DEPARTMENT OP THE INTERIOR,

BULLETIN

OF THE

UNITED STATES

GEOLOGICAL SURVEY

No. 135

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1895

UNITED STATES GEOLOGICAL SUEVEY

CHARLES D. WALCOTT, DIRECTOR

THE

CONSTITUTION OF THE SILICATES

BY

FRANK WIGGLESWORTH CLABKE

CHIEF CHEMIST

WASHINGTON

GOVERNMENT PRINTING-OFFICE

1895

CON TE N T S.

Chapter I. Introduction. ....................................

Chapter II. The theory of the silicates .......................

Chapter III. The orthosilicates of aluminum ..................

I. The uephelite type ..................................

II. The garnet-biotite type ............................'..

III. The feldspars and scapolites .........................

IV. The normal zeolites. .................................

V. The micas and. chlorites. .............................

VI. The tourmaline group ...............................

VII. Miscellaneous species ................................

Chapter IV. The orthosilicates of dyad bases ................

Chapter V. The orthosilicates of tetrad bases ...............

Chapter VI. The diorthosilicates ............................

Chapter VII. The ineta- and dimetasilicates ..................

Chapter VIII. Summary ............................--'-..-....

Index ........................................................

Page.

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THE CONSTITUTION OF THE SILICATES.

BY F. W. OLABKB.

OHAPTEE I.

INTRODUCTION.

Iii 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 of the rocks except the sandstones, quartzites, and carbonates; even these exceptions are commonly 

derivatives of the silicates, which break up under various conditions, 

yielding simpler bodies of their own class, together with free silica and 

limestone. From a geological 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 upon 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 the 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; isoinorphous 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 

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

7
8 ' THE CONSTITUTION OF THE SILICATES. [BULL. 125.

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 j>ossible so to investigate the 

constitution of the silicates as to bring many relations to light, developing formula} which express those relations and indicate profitable 

lines for future research.

The problem is open to attack from several points of view, and 

Yarious 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, while 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 quite limited in number, only 

a few hundred at most being known; whereas, if complexity 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 tlie commonest metals being almost universally distributed. The longer the 

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

silicate structures becomes.

Having established the empirical formula of a silicate, its physical 

properties may next be considered; and of these the crystalline form
CLAKKK.] INTRODUCTION. 9

and the specific gravity are the most important; Prom 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 secure 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, 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. Mineralogical literature is full of faulty records regarding alterations, and 

many diagnoses need to be revised. Pseudomorphs have been named 

by guesses, based upon their external appearance, and often a compact mica has been called steatite or serpentine. In every case an 

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. In each instance the supposed fact 

must 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 aualcite 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 

upon the question of chemical structure. With evidence of this sort 

relations appear which could not otherwise be recognized, and these 

relations may be closely correlated with observations of natural occurrences.

Evidence of the same or similar character is also furnished by the
10 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

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;, natrolite gives nepheline; talc, 

upon ignition, liberates silica; and the prolonged heating of ripidolite 

produces an insoluble residue having the empirical composition of spinel. All of these 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 upon 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 method 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 geological 

questions is limited in the extreme. 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.

In the development of constitutional formulae for the silicates it 

sometimes happens that alternatives offer 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
CLABKE.] INTRODUCTION. 11

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

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

hydrous silicates, and often arises from the difficulty of discriminating 

between water of crystallization, so called, and constitutional hydroxyl. 

If in every silicate analysis the state of hydration were carefully examined, the conditions of dehydration being observed at different temperatures, much of this obscurity would be avoided.

In discriminating between rival formulae, 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 tbis kind may render legitimate

assistance.
CHAPTER II.

THE THEORY OF THE SILICATES.

If all of 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 known. Their nature must be inferred from their salts, 

and especially from their ethers, and this side of the problem is the 

first to be considered.

Since silicon is quadrivalent, its orfcho acid is necessarily represented 

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

Si(OH)4 , or, structurally:

H 

I 

O

I 

H O Si O H

I 

O

I 

H

To this acid,orthosilicic acid, the normal silicic ethers 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

B"2 SiO4 Mg2SiO4

Rm4 (Si04 )3 Al4 (Si04 ) 3

RivSiO4 ZrSiO4

Any silicate in which the oxygen atoms outnumber the silicon atoms 

by more than four to one, as for example the compound Al2SiO5 , must 

be regarded as a basic salt.

By elimination of water orthosilicic-acid may be conceived as yielding, 

first, metasilicic acid, H2SiO3 , and, secondly, the anhydride, SiO2 , thus:

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

Corresponding to metasilicic acid many salts are known, but as yet 

no ethers have been certainly obtained. The ethers first described by 

Ebehnen were supposed to be metasilicates, but all recent investiga12
CLAKKE.] THEORY OF THE SILICATES. 13

tious have shown them to be ortho compounds, possibly more or less 

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

having the formula, (C2H5 )8Si4Oi2 , which is a polymer of a metasilicate ; 

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

for typical metasilicates are as follows :

Types. Examples.

lsra2SiO3 

MgSiO3

Al2 (Si03 ) 3* 

Rlv (SiO3 )2 Zr(Si03 )2*

By eliminating a molecule of water from two molecules of orthosilicic or metasilicic acid, a disilicic acid may be produced. From 

orthosilicic acid we have

Si=(OH)3

Si(OH)4 | 

 H2O = O 

Si(OH)4 |

or, H6 Si2 O7 ; a sexbasic acid of which several ethers have been obtained. 

It is, therefore, a well-established acid, and a number of silicates appear 

to be salts of it. It may be called diorthosilicic acid. From metasilicic 

acid, in the same way, we get

O=Si OH 

O=Si=(OH)2 |

 H2O = O

O=Si=(OH)2 |

O=Si OH

or dimetasilicic acid, H2Si2O5 . No ethers of this acid are known, but 

among its salts are the minerals petalite, milarite, ptilolite, and 

mordenite.

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

four molecules of orthosilicic acid, a series of tri- and tetrasilicic acids 

may be theoretically developed. These higher acids would present 

many possibilities for isomerism, and with their long chains of atoms 

would presumably be unstable. At all events, with a single exception, it is unnecessary to consider them in order to account for the 

natural silicates. The one exception is the tetrabasic acid H4Si3O8 , 

which appears most notably in eudidymite, the feldspars, the micas, 

and the scapolite group. The feldspars of this type, however, albite 

and orthoclase, are represented by Groth as mixed salts of two other 

silicic acids, when Si2O5+SiO3 =Si3O8 ; so that the trisilicic acid in 

question can be left out of consideration. But a general view of the 

whole field seems to point to its existence; at least greater simplicity 

is secured by retaining it in the scheme of silicic acids; and that policy

* Salts not actually known, but theoretically possible.
14 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

will be followed throughout this memoir. As opposed to Groth's 

we have also to consider the theory of Becker* that metasilicates may 

be really mixtures or compounds of ortho- and trisilicates, with H4SiO4 -f 

H4 Si3O8 =4:H2SiO3. This theory may easily be pushed to an extreme, 

for the weight of evidence shows that true metasilicates exist; but in 

many cases it has real significance; that is, mixed salts are known 

having pseudometasilicate ratios, and these are most simply explained 

by Becker's theory, which, in the case of the micas, had been independently developed by myself. As to the constitution of trisilicic acid, we 

have no positive evidence, and at least two independent structures are 

conceivable, thus :

0=Si-OH

I 

O

ISi=(OH)2

I 

O

I 

O=Si-CH

and

xox o 

(OH)2 =Si< >Si( >Si=(OH)2

XK MX

The first of these formulae has the closer analogy with the formulae 

of the other silicic acids, and is, therefore, in default of evidence, to be 

preferred. Furthermore, it indicates possibilities of isomerism, such 

as we find in the case of orthoclase and microcline, a consideration 

which is well worth bearing in mind.

So far, then, there are five silicic acids whose salts occur in the crust 

of the earth. They are:

Orthosilicic acid ............................................ H4 SiO4

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

Diorthosilicic acid.......................................... H6Si 2O7

Dimetasilicic acid............................«..'....,....... .H2Si2 05

Trisilicic acid..............................................H4Si3O8

and unless we admit the possible polymerization of metasilicic acid, 

as suggested by Troost and Hautefeuille's ether, no others need be 

invoked in order to account for the facts which are now available. 

The possibility of other silicic acids is an entirely distinct question.

If the natural silicates were simple normal salts of the five 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,

.. . . *Am. Jour. Sci., (3) XXXVIII, p. 154, 1889.
CLARKE.] THEORY OF THE SILICATES. 15

and the latter class offers the most serious difficulties. A basic inetasilicate, 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. Take for instance the 

formula Al2SiO5 , which represents the composition of three distinct 

minerals, andalusite, sillimanite, and kyanite, and it admits of several 

different structural expressions. As a basic metasilicate it may be 

written

Al^ Al\

No ' /SiO3 Al O Al

^S\rk ^\n ^ O / 

and as an orthosilicate it becomes either

0=Al-Si04 =Al, or Si0 4

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

Al2Si05, 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 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 clews 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.

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 apparent 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 j)oint has been made clear by 

the vapor density and cryoscopic investigations of recent years, and is
16 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

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 

A]4 (SiO4 )3 . To illustrate this rule for present purposes, the following 

examples will suffice:

Aluminum orthosilicate ............................ Al4 (SiO4 ) 3

Eucryptite...........'............................ Al3 (SiO4 ) 3Li3

Nephelite ......................................... Al3 (SiO4 )3Na3

Kaliophilite....................................... Al3 (SiO4 ) 3K3

Muscovite ....................................... . Al3 (SiO4 ) 3KH2

Paragonite........................................ Al3 (SiO4 ) 3NaH2

Andalusite......................................:. Al3 (SiO4 )3 (AlO)3

Topaz ............................................ Al3 (SiO4 ) 3 (AlF2 ) 3

Natrolite........................................... Al2 (SiO4 )3]S"a2H4

Biotite............................................ Al2 (SiO4 ) 3MgKH

Grossularite ...................................... Al2 (SiO4 ) 3Ca3

Prehnite.......................................... Al2 (SiO4 ) 3Ca2H2

Phlogopite........................................ Al(SiO4 ) 3Mg3KH2

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 into inuscovite, 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 exists in nature is still 

a matter of doubt. The rare mineral xenolite has the composition 

Al4 (SiO4 ) 3, but it needs further study before its integrity as a species 

can be fully recognized. At present the weight of evidence seems to 

be in its favor, but its occurrence is that of a secondary mineral. 

Among the products of primitive magmas it has not been observed. 

As regards its ultimate constitution or chemical structure, that question must also be left open. It can be written structurally in several 

ways; either with each aluminum atom linked with all the SiO4 groups 

or with only one atom so connected. In a sense this problem is analo-
CLAHKE.] THEORY OF THE SILICATES. 17

gous 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:

.8104= Al' SiO4=KHz 81O4=CaH

These expressions indicate the observed replaceability 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 

chapters. In the systematic discussion I shall begin with the orthosilicates, starting with those of aluminum, for the reason that they show 

most clearly the evidence relative to structure. In a classification of 

the silicates the starting point would be different, but this is not a classification. The trisilicates will be considered with the orthosilicates, for 

reasons which are obvious. In the feldspar, zeolite, and mica groups 

the ortho- and trisilicates can not well be separated. 

Bull. 125 2
OHAPTEE III.

THE ORTHOSILICATES OF ALUMINUM. 

I. THE NEPHELITE TYPE.

If, in accordance with the ideas. developed in the preceding chapter, 

we start out from the normal salt AL,(SiO4 ) 3 , the first and simplest 

replacement possible is that of a single aluminum atom by three 

monads, giving a compound of the general formula Al3 (SiO4) 3R'3. This 

formula represents several well-known minerals, and I propose to designate it as the nephelite type. At first sight it seems to be reducible 

to the simpler expression B'AlSiO4 , but that expression, as will be seen 

later, does not indicate all the known relations of the group.

The first three representatives of this type are as follows :

Eucryptite ........................................... Al3 (SiO4 )3Li3

Nephelite ............................................ .Al3 (SiO4 )3Na3

Kaliophilite .......................................... Ai3 (SiO4 ) 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 E/8Al8Si9O34 , in which a little potassium appears among the 

components of R', while there is an excess of silica over 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 (Si3 08 ) 3Na3. This replacement of SiO4 by Si3O8 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 "ill produce the divergence from normal composition shown in the analyses of natural nephelite.

By direct alteration nephelite passes easily into two zeolitic minerals, having the subjoined formulae:

Hydronephelite .............................. . Al3 (SiO4 ) 3Na2H.3H2O

Natrolite . .................................. .

In natrolite, two atoms of aluminum have been replaced by R'6 = 

Na2H4, and this species, with others of like origin, falls therefore into 

another group. Still it is desirable now to call attention to the fact> 

18
CLARKE.] NEPHELITE TYPE. 19

observed by Doelter,* that natrolite on fusion yields nephelite. The 

connection between the two species is unmistakable.

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, t 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 (SiO4 )3:N"a3

Muscovite.......................................... Al3 (SiO4 )3KH2

Paragonite. ........................................ Al3 (SiO4 ) 3j$TaH2

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 

i'act of alteration from one into another.

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 (SiO4 ) 3 (AlF2 ),

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

Here we encounter the evidently univalent atomic groups

/F 

-A1=O and -Al<

\F

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

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

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

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.

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

fundamental type Al3 (SiO4 ) 3E'3, the first substitution from the normal 

orthosilicate of aluminum. Leaving hydronephelite on one side, because 

of its water of crystallization, the remaining seven species fall into three 

subtypes, as follows:

Nepliellte. ( Muscovite. Topaz. 

,SiO4=Na3 ' ,SiO4EEKH2 ,SiO4=(AlF2) 3

Al-SiO4=Al Al-SiO4=Al 

\SiO4=Al \SiO^Al

symbols which clearly indicate the known chemical relations between 

the several minerals. In five of the eight examples the simplest pos-

* Allgem. Cbeiu. Mineralogie, p. 183. tBull. Soc. Min., XIH, p. 129, 1890.
'20 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

sible 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 minerals kornerupine and 

kryptotile, which will be considered later, are possibly also representatives of the nephelite type.

II. THE GARNET-BIOTITE TYPE.

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

from the normal salt Al4 (SiO4 ) 3 , in which two atoms of aluminum liave 

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

Al2 (SiO4 ) 3Ei0i;, and in this series bivalent elements or radicles frequently 

appear. In natrolite, E'G =Na2H4 ; in biotite, E'6 E"2E'2 ; and in garnet 

E'G are entirely replaced by E"3 . 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 three species seem definitely to occur, viz :

Natrolite .......................................... . Al2 (SiO4 ) 3Na2H4

Dumortierite ..................................... . Al2 (SiO4) 3 (AlO)6

Zunyite .......................................... . Al2 (SiO4 ) 3E'6*

Natrolite, as we have already seen, may be directly derived from 

nephelite by alteration. Dumortierite is analogous to andalusite, and, 

like the latter, is orthorhonibic. In zunyite E'6 is composed of the 

groups A1=F2 , A1=C12, and A1=(OH)2, with possibly a little 

 Al =O, and the mineral thus seems to be chemically related to durnortierite in the same way that topaz is related to andalusite. But topaz 

and audalusite have crystallographic analogy, while zunyite, being

isometric, is quite unlike its chemical congeners. The species evidently 

needs further study, but at present, if we unite the chlorine in it with 

the fluorine, it may be provisionally represented by the expression

,SiO4=(AlO2H2 ) 2 .AlF2 

Al-SiO4B(AlO2H2 )2 .AlF2

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.t

In the second Subtype, when E'fl is partly composed of bivalent and 

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

Prehnite ........................................ . Al2 (SiO4 ) 3Ca2H2

Biotite ......................................... . . Al2 (SiO4 ) 3Mg2HK

* Groth's formula, generalized. t Zeit. Kryst. Min., XVIII, p. 209.
CLARKE.] GARNET-BIOTITE TYPE. 21

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 still 

uncertain. Biotite will be more fully considered in the special chapter 

devoted to the mica group. Prehnite, according to Doelter,* decomposes ou fusion, yielding the same products as garnet.

Under the third subtype of this series Ave 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. Under the generic term garnet several 

species are included, all being isometric and strictly isomorphous, in 

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

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

we have :

Grossularite ........................................

Pyrope ............................................

Alniandite ......................................... . Al2 (SiO4 ) 3Fe

Spessartite ........................................

Andradite ..........................................

Ouvarovite .........................................

To these may be added schorlomifce, a garnet in which titanium occurs 

both as part of the acid, that is, with TiO4 replacing SiO4 , and also as 

Ti'" among the triad bases, equivalent to aluminum. The monoclinic 

partschinite, isomeric with spessartite, also falls into this group.

In the epidote group, at least four species appear, one, zoisite, being 

orthorhombic, while the other three are mouoclinic. These species are 

characterized by the bivalent group of atoms =A1 OH, thus:

Zoisite .............................. Al2 (SiO4 ) 3Ca2 (AlOH)

Epidote, a. .......................... ( Al2 (SiO4 ) 3Ca2 (AlOH)

Epidote, Z> ........................... ( Fe2 (SiO4 ) 3Ca2 (FeOH)

Piedmontite ......................... (AlMn)2 (SiO4 ) 3Ca2 (AlOH)

Allanite. . ... ^ ...................... . (AiCeFe)2 (SiO4 ) 3 (CaFe)2 (AlOH)

or, in general, as compared with garnet, *

Garnet. .................................... .K'"2 (SiO4 ) 3R"3

Epidote .................................... .K"'2 (SiO4 ) 3R"2 (R'"OH)

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

are often associated give emphasis to the formulae. For epidote, 

however, an alternative formula is possible, in which.the hydroxyl is 

supposed to be united with calcium rather than with aluminum. But 

epidote is a peculiarly stable mineral, while the group Ca OH would 

imply instability and easy alteration. The alternative formula, therefore, is improbable, although it would ally epidote more easily with 

anorthite, which is a product of its fusion.

* Allgem. Cliem. Mineralogie, p. 183.
22 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

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,* garnet 

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

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

Epidote, says Doelter, t yields linie-augite and anorthite, and prehnite 

behaves like garnet. These facts indicate analogies between the several 

species named, but are not easy of complete interpretation. The different products observed may be due in part to a direct splitting-up of 

the original insoluble mineral into soluble species like anorthite and a 

lime-olivine, and in part to secondary reactions taking place during the 

prolonged fusion and cooling of the resultant magma. In other words, 

two sets of phenomena are probably involved in the observations, and it 

is impossible at present to distinguish between them. The first set alone 

is immediately relevant to the constitutional question now before us.

In the four species sodalite, haiiynite, nosite, and lazurite we have a 

group of minerals which Brogger has classified as alkali garnets, £ 

Like garnet, they are ail isometric, and they are characterized by the 

presence of the bivalent groups =A1 01, =A1 SO4 Na, and 

=A1 S S Sv-isfa. There are also artificial products, ultramarines, 

in which the groups =A1 S S Na and =A1 S Na appear. 

Adopting Brogger's formulae, which are preferable to those formerly 

proposed by myself, § these species may be written as follows:

Sodalite................................. Al2 (SiO4 )3Na4 (A101)

Haiiynite ................................ Al2 (SiO4 ) 3Na2Ca( AlSO4Na)

Nosite.................................... Al2 (SiO4 ) 3Na4 (AlSO4Na)

Lazurite................................. Al2 (SiO4 ) 3Na4 (AlS3Na)

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

Through these species,- and especially through sodalite, a connection N 

with several other minerals is established. Sodalite occurs in elseolite 

syenytes, and is evidently derived from nephelite, and like the latter 

it yields natrolite, hydronephelite, and muscovite by alteration. 

Furthermore. C. and G. Friedel, || on heating j>owdercd muscovite with 

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

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 microsominite, are also, 

like sodalite, undoubtedly derivatives of nephelite, but their formulae 

are rather uncertain. Cancrinite approximates to the composition

* Allgem. Chem. Mineralogie, p. 182.

t Allgem. Chem. Mineralogie, p. 183.

t Brogger and Backstrom, Zeit. Kryst. Min., XVIII, p. 209.

§Bull. TJ. S. Geol. Survey No. 42, p. 38.

||Bull. Soc. Min., XIII, p. 183.
CLAEKE.] GARNET-BIOT1TE TYPE. 23

Al2 (SiO4 ) 3]Sra4H(AlCOa ), in which a little soda is replaced by lime, and 

the univalent group A1=CO3 may be partly substituted by Al=SiO3. 

Microsommite, according to the published analyses, varies in composition widely, containing always potassium, and having a notable proportion of chlorine and sulphuric acid among its constituents. If, however, we assume in it the univalent radicles A1=C12 and Al=SO4 ,its 

composition reduces easily to the form Al2 (SiO4 ) 3 (NaK)3Ca(Al(SO4 .Cl2)), 

like cancriuite, both species having then the composition of the general 

type Al2 (SiO4 ) 3B/6 . The theory as proposed, then, assumes univalent 

complex radicles for cancriuite and microsommite, and bivalent radicles 

for the sodalite group, thus :

In cancrinite group. In sodalite group.

-A1=C12 =A1-C1

-A1=SO4 =Al-S04 -Na

-Al=003 =Al-S3 -Na

-Al=SiO3 

and the typical structures are as follows :

Cancrinite. Sodalite.

Concerning these formulae and the peculiar uui- and bivalent radicles assumed in them, there are arguments both for and against. 

