, 1908.
112 THE CONSTITUTION" OF THE NATURAL SILICATES.

lite, which is in some respects allied to apophyllite, belongs in this 

class, and the two minerals appear to be similar in structure, thus:

Meliphanite. ' Zeophyllite. 

,0 Na /O H

I /O Ca F 

i< +H20 \0 Ca F

VH

Trisilicates of the form H4Si3 08 are numerous in the mineral 

kingdom and are especially represented by the alkali feldspars and 

their derivatives. They are common in isomorphous admixture with 

orthosilicates, forming the minerals which I have classed as pseudometasilicates, and only two remain to be noted here. These two 

silicates, eudidymite and epididymite, have the same empirical formula, HNaGlSi308 , which, doubled, becomes

Na2

a structure conformable to the type of several orthosilicates. The 

isomerism between eudidymite and epididymite is explainable by 

giving one species the constitution just written, whereas the other, in 

place of Na2 and H2 , would have the two groups NaH. It can also 

be ascribed to a different linking with the oxygen of the acid, and the 

empirical formula HNaGlSi308 can be put in two forms, thus : 

 0=Si O Na Si 0

A >

i< >G1 and Si

f MX I \0-Na

0=Si O H O=Si O H

a third isomer being also conceivable. The doubled formula, however, 

brings out analogies with bertrandite and other species and therefore, 

in default of evidence, is to be preferred.
CHAPTER V.

SILICATES OF TETRAD BASES, TITANOSILICATES, AND 

COLUMBO SILICATES.

On account of their relatively small number and general scarcity, 

the silicates containing the tetrad metals, tinanium, zirconium, and 

thorium, are difficult to interpret in any satisfactory manner. Evidence exists, however, which seems to show that they are explainable 

by the same principles which apply to aluminum and the dyads, and 

that the theory of substitution from normal salts is a good working

hypothesis to start from.

One definite normal salt is known in this series, the mineral zircon, 

ZrSi04 . As with the other inorganic silicates, the true molecular 

weight of this compound is unknown, and it can be inferred only from 

a study of its derivatives. If we assume it to be represented by the 

polymeric expression Zr4 (Si04 ) 4, it contains replaceable basic atoms, 

and a number of other zirconium silicates fall naturally into series 

derivable from this as the fundamental member. In this connection 

the mineral auerbachite is peculiarly suggestive, for its composition 

is best indicated by the formula Zr4 (Si308 )(Si04 ) 3 , which goes to show 

an important analogy between this group of silicates and those which 

have been previously considered. This formula, compared with 

Hermann's analysis of auerbachite, gives the following results:

'

SiOa ...................................................

Zr03 ...................................................

FeO...................................... ...............

HjO................... .................................

Hermann.

42.91

55.18

.93

.95

99.97

Calculated.

42.45

57.55

100. 00

Although zircon is a very stable and definite mineral, it alters by 

hydration into malacone, cyrtolite, and a variety of other indefinite 

substances which can not as yet be interpreted with any clearness. 

At the same time -other bases, such as lime and the rare earths, are 

taken up, producing mixtures of great complexity. Malacone is 

43633° Bull. 588 14 8 113
114 THE CONSTITUTION OF THE NATURAL SILICATES.

probably the first hydration derivative, and its relations to zircon, 

regarding the latter as Zr4 (Si04 ) 4 , may possibly be as follows:

Zircon . Malacone . 

Si04=Zr . ' OH

4

v > 4N . v 4v 

Si04=Zr Si04=Zr

The original cyrtolite from Rockport is near malacone, but the 

cyrtolite from Colorado, analyzed by Hillebrand, approximates to

-OH 

'/OR 

: \Si04=H(Zr02H2)

,Si04=H(Zr02H2 )

with part of the Zr02H2 replaced by other bases. A number of other 

altered zircons or derivatives of zircon have received specific names, 

but their nature is more or less doubtful. Two of them, however, are 

interesting and may represent distinct species. The anderbergite of 

Blomstrand, for instance, is very near

'Si04E=Zr OH

xSi04=Zr OH

and the alvite of Nordenskiold, analyzed by Lindstro'm, is well represented as a molecular mixture of two compounds,

Zr(Si04 ) 4 (ZrOH) 4 + Zr(Si04 ) 4 (G10H) 12 .

The alvite of Forbes and Dahll seems to be quite different, but the 

analysis of it is unsatisfactory. In Lindstrom's analysis one ZrOH 

group is replaced by R'", mainly Fe, Y, and Ce, and a little G10H is 

replaced by Ca. Many altered zircons, like these minerals, contain 

rare. earths in greater or less proportion, and this fact counts heavily 

against a common interpretation of zircon as a mixed oxide having 

the formula Zr02 -+ Si02 , or

/\ 0=Zr< >Si=0

This view, which is prevalent in Germany, is based upon the close 

morphologic relations between zircon, rutile, and cassiterite. The
SILICATES OF TETRAD BASES. 115

chemical relations, however, are at least equally important, and a 

good formula must express or suggest them all. The formula here 

proposed for malacone is not absolutely certain. The dehydration 

experiments of Zambonini * seem to show that it may be merely a 

hydrated zircon, that is, a zircon containing dissolved water in variable 

amounts. A broader study of malacone from several distinct localities seems to be desirable.

The foregoing formulae, taken by themselves, are entitled to little 

consideration, but they become more significant when studied in 

connection with other compounds later. It will be noticed that one 

atom of zirconium is represented as Unking four groups or radicles 

together, just as one aluminum atom has a similar triple function in 

the aluminous orthosilicates. This is practically equivalent to regarding the minerals under consideration as derivatives of a complex dodecabasic zircosilicic acid, H12ZrSi4012 , which is at least as probable an

interpretation as any other which has so far been advanced.

In eudialyte, elpidite, and catapleiite we have a group of zirconium 

silicates which form a highly suggestive series. Taking the simpler 

members first, they may be represented thus:

Elpidite. Lime catapleiite. Soda catapleiite.

OH /OH /OH

VOH //OH //OR

Zr Zr Zr

v\Si308=NaH2 \\OH \\OH

,Si308=NaH2 \Si308==CaNa \Si308=Na3

Connecting these formulae with zircon, we have the facts that Si308 

occurs in auerbachite, and that at Laven, according to Brogger,2 

zircon is found both intergrown with catapleiite and pseudomorphous 

'after it.

Here again a different interpretation of the minerals has been 

advanced by Zambonini,3 who regards their hydration as extrinsic 

and not constitutional. To catapleiite he assigns the formula 

Na2ZrSi309 .2H20, which, interpreted constitutionally, becomes

0=Zr=Si308=Na2 .2H20.

The calcium-tin silicate, stokesite, is similar, and it has the formula 

CaSnSi3O9 .2H20. Elpidite, according to Zambonini, is a metadisilicate,

Na2ZrSi6O15 .3H2O.

These formulae are legitimate and rational, and the only valid objection to them is that they fail to bring out clearly the relations between 

the several species and the fundamental compound, zircon.

i Contributo allo studio dei silicati idrati, p. 72,1908. 

a Zeitschr. Kryst. Min., vol. 16, p. 105,1890. 

8 Op. cit., pp. 54-64.
116 THE CONSTITUTION OP THE NATURAL SILICATES.

Eudialyte and eucolite are commonly regarded as metasilicates, 

with the compound ZrOCl2 as an admixture. But that compound is not 

found in nature by itself, and both minerals, unlike most of the true 

metasilicates, gelatinize with acids. Furthermore, the analyses of 

eudialyte and eucolite show a considerable range of. variation in the 

ratio Si : 0, although approximating somewhat nearly to the assumed 

Si03 . If now we treat eudialyte and eucolite as mixtures of trisilicates 

and orthosilicates, like the feldspars, scapolites, and some micas, all 

difficulties vanish, the chlorine becomes equivalent to hydroxyl, and 

the minerals fall into line with catapleiite and elpidite as the first 

members of the series. All varieties of eudialyte and eucolite are 

then interpretable as mixtures of the two molecules

Cl Cl

/Si0E=CaNa 

an

Si04=CaNa Si3O8=CaNa

commingled in ratios nearly but not exactly 1:1. Hydroxyl replaces 

chlorine to some extent, while 'iron and manganese partly replace 

calcium; but the ratios shown by the formulae are constant, and the 

structural analogies 'with the allied species are perfectly clear.

The so-called "zircon pyroxenes/ 7 rosenbuschite, wohlerite, lavenite, guarinite, and hiortdahlite all conform' to the type of expressions 

adopted here, although the analytical data are too scanty to yield 

positive conclusions. They can be given metasilicate ratios, following Brogger, by regarding the zirconium present in the form of a metazirconate. If this explanation is correct we should expect to find 

zirconates in nature, free from admixtures, but no such minerals are 

yet known. Artificial zirconates have,, indeed, been prepared; but 

zirconium is more markedly basic than acid in its functions, and the 

analogy furnished by the orthosilicate zircon has been my guide in 

the interpretation of these species. Furthermore, the ratio Si to 0 

in each mineral is 1 to 4 or nearly so, which places them among the 

orthosilicates.