The assumption of a group Al=SiO3 , equivalent to and replacing 

 A1=CO;5 , is clearly suggested by the experiments of Lemberg,* who, 

by the action of sodium silicate solution upon eloeolite, obtained a compound which he designates as a cancrinite containing Na2SiO3 in place 

of Na2CO3 . By similar reactions with sodium carbonate he produced a 

substance having the composition of true caucrinite. Hence, whatever 

the ultimate molecular structure of cancriuite may be, we are amply 

justified in assuming in it the replaceability of CO3 by SiO3 .

These experiments fairly represent a large number of like kind which 

are due' to Lemberg, and which may be found recorded in his various 

papers. Some of these will be cited later, but a reference to the work 

of his colaborer Thuguttt 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 scdiuin, 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

* Zeitscb. Deutsch. Geolog. Gesell., 1885, p. 962. t Miueralcheniischo Studien, Dorpat, 1891.
24 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

union. Following Leinberg, 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 ISFaCl 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 formulae 

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 oft' from sodalite the mechanism of the reaction is 

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

unstudied. Until these are understood the objections raised by Lcuiberg, 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 could occur. At all events the formula 

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 sodalitecancriuite minerals are derivable from nephelite, and that nephelite is 

again derivable from them, are unquestionable.

Taking now the formula of a typical garnet, Al2 (SiO4 ) 3Ca3, the question of its molecular structure remains to be considered. .Regarding it 

as a derivative of the normal salt Al4 (SiO4 ) 3 , it may be written in at 

least two ways, thus :

1. 2.

Ca3

--\

\SiO4 =Ca

\ftiO biu4= Ai Al ^\\ \'Si04 =Oa

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.
CLABKE.] GARNET-BIOTITE TYPE. 25

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

the second, expression none; in the first, at least, one calcium atom 

must link two SiO4 groups, while 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 :

SiO4=Ca SiO4=Al-OH

/ >A1-OH / >Ca

Al-SiO4=Ca Al-SiO4=Ca

\Si04 ~Al \SiO4 Al t

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

Additional evidence for the formula assigned to garnet is furnished 

by the composition of vesuvianite, which is most simply represented as 

formed by the coalescence of two garnet molecules, with elimination of 

silica. The following formula) express this relation, and also a relation to epidote, quite perfectly :

 Garnet. Vesuvianitc.

R Al-Si04=. B

If in vesuvianite E.//6 =Ca5Mg, this formula gives for the composition 

of the typical mineral :

SiO.3 ................................................ 38.38

A12 03 ................................................ 19. 55

CaO ................................................ 35.81

MgO...-- ....................................... .... 5.12

H2O ................................................ 1.14

100.00

This agrees well with the published analyses, if we remember that 

ferric iron may .partly replace aluminum, and that fluorine sometimes 

replaces hydroxyl. It is also, sustained by the facts that garnet and 

vesuviauite often occur in contact limestones under similar conditions
26 THE CONSTITUTION OF THE SILICATES. [HULL. 125.

of genesis; that vesuvianite alters into garnet, and also, like garnet, 

mto micas and chlorites; and that both species, as shown by Doelter,* 

yield essentially the same products upon fusion. The analogies connecting them are remarkably suggesti ve and complete, and the formulae 

here proposed render those analogies intelligible. In the Swedish 

"mangan-idocrase" a salt occurs which is doubtless the vesuvianite 

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

Two additional species, meliliteand gehlenite, which, like vesuviauite, 

are also tetragonal, may perhaps be best considered at this stage of 

the discussion. For melilite, the analyses are somewhat discordant; 

but on the whole they agree best with Groth's formula (CaMgNa^e 

{AlFe)2Si 5O] 9 . This may be written so as to resemble the formula of 

vesuviauite, but with the linking SiO4 group replaced by the bivalent 

SiO3 . For gehlenite, the empirical formula Ca3Al2Si2Oi 0 is commonly 

iissumed, but a careful comparison of the published analyses has convinced me that it is not entirely satisfactory. Most of the analyses 

show appreciable amounts of water, but this may be due to alteration; 

for the artificial gehlenite, prepared by the igneous method, is uonhydrated. A different formula, which relates gehleuite to melilite 

and vesuvianite, seems to be preferable; and such a formula is empirically Ca7MgAl4 Si5O24. In this the univalent group AK^ /]&" is to

be assumed, which, occurs also among the clintonite micas, and which, 

in natural gehlenite, may be replaced in part by A1=(OH)2 . .On 

this basis the three species are comparable as follows :

Fesuvianite. Melilite. Gehlenite.

* Al-SiO4= 

")Si04=Al-OH \Si03

 ) ' Al-Si04=

\

= 5

_

JLi

These symbols are not absolutely unobjectionable, but they are useful 

for purposes of correlation. Gehlenite alters into garnet, melilite sometimes occurs among the products of fusion of garnet, and the formulae 

suggest such relations. Furthermore, Lembergt has shown that gehlenite, heated to U00° with a solution of potassium carbonate, gives calcium carbonate and a product having the composition of a potash mica; 

while similar treatment with sodium carbonate converts the mineral 

into caucrinite. Gehlenite, garnet, cancrinite, and muscovite are therefore related to one another, and this fact is expressed by the formuhe 

proposed. In melilite the calcium is partly replaced by sodium, and

*Allgem. Chem. Mineralogie, p. 18:5. 1 Zeitsch. Deutsch. Geolog. Gesell., 1892, p. 237.
CLABKE.] GARNET-BIOTITE TYPE. 27

in some gehlenites obtained from furnace slags the same replacement 

has been observed.*

At the Orawitza locality, gehleuite occurs in rolled pebbles containing grains of vesuvianite. As this gehlenite contains little or no water, 

we may compare Janovsky's analysis of itt Avith the results obtained 

from calculation, both with the formula Ca3Al2Si^Oi0 , and with the new 

formula Al2 (SiO4 )5K"G (AlO2R")2 , when R"8 =Ca7Mg.

The results are as follows:

Si02 .... ............

AlsOs. ............

Fe.2 03 . ..............

FeO ................

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

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

Igu .................

Janovsky.

32.39

18.53

1.25

3.61

37. 65

6.69

.51

100. 63

Old 

formula.

30.77

) 26.15

\

? 43.08

100. 00

. New 

formula.

32. 05

21.80

41.88

4,27

100. 00

Neither formula compares perfectly with the analysis, but the new 

is rather better than the old. With some other analyses this advantage is reversed. We need new data, based upon purer and more definite material than seems to have been as yet collected. If the old 

formula should be sustained, then it might be written structurally in 

either of two ways, thus:

,O Al-O

Al SiO4= , Ca3 or

/Si04=Ca(AlO) 

Ns SiO4=Ca(AlO)

The first formula would serve to connect gehlenite with garnet and the 

micas, while the second represents a derivative of calcium orthosilicate 

and fails to indicate the various relationships of the mineral to other 

species of this group. Neither formula, hoAvever, is so suggestive as 

the one which is based upon the type of vesuvianite.

Another alternative is offered by the theory of Vogt,| who regards 

melilite and gehlenite as varying mixtures of two end products; one, 

the "gehlenite silicate," R///2R//3Si2O10, and the other, akevmannite, 

R"4 Si 3O10 . The last compound is a basic silicate, found in slags, Avhich 

needs further study. Vogt's theory is plausible, and, if established, Avill 

take the tAVO species in question, gehlenite and melilite, out of the present category as related minerals to vesuvianite and garnet.

The little-known mineral arctolite is possibly another member of

* See Hintze, Handbuch der Min., p. 310.

1 Dana's System of Mineralogy, 6th ed., p. 476.

| Neues Jahrb., J892, II, p. 73.
28 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

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

expressed by the formula

A]2 (SiO4 ) 3CaMgBv

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

this species, however, is not yet fully established.

III. THE FELDSPARS AND SCAPOLITES.

Although orthosilicic and trisilicic acids are technically distinct, and 

from a chemical 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 Tscherinak has met 

with general acceptance. In the case of the feldspars, Tscherinak was 

undoubtedly anticipated in great part by Hunt, Waltershausen, and 

others, but he gave to the theory its most precise statement, and to him 

its final establishment is due. According to this theory the triclinic 

plagioclase feldspars consist of albite, ALNaSi3O8 , and anorthite, Al2Ca 

(SiO4 )2 ; which, commingled in various proportions, give the intermediate 

oligoclase, labradorite, andesite, and so on. There are also.the triclinic 

microclme and its monoclinic equivalent, orthoclase, both represented 

by the formula AlKSi3O8 , and the barium feldspars, such as hyalophane, 

which may be regarded as mixtures of orthoclase with a barium salt 

analogous to anorthite in composition. Microcline and orthoclase, from 

a chemical point of view, may be considered as isomersj and so also 

may albite and soda-orthoclase; but the nature of the isomerisin is not 

clear. It may be due to the structure of the salt as such, or to the 

existence of two isomeric acids H4Si3O8 , and both possibilities are conceivable, For present purposes this problem may be left outstanding, 

and our attention can be confined to the two typical compounds, 

anorthite and albite. In the discussion, later, of the species eudidymite 

and epididymite, the question of isomeric trisilicates will come up.

For the scapolite series Tscherinak has elaborated a theory which is 

closely parallel to that of the feldspars. These tetragonal minerals 

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

compounds, meionite, AlB Ca4 Si(iO25, and marialite, Al3N"a4Si9O24Cl. 

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 

Tscherniak'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 into
CLARICE. ] FELDSPARS AND SCAPOLITES. 29

inuscovite, or pinite, which is only an impure pseudomorphous mica; 

and kaolin, crystallographically, has close relations with the mica 

family. Feldspars, scapolites, inuscovite, arid kaolin are therefore 

presumably connected, and the structural formulae of the minerals 

should render the relationship apparent.

The empirical formulae to be considered are now as follows: 

Albite ............................................. .AINaSiA,

Auorthite ........................................... Al2CaSi2O8

Meionite ............................................ A]6Ca4 SiuO25

Mafialite ......................................... . Al3Na4Si9O24Gl

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

Kaolin. ............................................ . Al2H4 Si2O9

For nmscovite the constitution has already been indicated; and this 

clew, 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 formulae of albite and anorthite, and we 

have the following expressions :

A Ibite. Anortldte.

, 38 

Al Si3O8=Al

,

Al 

Ca

Marialite.

/ >A1 01

Al Si3O8=Fa2

Muscovite.

, 

Al SiO4nAl

, 4-= 

Al SiO4=Al

Meionite.

xSiO4= 

l SiO4E

Ca

I 

O

I

Ca

I 

SiO4=Ca /

Al SiO4^

Kaolin. 

,OH

Al SiO4
30 THE CONSTITUTION OF THE SILICATES. [BULL.125.

On this basis anorthite becomes the calcium equivalent of nephelite, 

which latter species is also alterable into kaolin. Again, garnets are 

known to alter into feldspars and scapolite; and,according toBrauns,* 

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. I. have already called attention to the facts 

that anorthite and ineionite occur among the. products of fusion of 

garnet, and these data fit in well with the others. So far the forrnuke 

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.

Closely allied to the feldspar in its petrographic relations is the 

isometric mineral leucite, AlKSi2O6 . Empirically it seems to be a 

metasilicate, and is commonly so regarded, but it may easily be conceived *as a mixed salt, containing ortho- and trisilicate molecules. By 

alteration it yields orthoclase, nephelite, muscovite, and kaolin, and the 

pseudo-leucite of Magnet Cove has been shown by J. F. Williams to 

consist of orthoclase and elseolite intimately commingled.t 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 J have obtained leucite 

synthetically from muscovite as a starting point, orthoclase and nephelite being produced at the same time; and Leinberg, § in his 

experiments, has transformed leucite into sanidine, anorthite, and 

inicrosomniite, and also into andesine. In a later paper || Leinberg 

describes the action upon various silicates of the salt Na-2 SiO3 , SH,O, at 

200° under pressure, kaolin, albite, elreolite, leucite, and analcite all 

yielding a silicate-cancrinite containing SiO3 in place of CO3 . These 

facts connect the several species together, but to their explanation the

empirical expression AlKi3i2O6 gives no clew. A formula for leucite, 

to be satisfactory, must be a multiple df this, and several such multiples fulfill the conditions of the problem.

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

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

formula Al4K4Si8O24 . We then have, as a distinct possibility, the following series of molecules:

Garnet. Sodalite. Leucite.

SiO4=Na2 SiO4 = K2

1_C1 / >Al-SiO4~Al 

Al-Si3O8=K2

*Neues Jahrbnch, 1892, II, p. 1.

t Ann. Kept. Geol. Survey of Arkansas, 1890, II, p. 267 et seq.

I Bull. Soc. Min., XIII, p. 134, 1890.

§ Zeitsch. Geol. Gesell., XXVIII, pp. 611-615, 1876.

|| Zeitsch. Geol. Gesell., pp. 961-2, 1885.
CLARKE.] FELDSPARS AND SCAPOLITES. 31

Upon tbis basis leucite is clearly reduced to the uniform type of the 

minerals to which it is related crystallographically, 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.

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

empirically, has the composition AUsraSi2O6 ,H2O. 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 leucite, 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, aualcite may be written

= Na2

>Al-Si04EH(A10H) 

8=Na2 3H20

one-fourth of the water being regarded as constitutional. This last 

supposition, however, needs to be checked by careful experimeuts upon 

dehydration, for the data now on record are not absolutely conclusive.! 

A change in this detail would modify the' formula given, but would not 

affect its general type.

Still another isometric mineral, pollucite, should possibly be considered here. Two empirical formulas deserve to be regar'ded : one, based 

upon analyses of the Elba mineral, and given by Groth, H2Cs2Al2Si5O| 5 ; 

the other, deduced by Wells J for the Maine pollucite, HoCs4Al4SigO27. 

Both formulas give metasilicate ratios, but both are reducible to the 

type of leucite and analcite, thus:

JUlba. Maine.

Si3O8=Cs2 /Si3O8HCs2H

/ >Al-Si04=H2 (AlH202 ) ./«. 0 _r Al-Si,08=Cs2 . Al--Si3 08 ~Csa n

\Si3 0-Al \Si303=(A10)3

Neither formula can be regarded as conclusive, and their only value 

here is to show that pollucite is not necessarily a metasilicate, and -that 

it can be represented as similar in constitution to the minerals with 

which it is morphologically allied. Wells's analyses are probably the 

better; but the formula drawn from the Elba pollucite indicates the 

closer analogy to leucite and aualcite.

Although kaolin, mineralogically, is not a member of the feldspar 

group, it is properly discussable 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

* Dana'8 System of Mineralogy, 6th oil., p. 598.

t Compare Herscli, Der Wassergehalt der Zeolitho, Zurich,

J Am. Jour. Sci. (3) XLI, p. 213, 1891.

1887.
32 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

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 \vater 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) Si2O7 .H4 (AlO)2

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

(3) Si2O5.(AlH2O2 )2 . Groth's.

(4) H-0

Al=Si04=H2 

< 5) 0< Al=Si04=H2

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

trisilicate, thus :

,

(6) Al Si3O8=H.( A1H2O2 ) 2 

\Si308EE(AlH202 ) 3

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

two, the formula

/OH 

Al SiO4^H3

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 i* attainable. 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 

Al3Si2O7 as the result of ignition, but other interpretations are possible. The data are given here simply as data, which 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 formulas are in 

harmony with that chosen for kaolin.

IV. THE NORMAL ZEOLITES.

By this title is indicated a well-defined group of hydrous silicates, 

unmistakably related to nephelite and the feldspars, and for which the 

term feldspatMc zeolites would be equally appropriate. A few zeolitic 

species, ptilolite, mordemte,laubanite, etc., are excluded from the present
CLABKE.] . NORMAL ZEOLITES. 33

scheme, as being from a chemical point of view more properly placed 

elsewhere.

Three of the minerals in this group have already been mentioned, 

namely, hydronephelite, natrolite, and analcite. All of these occur as 

alteration products of eloeolite or nephelite,* and natrolite yields nephelite again upon fusion. Natrolite and analcite are both derivable by 

natural processes from albitejt analcite, as shown in the. preceding 

section, yields felclspathic pseudomorphs, and the relations of analcite 

to leucite are beyond controversy. From sodalite, both natrolite and 

hydronephelite may be generated, and from nephelite, by artificial 

means, Doelter lias produced analcite and natrolite.$ All of these relations, with others, both morphological and genetic, are covered by the 

formulfe which have already been developed, but which, in part at least, 

may be advantageously repeated here. Thus we have :

Nephelite. HydronepMite. Natrolite.

43 , 

Al Si04=Al Al Si04=Al. 3H2O Al SiO4=

Albite. Analcite.

Qi n VQ /SiO4==N"a2

^/bisUa^aa / >Al-Si04H(A10H)H

Al Si 3O8=Al Al Si;A=Na2 3H2O

For the full significance of these expressions the previous sections 

must be consulted; the analcite formula being correlated with the 

formula; of garnet, leucite, and th ,. minerals of the sodalite group.

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.§ The same investigator, after fusion of various zeolites and 

subsequent slow cooling, found anorthite among the substances produced by chabazite, heulandite, stilbite, scolecite, laumontite, and 

thomsonite.il In some cases anorthite was the chief product of fusion: 

in others it was subordinate to something else. Again, by various wet 

reactions, some of them unfortunately involving several stages, Lernberg 

has generated analcite 1] from chabazite, gmelinite, laumontite, harinotome, phillipsite, stilbite, and hi ulandite, in some cases studying several 

varieties of the same species. It is clear, therefore, that the connection between the zeolites and the feldspars is unquestionable, and it

* See Brogger, Zeit. Kryst. Min., XVI, p. 223 et seq.

tNoues Jahrb., 1892, II, p. 1; Brauns.

JNeues Jahrb., 1890, 1, p. 134.

§Nenea Jahrb., 1890, I, p. 128, et seq.

||Neues Jahrb., 1890, I, p. 118, and Allgem. Chein. Mineralogie, p. 183.

IT Zeitsch. Geolog. Gesell., 1885, p. 959 et seq.

. .Bull. 125 3
34 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

only remains to work out the details for each individual mineral. The 

fundamental relations between the two groups have been recognized 

by many writers; 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 in 

any case what part of the water is constitutional and what, crystalline 

is not easy, and no fixed criterion exists upon which judgment may be 

based. Water which is retained by a mineral above 300° of temperature is almost certainly constitutional; water lost at or below 100° is 

probably water of crystallization; but between the two there is a range 

of considerable vagueness. There is, however, one datum which has 

claims to high, consideration. Certain zeolites, at various temperatures between the limits indicated, lose water which is regained without change of crystalline character upon subsequent exposure of the 

minerals to moist air, and this, as has been urged by several authorities, should be regarded as water of crystallization. By considerations of this kind we can be guided to some extent, but beyond them 

each zeolite must be studied independently, in the light of such collateral evidence as may be available. For some species the evidence 

is full, for others it is meager; but such as it is it will appear in the 

systematic discussion following.

In a recent publication * I have shown how all of the normal zeolites 

may be formulated as substitution derivatives of the feldspars, although 

in some cases alternative formulte are readily conceivable. Thus, from 

anorthite several species are directly derivable, as follows :

Anortltite. Thomsonite.

,SiO4=Al ,SiO4=H2 .AlH2O2 

' Al-SiO4=Al Al-SiO4ZEAl

Ca Ca + 3H2O

Al-Si04=Al Al-SiO4=Al

Gismondite. Foresite.

Al SiO4 .=H2 .AlH2O2 A.1 SiO4=H3 

\SiO4=Ca

Ca +H2O

, 4= , 

Al SiO4=H2 .AlH2O2 Al SiO4=H3 

NN-SiO4=H2 .AlH2O2

Am. Jour. Sci. (3), XL VIII, p. 187, 1894.
OLABRE.] NORMAL ZEOLITES. 35

Of these formulae, the last one, that of foresite, is uncertain, on 

account of great variations in the published analyses. It is therefore 

to be regarded as approximate only, and as peculiarly subject to revision. For gisinondite the composition is empirically that given by 

Dana, and the calcium is partly replaceable by potassium. Gismondite is said to give off one-third of its water at 100°, and in the formula 

that amount is represented as water of crystallization. For thornsonite 

the evidence is much fuller. Here four-sevenths of the water is 

regarded as constitutional, a proportion which is fairly in accord with 

the data by Damour and by Hersch, as cited in Dana.* In nature 

thomsouite often occurs as a derivative of elseolite, while, according to 

Doelter (loc. cit.), it yields auorthite upon fusion. In some cases thomsonite, variety mesole, carries an excess of silica, which may be attributed to an admixture of trisilicate groups; that is, to Si3O8 replacing

SiG4 , Iii a tliomsonite derived from a plagioclase feldspar this should

be expected, and so also should the almost invariable replacement of 

a part of the calcium by sodium. Nonsodic tliomsonite exists, but is 

probably rare. A thomsonite containing Ca2Na2 in place of Ca3 would 

have the following theoretical composition:

SiO,............................................... 37.27

AljOj............................................... 31.68

CaO ............................................... 11.59

Na.2 0 .............................................. 6.42

H2 0 ............................................... 13.04

100.00 

fH2O (water of crystallization)..................... 5.59

This agrees well with published analyses, so that the formula proposed satisfies all the conditions imposed by the~evideuce and also by 

the general substitution theory.

In scolecite, Al4Ga2H8(SiO:t)6 .2H2O, we find a zeolite with relations to 

natrolite on one side and to anorthite on the other. Approximately 

one-third of its water is given off at temperatures below 300°, to be 

taken up again in moist air, and on fusion, according to Doelter, it yields 

auorthite. By the slow action of soda solutions it is transformed 

into natrolite and mesolite, while natrolite, by means of calcium 

chloride, is convertible into scolecite.t Mesolite, which is intermediate 

between natrolite and scolecite, may be regarded as a mixture of the 

two, or it may be formulated as an independent compound. Written 

on the anorthite type, scolecite and mesolite become

* Syat. Mini, Cth cd., p. COS. Compare Herscli, Der Wassergehnlt der Zeolithe, Zurich, 1887. 

tLemberg, Zcitsch. Geol. Gesell., XXVIII, p. 551, 1876.
36 THE CONSTITUTION OF THE 'SILICATES. [BULL. 125. 

Scolecite. Mesolite.

, 

Al-SiO4 ~H3 Al-SiO4~H3

Ca +2H2O Ca +H2O

. 

Al-Si64=H3 Al-SiO4=H3

the two species being possibly iniscible. On the other hand, the scolecite formula may be halved and correlated with that of natrolite, thus :

Natrolite. Scolecite. 

SiO "NTflTT SiO4=H2 //bl(J4 i>au2 / >Ca 

Al-SiO.~NaH2 Al-SiO4=H2 +H2O

Both formulae for scolecite satisfy the evidence, but between them it 

is difficult to decide. The existence of niesolite may perhaps turn the 

scale in favor of the anorthite structure.

By the action of barium chloride solution upon an artificial natrolite, 

Lemberg* obtained a barium compound which may be the equivalent 

of scolecite. It is also near the tetragonal edingtonite in composition. 

The evidence as to the constitution of edingtonite, however, is very 

imperfect, and dehydration data are entirely wanting. It may also be 

represented as the barium equivalent of the monoclinic gismondite, the 

difference in. form being due to a difference of combination on the part 

of the water. At all events, it is highly probable that edingtonite is a

derivative of a barium feldspar.

In the rhombohedral zeolites, gmelinite, levynite, and chabazite with 

its varieties, the ratios are approximately metasilicate, but somewhat 

variable. In this respect they resemble the feldspars, and the pseudometasilicate character is ascribable to mixtures of ortho- and trisilicates, with sometimes one and sometimes the other predominating. 

Gmelinite is essentially a sodium zeolite, with comparatively little or 

no lime in its conrposition, and when lime is present it can be 

attributed to a hydrous anorthite with 18 molecules of water. Apart 

from the lime, gmelinite is easily interpreted as a mixture of two silicates of precisely similar chemical type; one an albite, and the other 

its orthosilicic equivalent, thus:

/sf;, . x 

Al Si3O8^Al +9H2O Al SiO4=Al +9H2O

Zeitsch. Geol. Gesell., XXVIII, p. 553, 1876.
CLAHKE.] NORMAL ZEOLITES. 37

Calculating for mixtures in the ratios of 1:1 and 2:1, we get from these 

formulas the following composition of gmeliuite, in comparison with 

analyses by Bammelsberg and by Pirsson: *

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

Al»03 .~. .........

Fe,03 ............

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

NajO... ..........

K r\

H o

Rammelsberg.

46.48

20.64

3.78

7.19

1.74

20.41

100. 24

Pirsson.

50.67

18..50

.15

1.05

9.88

.16

20.15

100. 56

1:1.

46.89

19.92

12.11

21.08

100. 00

2:1.

50.89

18.21

11.27

19.63

100. 00

The water is probably all crystalline, thus distinguishing gmelinite 

from a sodium chabazite ; but the evidence on this point is incomplete. 

Pirssou's analysis may also be represented by a mixed_salt of this character :

,SiO4 =Na3 

Al-Si3O8=Al +9H2O

indicating a relationship to analcite. Such a relation is suggested by 

the experiments of Leinberg, t who not only derived analcite from the 

zeolites of this group, but also, starting from fused analcite, obtained 

a compound approximating to gmelinite in composition.

For chabazite the fundamental ratios are essentially the same as with 

gmelinite, but the proportions between lime and soda vary greatly. 

Potassium is also present to a noteworthy extent in some cases, indicating a partial derivation of the mineral from leucite, orthoclase, or microcline. As regards hydratiou, a part of the water in chabazite appears 

to be constitutional, probably two molecules out of the nine shown in 

the gmelinite formula, but this point is not absolutely certain. The 

evidence in its favor is strong, but not conclusive.

The analyses of chabazite are numerous. Some of them reduce easily 

to mixtures of anorthite+18 aq. and albite+9 aq., and some, show.- 

ing little lime, represent isomers of gmelinite. But many chabazites 

are very low in alkalies, while the ratio between Si3O8 and SiO4 remains 

practically constant. In these cases it seems necessary to assume the 

existence of a trisilicate anorthite, or calcium albite, which, by itself, 

is not known in nature. According to Doelter,J chabazite yields upon 

fusion a mixture of anorthite and silica, a fact which is in harmony

* Dana; 6th ed., p. 594.

t Zeitsch. Geol. Gesell., 1885, p, 972.

| Allgem. Cbem. Mineralogie, p. 183.
38 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

with the foregoing conjecture. Taking all the data into account, chabazite may be formulated as a mixture of the compound

Al-SiO4=H2 .AlH202

Ca +14H2O

with the salts

x

Al - Si3O8=H2 . A1H2O2 

\Si3083EAl

Al-SiO4^H2.AlH2O2 . +7H2O Al-Si3O8^H2,AlH2O2 . +7H2O

the latter being isomeric with the gmelinite molecules. A sodium 

chabazite agrees in empirical composition with gmelinite, and the calcium salt by itself has the following composition, as compared with 

chabazite from Aussig, as analyzed by Lemberg:*

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

A12 03 .............................

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

NajO.... ..........................

K2 0. ..............................

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

Found.

47.50

20.00

10.20

.23

1.24

21.40

100. 57

Calculated.

47.43

20.15

11. 07

21.35

100. 00

It is perfectly evident that the calcium chabazite may also be written 

as an equimolecular mixture derived from anorthite and its trisilicate 

equivalent; and in levynite, the last member of the rhombohedral group, 

we have a mineral of the same type, although differing in hydration. 

Instead of 18 molecules of water, as in chabazite, levynite contains 

only 15; but in both cases 4 molecules are to be regarded as constitutional, in accordance with the observations made by Damour. 

Computing from Hillebrand's analyses,t levynite contains the orthoand trisilicate molecules in the ratio of 3:2, giving the following empirical composition to the species:

3Al6 (SiO4 ) 6Ca3 . 15H2O + 2Al6 (Si3O3 )6Oa3 . 15H2O,

* Analysis No. 4 in Dana's table, Syst. Miii., p. 591. 

t Bull. U. S. Geol. Surv. No. 20, p. 38.
CLAHKE.] NORMAL ZEOLITES.

with the subjoined comparison between observation and theory:

39

Si02 ..............................

AliOg .............................

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

Na*O .............................

K2O ..............................

H.2 0 ..............................

Found.

47.76 

21.91 

11.12 

1.34 

.21 

18.65

99. 99

Calculated.

I

46.55 

21. 98 

12.07

19.40

100. 00

Here, as with the preceding minerals of this group, the accordance 

is quite satisfactory. 

One more point with regard to chabazite remains to be noted,

According to Kenngott,* a crystal of this mineral, heated to nearly 

the melting point of glass, retains its form and luster, but loses transparency. If, after cooling, such a dehydrated crystal be moistened 

with a drop of water, the latter is taken up with evolution of much 

heat. This observation may possibly imply that the constitutional 

hydroxyl of chabazite is united wholly or in part with calcium, in 

which case the formula as written by me should be correspondingly 

modified. CaOH would appear in it m place of A1H2O2 , but the fundamental type need not be thereby changed. The question thus raised 

suggests profitable lines for experimental investigation, which ought 

to shed much light upon the constitution of this and other allied 

minerals.

The supposition that the minerals of the chabazite group may be 

partly derived from a trisilicate anorthite is strongly sustained by the 

composition of stilbite, which, most typically, is represented by the 

empirical formula Ca3Al6 (Si3O8)fi + 18H2O. This requires

Si02 ............................................... 57.51

A12O3 .............................................. 16.29

CaO............................................... 8.94

H3 0 ............................................... 17.26

100.00

which agrees with many analyses. Some varieties of stilbite show less 

silica, which may be due to the presence of orthosilicate molecules, and 

in most cases a little soda or potash replaces lime. The latter fact represents the presence of albite, eheolite, or orthoclase among the parent 

minerals of the zeolite, and involves no change in the structure of the 

molecule. So far as can be determined from the observations of Hersch, 

about two-ninths of the water iu stilbite is retained at moderately high

' Quoted by Herscb, p. 36.
40 THE CONSTITUTION OF THE SILICATES. [BOLL. 125.

temperatures, and may therefore be regarded as constitutional. On 

this basis the formula of the mineral becomes

,Si308=Al 

Al Si308=H2:AlH2O8

Ca +14H2O

Si3O8=Ga 

1 Si3O8=H2.AlH2O2

which, in type, resembles that of chabazite, although the two species 

are crystallographically dissimilar.

To heulandite and epistilbite, two isomeric compounds, essentially the 

same formula belongs, but with less water of crystallization. Calculated with Al6 (Si3O8 )6Ca3.16H2O, we have for their composition

 SiO-j. ............................................... 58.63

Al,03 . ................ ............................. 16.61 ;

CaO ............................................... 9.12

H2O.... .......,...............................;.... 15.64 .

 ' 100- 00

in close agreement with analysis. Hersch's work on heulaudite shows 

about three-fourths of the water, or 12 molecules, to be easily 

expelled, leaving 4 molecules, as in stilbite, constitutional. Hence 

its structural formula may be written like that of stilbite, with 12

instead of 14 H20; or, Al4 (Si308 )6Ca3H4 (AlH202 )2 .12H20. Brewsterite 

appears to be like heulandite,"but with barium and strontium replacing 

calcium. ...."..

According to Doelter, stilbite and heulandite, on fusion, yield a pyroxenic mineral with silica and an amorphous ground mass,-and sometimes 

also anorthite. The same chemist* also effected the synthesis of heulandite from anorthite by digesting the latter with freshly precipitated 

silica and carbonated water at a temperature of 200°. These data are 

but partially interpreted by the constitutional formula here given, and 

for which alternative expressions are possible. A connection with 

anorthite is established, but the isomerism between heulandite and 

epistilbite, and the formation of a substance resembling pyroxene from 

stilbite and heulandite, remain to be accounted for. Possibly epistilbite

- Keues Jahrb.', 1890, I, p. 128.
CLARKE.] NOEMAL ZEOLITES. 41

may have the formula already given for this group, while stilbite and 

heulandite may contain the molecule

Al SiO4=

Oa

I 

Si3Oe=:Ca

Al Si3O8=H3

plus 8H2O in one species and CH2O in the other. This formula was 

adopted in my former paper on the zeolites, and its configuration, with

eight atoms of acid hydrogen, would indicate a complex breaking up 

upon complete dehydration. Between the two forms of expression, 

further investigation must decide. Both are in harmony with the fundamental theory, so that the questions at issue are mainly questions of 

detail.

To stilbite, morphologically, the species harmotome and phillipsite are closely allied. To harmotome may be assigned the formula 

Al6 (SiO4)2(Si3O8 )4Ba3 .14H2O, which requires

SiOi. .............................................. 45.23

Alj03 ....................'.-...-.................... 16.48

BaO ............................................... 24. ',2

H,O ............................................... 13.57

100.00

A little barium is commonly replaced by potassium or sodium, perhaps, or even probably as Al3 (Si3O8)3K3 .7H2O, the water being loosely 

combined and therefore all crystalline. In all probability harmotome 

is a derivative by hydration of the barium feldspar, hyalophane.

In phillipsite, which, since the investigations of Fresenius, has been 

generally recognized as the calcium equivalent of harmotome, we find 

a larger proportion of the bivalent metal replaced by potassium or 

sodium, and a more variable ratio between SiO4 and Si3O8 . A typical 

phillipsite approaches nearly to the formula AlG (SiO4 ) 3 (Si3O8 ) 3Ca2K2 . 

14H2O, which requires

Si02 ............................................... 48.52

Al,03 .............................................. 20.63

CaO .....................-...............:.....--.. 7.54

K2 0 ......................... ...................... 6.33

H2 0 ............................................... 1P.98

100.00
42 THE CONSTITUTION OF THE SILICATES. [BOLL. 125.

Here again no water is constitutional. In my former paper I assigned 

to both phillipsite and harmotome a hydration of 15 molecules, but 14 

agrees better with the published analyses. In some cases the calcium 

of phillipsite is lower and the alkaline metals higher than is shown by 

the ratio Ca2K2 , but the ratio CaK'4 does not appear to be reached, 

unless by the presence of H .among the components of B'4 The general formula for phillipsite and harmotome, as shown by all trustworthy 

analyses, is'Al6X6B//3 .14H2O, when X represents SiO4 +Si3O8 and R" 

is made to include the equivalent of the alkalies. Fusion experiments 

upon the two species seem to be wanting.

To laumontite, CaAl2Si4Oi 2 .4H2O, two constitutional formulae may 

be assigned, as follows :

,SiO4=Al ,SiO4=Al

Al SiO4HAl Al SiO4=H3 

Ca \SiO4=Ca

Ca +12H2O Oa +4H2O

38 , 

Al Si3O8=Al Al SiO4HH3

Both expressions give the same empirical composition, but the first 

is the more symmetrical. The second indicates a ratio between constitutional and crystalline water which agrees with the somewhat incomplete recorded data, but the first is adjustable in such a manner as to 

represent the same facts. According to Doelter,* laumontite upon 

fusion gives anorthite, together with a pyroxenic mineral and'an ainorphous ground mass, and this evidence may harmonize with either.

formula. A fuller experimental investigation of this species is evidently necessary.

For the.isometric faujasite the formula may be written structurally 

in two ways. One is in uniformity with the expressions assigned to the 

previous species, and is

Al4 (SiO4 )4 (Si3O8 )2Na2CaH8 .15H2O

The other is based upon the analcite formula, as developed in the preceding chapter, thus:

Si3O8

/ >Al-SiO4=H2 .AlH2O2 

Al Si3O8=Ca +15H2O

.Nsi3O8=H2 .AlH2Q8

Keues Jahrb., 1890, 1, p. 130.
CLAHKE.] NORMAL ZEOLITES. 43

From these, which are empirically identical, we get the subjoined 

comparison between theory and the analysis by Damour :*

SiOs.. .............................

AL2O3 .... ........... ..............

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

Na2 0.. ......... ..................

H2 0...... .........................

15HS 0 ...........................

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 

21.36

According to Jannasch,t faujasite loses 10.88 per cent of water at 

105°-110°. At 150° it loses 16.83, about 12 molecules, which is regained 

in moist air. Between 250° and 260° it loses 22.67 per cent, and the 

remainder of the water is expelled upon ignition. Of the two formulae 

given, the one corresponding to the leucite-analcite type is preferable, 

for it represents the morphological analogy between, the species. 

Furthermore, faujasite on treatment with hydrochloric acid is decomposed, but does not gelatinize, a fact which may indicate a predominance of trisilicate over orthosilicate groups. This datum, however, is 

of questionable significance.

One more zeolite, the little-known species offretite, remains to be 

considered in this connection. This may be represented as a mixture of 

two molecules,

Al Si3O8=Al. 8H2O, and Al SiO4=Al. 8H2O,

in the ratio of 2 :1. This gives, in comparison with Gonnard's analysis, 

the following composition:

Si03 . .................. ............

AljOs.... ..........................

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

KsO. ...............................

HaO. ..............................

Found, i

52.47

19.06

2.43

7.72

18.90

100. 58

Calculated.

52.02

18.95

2.30

7.77

18.96

100. 00

This species is evidently a derivative of orthoclase and anorthite, 

and the calcium term in the foregoing formula may evidently be written on the anorthite type, namely, as Al^SiO^oCaaHa.ieHgO. The

* Dana's System, 6th ed., p. 598. 

tNeues Jahrb., 1887, II, p. 24. 

} Dana's System, 6tli ed., p. 1043.
44 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

character of its hydration, other than is shown in the formula as given, 

remains to be determined, The mineral is decomposable by acids with 

difficulty.

From the evidence given in the foregoing pages the constitutional 

and genetic connection between the normal zeolites and the feldspars 

seems to be perfectly clear, and it ought to be easily emphasized by 

investigations along petrographic lines. 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 laumoutite, heulandite, stilbite, and analcite alter into albite or orthoclase; laumontite 

and stilbite into analcite; chabazite intouatrolite; and gismondite into 

phillipsite. So also alterations into prehnite are recorded on the part 

of iaumontite, scolecite, mesolite, natrolite, and analcite; and the 

identity of chemical type seems to be almost unquestionable. From 

the formula; here developed all of these alterations become intelligible, 

and the theory of substitution from normal salts is very emphatically 

sustained. For the group of zeolites it only remains to summarize the 

expressions which have been adopted.

For starting points the following minerals are taken : 

Nephelite................................ '............ Al3 (SiO4)3lsra3

Albite and orthoclase................................. Al^SisOs^It's

Anorthite....'..................................... -. .. Al6 (SiO4 ) 6Ca3

Trisilicic anorthite.................................... Al6 (Si3O8 )6Ca3

The last compound is purely hypothetical. To these may be added 

leucite, from which analcite and faujasite are derivable, and the bariumstrontium feldspars which give rise to brewsterite, edingtouite, and 

harmotome. Brewsterite> as the equivalent of heulandite, and harniotome, as a barium phillipsite, need not be considered further; and as 

for edingtonite, its constitution is very uncertain. The remaining zeolites are as follows, with the difference between water of constitution 

and water of crystallization disregarded, except in the few cases in 

which the distinction affects the integrity of the molecular type. The 

symbol X is used to express SiO4 or Si3O8 indiscriminately:

TypeM3XsR'5 . 

Hydronephelite ...................... Al3 (SiO4 ) 3Na2H, 3H2O

Offretite.............................. Al3 (SiO4 )(Si3O8 )2E/3, 8H2O

Gmelinite......... .................. Al3 (SiO4 )(Si3O8 )2Na3 , 9H2O

Type Al0XK Ca^.

Thomsonite ...............'........... Al6 (SiO4)6Ca3, 7H2O

Gismondite .......................... Al6 (SiO4 )6Ca3 ,12H2O

Laumontite ..........................Al6 (SiO4 )3 (Si3O8 ) 3Ca3 ,12H2O

Phillipsite ........................... Al6 (SiO4 )3 (Si3O8 ) 3Ca2K2 ,14H2O

Levynite.............................. Al6X6Ca3 ,15H2O

Heulandite, epistilbite ................ Al6 (Si3O8 ) 6Ca3,16H2O

Stilbite .............................. Al6(Si3O8 ) 6Ca3 ,18H2O

Chabazite............................ Al6(SiO4 )3Si3O8 ) 3Ca3 ,18H2O
CLAKKE.] . MICAS AND CHLORITES. 45

Type AkXzR'6.

Natrolite ........................... .

Prehuite ............................ . Al2 (SiO4 ) 3Ca5>H2

Scolecite ............................ . Al2 (SiO4 )3CaH4, H2O

Type AliXeCctc.

Mesolite ............................ . Al4 (SiO4 )6Ca]S'a2H 8,H2O

Foresite ............................ . Al4 (SiO4 )6CaH10 , H2O

Some of these formulae are conditioned by the replacements and 

alternatives cited in the previous pages and in my former paper upon 

the zeolites, and they are further modified by the character of the 

hydration. The expressions for analcite and faujasite, as based upon 

the leucite type, need not be repeated.

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 studying 

the mechanism of that change this possibility ought to be considered.

V. THE MICAS AND CHLOEITES.

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 arise from the alteration of 

other silicates, while these in some cases have been synthetically

derived from micaceous material. f 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(SiO4) 3

Muscovite. ....................................... Al3 (SiO4 )3KH2

Normal biotite ................................... . Al2 (SiO4 ) 3Mg2KH

Normal phlogopite .......................... ..... . Al(SiO4 )3Mg3KH2

No further substitution of the same order is possible, for the reason 

that its formation would remove the linking atom of aluminum, and 

break up the fundamental molecule.

Muscovite, the first species in the foregoing series, occurs iu nature 

as an independent mineral, and also as an alteration product of 

nephelite, eucryptite, topaz, audalusite, the feldspars, the scapolites, 

and various other natural silicates. All of these alterations become
46 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

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 (SiO4 ) 3NaH2 . Secondly, in fuchsite, the chromic 

mica, a chromium salt partly replaces the aluminum compound, and 

similar ferric replacements are also known. En all of these variations, 

however, there is no change of ratios, and the typical formula remains 

undisturbed. In the barium niica, cellacherite, there is possibly a 

molecule of the type Al3 (SiO4 ) 3K//E/, but this is uncertain. Other 

interpretations of the barium micas are admissible.

The most important variation in muscovite is in the direction of 

increased silica. Normal muscovite contains 45.3 per cent of SiO2 , 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 (Si3O8 )3KH2 is identical in type with ordinary orthosilicate 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 (SiO4 ) 3B/3 +Al3 (Si3O8 ) 3K/3, in which the latter molecule varies from zero to fifty per cent, and with ferric iron or chromium 

sometimes replacing aluminum.

With the biotites and phlogopites the variability of composition is 

even greater than in the case of muscovite. For the typical molecules 

the composition would be

.

Si03 ..........

A12 03 .. .......

MgO.... ......

K3 O ..........

H,O ..........

Al2 (Si04 ) 3Mg2KH.

' 43. 06

24. 40

19.14

11.25

2.15

100. 00

Al(Si04 ) 3Mg3KH2 .