The simplest of these species, and probably the most definite, are 

rosenbuschite and guarinite. Guarinite, which originally was 

described as an isomer of titanite, has recently been reexamined by 

Zambonini and Prior 1 and found to be identical or nearly so with 

hiortdahlite, although the two minerals differ somewhat in composition. .Zambonini and Prior represent guarinite as a mixed salt of 

calcium metasilicate and sodium zirconate, with some calcium 

fluoride, but the following formula, which gives the same ratios, is 

more probable :

i Mineralog. Mag., vol. 15, p. 259, 1909.
SILICATES OF TETRAD BASES. 117

x 4= 34v 

F Zr Si04==Ca3E=Si04 Zr F

Na Na

In hiortdahlite, as represented by Cleve's analysis, the same molecule 

appears, plus a less condensed molecule :

/Si04E=Ca3EESiO4\

F2=Zr< >Zr F2

\Si04 Ca SiO/

NaH NaH

the two being commingled in the ratio 1:1. Hydroxyl may replace 

fluorine to some extent in either case.

These formulae compare with the actual analyses as follows :

SiOa ...........................

ZrO,.... :.......... ............ Tic-;.............. .............

Cba06 ------------------------

Ta O

Fe203 --------------------------

FeO..........................

MnO .........................

CaO............ ...............

MgO................ ...........

K20..........................

Na20.......... ................

H20.... ......................

F.............................

Less 0........................

Guarinite.

Found.

30. 53 

19.70

1.68 

.21

1.91 

1.56 

35.80 

.57 

.43 

6.13

1.28

99.80 

.54

99.26

Calculated.

31.81 

21.44

39. 36

5.45

3.34

101. 40 

1.40

100. 00

Hiortdahlite.

Found.

31.60 

21.48 

1.50

.34 

.94 

.96 

32.53 

.10

6.53 

.58 

5.83

102. 39 

2.45

99.94

Calculated.

30.56 

j 24. 77

34. 11

6.30 

.91

5.78

102. 43 

2.43

100. 00

To rosenbuschite the following formula may be assigned:

,Si04=Ca3==SiO4v

F Zr SiO4=ECa3EEESi04 Zr F

\Si04 Ti SioX

Na, F, Na,
118 THE CONSTITUTION' OF THE NATURAL SILICATES.

This composition compares with Cleve's analysis as follows:

SiOa .. .................................................

Zr02 ................................................:..

Ti02 ...................................................

J?e£)3 ... ................................................

La203 . .................................................

MnO........ ...........................................

CaO. ..................................................

Na20. .................................................

F.....................................................

Less 0 ................................................

Found.

- 31.36

20.10

6.85

LOO

.33

1.39

24.87

9.93

5.83

101. 66

2.47

99. 19

Calculated.

- 30.30

20.54

6.73

28.29

10. 44

6.40

102. 70 

2.70

100. 00

The formulation of wohlerite is complicated by the presence of the 

quinquivalent element columbium, which may be either in the dyad 

group, =CbOF, or in the trivalent, =Cb=0. This alternative leads 

to two possible types of structure, one of them like those immediately 

preceding, the other resembling that of eudialyte, thus:

Si04=Ca

/ \C bOF

Zr Si0=C

Si04=Ca

ZrF

I. 4 Zr Si04=Ca + 1 Zr Si04=Ca

sSiO4=CaNa Si04=CaNa 

with small amounts of the groups R'"F replacing ZrF2 .

x 43 

4 0=Cb Si04EEECa3=SiO4 Cb=O

II.

i04=Zr =SiO

Na Na 

+

xSi04=Ca3=Si 

1 F Zr Si04=Ca3=Si04 Zr F 

^iO, Zr SiO/

Na F Na

On comparing these alternative formulae with Cleve's analysis of 

wohlerite we have:
SILICATES OF TETRAD BASES. 119

Si02 ......................................

Ti02 .... ..................................

Zr02 . ......................................

Cba08 ......................................

Fea08 ................... :.................

Ce203 ...................................... Feo..... ....:..................:..........

MnO. .....................................

MgO......................................

CaO ......................................

Na20 .....................................

HaO....... ................................

F.. ..........................................

Found.

30.14 

.42 

16.12 

12.85 

.48 

.66 

1.26 

1.00 

.12 

26.97 

7.50 

.74 

2.98

101. 24

1.24

100. 00

Calculated.

I.

30. 27 

} 18. 46 

13. 52

28. 26 

7.83

2.87

101.21

1.21

100. 00

II.

30.48 

19. 20 

12. 87

28.39

7.47

2.75

101.16 

1.16

100. 00

These comparisons between observation and theory are as close as 

could be expected when we consider the evident impurities of the 

several minerals. Lavenite can be formulated in a similar manner, 

but the result is doubtful. The mineral contains impurities which 

can not be easily disposed of by mere calculation. New analyses are 

needed on purer material.

It is worth noting, before going further, that these new formulae for 

the "zircon pyroxenes" are curiously similar to those developed for 

garnet, epidote, meionite, spodumene, and similar minerals. In these 

zircosilicates the trivalent radicles=Zr F and =Cb=0 play the same 

part that aluminum and ferric iron play elsewhere. The analogy is 

suggestive but may be nothing more.