43.27

12.26

28.85

11.29

4.33

100. 00

To these types but few natural micas actually correspond, although 

intermediate mixtures are very common. Furthermore, ferrous salts 

frequently replace the magnesium compounds, and ferric molecules 

replace those of aluminum. Manganese, also, sometimes occurs among 

the dyad bases, and occasionally trisilicates in small amount are commingled with the ortho-salts. Because of these complications, the 

reduction of a given analysis to a specific formula may be very difficult, 

and this difficulty is often increased by uncertainty in the determination 

of water, or of the state of oxidation of the iron. In spite of these 

difficulties, however, the reduction is generally possible, and the ferro-
CLARKE.] MICAS AND CHLORITES. 47

magnesian micas, with some exceptions to be noted further on, are 

rem-esentable as mixtures of the two types shown in the formulae as 

given. The iron mica siderophyllite agrees closely in composition 

with a normal ferrous biotite, Al2 (SiO4 ) 3Fe2KH. Aspidolite appears 

to be a sodium phlogopite, but needs reanalysis. In exceptional cases, 

nmscovitic types seem to exist among these micas, and certain lepidomelanes approximate to the general formula Al3 (SiO4 ) 3Fe"K+Fe'"3 

(SiO4 ) 3Fe"K. Another unusual mica is Breithaupt's alurgite, which, 

as analyzed by Penfield,* corresponds to a mixture of molecules 

 2. Al3 (Si3O8)3KH2 

3. Al2 (SiO4 ) 3K2H4 

3. Al2 (SiO4 ) 3MgaKH,

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(SiO4 ) 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 magnesiau micas fluorine is found, and the iron micas 

frequently contain oxygen in excess of the amount necessary to convert 

all of 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 admixtures of micas belonging to the clintonite group. This 

group, the members of which resemble biotite morphologically, are 

extremely basic, and are representable by the general formula

Al O

in which B/3 may be replaced by Al, or by R"R', R" being magnesium, 

calcium, or ferrous iron. The univalent group A1O2R/' may also 

appear in R', giving as the extreme end of the clintonite series the 

compound (AlO2R")4SiO4 . In seybertite and brandisite we have nearly

/°>Mg /°>0a 

2A1-0 b +3A1 O

. X\SiO4=(AlO2Mg)3 \SiO4=H3 

while xanthophyllite t is .

/°>Ca ' /°>Mg /°>Ca 

5 Al O +5 Al O +1 Al O

2 \SiO4=(AlO2Mg)3 X-SiO4 

The xanthophyllite, upon ignition, decomposes into a portion soluble 

and a portion insoluble in hydrochloric acid. The insoluble portion 

has the composition of spinel, a mineral which is generally found

* Am. Jour. Sci., (3) XL VI, p. 289. 1803.

t See Clarko and Schneider, U. S. G. S. Bulletin No. 113, p. 27.
48 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

accompanying clintouite micas, and of which the formation is rendered 

intelligible by the formulae. Whether chloritoid belongs here or npl is 

doubtful, but if it does its formula may be written

/°>Fe 

Al O

Ottrelite may possibly be essentially the trisilicate equivalent of 

chloritoid,

/°>Fe Al-O

and Damour's venasquite approaches this composition. Variations in 

composition in ottrelite correspond to mixtures of this molecule with 

small quantities of chloritoid. Venasquite, however, is also reducible 

to the normal mica type, and agrees sharply with

2 Al:i(SiO4 )3H3 +

3 Al2 (SiO4 ) 3Fe2H2

It may be that ottrelite and its .varieties represent the first stage 

intermediate between micas and vermiculites ; the alkalies of mica having been replaced by water, all E' becoming H.

Although fluorine is found in micas of all classes, it is chiefly characteristic of lepidolite, zinnwaldite, and phlogopite, and is generally connected with a low oxygen ratio. Indeed, the extreme end of the 

lepidolite series, polylithionite, is entirely trisilicate in character, and 

its existence fully justifies the assumption that trisilicates occur in 

other micas. Furthermore, polylithionite furnishes a clew to the entire 

lepidolite or lithia mica series, and its formula is easily written

/F xSi0

5 Al-F + 1 Al-Si308=Na2K

the dominant molecule being akin to clintonite in general constitution. 

Ordinary lepidolite contains the same molecule, mingled with molecules 

of the muscovite type, and an average example is represented by

JF X ,SiO4=KH2 

2 Al-F +1 Al-F +2 Al SiO4=Al

In cryophyllite we have similar molecules, plus molecules of an iron 

biotite, Al2 (SiO4 ) 3Fe2H2 . Oryophyllite is associated most intimately 

with an iron mica of that general character, and lepidolite, similarly, 

is always accompanied by muscovite. In zinnwaldite the molecule /F

Al-F
CLAHKE.] MICAS AND CHLOEITES. 49

probably occurs, commingled with others of.the ordinary lepidolite and 

| muscovite types. In short, all the lithia micas contain compounds of

the type

i ,F

Al F

with members of the normal mica series.* The fluoriferous phlogopites 

may contain molecules of the same character, especially AlF2 .Si3O8 .MgK, 

or they may be of the normal type with univalent Mg F among 

the monad bases. Between these alternatives it is difficult to decide, 

and more evidence is necessary.

By alteration lepidolite passes into cookeite, a mineral which well 

illustrates the equivalent character of hydroxyl and fluorine. Penfield's 

analysis gives for cookeite a general formula,

OH 

Al OH

and in detail a mixture of molecules,

10Al(OH)2SiO4 .Li3 

+14Al(OH)2SiO4 .H3 

+33Al(OH)2SiO4 .Al

The identity of type between cookeite and lepidolite is perfectly clear, 

and the analogy with the clintonite micas is still more apparent.

In a similar way the other micas alter by hydration, 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. HydroMotite.

,SiO4=MgK ,SiO4=MgH 

Al SiO4=MgH Al SiO4=MgH +3H2O

Phlogopite. Hydrophlogopite.

, 4 , 4 MgH

Al SiO4=MgH Al SiO4=MgH +3H2O

Clintonite. Hydroclintonite.

/°\Mg /°Vg

Al (X Al (X +3H2O

Bull. 125

For fuller discussion, see Bulletin U. S. Gr. S. "No. 113, p. 22. 

.1
50 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

The micas in question occur in nature in great variety of admixture, 

and the corresponding veriniculites show a parallel complexity. In 

every case, however, the alteration of the one series into the other follows the line indicated by the formulae, and the verrniculite is simply 

the mica with H in place of K or Ua, 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 veriniculites and the zeolites. Some vermiculites, however, 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 original mineral and the final product. 

Among the more characteristic vermiculites, kerrite, protovermiculite, 

hallite, and lennilite appear to be essentially trihydrated mixtures of 

the hydrophlogopite and hydroclintonite types, commingled in ratios 

nearly as follows :

Hydroplilog. Hydrocliiit.

Kerrite ...................................... 5 : 1

Protovermiculite .............................. 1 : 1

Hallite....................................... 7 : 18

Lennilite is near hallite, and »both may contain small admixtures of 

chloritic material. The kerrite is quite near the typical hydrophlogopite alone.

Jefferisite resembles protovermiculite in ratios, but with a hydrobiotite in place of the hydrophlogopite; that is,

SiO^MgH O 

Al SiO4EEMgH. 3H2O + Al O 5 3H2O

and vaalite is very nearly

,Si04EEMgH / 

5 Al Si04=MgH. 3H20 + 2 Fe O 3H2O

Maconite and lucasite are probably micas which have been partly 

vermiculitized, and an essential portion of the first-named mineral 

appears to be a derivative of muscovite. Eoseite is very near a mixture 

of partially muscovitic character, viz :

Si04=H3 O

7 Al SiO4=Al. 3H2O + 9 Al O 3H2O 

^SiO4 =Al \Si04=MgH

Each of these expressions gives the composition of its respective mineral very closely, and so strengthens the general mica theory. The 

character of the alteration from a mica is simple, and is what is most 

reasonably to be expected.
CLAEKE.] MICAS AND CHLORITES. 51

From some points of view, and especially with reference to its crystalline form, kaolin may be regarded as a member of the mica group. 

As such it would form a connecting link between the micas proper and 

the clintonite group, and with it the calcium mica, rnargarite, can be 

correlated. Furthermore, margarite yields an alteration product, dudleyite, which falls into line with the other two species, thus :

Kaolin. Margarite. Dudleyite.

X)H X)H ,OH

Al-SiO4=H3 Al-SiO4=CaH Al-SiO4=CaH

\SiO4=Al \SiO4=AlO 3 N\SiO4=AlOH.AlH2O2

These expressions are suggestive, but not altogether conclusive. They 

represent the known facts fairly well, however, and can serve their

purpose for the time being1 . Several other micaceous minerals are

altogether doubtful in character, and need only passing mention. Thus 

helvetan may be a trisilicate of the biotite type. Caswellite,* an alteration product of a manganesian biotite, may be written as a member of 

the clintonite group. Willcoxite seems to be an alkaline clintonite, and 

corresponds very closely to

/°>Mg /°>Mg 

Al-0 fe + Al O *

^SiC^Al \Si04=NaH2

There are also several vermiculitic minerals which probably belong in 

the chlorite group. All of these supposed species need further investigation.

In roscoelite, which is a vanadium mica, the ratios are quite uncertain. According to Genth, the mineral contains V2O3? while Eoscoe 

gives the vanadium as V2O5 ; and in other respects the analyses are discordant. If Genth's analysis is accepted, roscoelite has essentially the 

ratios of a phengitic muscovite, with half of the aluminum replaced by 

vanadium. This seems to be probable, but the species needs reanalysis.

Ganophyllite, described by Hamberg in 1890, t and regarded by the 

discoverer as a manganese zeolite, should probably be classed with the 

micas. Like the micas, it is monoclinic, highly cleaveable, and gives a 

percussion figure consisting of a six-rayed star. In all its physical 

characteristics it resembles the micas, with one of which, manganophyD, 

it is associated. It loses water, however, in vacuo, regaining the amount 

lost on exposure to moist air, and on this account it may be compared 

either with the zeolites or with the vermiculite group.

* Chester, Trans. N. Y. Acad. Sci., XIII, p. 97, 1894. 

tGeol. Fciren. Porhandl., XII, p. 586.
52 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

On the latter basis it agrees in composition with a hydrated manganese phlogopite, having the subjoined admixture of molecules:

1. Al(Si3O8 ) 3Mn3H3.2aq.

2. Al(SiO4 ) 3Mn3KH2 .2aq.

3. Al(SiO4 ) 3Mn3lsraH2.2aq.

or, in percentages, as follows :

SiOs ... ............................

A12 0........ ........................

Fe2 0, .............................

MnU ..............................

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

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

PbO ..............................

K2 0 ...............................

Na,0 ..............................

'HaO. ..............................

Found.

39.67

7.95

.90

35.15

1.11

.20

.20

2.70

2.18

9.79

99.85

Calculated.

39.99

> 8.81

"1 36. 80

f

1

2.33

2.29

9.78

100. 00

Of this water 6.22 per cent is crystalline and 3.56 constitutional.

The associated manganophyll has also phlogopite ratios, but varies 

so widely in composition as to show that it is, like many other micas, 

a mixture. Hamberg gives two analyses of this mineral j the first 

variety, of a reddish-brown color, agrees with

,SiO4=MnK

Fe-SiO4=MnH

\SiO4=MnH

The second variety, which is dark-brown, may be written

/VJl.\J\ .UJLli-LV. .Ml 7^ IVI g K,

4Al-SiO4=MnH + 5Al-SiO4=MgH

These manganophylls are from Pajsberg, but the manganophyll from 

Longban, analyzed by Flink, has ratios which indicate the presence of 

biotitic molecules somewhat similar to those found in alurgite. Manganophyll, however, contains among its constituents a true manganese 

phlogopite, of which ganophyllite is most probably the hydrate. On 

this basis both minerals are most easily explained, and all their relations are accounted for.

Between the micas and the chlorites the relations are exceedingly 

close, as regards both composition and physical properties, and from 

certain points of view the vermiculites are to be regarded as a transitional series connecting the two groups. All the species are highly
CLAHKE.] MICAS AND CHLORITES. 53

foliated, all or nearly all are monoclinic, and to each of the definite 

ferromagnesian micas a chlorite, higher in magnesia and water, seems 

to correspond. In former publications* I have sought to show that the 

chlorites may be interpreted as derived from magnesium orthosilicate 

in the same way that the micas are derived from the normal aluminum 

salt; but fuller consideration leads me to a more satisfactory generalization, which unites both series under one set of fundamental formula. 

Returning to the normal micas and ignoring the replacements of 

magnesium and aluminum by other equivalent bases, we have the 

followiug formulae already developed:

Biotite.

,SiO4=MgK 

Al-SiO4=MgH

Phloyopite.

,SiO4=MgK 

Al-Si04=MgH

Clintonite.

By admixtures of these types all of the ferromagnesian micas are 

completely explained, and for the orthochlorite series the following 

parallel expressions are similarly sufficient:

Biotite-chlorite. 

xSiO4=(MgOH)2H

Al-SiO4EE(MgOH)2H

Chloritoid type.

/°>Mg 

Al-O *

Phlogopite-clilorite.

,SiO4=(MgOH)2H 

Al-SiO4=(MgOH)2H 

\SiO4=(MgOH)2H

An average penniue consists of the first and second of these types 

in the ratio 1:1, and an average clinochlore has the same molecules in 

the ratio 2:1. From these ratios the followiug composition is deduced:

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

Al,03 .... ..........................

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

H8 0.... ...........................

Pennine.

34.35

14.60

38.17

12.88

100. 00

Clinoclilore.

34.97

16.51

36.27

12.25

100. 00

In both minerals, as among the corresponding micas, admixtures of 

molecules of the third type occur, giving a range of variation which 

is shown in the published analyses. Thus, for leuchtenbergite, we 

have, with the molecules given above, a large admixture of the 

compound

/°>Mg Al-O

' Bull. H. S. Geol. Survey, No. 78, p. 22; Bull. No. 90, p. 17 ; Bull. No. 113, pp. 11 and 27.
54 THE CONSTITUTION OF THE SILICATES. [BULL.125.

while iii prochlorite and corundophilite the latter compound predominates. Prochlorite has the last formula, with trifling admixtures of 

chloritoid or ottrelite, and corundophilite agrees with the composition

/°>Mg /°>Mg 

7A1-O + 33A1-O

XxSiO4=(AlH2O2 )2H \SiO4IZ(MgOH)H2 

In both prochlorite and corundophilite there are large replacements of 

magnesium by iron.

To these formulae some of the rarer or obscurer chlorites offer interesting variations, which, however, are all simple and intelligible. 

Thus, rumpfite is represented by the formula

/°>Mg . /OH 

3A1-O + 2A1-OH

\SiO4=H(AlOH) \SiO4=H3

the first compound being a magnesian chloritoid, and the second a 

molecule already indicated in cookeite. The original delessite is very 

near

/ Al-0

while the Scotch delessites, analyzed by Heddle, agree sharply with 

the formula

,SiO4=(MgOH)2H

Al-SiO4E(MgOH)2H + 2H2O

with one-fourth of the Mg replaced by Fe. In the last case the discrimination between hydroxyl and water of crystallization is shown by

the analyses, and it is very probable that an analytical revision of the

chlorites would bring other instances of the same kind to light. 

For melanolite we have very closely the composition

,SiO4=(Fe"OH)H2 

Fe SiO4=(Fe"OH)H2

and for euralite

,Si04=(MgOH)2H ,SiO4=(MgOH)H8 

7A1 SiO4=(MgOH)3H + 3A1 SiO4=(MgOH)H2

Trisilicate molecules, as among the micas, also appear in the chlorite 

series, and stilpnomelane is a good example of this kind. Its formula 

is very nearly

/Si3 08=(FeOH)2H

Al Si3O8=(FeOH)2H 

\Si3O8=(FeOH)2H
CLAKKE.] MICAS AND CHLORITES. 55

with very little SiO4 , and epichlorite is a mixture of this salt with the 

equivalent Al(SiO4 ) 3 (MgOH)6H3 , the two compounds being commingled 

in nearly equal percentages, or about half and half.

Some of the chlorites, also, are parallel with kaolin and margarite in 

constitution. Aphrosiderite and strigovite, for example, are' well 

represented, as follows :

Kaolin. Strigovite. Aplirosiderite.

Al SiO4=H3 Al SiO4=(FeOH)2H Al SiO4=(FeOH)3

with much ferric iron in strigovite in place of aluminum. In diabantite we find a composition indicative of a mixture between a molecule 

of this type and one in the normal series. The diabantite from Connecticut, for instance, is very near

,Si04=(MgOH)2H xOH 

Al Si04=(MgOH)8H + 2A1 SiO4~FeH

For the minerals daphnite, metachlorite, klementite, chainoisite, thuringite, and cronstedtite, the formulae are more or less doubtful, for the 

reason that alternatives are possible. They all, however, reduce to 

mixtures of the chloritoid or clintonite type, of the forms

A. B. C.

,0 /O X)

\Si04=H.(AlOH)

Using the letters underneath the formulae as symbols, the several 

species become 

Chamoisite. ............................ BjG,, with all R"=Fe

Metachlorite ...................... .... B3Cl? with all K"=Fe

Thuringite ..... ............... r. ..... . A,B,0,, with all K/; =Fe

Klementitf ............................ . AiB2G^ with B" mostly Mg

Daphnite ............................. .AiB5C4, with K" mostly'Fe

In cronstedtite there is a range from AiBjCi, as in thuringite, to 

A^, and all the aluminum is replaced by ferric iron, while all E"=Fe. 

These expressions give the composition of the several species very 

closely, and serve to correlate them with delessite, rurapfite, prochlorite. 

and corundophilite. All of the valid evidence is satisfied, and the 

micas, clintonites, vermiculites, and chlorites form one systematic group. 

The facts that garnet and vesuviauite alter into chlorites, and that 

chloritic pseudomorphs after feldspar are known, serve to still more 

closely connect the formulae here adopted with the similar formulae of 

the preceding groups of minerals.
56 THE CONSTITUTION OF THE SILICATES. [BULL. 325.

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, A1OH, and A1O2H2, ought to generate 

spinel, and the appearance of a compound of this character is evidence 

in favor of the formulae.

VI. THE TOURMALINE GROUP.

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 cases. 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, litbia 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. These considerations lead directly to a system of

formula;, which, like those of the micas, are derivable from molecules 

of the normal aluminum orthosilicate, and which express completely 

all the known chemical relations of tourmaline.

On studying carefully all the more recent and more trustworthy 

analyses of tourmaline, a constant ratio appears connecting silicon, 

boron, and oxygen, which is represented by the figures Si6B3O3i. The 

variations from constancy are almost all within the limits of accuracy 

of the analyses, but occasionally a small amount of fluorine is noted, 

which appears to replace the univalent acid group BO2. Aluminum, 

with respect to silicon, varies widely, the ratio ranging from Al8Si6 to 

Alr,Si6, while the other bases vary reciprocally with aluminum and in 

the opposite direction. The tourmalines high in aluminum approach, 

like muscovite. most nearly to the normal ortho-salt; those lowest in 

aluminum are high in magnesium, like phlogopite; and, like biotite in 

the mica group, the iron tourmalines lie between these extremes.
CLARKE.] TOUEMALINE GEOUP. 57

Taking now the smallest amount of aluminum found in the formula 

of any tourmaline, all varieties of the species may be regarded as containing the nucleus Al5 (SiO4 )G (BO3 )(BO2 ) 2, in which 14 units of affinity 

are still unsatisfied, and among which all of the essential variations 

in composition take place. The orthoboric group, BO3, is obviously 

trivalent, BO2 being univalent, and the radicle of metabolic acid.

From this nucleus four types of tourmaline are derivable, thus:

1 2

Al SiO4=Al

=Al BO2

I 

Al BO3=NaH

,SiO4=Al BO2

Al-SiO4=Al

3.

,SiO4 -MgH 

Al Si04=MgH

Al-SiO4=MgH

^SiO^Al BO2

I 

Al BO3=NaH

SiO4=Al BO2 

Al-SiO4IEAl

4.

iO4=Al BO2

I 

Al BO3=NaH

I 

,SiO4=Al BO2

Al SiO4EIMgH

Al-SiO4=MgH

\SiO4=Al BO2

I 

Al BO3=ISraH

I 

,SiO4=Al BO2

Al-SiO4=MgH

and to all of these, known varieties of the mineral correspond. The 

only notable variations are due to Or or Fe replacing Al, Fe or Mn 

replacing Mg, small quantities of F replacing BO2, and in No. 4, Ca 

sometimes replacing NaH. If in No. 1 E'3 =LiH2, and in No. 2 Fe2 

replaces Mg2, these formulae give the subjoined percentage compositions:

Si02 ..........................

BaOa...... ....................

A12O:J .........................

PeO...... .....................

NazO.. ....... ................

LiaO ..........................

H2 0...... .....................

1

38.05

3 1.10

43.13

3.28

1.58

2.86

100. 00

2

35.16

10.25

34. 86

14.06

3.03

2.64

100. 00
58 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

These figures agree well with the best analyses of the lithia tourmalines and the ordinary black tourmalines which are found in the same 

localities; divergencies being due to trifling admixtures of one with 

the other, and.to the presence of fluorine as previously noted. 

Formula No. 3 corresponds to

SiO2 ................................................ 37.58

B2O:i ................................................ 10.96

A12 0.5 ........................ ...................... 31. 94

MgO................................................ 12.52

Na2O ............................................... 3.24

H2 0 ................................................ 3.76

100. 00

which is very near the composition of the dark-brown tourmalines from 

Orford, N. H., and Monroe, Conn., except that in them a little Fe 

replaces Mg, and a little Ca takes the place of NaH.

The brown tourmalines of Hamburg, N. J., and Gouverneur, N. Y., 

with the white tourmaline from DeKalb, all approximate toward formula 

No. 4, but with admixtures of No. 3, and with Ca in place of NaH. The

mixture

,SiO4ZIMgH ,SiO4=MgH

Al SiO4=Al Al SiO4=MgH 

\8iO4=Al BO2 \SiO4=Al BO2

Al BO3=NaH + 2 Al BO3=Ca

I I

.SiO4=Al BOa /SiO4=Al BO8

Al SiO4=Al Al Si04=MgH

gives the following percentage composition:

SiOB .... ............................................ 37.19

B.2 0S ............................................... 10. 85

A12 0:, .............................................. 29.86

MgO ............................................... 13. 77

CaO ............................................... 3. 86

Na 2 0 ............................................. 1. 06

H2U ............................................... 3 41

100.00

which represents the DeKalb tourmaline fairly well. In the Hamburg mineral there is more of the calcium compound, and in that from 

Gouverneur rather less. The black tourmaline from Pierrepont gives 

formula No. 4 very closely, except that about three-tenths of the Mg 

has been replaced by Fe, and one-half of the NaH by Ca. Calculating 

fqr the following mixture of molecules:

3Al5 (SiO4 )6 (BO2 )2BO3NaFe4H5

2Al5 (SiO4 )6 (BO2 )2BO3NaMg4H5

5Al5 (SiO4)6(BO2 ) 2BO3CaMg4H4
CLABKE.] TOURMALINE GROUP.

we have the following comparison with Biggs's analysis of Pierrepout 

tourmaline:

SiO,.... ...........................

TiOo ..............................

B,0, ..............................

F. ................................

A12 0.<-- ............................

Fe,0:t .............................

FeO... ............................

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

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

Na,O. .............................

K2 0. ..............................

H2 0. ..............................

Found.

35. 61

.55

10.15

.27

25.29

.44

8.19

11.07

3,31

1.51

.20

3.34

99.93

Calculated.

) 35. 91

> 10. 48

) 25. 44

8.62

11.17

2.79

j 1.55

4.04

100. 00

Tourmalines which do not correspond to any one of the four types 

given agree with mixtures of them, and all of the analyses of this 

mineral published by Jannasch and Oalb or by Biggs can be reduced 

to suitable formulae. These formulas suggest an end product,

Al9 (SiO4 )6 (BO2 )2BO3NaH

which may possibly exist, but is not known* They also indicate the 

obvious relationship of tourmaline to the micas, and express the ready 

alterability of the former into the latter. 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,* who investigated the action of alkaline solutions upon tourmaline, are in accord with these suppositions.

Although otherwise interpreted by Brogger, the minerals cappeliniter 

melanocerite, karyocerite, and tritoniite seem to be structurally akin to 

tourmaline. This view of their nature has already been suggested by 

Wiikjt 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 trivaleut, 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.

* Zeitsch. Deutscli. Geol. Gcsell., 1892, p. 239. 

t Zeitsoh. Kryst. Min., XXIII, pp. 421,422,1894.
60 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

Thus, if we compute the atomic ratios from the analyses cited by 

Brogger, * the following relation appears:

Melanocerite .............

Karyocerite ..............

Si.

236

218

216

226

B.

488

92

134

210

K*

304

296

226

B + F.

488

396

430

436

That is, Si:B + F::l: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 member of the group, cappelinite, is a borosilicate of yttrium 

and barium, and approximates in composition to

,BO2 /BO2 ' 

Y BO2 + 4 Y BO2

With the earths of uncertain molecular weight designated as "yttria," 

are a little lanthanum oxide and trifling quantities of ThO2 and OeO2, 

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 CeO2 , ThO2 , and ZrO2 . In- eudialyte and catapleiite we have two 

rhombohedral silicates of zirconia, which help to explain these compounds. Catapleiite probably has the constitution (OH)3Zr.Si3O8 .R/3.

If we regard the tetrad bases in the cappeliiiite group as forming 

orthosilicates of this same type, the remainder 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

+2R"' BO3 =Ca +7R'" F

Karyocerite may be written similarly, and tritomite becomes

EIV <OH +E'"-B02

\SiO4=H2 .R'"F2

" Zeit. Kryst. Min., XVI, pp. 462-469.
CLARKE.] TOURMALINE GROUP. 61

These formulas 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.

There is one other borosilicate containing aluminum, axinite, whose 

relations are uncertain. Physically it differs widely from tourmaline, 

being triclinic, but on account of its composition it may fairly be considered here. The analyses of it vary to a considerable extent, partly 

because of uncertainty in the determinations of water and boron, but 

also in other respects which indicate a mixture of molecules. Some 

analyses give orthosilicate ratios, provided that the boron is regarded 

as basic and equivalent to aluminum, but others show much too little 

oxygen. Boron, however, is essentially an acid-forming element, and

it seems more probable that in axinite it has acid functions. The most

typical axiuite is that from Bourg d'Oisans, and of this the best 

analysis is by Jannasch and Locke.* From this the following formula

is easily derived:

,BO3=A1-OH

\Si30 - { Ca3

in which the boron is regarded as part of an orthoborate molecule, 

derived from the acid H3BO3 .

This may be compared with Jannasch and Locke's analysis directly, 

and with the latter reduced by recalculating Fe2O3 into A12O3, the other 

bases all into their equivalents in CaO, and then bringing to 100 

per cent, thus:

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

BaO,.. ............ ....

AlaOa.. ...............

FezO:,.----. ...........

FeO. ....... ..........

MnO .................

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

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

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

NajO... ..............

HsO ..................

Found.

42.88

6.02

18.24

.62

7.10

1.06

19.89

2.23

.11

.36

2.14

100. 65

Reduced.

43.13

6.06

) 18. 73

29.93

2.15

100. 00

Calculated.

43.32

6.32

18.41

30.33

1.62

100. 00

The formula may also be written

JBO3 =A1OH 

Al-SiO4=

\Si04EE

,BO3=A1OF

+

' Zeitsch. Anorgan. Chern., VI, p. 57, 1894.
THE CONSTITUTION OF. THE SILICATES [BULL. 125.

in which form, it compares better with other analyses. Thus Baumert's 

analysis of axinite from the Kadauthal* gives

X)H

Al Si04= Al Si308 EE

and, a comparison as follows:

Si02 ...... ............

BjOs--.. ..............

AljOs. ................

Fe3Oa...... ...........

FeO........ ..........

MnO .................

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

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

H2 0 ..................

Found.

40.76

4.76

12.47

2.17

3.60

2.84

30.21

2.00

1.22

100. 03

Reduced.

40. 92

4.78

> 13. 91

1

1

f 39. 17

1.22

100. 00

Calculated.

41.32

4.82

14.05

38.57

1.24

100. 00

Genth's analyses * of the axinite from Franklin, ST. J., give similar 

ratios, minus the boron-free molecule required in Baumert's case. 

Approximately they give

4 Al-SiO4= Al-Si04= 

\'SiO4=

,BO3=Mn 

Al-SiO4= , 

\Si04=J Mn3

with rather less than naif the Si04 replaced by SisOg, In general,

when S=Si308+SiO4 all axinite except that analyzed by Bauinert 

gives the general formula

Al X=

in which E" is partly A1OH, with Ca, Mg, Mn, or Fe. Until further 

 evidence is available this expression may be regarded as valid, but it 

represents only the composition of the mineral, and no other relations. 

It is, however, in conformity with the general theory of substitution. 

Other formulae, which represent axinite either as rnetasilicate or as 

orthodisilicate, are possible, but none of them has any advantage over 

the one proposed.

Dana's System of Min., 6th ed., p. 529.
CLARKE.] MISCELLANEOUS SPECIES. 63 

VII. MISCELLANEOUS SPECIES.

Among the ortho- 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, dumortierite, and sillirnanite, and which, like them, is orthorhpmbic. Like andalusite, furthermore, staurolite alters into 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,* who adopt Grotli's 

formula HAlsFe^SigOia. This, structurally, is best written

,O H

Al Si04=(A10)2

\mn>Fe XSl°<=(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 one 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 

that the ideal staurolite is not yet known. The expressions proposed 

are as follows:

Andalusite. Dumortierite. Staurolite.

,SiO4=(A10)3 ,SiO4=(AlO)3 xSiO4=(A!O) 3

Al SiO4EAl Al SiO4=(AlO)3 Al SiO4=(AlO)3

Fe 

I 

,SiO4=Fe

Al SiO4=(AlO)3

*Am. Jour. Sci., (3) XL VII, p. 81, 1894.
64 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

This formula, in contrast with that of Penfleld and Pratt, and with 

their reduced analysis of staurolite from Lisbon, N. H., gives the subjoined percentage composition:

Si02 .... ..............

AL,03 ...... ...........

FeO ..................

H2 0 ..................

Lisbon.

27.44

55.16

15.72

1 fi8

100. 00

P. and P.

26.32

55.92

15.79

1 Q7

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

Ca 

I 

' Si04=Mn

This, in comparison with Flink's analysis, gives

SiO,.... ...........................

A130...... ..........................

MnO ..............................

MgU ...... ........................

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

K2O.... ...........................

Na2 0.... ..........................

H2O.'.. ............................

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

V

3.91

100. 00

a result which is fairly satisfactory. More data relative to harstigite 

are evidently needed.
CLABKE.] MISCELLANEOUS SPECIES. 65

In order to interpret the clintonite micas, the highly basic compound

/°>Mg Al-O

was assumed. This formula also gives, empirically, the percentage 

composition of the species kornerupine or prismatine, which, unlike 

the micas, is orthorhombic, and also insoluble in acids. On these 

grounds it may be compared with andalusite and staurolite; and a clew 

to its constitution is found in the existence of an alteration product of 

prismatme, kryptotile, having the composition HAlSiO4 . Tripling the 

formulae, we have

Andalusite. Kornerupine. Jfryptolile. 

/SiO4=(AlO)3 .SiO4=(A102Mg)3 /SiO4 =H3

Al-SiO4=Al Al-SiO4=Al Al-SiO4ZlAl

\Si04=Al \Si04=Al \Si04EAl 

symbols which express the observed relationship between these and 

other species. Further than this the formulae have no real significance, 

and alternative expressions are considered later in connection with the 

pyroxenes and amphiboles. Associated with kornerupine the still 

more basic sapphirine, MgsAl^Si^?, is found. To the constitution of 

this species there is no good clew, and any attempt at structural formulation is useless. More than one structural formula is possible for 

sapphirine, but all are at present meaningless, except in so far as they 

show that the compound is stoichiometrically conceivable. I

A peculiarly difficult group of minerals to interpret constitutionally 

is the group of clays. One member of this group, kaolin, has already 

been considered,- and this member has the advantage of 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 to correlate with other species. They 

represent undoubtedly 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. Their formulae, therefore, are merely tentative, and serve only as a first step 

toward a 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 ivoerthite.

,SiO4=Al .OH 

Al SiO4=Al Al SiO4=Al

Kaolin. Montmorillonite. Newtonite.

,OK .OH .OH 

Al SiO4 zEH3 Al SiO4=H3 Al OH

\Si04=Al 

Bull. 125 5
66 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

Woerthite is an altered sillinianite, and westanite is perhaps a similar 

denvativeof 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 SiO4=H3 is indicative of loosely combined water, 

while in these particular clays th'e 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 Al:,(SiO4 ) 3H3 +2H2O, 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 have similar structure. Allophane is perhaps (AlO2H2 )2H2SiO4 -f 3H2O, or it may be written 

analogously to andalusite,

Al3 (SiO4 ) 3 (AlO2H2 ) 3 .12H2O.

Neither formula is sustained by any good evidence. Other clays are 

possibly as follows : 

Samoite ...................................... . Al4 (SiO4 ) 3 . *OH2O

Cimolite ....................................... . Al4 (Si3O8 )3 .6H2O

Collyrite ...................................... . (AlO)4SiO4 .6H2O

Schrotterite. ................................... (AlO2H2 )4SiO4 .6H2O

None of these formula 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 (Si3O8 ) 3 .2H2O; and 

chloropal by the expression Fe2 (SiO4 ) 3H6 . Hisingerite seems to range 

from* a ferric kaolin to a ferric halloysite, and farther 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, etc., are also of uncertain character. 

To cerite, which is of doubtful composition, the provisional formula

Si04=CeO.H2 

Ce SiO4=CeO.H2

may be assigned. Other earth metals lanthanum, the two didymiuins, etc. replace a considerable part of the cerium. With cerium 

only the formula requires

SiO2 ................................................ 20.22

CejOs....... ........................................ 73.71

H2 0. ................................................ 6.07

100. 00

Cenosite, H4Ca2Y2CSi4Oi7 , and ardennite, H5Mu4Al4VSi4O23 , can each 

be written structurally in more than one way, with no determining 

evidence upon which to base a decision. They may be orthosilicates or
CL*auB.] MISCELLANEOUS SPECIES. 67

diorthosilicates; the vanadium in ardennite may be combined as VO3 or 

as VO4 , or it may form part of a complex vanadio-silicic acid, and between 

these alternatives there is no present means of deciding. To carpholite 

the formula H4MnAl2Si2Oio is commonly assigned, but the analyses vary 

from this composition too widely. Even with this formula carpholite 

may be either an orthosilicate or a basic nietasilicate, and we can only 

say with regard to this species that further investigation is necessary.

Spodumene, acmite, jadeite, and the corresponding members of the 

amphibole group will be considered in connection with the metasilicates. 

It is quite possible that these minerals are really pseudo metasilicates, and that they should properly be discussed now. Their study is 

deferred on inineralogical grounds solely.

Three other salts of triad bases may be noted here, as having more 

analogy with the salts of aluminum than with any other silicates. They 

are pseudobrookite, an orthotitanate of iron, Fe4 (Ti04 ) 3 , and the bismuth 

silicates, eulytite and agricolite, Bi4 (SiO4 ) 3 . The two last-named species 

differ in form but are identical in empirical composition. They, therefore, suggest two types of chemical structure among the silicates of 

trivalent bases.
OHAPTEE IY.

THE ORTHOSILICATES OF DYAD BASES.

Although the orthosilicates of the dyad metals are presumably simpler than those of aluminum, the problem of their constitution, studied 

in the light of mineralogical evidence, is peculiarly difficult. Starting 

points exist, in the salts of magnesium, iron, manganese, zinc, and 

gluctnum, but the derivatives are fewer than in the case of aluminum, 

and the evidence upon which to base argument is proportionally limited.

Expressed in the simplest terms, the normal orthosilicates of this 

group are represented by the general formula R2SiO4. To this type 

the following minerals correspond:

Forsterite. .............................................Mg2SiO4

Fayalite ................................................. Fe2SiO4

Tephroite ...............................................Mn2 8iO4

Willemite...............................................Zn2SiO4

Phenakite.............................................. .Gl2SiO4

Monticellite............................................. CaMgSiO4

Knebelite...............................................MnFeSiO4

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 (SiO4)3. So also, allied to knebelite, we 

have igelstroiuite, Fe4Mn2 (SiO4 ) 3 : and in trimerite we find the salt 

Gl3Mn2CaCSiO4 )3 .

Upon studying closely the derivatives of these normal salts, 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 upon this basis it becomes possible to develop a system 

of formulas 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 formulas are apparently clear. For 

instance, a good example is furnished by the chondrodite group, for

68
CLARKE.) ORTHOSILICATES OF DYAD BASES. 69

/ which we have the empirical formulae established by Penfield and Howe.* 

Structurally written these become

Clinohumite. Humite. Chondrodite.

Mg Mg Mg

/

kjlvy4\ xkJl\_7,j\ xKJlV/^x. 

^6v >Mg Mg< >Mg Mg< >Mg

>SiO4< >SiO4< \SiO/ 

Mg< >Mg Mg< >Mg ||

>Si04< \SiO/ (MgF)2 . Mg/ >Mg || 

\SiO/ (MgF)3

II 

(MgF),.

or derivatives respectively of the salts Mg8 (SiO4 )4 , Mg6 (SiO4 ) 3, and 

Mg4 (SiOJ2, with one atom of magnesium in each case replaced by the

two univalent Mg F groups. The clinohumite occurs in association 

with forsterite, and the two species have, nearly the same specific gravity. Hence forsterite may be Mg8 (SiO4 )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 effected, 

would go far toward settling the question.

In the case of phenakite the triple formula Gl6 (SiO4 )3 is rendered 

probable by the existence of trimerite, Gl3Mn2Ca(SiO4 )3. It is also 

emphasized by the species helvite and danalite, which contain sulphur, 

probably combined in the dyad group E S E . Both of these 

species, in all of their known occurrences, agree with the general 

formula

Gl

Si04 

E E

E S

in which E maybe either Fe", Mn, or Zn. The E is variable, but the 

other constituents are constant. In helvite, Mn and Fe occur, and in 

daualite zinc appears. In the Colorado daualite Zn predominates 

largely over Fe, and there is very little Mn. The Eockport danalite 

has Fe in excess of Zn, and rather more Mn. The Cornish danalite 

is very low in Zn, and the Fe largely exceeds the Mn. The ratio 

Gl:SiO4 ::3:3, however, holds for all.

If phenakite is Gl6 (SiO4)3, then willemite, which is morphologically 

similar, is probably Zn6 (SiO4 ) 3, with zinc partly replaced by manganese 

in the variety known as troostite.

* Am. Jour. Sci. (3), XL VII, p. 188, 1894.
70 THE CONSTITUTION OF THE SILTCAT3S. [BULL. 125.

The three orthorhombic species, bertrandite, calamine, and ilvaite, 

may be most conveniently represented as derivatives of the bipolymer 

B4(SiO4)2, like chondrodite. Grystallographically, ilvaite resembles 

humite, but bertraudite and calamine are related to each other. The 

simplest analogous formulae for the three minerals are as follows:

Bertrandite. Calcimine. Ilvaite.

Gl H2 Ca

/

KJlVx^v yKJ-l-V/^v 

\Zn Fe/ >Fe

H/>J1V*\Q1__OHj

(ZnOH)2 . Al OH.

Oalamine may also be written as a metasilicate, (ZnOH)2SiO3 , 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.

To the- datolite group a similar constitution is ascribable. The 

species, reduced to their simplest empirical expressions, are these:

Datolite.............................................HCaBSiO5

Homilite ........................................... .Ca2FeB2 Si2O10

Euclase .............................................HGlAlSiO5

Gadolinite..... .................................... .Gl2FeY2Si2O10

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 formulae follow:

Datolite. Euclase.

H TT 

2 -H-2

II II

xSi04V /Si04V

Ca< >Ca Gl< >G1

\SiO/ \SiO/

(BO), (A10),

Homilite. Gadolinite.

Fe Fe

 / 4X / 4X

Ca< >Ca Gl< >G1

\SiO/ \SiO/

II II

(B0)2 (Y0)a

Gadolinite alters with great ease, passing by hydration into a brownish-red earthy substance. The analyses of this substance, which is
CLARKE.] ORTHOSILICATES OF DYAD BASES. 71

probably never pure and definite, are not altogether satisfactory; but 

they indicate in a general way a transformation into the compound

Fe'"OH

II

S1O4

(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. Prom them, however, chain-like molecules are derivable, and these seem to exist in the 

cases of 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 K8 (SiO4 )4 

are probably the best to represent existing evidence:

Frwdelite.

/SiO4=H2 (MnCl). 

Mu<

>SiO4=H2 >SiO4=H2 

Mn< Cu<

>Si04=H2 >Si04=H 

Mn< . Cu<

\ \

Pyrosmalite is like friedelite, but with a large part of the manganese 

replaced by iron. Possibly karyopilite may be similar, haviug the 

formula

/SiO4^H2 (MnOH) 

Mn<

x/SiO4=H2 

Mn<

\SiO4=HMn.

These formulae are purely tentative and need additional support. By 

syuthetic and genetic investigations they may be supported or overthrown. That they sustain one another, and fit in well with the formulae 

of the preceding species, is all that can be said iu their favor.

For serpentine, H4Mg3Si2O», several formula) 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 no direct evidence, but still it should not be left 

entirely out of account.

By some authorities serpentine is regarded as an or tho silicate, and 

by others as a salt of the acid HG Si2O7 . On the latter supposition it
72 ' THE CONSTITUTION OF THE SILICATES. [nutL.125.

becomes Mg=Si2O7==H2 (MgOH)2, which may be derived either from 

2 Mg2SiO4 or 2 MgSiO3, with loss of magnesium in one case and gain in 

the other. On the orthosilica.te basis it is simply derivable from the 

polymei Mg4 (SiO4 )2, and is related to the intermediate alteration product villarsite, as follows:

Mg4 (Si04 )i. Villarsite. Serpentine.

Mg Mg H2

/ 4X / 4X

Mg Mg< >Mg Mg< >Mg

\SiO/ XSiO/ \SiO/

Mg H Mg OH H Mg OH

On this scheme the formula for serpentine corresponds with that of 

choudrodite; and the fact that the latter mineral alters readily into 

serpentine is strong evidence in its favor. If pyroxene is written 

structurally

O

O Si O,

>Mg 

O Si (K

O

the derivation of serpentine from it by action of infiltrating magnesian 

solutions becomes easy to understand, and the orthosilicate formula for 

serpentine is rendered still more probable. In short, that formula best 

indicates the genetic relationships of serpentine, and on these 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,* 

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, while 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 Lindnert have shown that the 

expectation was not well founded. I still believe, however, that the 

reaction discriminates between those magnesian 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

 » Bull. TJ. S. Geol. Survey No. 78, p. 11 ; Bull. No. 90, p. 11, and Bull. No. 113, pp. 27 and 34. 

t A. Liudner, Inaugural Dissertation, Breslau, 1893.
CLARKE.] ORTH08ILICATES OF DYAD BASES. 73

.upon by the reagent. Brauns's objections* to this supposition, on the 

ground that the dry hydrochloric acid becomes moist, are not well 

taken, for the reaction was always applied 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, while 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 Mg3Si2O7 is left" behind. According to Baininelsberg t 

the water is given off in two portions one-half upon weak ignition, the 

other after heating more strongly. On the orthosilicate theory these 

stages may be represented thus:

Seri)entine. First stage. . Second stage. 