The typical silicate of thorium, thorite, or orangite is an unsatisfactory species on account of its wide variations in composition. It is 

commonly supposed that the mineral, as it exists in nature, has been 

derived from an original ThSi04 by hydration, and that ThSi04 was 

isomorphous with zircon. The nearest approach to the type is found 

in orangite, which may perhaps be regarded as a thorium cyrtolite 

or thorium malacone.

Yttrialite is another thoriferous mineral, which, however, seems to 

be a mixture of two salts. Its empirical formula is that of an orthotrisilicate, and as given by Hillebrand's l analysis, it is not far from

i2O7R + 4Y2Si207 ,

i U. 8. Geol. Survey Bull. 262, p. 61,1905.
120 THE CONSTITUTION OF THE NATURAL SILICATES.

R being Fe, Mn, Pb, and Ca. The second of the two compounds is 

analogous to thortveitite, Sc2Si207 . The actual mineral contains 

many impurities, so that its assumed constitution needs to be verified 

by analyses of better material, if that should ever be found. Steenstrupine is still another silicate containing variable quantities of 

thorium, but the analyses are discordant and unreducible to any 

simple formula.

Mackintoshite, a silicate of thorium and uranium, may perhaps be 

represented by the formula UO2 .2Th02 .3SiO2 .3H20, and thorogummite appears to be a hydration derivative of it. Both minerals are 

closely related to thorite, which sometimes contains noteworthy quantities of uranium. The corresponding salts of quadrivalent uranium 

and thorium are probably isomorphous. Uranium functions as a 

hexad element in uranophane, Ca0.2U03 .2Si02 .6H20, and the mineral 

is possibly a basic orthodisilicate, containing the dyad radicle U02 . 

Its formula then becomes Ca(U02 )2Si207 .6H20. Nsegite is a complex 

zircosilicate, containing uranium, thorium, yttrium, and columbium, 

to which no definite formula can as yet be assigned.

To the titanium silicates astrophyllite, johnstrupite, and rinkite 

formulae, like those given to zircon and its derivatives, are assignable. 

Indeed, this has already been done for astrophyllite by Brogger, who 

writes the formula TiCSiOJJR/^RV This seems to be the dominant 

molecule in astrophyllite, which, however, varies in composition. To 

the Colorado mineral we may more precisely give the formula

SiO4=FeH OH 

/Si04E=FeK /Si0 " 4=FeH

Si04=FeH Si04=FeH 

which requires the following percentage composition:

Si02 . ......................................................... 34. 30

Ti02 .. ....................................................... 12.20

FeO. ..................................... r ................. 41. 16

K20........................................................ 5.36

Na20. ....................................................... 3. 55

ILO................. ....................................... 3.43

100.00

Some iron is replaced by manganese, and ferric iron, perhaps as 

=Fe OH, is also present. In the fluoriferous astrophyllites the 

fluorine should replace hydroxyl.
SILICATES OF TETKAD BASES. 121

Johnstrupite and rinkite are both fluoriferous, and both contain 

earths of the cerium groups. In Johnstrupite, a little Zr02 , Th02 , and 

Ce02 replace some Ti02 . For Johnstrupite the expression

Si04==Ca(CeF2 ) 

./Si04==CaNa

SiO4=CaH

agrees well with the ratios given by analysis. In rinkite we have, 

with great probability, the mixture

Si04=CaNa Si04=Ti F 

/Si04=CaNa ./Si04=Ca(CeF2 )

Si04=CaNa SiO4=Ca(CeF2 )

Even the complex mosandrite reduces to the same general type, 

agreeing very closely with

OH F 

/Si04=HNa(CeO2H2 ) /Si04=CaH

SiO4=HNa(CeOaH2 ) Si04=CaH

in which R = Ceiv : Zr : Ti : : 1 : 2 : 2. For each of these species the 

published analyses agree well with the composition calculated from 

these formulae.

Among the titanosilicates, neptunite appears to be a meta-compound, analogous in general structure to astrophyllite. Its formula, 

which is sharply in accord with the analysis, is

Si03 E' 

/SiO.

Si08 E'

where E' = Na, K, and E" = Fe, Mn.

There is still another group of titaniferous silicates which seems to 

be unconformable with the foregoing scheme of interpretation. 

Titanite, the typical member of the group, has the empirical formula 

CaTiSi05 , for which two distinct structures have been proposed. 

One regards the mineral as the calcium salt of an acid, H2TiSi05 , 

analogous to H2Si205 , and the other treats it as a basic orthosilicate,

Ca=Si04=TiO.
122 THE CONSTITUTION OF THE NATURAL SILICATES.