H2

/Si04X Mg/ . *\Mg Mg/ 3\Mg 

Mg/ ^ \Mg \SiO/ \SiO/

/Si°4\ H// Nttg OH J 

H/ \Mg-OH Mg

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 JVIg2H2Si2O7 is commonly assigned. Although this expression suggests a diorthosilicate, it may also be written

-Mg

H2

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

* Neues Jakrbuch, 1894. T, p. 205.

t Handbuch der Minei'ilclieuiie, 2 Aufl., p. 506.
74 THE CONSTITUTION OF THE SILICATES. [BOLL. 125.

Several other hydrous silicates of magnesium seem to belong in this 

group, but their nature is altogether doubtful. Thus we have

Aphrodite. ...................;..-............. .Mg2H4 (SiO4 )2

Kerolite ....................................... -Mg(SiO4 )2H5 (MgOH)

Deweylite is possibly

/SiO4=H3

Mg<

>SiO4=(MgOH)2 + 2H2O 

Mg

and genthite is similar, with part of the magnesium replaced by nickel. 

Sepiolite may be a trisilicate, with the formula H2=Si3O8 =(MgOH)2 , in 

which case conarite is the corresponding nickel salt. These symbols 

are to be regarded as suggestions only. Two other trisilicates eudidymite and epididymite should also be noted at this point. The two 

species have the same empirical formula, HNaGlSi3O8 , which, doubled, 

becomes

^Si308

II

H2

a structure conformable to the type of the orthosilicates already considered. The isomerism between eudidymite and epididymite is 

explainable by giving one species the constitution just written, while 

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 HNaGlSi 308 can be put in two forms, 

thus:

O=Si O Na 0=Si O

I I

o o 0

I I ^

I o I /

Si< >G1 and Si( 4 ° ox°-*a 

i i

O=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 Y.

THE ORTHOSILICATES OF TETRAD BASES.

On account of their relatively small number and general scarcity, the 

silicates of the tetrad metalSj titanium, 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, 

ZrSiOj, As with the otber 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 (SiO4 )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 (Si3O8 )(SiOj)3 j which goes to shoV 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:

Si02 ...... .........................

Zr02 ... ...........................

FeO... ............................

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

Hermann.

42.91

55.18

.93

£)<!

99.97

Calculated.

42.45

57. 55

100. 00

Although zircon is a very stable and definite mineral, it alters by 

hydration iuto 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 probably the 

first hydration derivative, and its relations to zircon, regarding the 

latter as Zr4 (SiO4 ) 4, may possibly be as follows:

Zircon. Malacone. 

EZr OH

75

SiO4==Zr
76 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

The original cyrtolite from Eockport is near malacone, but the cyrtolite from Colorado, analyzed by Hillebrand, approximates to

/OH

\ \SiO4=H(ZrO2H2 ) 

c \SiO4=H(ZrO2H2 )

with part of the ZrO2H2 replaced by other bases. These 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 linking four groups or radicles together; just as one 

aluminum atom has a similar triple function in the aluminous orthosilicates. This mode of linkage will be followed throughout this group 

of compounds.

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

. OH .//OH //OH Zr Zr / Zr / \Si3O8=NaH3

Connecting these formulae with zircon, we have the facts that Si3 O8 

occurs in auerbachite, and that at Laven, according to Brogger,* zircon 

is found both intergrown with catapleiite and pseudoinorphous after it. 

Eudialyte and eucolite are commonly regarded as metasilicates, with 

the compound ZrOCl2 as an admixture. But the latter 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 : O, 

although approximating somewhat nearly to the assumed SiO3. If 

now we treat eudialyte and eucolite as mixtures of tri- and orthosilicates, like the feldspars, ssapolites, 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

* Zeitscli. Kryat. Min., XVI, p. 105.
CLARKE.] ORTHOSILICATES OF TETRAD BASES. 77

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 remaining zirconium silicates, wohlerite, rosenbuschite, lavenite, 

and hiortdahlite, are difficult to formulate conclusively. Brogger 

regards them as rneta compounds, and as such they are classed with 

the pyroxenes; but the ratio Si:O is in each case very nearly 1:4. In 

wohlerite, which contains columbium, the latter pentavalent element 

may be regarded as forming the dyad group =CbOF, and as equivalent 

to the similar group =ZrF2 . On this supposition wohlerite can be 

written

.SiO4=Ca SiO4=Ca

/ \ 

/

CbOF ZrF2

4 Zr Si04=0a +1 Zr Si04=Ca

SiO4 Catta

with small amounts of the groups B'"F replacing ZrF2 . As thus written the calculated composition agrees fairly with Cleve's analysis, thus:

Si02 ..............................

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

Zr02 ...... ........................

Cb205...... .......................

PesOa .............................

PA.O

FeO ..............................

MnO.... ..........................

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

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

Na*O .............................

H

r\

F...... ...........................

T i Afla O

Cleve.*

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.

30.27

5 18. 46

13.52

28.26

7.83

2.87

101.21 

1.21

100. 00

* Zeit. Kryst. Hin., XVI, p. 347.

The other "zircon pyroxenes" can be interpreted in a somewhat similar manner, but the results are less satisfactory. The analyses are 

complicated by the presence of small amounts of titanium, which may 

be either acid or basic, and more data are needed before formulation, 

can become useful.

The typical silicate of thorium, thorite, or orangite is another unsatisfactory species on account of its wide variations in composition. It is 

commonly supposed that the mineral, as it exists in nature, has been
78 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

derived from an original ThSiO4 by hydration; and that that ThSiO4 

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 approximates to the composition

Th(Si3 08 )4Fe"2 (YO)8 .

and in steenstrupine a thorium silicate exists which may be Th(SiO4 )4 

Ca4Na4 . Further discussion of these minerals at present would be 

fruitless. The data are wholly inadequate.

To the titanium silicates astrophyllite, johnstrupite, and rinkite similar formulae to the foregoing are assignable. Indeed, this has already 

been done for astrophyllite by Brogger, who writes the formula Ti(Si04 ) 4 

K"4B'4. 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

SiO4EEFeH I 

a

SiO4=FeH 

which requires

SiOs ............................................... 34.30

 Ti02 .. .............................................. 12.20

FeO ............................................... 41.16

KaO ............................................... 5.36

Na2O .............................................. 3.55

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

Johnstrupite and rinkite are both fluoriferous, and both contain 

earths of the cerium groups. In johnstrupite, a little ZrO2, ThO2, and 

Ce02 replace some TiO2 . For johnstrupite the expression

SiO4=Ca(CeF2 )

iO4 CaNa 

SiO4=OaH

agrees well with the ratios gFven by analysis. In rinkite we have, with 

great probability, the mixture

SiO4=Ti-F

. /SiO4HCa(CeF2 ) 

3 Tl \SiO4=CalsTa + 2 Ti^SiO4=Ca(CeF2 )

SiO4=Ca(CeF2 )
CLARKB.] OKTHOSILICATES OF TETRAD BASES. 79

Even the complex mosandrite reduces to the same general type 

agreeing very closely with

/OH /F 

/ SiO4=HNa(CeO2H2 ) / SiO4=CaH 

1 \< SiO4=HNa(OeO2H2 ) + 4 ^ SiO4=CaH 

v c,^ ^^..^.^^ \SiO4=CaH

in which E=Celv : Zr : Ti ::1 : 2 : 2. For each of these species the 

published analyses agree well with the composition calculated from 

these formulae.

There is still another group of titaniferous silicates, represented by 

sphene, keilhauite, and tscheffkinite, which seems to be unconformable 

with the foregoing scheme of interpretation. Sphene, 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, H2TiSiOs, analogous to H2Si205 , while 

the other treats it as a basic orthosilicate,

Ca=SiO4=TiO.

A careful study of the recorded analyses of sphene leads me to prefer 

the latter expression, for the actual ratios vary in a way which indicates 

a replacement, sometimes of Ca and sometimes of TiO by other bases. 

On the meta formula, only the calcium should be replaceable. This 

variability of ratio is well shown by some of the varieties of sphene, 

such as grothite, alshedite, and eucolite-titanite, but the data are not 

absolutely conclusive. If, however, sphene is a basic orthosilicate, it 

should be placed in the preceding chapter with datolite and the other 

calcium salts. On the other hand, the acid character of thetitaniuni is 

suggested by the remarkable hydration derivative of sphene, xanthitane, of which the composition is approximately represented by the 

formula Al=Ti2O7=H3.

For tscheffkinite, as shown by Eakins' analyses, the composition is 

approximately (FeCa)3Ce6Ti4SiGO32 , but the constitution of the mineral 

is very doubtful. Keilhauite appears to be like sphene, with Ca or TiO 

replaced by R"'OH or E///O2H2 , but the analyses are widely discordant. 

The orthorhornbic guarinite is generally assumed to be isomeric with 

spheue, although there is but one incomplete analysis from which to 

calculate.

Some light may possibly be shed upon the sphene groups of mineral 

by a study of the columbo-titanates seschynite and polymignite. The 

composition of seschynite is very well represented by a mixture of 

molecules

2 O=Ti=TiO4 =Ca

2 (CbO3 )2=Ti=TiO4 =Fe

3 (CbO3 )2=Th=TiO4 =(CeO)2
80 THE CONSTITUTION OF THE SILICATES, 

which compares with Eammelsberg's analysis as follows:

[BULL. 125.

Ti03 ..............................

ThOa ...... ........................

CbaOB .... .........................

(Ce, La, Di)203 ....................

(Y,Er)2 03 .........................

FeO.... ...........................

Ca.. ..............................

Found.

21.20

17.55

32.51

19.41

3.10

3.34

2.50

9$. 61

Calculated.

20.70

18.63

31.52

> 23. 14

3.38

2.63

100. 00

For polymignite a similar expression gives equally good results. It is

5 (CbO3 )2 =Zr=TiO4=(Fe/// O)2 

4 O=Zr=TiO4 .-=(CeO)2 

15 O=Zr=TiO4 =Ca 

1 O=Th=TiO4 =Na2

The similarity in type of these expressions to the orthosilicate formula for sphene is obvious. It is also obvious that the entire group is 

greatly in need of further study.
CHAPTEE VI.

THE DIORTHOSILICATES.

Although the existence of the sexbasic acid H6Si2O7 has been well 

established by the preparation of its ethers, its metallic salts are little 

known and uncertain. I have already shown, in the case of serpentine, 

that a mineral may be apparently a diorthosilicate, and yet equally 

well explainable otherwise; and what is true in that instance may be 

true in others. For the following species the diorthosilicate formula 

seem to be the best and simplest, even though they are not wholly free 

from objection. They fit existing evidence, but are iiot absolutely conclusive.

The typical member of this group of minerals is the hexagonal lead 

silicate, barysilite, Pb3Si2O7 . The artificial compound from the slags of 

Boimeterre, Mo., described by Dana and Penfield, is near this, and may 

have the composition Pb2CaSi2O7 . The tetragonal ganomalite is 

another similar compound, approximating to 3Pb3Si2O7 -f2Ca3Si2O7, 

although the latest analysis agrees rather better with Pb3Si2O7 +Ca2SiO4 . 

Two other ortnorhornbic species, kentrolite and melanotekite, may be 

regarded as basic salts derived from .barysilite, thus:

Barysilite .......................................Pb2Si2O7Pb

Melanotekite .....................................Pb2 Si 2O7 (Fe" /O)2

Kentrolite........................................ Pb2Si2O7 (Mu'"O)2

The group of zeolitic calcium silicates, okenite, gyrolite, and apophyllite, are unquestionably related to one another, and are best represented 

as salts of H6Si2O7. In nature, gyrolite may be derived from apophyllite, and apophyllite also from gyrolite, while Doelter has generated 

apophyllite from okenite by artificial means.* First, by heating okeuite with potassium silicate and water to 200°, crystals of apophyllite 

were obtained. Secondly, by heating okenite with aluminum chloride, 

sodium carbonate, and carbonated water together at 220°, upophyllite, 

analcite, and chabazite were produced. The most satisfactory general 

formulae for the three species are these:

Okenite.

xSi2 07.H5 

Ca< 

>Si2O7 .CaHa 

Oa< 

\Si207.H5

Gyrolite.

/Si2O7.CaH3 

Ca< 

>Si2 07 .H4 

Ca< 

\Si2O7 .CaH3

Apophyllite.

/Si2O7 .H4 (CaOH) 

Ca< 

>Si2O7.H4 

Ca< 

\Si207.H4 (CaOH)

* Neues Jahrb., 1890,1, p. 118.

81

Bull. 125 6
82 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

In apophyllite fluorine may partly replace bydroxyl, and K may 

replace the univalent CaOH. With K : CaOH : : 1:1 the composition 

of apophyllite becomes

SiO,............................................... 52.03

CaO.............................. ...........:..... 24.27

K2 0 ............................................... 6.79

H2 0 ............................................... 16.91

100. 00

'The uncertain mineral plpmbierite may be a fourth member of this 

group, with the formula

/Si2O7 .CaH3 

Ca<

)Si2O7.Ca2 + 9 aq. Ca/

\Si2O7.CaH3

To the calcium-manganese silicate, inesite, various formulas are 

assignable. By Flink it is regarded as 2(CaMn)SiO3. H2O. But part 

of the water is stable at temperatures above 300°; and this fact is 

expressed by Schueider's formula (CaMn)Si3O8 (MnOH)2 . H2O. Both 

formulae agree with the analyses approximately, but the latest analysis, 

that of Luudell,* is better represented by the following mixture:

/Si2O7 .CaH3 /Si2O7.MuH3 

Ca< Mh<

>Si2O7 .Ca2 +2H2O, and >Si2O7 .Mn2 +2H2O 

Ca< Mn

\Si2O7.OaH3 \Si2O7.MnH3

which requires :

Si02 ........................

PbO ........................

MnO ........................

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

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

H,0... ......................

Found, 

 Lundell.

42. 92

.73

36.31

.37

8.68

10.48

99. 49 -

Calculated.

42. 18

I 37. 44

( 9.84

10.54

100. 00

With iuesite another manganese silicate, bemeutite, is perhaps 

related, having the formula

Mn 

Mn

,Si2O7 .MnH3

* See Hatubere, Geol. Foren. Forbandl., XVI, p. 325.
DIORTHOSILICATES. 83

and the magnesiau spadaite may be similar, with the composition

Si207 .MgH3

Mg<

>Si207 .MgH2

Mg<

\Si2O7 .MgH3 .

Another maguesian silicate, saponite, is perhaps normally H4 (MgOH)2 

Si2O7, although the analyses all show admixtures of some aluminous 

compound.

Among the silicates of aluminum, salts of diorthosilicic ac,id are 

very rare. The only one which seems to be at all well denned is iolite, 

which agrees best with the formula

,Si2O7 .AlMg 

Al Si207 .AlMg

\Si207 .(A10H)2 

Al Si2O7 .AlMg

\Si2O7 .AlFe 

which requires

Si03 ...... .......................................... 49.26

A12 03 ............................................... 33.50'

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

FeO

9.85 

. 5. 91 

. 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 Si2O7 in iolite is split into 

two orthosilicic radicles, yielding two molecules of the type

xSiO4=H2 . A1O2H2 

Al Si2O7mAlMg

from which the final transition into a mica is easy. If we take Bammelsberg's analysis of chlorophyllite, recalculate the ferric oxide into 

alumina and lime into magnesia, reducing afterwards to 100 per cent, 

we get the following comparison between observed fact and the composition of chlorophyllite computed from the foregoing formula :

SiOB...... ............

AlsOs... ..............

FesOa.... .............

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

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

H2O ..................

Found.

46.31

25.17

10.99

10.91

.58

6.70

100. 66

Reduced.

47.99

> 33. 34

> 11. 74

6.93

100. 00

Calculated.

48.39

32. 90 

12.90

5.81

100. 00
84 THE CONSTITUTION OF THE SILICATES. [BULL. 122.

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 aluminous silicates possibly belong in this group. One, 

barylite, is near

xSi2O7 .A!Ba

Al-Si2O7 .AlBa 

\Si2O7 .AlBa 

which requires

Si02 ....................... 35.19

Al.:03 ...................... 19.94

BaO...............:....... 44.87

100.00

The other silicate, sphenoclase, is approximately Al2 (Si2O7 ) 3Ca6 ; which 

may be analogous to barylite in structure, or written as a calcium 

salt similar in type to okeriite. Both barylite and sphenoclase, however, are uncertain, and their relations are not definitely known. The 

species glaucouite, pholidolite, and celadonite, all of doubtful composition, seem also to have diorthosilicate ratios.

To the rare mineral rowlandite, in accordance with Hillebrand's 

analysis,* a diorthosilicate formula belongs, viz:

Y F

II

/Si2O7=Y 

Fe<

\Si2O7=Y

II Y F

which expresses the composition of the species very closely.

* Bull. TJ. S. Geol. Survey JTo. 113, p. 45.
CHAPTER VII.

THE META AND DIMETASILICATES.

Although the metasilicates appear at first sight to be extremely simple, they are actually quite difficult to interpret. It is easy enough to 

deduce their empirical formulae, and to write them afterwards in structural terms j but this is not sufficient. The structural formulae must

express all known relations in the case of each species, and ill attempting to satisfy the established conditions the difficulties begin to appear. 

In the first place, metasilicic acid itself is defectively known, and no 

ether of the form R2SiO3 has yet been certainly obtained. Troost and 

Hautfeuille's ether (C2H5 )8Si4O|2 suggests the possibility that metasilicic acid, like metaphosphoric acid, may polymerize, but an attempt 

to draw general conclusions on so important a supposition from one 

datum only would be most unwise. The possibility of polymeric acids, 

however, must be recognized.

Again, as we have repeatedly seen, a mineral may be apparently a 

metasilicate and yet really a mixture of ortho- and trisilicates. Even 

a basic trisilicate can have seemingly metasilicate ratios. All of these 

considerations complicate the identification and study of the true metasilicates to such an extent that only provisional conclusions can be 

drawn from the data now on hand.

A crystallized silicate of sodium, ]STa2SiO3.8H2O, 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 Ca2Si2O6 .5H2O. This, minus the 

water, is analogous to the mineral wollastonite, from which another 

mineral, pectolite, is derived. If wollastonite, instead of the formula 

Ca2Si2O6 , be given the formula Ca3Si3O9, it may be compared structurally with pectolite, as follows:

Wollastonite. Pectolite.

/SiOsx /SiO3 Na 

Ca/ X 4\Ca/a<

>SiO3 >Ca >SiO

a< / Oa< \SiO/ \SiOSiOs H

It must be remembered that the molecular weights of the inorganic 

silicates are not known, but only assumed; and the problem suggested

85
86 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

by the foregoing expressions is to find a set of structural formulae 

which shall represent all of 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, while 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.

Empirically, however, the nonaluminous pyroxenes resemble wollastonite in their ratios. Thus we have, according to the commonly 

accepted formulae developed by Tschermak, Doelter, and others: 

Enstatite, orthorhoinbic ................................ Mg2Si 2O6

Diopside, monoclinic ................................... CaMgSi2O6

Hedenbergite, mouoclinic ................................ CaFeSi2O6

Ehodonite, triclinic . .................................... Mu2Si2O6

There is also a great variety of other intermediate species or isomorphous mixtures in the pyroxene series, such as bronzite, hypersthene, 

schefterite, sahlite, jeifersonite, and fowlerite, in which we find, variously 

replacing one another, salts of magnesium, calcium, iron, manganese, and 

zinc. All of these minerals, however, conform to the general formula 

ESiO3 , or R2Si2O6 , which adequately expresses their constitution so far 

as they alone are concerned. This, structurally, can be written

/ 3x 

B R

which would be satisfactory if the pyroxene series ended here and if 

the amphiboles were unknown. 

Going a step farther we find in augite a pyroxene containing aluminum, and having an oxygen ratio greater than Si: 03 . 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 R2Si2O6, the basic salt RAlAlSi06 ; which, as shown by 

Groth, is empirically analogous to kornerupiue. It is also similar in 

composition to the silicate which is characteristic of the clintouite 

micas; but the two are probably not identical. A inetasilicate isomer 

of the clintonite molecule might be written either as

/Si03-Al=0 /SKV-A1-0

Mg< or as Al O

\0 Al=0 \0 >Mg

having in the first form some resemblance in structure to the metasihcate formula for the noualuminous pyroxenes. The compound, however, 

unless represented by kornerupiue, is not known by itself, but is 

assumed as existent in mixtures ; its reality, therefore, is still question-
CLARKK.] META- AND DIMETASILICATES. 87

able. Its simplest representation would be as a basic orthosilicate, 

Mg(SiO4 )(AlO)2, and it will be seen later that a polymer of this form is 

far more satisfactory than any metasilicate expression.