A careful study of the recorded analyses of titanite leads me to prefer 

the orthosilicate expression, for the actual ratios vary in a way which 

indicates a replacement, sometimes of Ca and sometimes of TiO by 

other bases. According to the other formula, only the calcium should 

be replaceable. This variability of ratio is well shown by some of 

the varieties of titanite, such as grothite, alshedite, and eucolitetitanite, but the data are not absolutely conclusive. If, however, 

titanite is a basic orthosilicate, it should be classed with the orthosalts 

of dyad bases. On the other hand, the acid character of the titanium 

is suggested by the remarkable hydration derivative of titanite, 

xanthitane, of which the composition is approximately represented 

by the formula Al=Ti207=H3 .

For tscheffkinite, as shown by Eakins's analyses, the composition is 

approximately (FeCa) 3Ce6Ti4Si6032 , but, the constitution of the 

mineral is very doubtful. Keilhauite appears to be like titanite, 

.with Ca or TiO replaced by K/"OH or R'"02H2 , but the analyses are 

widely discordant.

Benitoite, having the simple and definite formula BaTiSi309, may 

be either a metasilicate or else a trisilicate,

Ba = Si308 =TiO.

The trisilicate formula is analogous to that of titanite and is therefore to be preferred.

Nasarsukite is a mineral of rather uncertain relations, but apparently a trisilicate

in which about one-sixth of the sodium is replaced by the univalent 

group Fe=0. The ideal sodium salt compares with the analysis 

of the actual mineral by Christensen as follows:

SiOa .. ..................................................

Ti02 ......... ..........................................

Fe2Q3 ... .......................................... -^...

Al O

MnO.. .................................................

MgO.. .................................................

Na20. ..................................................

H.,0............ ........................................

F..., .................................................

Less 0 .................................................

Found.

61.63

14.00

6.30

.28

.47

.24

16.12

.29

.71

100. 04 

.30

99.74

Calculated.

63. 83

14.19

1 21. 98

100. 00
SILICATES OF TETRAD BASES. 123

Leucosphenite, which is related morphologically to eudidymite, is 

probably also a trisilicate, although its formula has been written

Na4Ba(TiO) 2 (Si22^5/5' 05 )

This formula is difficult to interpret structurally, and a more rational 

expression, which also fits the analysis fairly well, is as follows:

Na8Ba2Ti4 (Si308 ) 7 . - ,

Lorenzenite, an orthotrisilicate, appears to be a crystalline mixture 

of two salts, thus:

4 Na2 (TiO)2Si207 + Na2 (ZrO)2Si207 .

Several other minerals of more obscure character remain to be 

mentioned here. Molengraafite 1 is essentially a titanosilicate of 

sodium and calcium, which may perhaps have the empirical formula 

HNa3Ca4Ti3Si4020 , if replacements and obvious impurities are left 

out of account. Its true character is quite uncertain. Epistolite is 

a complex columbosilicate, which is not far from H13Na6Cb3Si5 027 . 

Chalcolamprite and endiolite are also columbosilicates of doubtful 

constitution. Possibly the phosphato-silicates erikite and britholite, 

and perhaps also steenstrupine, should be classed with them.

i See Brouwer, Centralb. Mineralogie, 1911, p. 129.
APPENDIX.

A number of well-defined silicates, of uncertain constitution, are 

difficult to place in any of the classes covered by preceding chapters. 

They may be briefly summed up as follows:

Thaumasite. Empirical formula: CaSi03 .CaSO4.CaC03 .15H20.

This can be written structurally, regarding the water as extrinsic, 

as in some respects analogous to wollastonite, but such a formula 

would lack the evidence necessary to sustain it.

Tseniolite. A silicate of magnesia and the alkalies of uncertain 

composition. The one analysis of it is incomplete.

Bakerite. A borosilicate of calcium. The empirical formula is 

8CaO.5B2O3 .6Si02 .6H20.

Roeblingite.- This species is unique in that it contains a sulphite 

radicle. It is regarded by Penfield and Foote as a mixed silicate and 

basic sulphite, 5 H2CaSi04 + 2 CaPbO(SO3 ).

This can be given a structural formula, which, however, would have 

little real significance.

Langbanite. A silicate of manganese and iron containing antimony. 

Allied on crystallographic grounds to hematite and ilmenite.

A multitude of other silicates have been described as species, but 

without, as yet, securing full recognition. Some of these are doubtless mixtures or impure varieties of well-known minerals, but. others 

may be ultimately established as good and definite compounds. A 

discussion of the analyses, without experimental investigation of the 

various minerals, would have very uncertain value. I therefore 

omit these doubtful species from consideration. 

124
INDEX TO SPECIES.

A. Page. 

Acmite. ...............\ ................. 97,98,103

Actinolite.............:................... 100,106

JEnigmatite................................. 103

Agnolite..................................... 106

Agricolite. ............'...................... 86

Akermanite................................. 34

Alamosite................................... 106

Albite........................ 36,37,40,42,43,48,98

Alexandrolito............................... 84

Alipite.............'........................ 100

Alianite...................................... 25

Allophane................................... 84

Almandite.................................. 25

Alshedite.................................... 122

Alurgite..................................... 54

Alvito....................................... 114

Amesite..................................... 65

Amphibolo............. 99,100,101,102,103,104,105

Analcite. ..... 37,38,39,40,41,42,43,46,47,48,98,108

Anaphorite.................................. 104

Andalusito....... 15,19,22,24,31,51,72,74,75,77,84

Anderbergite ............................... 114

Andradite................................... 25

Annite...................................... 55

Anorthite.................... 26,30,32,36,37,41,46

Anthophyllito.............................. 100,105

Anthosiderite............................... 84

Antigorite................................... 92

Aphrodite................................... 94

Aphrosiderite................::............. 64

Apophyllito................................. 108

Arctolito.................................... 34

Ardennito................................... 78,79

Arfvedsonito................................ 103

Arizonite.................................... 86

Aspidolite................................... 53

Astochite.................................. 100,101

Astrolite.................................... 81

Astrophyllito................................ 120

Auerbachite ............................... 113,115

Augite................................... 26,96,99

Axinite..................................... 65,72

B.

Babingtonito................................ 98

Bakerite.................................... 124

Barkevikito................................. 103

Barsowite................................... 74

Barylite..................................... 80,81

Barysilite................................... 107

Batavite.................................... 77

Bavenite.................................... 49

Beckelite.................................... 85

Bementite................................... 92

Benitoite.................................... 122

Bertrandite................................ 89,110

Beryl....................................... . 81

Biotite....................... 24,51,52,53,54,58,66

Bityite...................................... 78

Brewsterite................................. 46

Britholite.................................../ 85

Bronzite.................................... 96

Brunsvigite................................. 60,61

C. 

Calamine.................................... 89,90

Cancrinite................................ 27,28,34

Cappelinite.................................. 70,71

Carnegieito............................ 21,23,45,48

Caswellite................................... 59

Catapleiite.............................. 71,115,116

Celadonite................................... 86

Celsian...................................... 35,46

Cenosite..................................... 85

Cerite....................................... 84

Chabazite............................ 41,47,48,108

Chalcolarnprito.............................. 123

Chamosite................................... 65

Chloritoid................................... 56,57

Chloropal.................................... 84

ChJorophajito............................... 65

Chlorophyllitc............................... 79,80

Chondrodite.................................. 88,93

Chrysocolla.................................. 106

Chrysotile................................... 92

Cimolite...................................... 84

Clinochlore.................................. 59,65

Clinohedrite................................. 90

Clinohumite.................:............... 88

Clintonite................................... 56

Connarite................................... Ill

Cookeite..................................... 57,78

Corundophilite.............................. 62,65

Cossyrite.................................... 103

Crocidolite.................................. 102

Cronstedtito................................. 60

Crossite..................................... 102

Cryophyllite................................ 55

Cummingtonite.............................. 109

Cuspidine................................. 109,110

Custerite.................................... 107

Cyrtolite. ................................. 113,114

D. 

Danalite..................................... 89

Danburite................................... 74

Dannemorite................................ 100

Daphnite.................................... 64

Datolite..................................... 90

Delessite.................................... 60,61

Deweylite................................... ill

125
126 INDEX TO SPECIES.

Diabantite.................................. 62,63

Didymolite.................................. 78

Diopside................................ 96,99,100

Dioptase.................................... 91

Dudleyite................................... 57

Dumortierite................................ 65,73

E. 

Edingtonite.............................. 42,43,46

Ekmanite................................... 63,64

Elseolite............................... 22,27,28,37

Elpidite................................... 115,116

Esndeiolite................................... 123

Esnstatite................................. 94,96,99

Epichlorite.................................. 64

Epididymite................................ 112

Epidote............................ 24,25,29,31,36

Epiphanite.................................. 65

Epistilbite................................. 46

Epistolite.................................... 123

Erikite..................................... 85,123

Erionite..................................... 46

Euclase..................................... 90

Eucolite..................................... 116

Eucolite-titanite............................. 122

Eucryptite............................ 19,21,51,98

Eudialyte................. ; ............ 71,115,116

Eudidymite................................. 112

Eulytite..................................... 86

Euphyllite........... v ...................'.. 52

Euralite..................................... 65

F. 

Fassaite............ '.. ...................... 33,105

Faujasite.................................... 42,43

Fayalite..................................... 87

Foresite..................................... 49,50

Forsterite................................... 87,88

Fowlerite...............'..................... 96

Friedelite................................... 91,92

Fuchsite.................................... 01

G. 