Still another series of silicates containing triad bases and also alkalies are classed with the pyroxenes, as follows :

Spodnmene ........................ .................. AlLiSi2O8

Jadeite ..................................... ......... AlNaSi2O6

Acrnite .............................. ................ Fe'"FaSi2O6

and their empirical formula} are fairly satisfactory. Structurally, these 

expressions become, as metasilicates,

, K'"f

\SiO3 K' -

and babingtonite, which contains no alkalies, is similar, thus :

, Fe'"

/sv

Fe" + 3 K"< >R"

I \8iO/

/Si03 Fe'"/

K" 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.spoduineue, as shown by the elaborate research 

of Brush and Dana, splits upon alteration into a 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 pseudoinetasilicate, 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 ..............'.................................. . AlKSi2O6

Spodumene ............................................ . AlNaSiaOe

Like spodumene, leucite alters into a feldspar and a member of the 

nepheline group; but it differs from spodumene inform and in density. 

The specific gravity of the isometric leucite is 2.5, that of the mcnocliuic spodumene is nearly 3.2, and hence we may reasonably infer that 

the latter species has the larger and more condensed molecule. In 

order to explain the relations of leucite, its empirical formula was
88 THE CONSTITUTION OF $HE SILICATES. [BULL. 125.

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

Al=Si3O8 Al< \A1 SiO4=Al 

\Si3O8=Al SiO/

I li 

Li Li2

From this grouping of atoms the transition into Al3 (Si3O8 )3Na3 4- 

Al3(SiO4 )3Li3 is hardly more than a simple case of 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.

If the formula just developed for spodumene should be sustained, it 

would seem necessary to adjust the other pyroxenes with it. In the 

case of Tschermak's aluminous constituent of augite this adjustment 

is easily made by taking the formula Al2MgSiO6 six times, as in the 

case of spoduraene. The parallelism between the two species is then 

representable as follows : 

Spodumene ............................. . Al^SijOsMSiO^Lig

Al. augite .............................. .Al6 (SiO4 ) 3 (SiO4 ) 3 (AlO2Mg)6

the uuivalent AlO2Mg 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

B E

I! II

/Si3 08 11 Si04v

K /K 

\SiA- R SiO/ '

II II 

R E

with four atoms of R given linking functions, while the other four are, 

so to speak, replaceably combined. ° On this basis we should write

Diopside ..................................... .Mg4Ca4 (Si3O8 )2 (SiOJ2

Hedenbergite. . ............................... .Fe4Ca4 (Si3O8 )2 (SiO4 )2
CLARKE.] META- AND DIMETAS1LICATES. 89

The formula E"4 (AlO)8 (SiO4 )2(SiO4 )2 would be exactly parallel with these, 

and affords another expression for Tschennak's compound Al2ESiO°. 

On the ground of simplicity this is preferable to the more complex 

expression based on the formula of spodumene.

The so-called "zircon pyroxenes," rosen buschite, wohlerite, laveuite, 

and hiortdahlite, I have already mentioned among the orthosilicates of 

zirconium. They can be given metasilicate ratios, following Brogger, 

by regarding the zirconium as present in the form of a metazircouate. 

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. Eosenbuschite, however, can be represented 

as a definite compound having structural analogies with the pyroxenes,

as here formulated, with the composition

Na TiF2 Ka

,SiO4 Ca SiO4 Ca SiO4 ,

F Zrf

\SiO4 Ca SiO4 Ca SiO/

 F

Ca Ka2 Ca 

which compares well with Cleve's analysis,* thus-.

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

Zr02 ..............................

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

Fe2 03 .............................

MnO.... .................. ........

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

Na»0... ...........................

r .................................

Found.

31.36

20.10

6.85

1.00

.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

This mode of interpreting the pyroxenes is so remote from our usual 

conceptions that I bring it forward 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 as double

*Zeit. Kryst. Min., XVI, p. 383.
90 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

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 .............................................. CaMgSi2O6

Treniolite. ............................................. . CaMg3Si4O12

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 Ca2Mg2Si4O12 . 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 comptex 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 orthorhoinbic anthophyllite is the equivalent or isorner of enstatite and hyper sthene. Then follows a inouoclinic series, containing tremolite, actinolite, cummingtonite, dannemorite,etc.,all being represented by the general empirical formula ESiO3, 

with calcium, magnesium, iron, or manganese as the bivalent metal. 

In griinerite the salt FeSiO3 exists by itself, and in richterite and 

astochite alkalioe 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:

Antho2)hyllite. Tremolite. 

Mg Mg

I! ' II

/Si04v x Si04x

Mg< >Mg Mg< , >Mg

XSi,0/ xSi 30/

Mg Ca 

Richterite becomes a mixture of salts,

E E

/SiO4v x4x

E< >E + E< >E

\ \

E 

commingled in ratios near 1:1, and astochite is similar, but with NaH
CLAHKE.] META- AND DIMETASILICATES. 91

in place of Na2 . Potassium may also partly replace sodium. Another 

alkaline amphibole of doubtful character, waldheiinite, approximates to

E

/ 38x

E< >E

\Si30/

which is the formula of a trisilicate pure and simple, with E=Ca, Fe, 

Mg. The existence of this compound is strong evidence in favor of 

the pseudornetasilicate theory; and, as will be seen later, it does not 

stand alone. The formulae as written suggest a close analogy between 

the pyroxene-amphibole minerals and the olivine group, which may 

have some future significance.

Among the amphiboles, as among the pyroxenes, aluminous and 

ferric compounds are common, and 'with these the minerals approach 

to 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 Al2RSiO6 or Al4E2Si2O12 . An alternative to this view is offered by Scharizer,* who shows that the hornblendes can be explained as mixtures of actinolite, E4Si4Oi2 , with an 

orthosilicate called syntagmatite (K/2E")3Al2 (SiO4 )3, whose ratios are 

similar to those of garnet. An arnphibole from Jan Mayen's Island 

approaches very nearly to syntagmatite in composition. 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

A1O2 Mg . (A1O)8

I II

.SiO, .SiO,

Alf >A1 or as Mg< >Mg

\3iO/ \SiO/

AlO2 Mg (A1O)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 the latter and identical with it in type.

No amphibole is yet known which, by itself, corresponds precisely in 

its constitution to acinite and spodumene: In 'glaucophane we find a 

species, which, as a metasilicate, may be written ALN"aSi2O6+(MgFe) 

SiO3 ; and in crocidolite another similar salt,. Fe///NaSi2O6 +FeSiO3 . 

Crocidolite alters easily; and one of the products of alteration, which

*Neues Jalirbnch, 1884, (2), p. 143.
92 THE CONSTITUTION OF THE SILICATES. I BULL. 125.

has been named griqual an elite, is very near Fe" /HSi2O6, the equivalent 

of acmite in general type. This last compound can be written

H

Fe Fe

H

and so adjusted as an ainphibole-hydrogen-acinite to the remainder of 

the group; but glaucophane and crocidolite are best, formulated as 

follows :

Glaucophane. Crocidolite.

(AlO) a (Fe'"O)2

II ' Jl

/Si3O8\ /SinOgx

Mg< >Mg Fe< . >Fe

 \Si 30/ \Si308/

which makes them, as trisilicates, precisely equivalent in structure to 

the normal ainphiboles. These compounds, and their corresponding 

orthosilicates, commingled with salts like trernolite or actinolite, give 

mixtures which conform in composition to the aluminous hornblendes. 

Closely allied to crocidolite and glaucophane is the recently described 

amphibole crossite;* in which the ratio of Si:O is distinctly less than 

1:3. This species may be written as a mixture of the two molecules 

E (Fe'"O)2

,.Si 3O8\ 

E E 1 E E

with some Al in place of Fe'", and E being=Fe, Mg, Ca. Gastaldite 

and' riebeckite are similar species, but the analyses are not perfectly 

conclusive. It is, perhaps, necessary with these minerals to assume 

the presence of acmite like molecules, riebeckite being empirically 

near 2Fe"/NaSi2O6+FeSiO3 .

Arfvedsonite, in which E" is mainly Fe, may be represented quite 

closely as a mixture of this order:

E E

/Si04X /SiO.

7 EC >E + 3 E< >E

V XSi3O/

Na, ____________(AlO)a______

r Palache, Geol. Bulletin, Univ. of California, I, p. 181. 1894.
CLARKE.] META- AND DIMETASILICATES. 93

a little aluminum being replaced by iron. Barkevikile, a more basic 

species still, has its composition well indicated by the following 

expression, with some Fe replacing Al as in arfvedsonite:

Fe (A10),

3 Fe + 2 Ca/ 1 %

\Si,O/

Ca

(A10),

Finally, in the triclinic aenigmatite, if we assume that the titanium 

(7.57 per cent TiO2 ) is equivalent to silica, we find the mixture

Fe Fe

2 F Fe + 1 F<

\Si,

Fe

(BO),

with RO = A1O: FeO:: 1:1 almost exactly, and Si: Ti:: 7:1. Calculating 

with these ratios, asnigmatite has the following composition in comparison with Forsberg's analysis :*

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

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

Fe2 03 -. ...........................

AlsOa-.-- .........................

FeO..............................

MnO...... ........................

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

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

Na2O. ............................

K20...;.. ........................

Found.

Q7 Q2

7.57

5.81

3.23

35.88

1.00

1.36

.33

6.58

.51

100. 19

Calculated.

38. 11

7.26

4.85

3.08

I 39. 20

/*

) 7.50

'

100. 00

The variations here are plainly due to the replacements of Fe by Mn, 

Ca, and Mg, and of Ka by K.

A careful study of the best analyses in the pyroxene and aniphibole 

groups will strengthen very materially the view here developed that 

the species are not true metasilicates. Although in most cases the 

approximation to inetasilicate 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. On this

*Zeit. Kryst. Min., XVI, p. 428.
94 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

theory, if we represent Si3O8 and SiO4 groups indiscriminately by the 

general symbol X, all of the amphiboles are covered by the-following 

typical symbols, in which E" stands for any dyad metal, and E' for K, 

Na, H, A1O, orFe'"O:

E"4X2

.K 3-X_2-K 2

or, structurally,

E" E" B'

\x 

II II II

B" B'2 B'2

In a similar way, all the pyroxenes, except the acmite-spoduinene 

group, which has the special formula discussed previously, may be 

represented as formed by mixtures of

B"4X4E"4

B"4X4 (B'"0)8

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 olivine minerals are represented by similar orthosilicate formulae, E4 (SiO4)2 being the constitutional equivalent of E4 (SiO4 )(Si3O8 ). Pseudomorphs of pyroxene 

(fassaite) after inonticellite, have been found at Monzoni, and are well 

known. Furthermore, Becke* has described pseudomorphs of anthophyllite and actinolite after olivine, GO 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.

By the hydration of pyroxene or arnphibole either serpentine or talc 

may be generated. The latter species has the composition H2Mg3Si4O]2, 

and may be written structurally like amphibole either

Mg H2

/ 4x / 4X 

Mg/ >Mg or Mg<( >M

H2 Mg

Both expressions are in accord with the fact recorded by Schneider 

and inyself,t that upon the ignition of talc one-fourth of the silica is

* Min. Pet. Mitth. (N. F.), 4, pp. 355 and 450. 

tBull. U. S. Geol. Survey No. 78, p. 13.
CLARKE.] META- AND DIMETASILICATES. 95

set free quantitatively. This would give the ignited residue the composition shown by the subjoined alternative formulae:

Mg

Mg<( >Mg and Mg<^ 

XSi205 / XSi30

II 

Mg

and of these the first would seem to accord the better with the remark. 

able stability and insolubility of the material. A metasilicate formula, 

H2Mg3 (SiO3)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

t'he.species,

Pyrophyllite and kyanite are two aluminous metasilicates which SCem 

to be related ; at all events the former is not infrequently the gangue 

or matrix of the latter. Empirically they are 

Pyrophyllite ............................................ .AlHSSi2O6

Kyanite ............................................ ... Al2SiO5

or, structurally,

.Si03 ,0 

Alf and Al{

\Si03 H xSiO3 A1=O

To these formulae I attach very little significance, except in so far as 

they represent a single observed relation. Probably the true formulae 

should be multiples of these 5 for kyanite is much denser than its isorner, 

andalusite, Al3 (SiO4 )3(AlO)3 . Both kyanite and andalusite, at very high 

temperatures, are transformed, with disengagement of heat, into a third 

isorner, sillimanite,* a phenomenon which indicates dissociation. Sillimanite doubtless has the smallest molecule of the three species, and 

may be AlEESiO4 A1O, but even this is so purely hypothetical that it 

would be useless at present to discuss the species farther.

With beryl, Gl3Al2SiBO| 8 , 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,

Al-SiO3 G1

^SiO, ||

I /Si3(V

Gl and Gl< >G1

I XSi30/

xSi03 II

* Vernadsky, Bull. Soc. Min., XII, p. 447, and XIII, p. 256.
96 THE CONSTITUTION OF THE SILICATES [BULL. 125.

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* has effected the synthesis of beryl by precipitating a mixture of glucinum and aluminum sulphates with a solution 

of sodium inetasilicate^ 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.

Two other minerals containing glucinum, leucophanite, and meliphanite, appear to be essentially metasilicates with the subjoined formulas:

Leucophanite......................................NaCaGlFSi2O6

Meliphanite ....................................... NaCa2Gl2FSi:,O]o

These are capable of several interpretations. The simplest regards 

meliphanite as a mixed meta- and orthosilicate, when both species can 

be written

SiOr- Gl F

/Si03 Gl F Ca< 

Ca< >SiO4=Gl \SiO3^Na Ca<

\SiO3 Na

or else with the glucinum as the linking element, and with the calcium 

united to fluorine. This involves no change of type, but only an 

exchange of position between Ca and Gl. Brogger,t to whom the 

empirical formulae are due, also represents meliphanite as a basic 

metasilicate

SiO3 Na 

C

Nsi03 Gl/

\SiO3 Gl F

which is also justifiable. Between the several alternatives there is no 

ground for deciding. The stability of meliphanite toward ordinary 

acids would seem to favor the formula first given, for a basic calcium 

salt would probably be more easily decomposable.

Danburite, CaB2Si2O8 , is sometimes regarded as the equivalent of 

barsowite, CaAl2Si2O8 , a doubtful isomer of anorthite. But barsowite

* Neuea Jahrbuch., 1894»I, p. 275. 

t Zeit. Kryst. Min., XVI, pp. 289-291.
CIABKB.] META- AND DIMETASILICATES. 97

gelatinizes with hydrochloric acid, while danburite is attacked by the 

reagent only after ignition. Possibly the difference may be as follows :

/SiO4=Al /SiO3 B=O

Ca< Ca<

\SiO4=Al \SiO3 B=O

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.

Chrysocolla is probably a metasilicate, and perhaps, empirically, 

CuSiO3.2H2O. It can not be well regarded as impure dioptase,for the 

latter gelatinizes with hydrochloric acid, while chrysocolla does not. 

The Species, which may be a mixture of compounds, needs careful 

investigation.

Among the titanosilicates, the recently described neptunite* appears 

to be a uieta compound,

R'

sio3 >E

SiOr- K'

with R'=Na, K, and R"=Fe, Mn. The analogy between this formula 

and the formulae of the titanium orthosilicates is its sole justification. 

The ratios given by analysis, however, are sharply in accord with the 

proposed expression.

The salts which are fairly ascribable to dimetasilicic acid, H2Si2O5, 

are few in number and comparatively rare. Among them, petalite, 

AlLi(Si2O5)2, is the best known and most characteristic. Its formula, as 

here written, is closely 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 to

.(OH), 

Alf

\Si2O5 H 

which requires

SiOz ............................................ 60.60

AlaOs ........................................... 25.75

HsO ............................................ 13.65

100. 00

The actual hydrocastorite contains about 4.3 per cent of lime, and 

is doubtless impure. By Doeltert petalite is interpreted somewhat

* Flink. Geol. Foren. Forhancft., XV, p. 196. 

tMin.Pet.Mitth., 1878, I,.p. 529.

Bull. 125 7
98 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

differently, being given the empirical formula A^L^SiioO^; but the 

formula here adopted is the one most generally received.

Milarite, "HKCa2Al2(Si2O5 )6, is, like beryl, hexagonal; .and its formula 

is analogously to be written

/Si205

Al-Si205 >0a

2O5 

I 

Ca

Al Si2O5 H 

^Si.Os K

For this species the only evidence is that of its composition., Its genesis and its possible alterations are unknown.

The two closely allied zeolitic minerals, mordenite and ptilolite, are 

also to be classed as dimetasilicates. Their constitution, which I have 

fully discussed elsewhere,* is easily expressed by regarding both minerals as mixtures of the two molecules

/Si205 /Si205 -K'

/ >Ca /

Al Si2O5 Al-Si2O5 -K'

Nsi2O5 +6H2O and Nsi2O5 +6H2O

Al-Si205 -H Al-Si2O5 -H

 \Si2O5 -H \Si2O5 -H

in which R'=Na orK. In one-occurrence of'ptilolite the vater 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 mordemte, and is easily interpreted thus :

>Can

Al-SiO3

3 +6H2O 

Al-SiO3

Pilinite, a similar mineral, seems to be Al2 (SiO3 )5Ca2.H2O, 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.

The rare mineral hyalotekite, which contains, boron, agrees very 

closely with the empirical formula E'^BFSieOn, if we regard the water

* Am. Jour. Sci., August, 1892.
CLAKKE.] META- AND DIMETASILICATES. 99

in it as hydroxyl replacing fluorine. This can be interpreted either as 

a dimetasilicate or as a trisilicate, with the following alternative formulae:

E 

5 R BO2 ' ||

25 or

<

xSi2O5 R-F O=B

38\

\K 

Si308<

\R F

Its association with feldspar and schefferite rather favors the trisilicate formula, but the two are empirically identical. If we reduce 

Lindstrom's analysis to 100 per cent, after calculating the water 

(ignition) into its equivalent of fluorine, rejecting as impurities the 

traces of A12O3 and Fe2O3, and consolidating like bases, we get the

following comparison with theory:

SiOj. .................

B r\

PbO...... ............

CuO.. ................

MnO.... ..............

G1O ..................

Na2O .................

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

BaO ..................

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

K2 0 ..................

Al,0s ............. ....

FejOa ..............

F... ..................

Cl ....................

Ign...... .............

Found.

39.47

3.73

25.11

.09

.29

.75

.17

7.82

20.08

.09

.89

.18

.06

.99)

.06V

.59^

100. 37

Reduced.

38.60

3.63

25.71

9.43

21.32

2.26

100.95 Qfi

100. 00

Calculated.

38.10

3.71

26.22

9.22

21.59

2.01

100. 85 

.85

100. 00

In computing, E" 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.

There still remain a few natural silicates which are definitely recognized as species, but which do not fall distinctly into any one of the 

chapters of this work. Their constitution is obscure, and I shall not 

attempt to discuss them. Still, they may properly be recorded at this 

point for the sake of completeness.

Guspidine. A fluosilicate of calcium, supposed to be orthosilicate in 

structure. It may be allied to chondrodite, but it has not yet been completely analyzed.
100 . THE CONSTITUTION OF THE SILICATES. [BULL. 125.

Ldngbanite. A silicate of manganese and iron containing antimony. 

Allied on crystallographic grounds to the oxides hematite and ilmenite.

Uranophane. Composition represented by the empirical formula 

CaU2Si2O M .6H2O.

Thaumasite. CaSiO3?CaSO4,OaCO3,15H2O. To the molecular constitution of this extraordinary substance there is as yet no tangible clew. 

A structural formula can be assigned to it, but would be meaningless.

Neotocite. Hydrous silicates of manganese and iron, of variable 

composition. Several distinct compounds are probably covered by this 

name.

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; while others may 

be ultimately established as good and definite compounds. A discussion of the records, without experimental investigation of the various 

minerals, would have very uncertain value. I therefore omit these 

doubtful species from consideration.
CHAPTEE VIII. 

SUMMARY.

The formulae developed in the body of this memoir are so varied and 

so interlinked, one with another, that a summary of them in concise 

form seems to be desirable. I accordingly append such a summary, 

with occasional notes intended to make the subject, if possible, more 

clear.

1, Ortho- and trisilicates of triad bases.

A. Normal salts.

Xenolite ....................................... . Al4 (Si04) 3 ( 9)

Samoite ...................................... . . Al4 (SiO4 ) 3.10H2O(?)

Ciinolite ........................................ . Al4 (Si308 ) 3.6H2OC?)

Anthosiderite .................................. .Fe4 (Si3O8 ) 3.2H2O(?)

Pseudobrook'ite ................................ .Fe4 (TiO4 ) 3

Eulytite .....................................

Agricolite .............:.....................

Of these species the last three are definitely established, but the 

others are more or less uncertain.

B. First siibstittition series.

/X=R'3

Type R X=R'"

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

Nephelite^ .................. ........... . Al3 (SiO4 ) 3Na3

Kaliophilite". ............................ .Al3 (SiO4)3K3

Kryptotile. .............................. . Al3 (SiO4)3H3C&)

Albite. .................................. . Al3 (Si3O8 ) 3Na3

Orthoclase. ....................'.......... . Al3 (Si3O8 ) 3K3

Paragonite .............................. . Al3 (SiO4 ) 3NaH2

Muscovite ................................ Al3 (SiO4 ) 3KH2

Andalusite .............................) .,, .._ ,^

Sillimanite........... .................. | Al3 (SiO4 ) 3 (AlO)3

Topaz ................................... . Al3 (SiO4)3 (AlF2) 3

Kornerupine* ........................... . . Al3(SiO4 ) 3(AlO2Mg)3

Rectorite ............................... . . Al3 (SiO4 ) 3H3.2H2

Hydroiiephelite ...........................