Gadolinite................................... 90,91

Ganomalite................................ 107,108

'Ganophyllite............................. 49,50,53

Garnet................. 24,25,26,29,30,32,34,56,65

Garnierite.................................... . 94

Gastaldite........:.......................... 103

Gehlenite.............................. 31,32,33,34

Genthite.................................... Ill

Gisinondite.................................. 45,48

Glaucochroite............................... 87

Glauconite.................................. 86

Glaucophane.............................. 101,102

Gmelinite................................... 41,47

Gonnardite.................................. 50

Grandidierite................................ 76,77

Griqualandite............................... 102

Grochauite.................................. 61

Grossularite................................. 25,33

Grothite .................................... 122

Griinerite................................... 100

Guarinite................................. 116,117

Gyrolite..................................... 108

H. 

Hallite...................................... 59

Halloysite................................... 83

Page. 

Hancockite.................................. 25

Hardystonite................................ 107

Harmotome........................... 41,43,44,46

Harstigite................................... 75

Hastingsite................................ 101,103

Haughtonite ................................ 53

Hauynite............'........................ 26

Eedenbergite................................ 96,99

Hellandite................................... 85

Helvite...................................... 89

Heulandite............................... 41,46,48

Eibscbite................................... 45,76

Hillebrandite........................... 89,106,107

Hiortdablite............................... 116,117

Hisingerite.................. ............... 84

Homilite.................................... 90,91

Hullite..........;........................... 65

Humite..................................... 88

Hyalophane................................. 35,46

Hyalotekite............................... 110, 111

Hydrobiotite................................ 58

Hydrocastorite.............................. 82

Hydronephelite....................... 26,40,42,43

Hydrophlogopite............................ 58

Hypersthene................................ 96

I. 

Igelstromite................................. 87 -

Ilvaite....................................... 89,90

Inesite...................................... 109

lolite........................................ % 79

Irvingite.................................... 55

J. 

Jadeite...................................... 97,98

Jefierisite.................................... 58

Jeffersonite................................... 96

Johnstrupite.........................'..... 120,121

K. 

Kaersutite..................................: 104

Kainosite.. .................................. 85

Kaliopbilite.............................. 19,21,22

Eaolinite............... 36,37,39,40,57,58,64,83,84

Karyocerite.................................. 70,71

Karyopilite.................................. 91

Keilbauite................................... 122

Kentrolite................................... 85

Kerolite..................................... 94

Kerrite...................................... 58

Klementite.................................. 65

Knebelite.................................. 1 87

Kornerupine............................. 76,77,96

Kotschubeite................................ 60

Kryptotile.......................... 19,22,52,76,83

Kyanite............................... 15,23,24,31

L. 

Lagoriolite.................................. 24,25

Langbanite.................................. 124

Lasallite..................................... 80

Laubanite................................... 49

Laumontite........................... 41,46,47,48

Lavenite.................................. 116,119

Lawsonite................................... 45,76

Lazurite..................................... 26

Lennilite.................................... 58

Lepidolite................................... 54,55

Lepidomorphite............................. 52

Leuchtenbergite............................. 59,65
INDEX TO SPECIES. 127

Leucite............................. 37,38,40,43,98

Leucophanite................................ 106

Leucophoenicite.............................. 88

Leucosphenite............................... 123

Leverrierite................................. 22,51

Levynite.. ................................... 47,48

Linosite..................................... 104

Lorenzenite................................. 123

Lucasite..................................... 58

M. 

Maokintoshlte............................... 120

Maconite.................................... - 58

Malacon .............................. 113,114,115

Manandonite............................. 65,73,78

Manganidocrase............................. 31

Manganophyllite............................ 53

Margarite................................... 57

Marialite.................................... 36,37

Meionite.................................. 23,36,37

Melanocerite................................ 70,71

Melanolite................................... 60

Melanotekite................................ 85

Melilite................................... 26,31,33

Melite....................................... 84

Meliphanite................................. 112

Mesolite..................................... 42,48

Metachlorite................................ 61

Microsommite............................... 27,37

Milarite..................................... 82

Minguetite.................................. 64

Molengraaflte................................ 123

Molybdophyllite............................ 89

Monticellite............................. 87,99,105

Montmorillonite............................. 83

Mordenite................................... 48

Mosandrite.................................. 121

Mullerite.................................... 84

Muscovite................................... 19,

22,23,26,34,36,37,51,52,55,66,72,74

N. 

Nasgite...................................... 120

Nasarsukite.................................. 122

Nasonite.. ................................ 107,108

Natrolite........................ 26,40,42,43; 46,48

Nephelite......... 19,21,23,27,28,36,37,40,42,46,51

Nepouite................;................... 94"

Neptunite................................... 121

Newtonite................................... 83

Nontronite.................................. 84

Noselite.".................................... 26

Noumeite................................... 94

0. 

Offretite.................................... 50

Okenite...................................... 108

Olivine............................... 26,94', 99,105

Orangite.............. '........................ 119

Orthoclase................................... 37,48

Ottrelite .................................... 56

Ouvarovite ............................ 1.... 25

P. 

Painterite................................... 59

Paragonite............................ 19,22,51,52

Partschinite................................. 25,29

Pattersonite................................. 59

Page. 

Pectolite.................................... 92

Pennine ................................. 59,60,65

Petalite..................................... 82

Phenakite .................................. 87,89

Phengite.................................... 52

Philadelphite................................ 58

Philipstadite................................ 103

Phillipsite.......................... 41,43,44,45,48

Phlogopite......................... 51,53,54,59,66

Pholidolite.................................. 107

Picrosmine.................................. 94

Piedmontite................................. 25

Pilinite......................:............... 49

Pilolite...................................... 80

Plancb.e'ite................................... 106

Plombierite................................. 108

Pollucite.................................... 39

Polylithionite............................... 55

Prehnite......................... 24,29,34,36,38,48

Prismatine.................................. 76,77

Prochlorite............................... 61,62,65

Prolectite.................................... 88

Protovermiculito............................ 58

Pseudobrookite.............................. 86

Pseudodeweylite.......................... 107, 111

Pseudoleucite............................... 37

Pseudonatrolite............................. 50

Pseudonephelite............................. 22

Ptilolite..................................... 49

Pycnochlorite............................... 65

Pyrope...................................... 25

Pyrophyllite................................ 98

Pyrosclerite................................. 59

Pyrosmalite................................. 91

Pyroxene ................. 95,96,97,99,100,104,105

R. 

Rectorite.................................... 22,83

Rhodonite................................... 96

Rhodusite................................... 102

Rho'nite..................................... 104

Richterite.................................... 100

Riebeckite................................... 103

Rinkite.................................... 120,121

Rivaite...................................... 110

Roeblingite.................................. 124

Roscoelite................................... 51

Roseite ............ ; ........................ 58

Rosenbuschite............................. 116,118

Rubellite.................................... 68

Rumpflte................................... 60

S. . 

Sahlite............................."......... 96.

Samoite..................................... 84

Saponite.................................... 109

Sapphirine................................... 73

Sarcolite.................................... 24

Scapolite........................... 31,33,34,36,51

Schefferite................................. 96,110

Schizolite.................................... 96

Schorlomite................................. 25

Schrotterite....................... v ........ 84

Scolecite................................. 42,43,48

Serendibite............................... 65,73,78

Sericite .................i................... 51

Serpentine............................. 65,92,93,94
128 INDEX TO SPECIES.

Page. 

Seybertite................................... 56

Sheridanite.................................. ,64

Siderophyllite............................... 53

Sillimanite......................... 15,23,24,31,83

Sodalite...........'........... 26,27,28,29,37,40,42

Soretite..................................... 103

Spadaite.................................... 119

Spessarite. 1................................. 25,29

Sphenoelase................................. 81

Spinel....................................... 56,65

Spodiophyllite.............................. 107

Spodumene.............................. 97,98,99

Spurrite..................................... 89

Staurolite................................... 74,75.

Steenstrupine............................... 123

Stellerite.................................... 50

Stilbite............................. 41,43,44,45,48

Stilpnomelane............................... 63,64

Stokesite.................................... 115

Strigovite................................... 64

Syntagmatite................................ 101

T. 

Tseniolite................................... 124

Talc........................................ 105,106

Talc-chlorite................................. 63

Taramellite.................................. 107

Tawmawite................................. 25

Tephroite................................... 87

Termierite................................... 84

Thalenite-.l................................. 85

Thaumasite................................. 124

Thomsonite.........................'........ 45

Thorite...................................... 119

Thorogummite.............................. 120

Thortveitite................................. 85

Thuringite.................................. 62,63

Titanite................................... 121,123

Page. 

Topaz................................. 19,22,23,51

Tourmaline..................... 65,66,67,68,69,78

Tremolite............................. 100,101,105

Trimerite.................................... 87,89

Tritomite................................... 70,71

Troostite.................................... 89

TschefEkinite................................ 122

U. 

Uranophane................................. 120

Vaalite...................................... 59

Vermiculite................................. 58,59

Vesuvianite........................... 30,31,34,65

Villarsite.................................... 92

W. 

Waldheimite................................ 101

Weinbergerite............................... 107

Wellsite..................................... 43,44

Westanite................................... 83

Willcoxite................................... 57

Willemite................................... 87,89

Woerthite................................... 83

Wohlerite............................. 116,118,119

Wolchonskoite.............................. 86

Wollastonite................................ 95,96

X. 

Xantliophyllite.............................. 56,65

Y.

Yttrialite. 119

Z.

Zeophyllite................................ Ill, 112

Zinnwaldite................................. 54,55

Zircon................................. 113,114,115

Zoisite....................................... 25,29

Zungite..................................... 24,29

o