 Doubtful. Compare with formula* given for pyroxenes and amphiboles.

101
102 THE CONSTITUTION OF THE SILICATES. [BOLL. 125.

Offreti r2Al3 (Si308 ) 3K2H.8H20-1 unretite ................................ J

_1Al3(iSio4 )3CaH.SH2O

E X=E'"

\X=E'2

Double type E"

E X=E'"

.\XHE'" 

Anortbite ............................ . Al6 (SiO4 ) 6Ca3

Meionite .............................. Al6 (SiO4 )6Ga2 .(OaOCa)

Thomsonite .......................... . Al6 (SiO4 )6Ca3 .7H2O

Gismondite. .......................... . Al6 (SiO4 )6Ca3.12H2O

Phillipsite ........................... . Al6(SiO4 ) 3 (Si3O8 ) 3Ca2K2 .14H2O

Levynite ............................. . Al6X6Oa3.15H2O

Stilbite .............................. .AlG (Sl3O8 )6Oa3 .18H2O

Chabazite ............................ . Al6 (SiO4) 3 (Si3O8 ) 3Ca3.lSH2O

Also the barium feldspars, and the barium-strontium zeolites harmo 

tome and brewsterite. X represents the tetrad groups SiO4 and Si3O8 

indiscriminately.

C. Second substitution series.

/XEEE'3

Type R-XEIR'a 

\K=E'" 

Natrolite ............................ . Al2 (SiO4 )3Na2H4 ,

Dumortierite ......................... . Al2 (Si04 ) 3 (A10)6

Zunyite ............................. . Al2 (SiO4)3E'6

Chloropal ........................... .Fea(SiO4 ) 3H6

Cancrinite ........................... . Al2 (SiO4 )3NaJ H(AlCO3)

Microsommite . ....................... . Al2 (SiO4 ) 3CaR'4

Prehnite ............................ . Al2 (SiO4 ) 3Ca2H2

Arctolite ............................ . Al2 (SiO4 )3CaMgH3

Biotite .............................. . Al2 (SiO4 ) 3Mg2KH

Siderophyllite ....................... .Al2 (SiO4 ) 3Fe2KH- '

Biotite-vermiculite ................... . Al2 (SiO4 ) 3Mg2H2.3H2O

Biotite-chlorite ........................ Al2 (SiO4 ) 3 (MgOH)4H2

Scolecite. ............................ .Al2 (SiO4 )3CaH4.H2O

Sodalite ............................. . Al2 (SiO4 ) 3Na4 (A101)
CLARKE.J SUMMARY. 103

Haiiynite ............................ . Al2 (SiO4) 3Na2Ca(AlSO4Na)

Nosite ............................... . Al2 (SiO4 )3Na4 (AlSO4Na)

Lazurite. ............................ . Al2 (SiO4)3Na4(AlS3Na)

Garnet group ............................. ..E'"2 (SiO4) 3E','3

Epidote group ................... r ... ,E'//2 (SiO4)3B"2 (B///OH)

Leucite ................................ Al2(SiO4)(Si3O8 )2K4 ( AlSiO4Al)

Analcite. ........ .................... Similar to leucite.

Pollucite ......................... .... Similar to leucite. . Marialite ............................ . Al2(Si3O8)3Na4(AlCl)

Double type, R'"tX6R"e. Mesolite ................................... . Al4(Si04)6CaNa2H8.H20

Foresite ................................... . Al4 (SiO4)6CaHi0 .H2O

Conjugate type, R"'tXBR"7.

Yesuvianite. ............................... .Al2(SiO4 )5E"6(AlOH)

Gelllenite .................................. . Al2 (SiO4 ) 5E"6 (AlO2Mg)2

Melilite ..................................... Al2 (Si04)4 (Sl03)ft"

D. Third substitution series.

Type E X=E'

Phlogopite .............................. . Al(SiO4)3Mg3KH2

Phlogopite-vermiculite ................... . Al(SiO4) 3Mg3H3 .3H2O

Phlogopite-chlorite ...:.................... Al(SiO4 ) 3 (MgOH)6H3

Manganopbyll ............................ (AlFe)(SiO4) 3 (MnMg)3KH2

Ganophyllite ............................ . AlX3Mn3E'3.2H2O

Stilpnonielane ........................... . Al(Si3O8)3 (FeOH)6H3

Faujasite. ............................... . AltSiA^E'sE'^lSB^O

Cerite .................................. .Ce(SiO4 )3 (CeO)3H6

Double type, R"l2X6R'iS .

Staurolite. ................................... . Al2 (SiO4)6Fe"3(AlO)12

Harstigite ................................... . Al2 (SiO4 )6Ca5Mn2H4

E. Basic salts.

Westanite ............................... . AlOH(SiO4)2Al2

Kaolin. .................................. .AlOH(SiO4)2AlH3

Moutmorillouite .......................... . AlOH(SiO4)2H6

Newtonite ............................... . Al(OH)2(SiO4 )H3

Margarite ................................ . AlOH(SiO4 )2CaH(AiO) 3

Strigovite. ............................... . AlOH(SiO4 )2AlH(FeOH)2

Aphrosiderite ............................. AlOH(SiO4) 2Al(FeOH)3

Lepidolite type. .......................... . AlF2(Si3O8 )E/3

Cookeitetype. ............................ . Al(OH)2 (SiO4 )E'3

Clintonite type ........................... . AlO2Mg(SiO4 )E'3

Clintonite-vermiculite. .................... .AlO2Mg(SiO4 )E'3 .3H2O

Chloritoid. .............................:. . AlO2Fe(SiO4)H(AlOH)

Ottrelite .............................:... . AlO2Fe(Si3O8)H(AlOH)
104 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

With many members of the chlorite group, such as prochlorite, 

leuchtenbergite, etc.

F. JBorosilicates, etc.

Tourmaline, A....................... . Al8 (Si04)6(BO3)(BO2)2B'5

Tourmaline, B....................... . Al7 (SiO4 )6 (BO3 )(BO2 )2Mg2R'4

Tourmaline, 0....................... . Al6 (SiO4 )6 (BO3 )(BO2 )2Mg3R'5

Tourmaline, D....................... . Al5 (SiO4 )6 (BO3)(BO2 ) 2Mg4R'6

Axinite............................. . A](SiO4)(Si3O8 )(BO3 )Ca3 (AlOH)

And the species cappelinite, karyocerite, melanocerite, and tritomite.

G-. Complex silicates. 

Spodumene ................................ . Al6 (Si3O8 ) 3 (SiO4 ) 3Li6

Jadeite ..................................... Al6 (Si3O8 ) 3 (SiO4 ) 3Na6

Acmite .....................................Fe6 (Si3O8 )3 (SiO4 ) 3Na6

And in babingtouite the molecule..............Fe'"6 (Si3O8 ) 3 (SiO4 ) 3Fe"3

To this group of triad silicates, the largest and most important group 

of all, other species belong which I have not clearly interpreted. Such 

species are halloysite, allophane, collyrite, schrotterite, sapphirin, 

ardennite, carpholite, cenosite, sarcolite, etc. The formulae of the tabulated species must all be construed in a general way that is, with 

common replacements of aluminum by ferric iron, magnesium by ferrous 

iron, and so on. In every case details are suppressed, and these are to 

be found in the text. The same rule applies to the species in the groups 

which follow:

2. Ortho- and trisilicates of dyad bases.

Forsterite...............................n.Mg2SiO4

Fayalite................................n.Fe2SiO4

Tephroite ..............................n.Mn2SiO4

Willemite ..............................n,Zn2 8i04

Phenakite .............................. n.G!2SiO4

Monticellite ............................ CaMgSiO4

Knebelite .............................. MnFeSi04

Hyalosiderite........................... Mg4Fe2 (SiO4 )3

Igelstroinite............................ Fe4Mn2 (SiO4 ) 3

Trimerite............................... Gl3Mn2Ca(SiO4 )3

Choudrodite............................Mg3 (SiO4)2 (MgF)2

Humite ................................Mg5(SiO4 ) 3 (MgF)2

Clinohumite ............................Mg7 (SiO4 )4 (MgF)2

Helvite .............................

Danalite....:.......................

Bertrandite ............................ Gl3 (SiO4)2H(GlOH)

Calamine. ...°......................... ..Zn2 (SiO4 )2H2 (ZnOH)2

Lievrite................................Fe2Ca(SiO4)2 (AlOH)

Datolite................................ .Ca2 (SiO4 )2H2 (BO)2

Homilite ............................... Ca2Fe(SiO4 )2 (BO)2
CLABKB.] SUMMARY. 105

Euclase .............................. . . G-l2 (SiO4 )2H2 (AlO)2

Gadolinite ............................ .Gl2Fe(SiO4 )2 (YO)2

Dioptase .............................. .n.CuH2SiO4

Karyopilite ........................... .Mn3 (SiO4 ) 3H5 (MnOH)

Villarsite ............................. ,Mg3 (SiO4 )2H(MgOH)

Serpentine ............................ .Mg2 (SiO4)2H3(MgOH)

Picrosmine ............................ .Mg2 (SiO3 )(SiO4)H2

Aphrodite. ............................ .Mg2(SiO4)2H4

Kerolite .............................. .Mg(SiO4 ) 2H5 (MgOH)

Deweylite. .............................. Mg2 (SiO4 ) 3HG (MgOH)2 .2H20

Eudidymite ..........................)

Bpididymite..........................* 11^0181'0'

3. Ortho- and trisilicates of tetrad bases.

Zircon ..................................... n.ZrSiO4

Thorite .................................. . .n.ThSiO4(?)

Auerbachite ................................ Zr4 (Si04) 3 (Si308 )

Malacone. ................................. .Zr3 (SiO4 ) 3 (OH)H

Eudialyte ................................. . . ZrX3Cl.Ca3Na3

Elpidite. ................................. . .Zr(Si3O8 )2 (OH)2Na2H4

Oatapleiite ................................ .Zr(Si3 08 )(OH)3Na3

Wohlerite ................................. .Zr(SiO4 )4E//5Na2

Yttrialite. . ................................ .Th(Si3O8 ) 4Fe//2 (YO)8

Astrophyllite .............................. .^(SiO.J.Fe'^E',

Johnstrupite ............................. . .Ti(SiO4 )4Ca4 (CeF2 )Na2H

Sphene. ................................... .Ca(SiO4 )(TiO)

With rosenbuschite, lavenite, and hiortdahlite, allied to wohlerite; 

rinkite and mosandrite, allied to Johnstrupite; and keilhauite and 

tscheff kinite, allied to sphene.

4. The diorthosilicates. 

Barysilite. .................................. .Pb3 (Si2O7 )

Melanotekite. ............................... .Pb2 (Si2O7 )(Fe///O)2

Keutrolite .................................. .Pb2 (Si2O7 )(Mn///O)2

Ganomalite ................................ . . (PbCa)3 (Si2O7 )

Okenite ..................................... Ca3 (Si2O7 ) 3H12

Gyrolite ................................'..... Ca4 (Si2O7 )3Hi0

Apophyllite .................................. Ca2(Si2O7 )3H]2 (CaOH)2

Plombierite ................................. .Ca6(Si2O7 )3H6. 9H2O

Inesite .................................... . . E6 (Si2O7 )3H6. 2H2O

Bementite ................................... Mn6 (Si2O7)3H6

Spadaite. ................................... .Mg5 (Si2O7 ) 3H8

Saponite ................................... .(MgOH)2H4 (Si2O7 ).(?)

lolite. ...................................... . Al0 (Si2O7 )5Mg4 (AlOH)2
106 THE CONSTITUTION OF THE SILICATES. [BULL. 125.

Barylite .................................... . Al4 (Si2O7 ) 3Ba3

Sphenoclase ................................ . Al2(Si2O7 ) 3Ca6

Bowlaudite ................................. .Fe"(Si2O7 )2Y2(YF)2

5. The metasilicates.

Laubaiiite. .............................. . Al2(SiO3 )5Ca2.6H2O

Talc* ................................... .Mg3 (Si03 )4H2 (?)

Pyrophyllite ............................ . Al(SiO3 )2H

Kyanite. ................................ .AlO(SiO3)(AlO)

Beryl......... ........................... Al2Gl3(Si03)6

Leueophanite ............................ Ca(SiO3) 2Na(GlF)

Meliphanite. ............................ .Ca3(SiO3 )2 (SiO4 )GlNa(GlF)

Wollastouite ............................ .Ca3 (SiO3 ) 3

Pectolite. ................................ Ca2(SiO3 ) 3NaH

Danburite. ............................. . .Ca(SiO3 )2 (BO)2

Chrysocolla ............................ . .CuSiO3 .2H2O(?)

Neptunite ............................... .Ti(SiO3 )4B"B'2

Leucite, pollucite, analcite, eudialyte, etc., have been treated as 

pseudometasilicates. To these species must be added the pyroxenes 

and the amphiboles, with the following general formulae : 

Pyroxene .......................................... .B//8 (SiO4 )2 (Si3O8 )2

Amphibole ....................................... .B"4 (SiO4 )(Si3O8 )

Glaucophane and crocidolite may be written

or all trisilicate, and crossite, waldheimite, etc., seem to be similar. 

The evidence for these expressions appears in the text.

6. The dimetasilicates. .

Petalite .................................... . Al(Si205 )2Li

Milarite. ................................... . Al2 (Si2O5 )6HKCa2

Mordenite )............................... rAl2 (Si205 )5CaH2.6H2O-i

Ptilolite ) ............................... LAl2 (Si2O5)5R'2H2.6H2O J

Hyalotekite. ........................ .... . . . .B//2 (Si05 ) 3 (BF)(BBO2)

* Possibly a pseudometasilicate.
INDEX TO SPECIES.

. A.

Acmito, 67, 87, 88, 91, 92,94,104. 

Actinolite, 90, 91,94. 

2Enigmatite, 93. 

JEschynite, 79. 

Agricolite, 67,101.

Atomannite, 27.

Albito, 13, 28,29, 30, 33, 36, 37,39,44, 87, 88,101.

Allanite, 21.

Allophaue, 66,104.

Almandito, 21.

Alshedite, 79.

Alnrgite, 47,52.

Amphibole, 71,90,91, 93,94,106.

Analcite, 30, 31, 33, 37,42,44,45, 81, 88,103,106.

Andalusite, 15,16,19, 45, 63, 65,66,95,101.

Andesite, 28, 30.

Andradite, 21.

Anorthite, 21, 22, 28, 29, 30, 33,34, 35, 37, 40, 42,43,44,

96,102.

Anthophyllite, 90, 94. 

Anthosiderite, 66, 101. 

Aphrodite, 74,105. 

Aphrosiderito, 55,103. 

Apophyllite, 81, 82,105. 

Arctolite, 27,102. 

Ardennite, 66, 67, 104. 

Arfvedsonite, 92, 93. 

Aspidolite, 47. 

Astochite, 90, 91. 

Astrophyllite, 78, 105. 

Aucrbachite, 75, 76, 105. 

Augite, 22, 86. 

Axiuito, 61, 62, 104.

B.

Babingtonite, 87,104.

Barkevikite, 93.

Barsowite, 96.

Barylite, 84, 106.

Bairysilite, 81,105.

Bementito, 82, 105.

Berfcrandite, 70, 74, 105.

Beryl, 95, 96,106.

Biotite, 16, 20,45, 46, 47, 49, 51, 52, 53, 56, 102.

Biotite-cblorite, 53,102.

Brandisite, 47. , , Brewsterite, 40, 44, 102.

Brouzite, 86.

C.

Calamine, 70,104. 

Cancrinite, 22, 23, 20, 30,102. 

Sappelinite, 59, 60, 104.

Carphblite, 67, 104.

Caswellite, 51.

Catapleiite, 60, 76, 105.

Celadonite, 84.

Cenosite, 66, 104.

Cerite, 66, 103.

Chabazite, 33, 36, 37, 38, 39, 44, 81, 102.

Charaoisito, 55.

Chloritoid, 48, 54, 55, 103.

Chloropal, 102.

Chlorophyllite, 83.

Chondrodite, 69, 70, 104.

Chrysocolla, 97,106.

Chrysolite, 68.

Cimolite, 66, 101.

Clinochlore, 53, 56.

Clinohumite, 69, 104.

Clintonite, 26, 47, 48, 49, 51, 53, 55, 56, 65, 66, 1CG

Collyrite, 66, 104.

Conarite, 74.

Cookeite, 49, 54, 66, 103.

Corundophilite, 54, 55.

Crocidolite, 91, 92, 106.

Cronstedtite, 55.

Crossite, 92, 106.

Cryophyllite, 48.

Cummin gtonite, 90.

Cuspidine, 99.

Cyrtolite, 75, 76, 78.

D.

Danalite, 69, 104. 

Danburite, 96, 97, 106. 

bannemorite, 90. 

Daphiiite, 55. 

Datolite, 70, 79, 104. 

Delessite, 54, 55. 

Deweylite, 74, 105. 

Diabautite, 55. 

Diopside, 86, 88, 90. 

Dioptase, 71. 

Duttleyite, 51. 

Dumortierite, 20, 63, 102.

fedin gtonite, 36, 44. 

Elrcolite, 23, 30, 33, 35, 39. 

Elpidite, 76, 105. 

Enstatite, 86, 90. 

Epiohlorite, 55. 

Epididymite, 74, 105. 

Epidote, 21, 22, 30, 91,103. 

Epistilbite, 40, 44, 102. 

Euclase, 70, 105.

TOT
108 INDEX TO SPECIES.

Eucolite, 76. 

Eucolite-titanite, 79. 

Eucryptite, 16,18,19,45,87,88,101. 

Eudialyte,67,J05,106. 

Eudidymite.13,74,105. 

Eulytite, 67,101. 

Euralite, 54.

F.

Faujasite, 42,43,44,45,103. 

Fayalite, 68,104. 

Foresite, 34,35,45,103. 

Forsterite, 68,104. 

Fowlerite, 86. 

Friedelite, 71,105. 

Fucbsite, 46.

 Gadolinite, 70,105. 

Ganoinalite, 81,105.

 Ganopbyllite, 51, 52,103.

 Garnet, 20,21,22,24,26,27,30,33,55,103.

 Gastaldite, 92.

 Geblenite, 26,27,103. 

Genthite, 74.

Gismondite, 34,35,44,102. 

Glauconite, 84. 

Glaucophane, 91,92,106. 

Gmelinite, 33,36,37,38,44,102. 

Griqnalandite, 92. 

Grossularite, 16,21. 

Grothite, 79. 

Guarinite, 81. 

Gyrolite, 81,105.

H.

Hallite, 50. 

Halloyaite, 66,104.' 

Harrnotome, 31,41, 42,44,102. 

Harstigite, 64,103. 

Haiiynite 22,103. 

Hedenbergite, 86,88. 

Helvite, 69,104.

Heulandite.33,40,41,44,102.

Hiortdahlite, 77, 89,105. 

Hisingerite, 66. 

Homilite, 70,104. 

Hornblende, 91. 

Humite, 69,70,104. 

Hyalophane, 28,41. 

Hyalosiderite, 68,104. 

Hyalotekite, 97,98,106. 

Hydrobiotite, 49,50. 

Hydrocastorite, 97. 

Hydroclintonite, 49,50. 

Hydronephelite, 18,22,33,44,10L 

Hydrophiogopite, 49,50. 

Hypersthene, 86,90.

I.

Igelstromite, 68,104. 

Ilvaite, 70. 

Inesite, 82,105. 

lolite, 83,105.

Jadeite, 67,87,88,104. 

Jefferisite, 50.

Jeffersonite, 86. 

Jobnstrupite, 78,105.

K.

Kaliophilite, 16,18.10L

Kaolin, 23, 28, 29, 30, 31,32,45, 51,55, 65, 66,103.

Karyocerite, 59,60,104.

Karyopilite, 71,105.

Keilhauite, 79,105.

Kentrolite, 81,105.

Kerolite, 74.

Kerrite, 50.

Klementite, 55.

Kuebelite, 68,104.

Kornerupine, 20,65,86.

Kryptotile, 20,65,66,101.

Kyanite, 15,95,106.

L.

Labradorite, 28. 

Langbanite. 100. 

Laubanite, 32,97,106. 

Laumontite, 33,42,44. 

Lavenite, 77,89,105. 

Lazurite, 22,103. 

Lennilite, 50. 

Lepidolite, 48,49, 56,103. 

Lepidomorphite, 46. 

Leucbtenbergite, 53,56,104. 

Leucite, 30,31,33,37,44,45, 87,103,106. 

Leucophanite, 96,106. 

Levynite, 36,38,44,102. 

Lucasite, 50.

M.

Maconite, 50. 

Malacone, 75,76,78,105. 

Mangan-idocrase, 26. 

Manganophyll, 51,52,103. 

Margarite, 51, 55,103. 

Marialite, 28, 29,103. 

Meionite, 22,28,29,30,102. 

Melanocerite, 59,60,104. 

Melanolite, 54. 

Melanotekite, 81,105. 

Melilite, 22,26,27,103. 

Meliphanite, 96,106. 

Mesolite, 35,36,44,45,103. 

Metachlorite, 55. 

Microcline, 28, 37. 

Microsommite, 22,23,30,102. , 

Milarite, 13,98,106. 

Monticellite, 68,94,104. 

Montmorillonite, 65. 

Mordenite, 13,32,97,106. 

Mosandrite, 79,105.

Muscovite, 16,19,22,26,29,30,45,46,48,50,51,56,63, 

64,101.

N.

Natroli'te, 16,18,20, 22, 33, 35, 36,44,45,102.

Neotocite, 100.

Nepbelite, 16,18,19,22,30,33,44,45,101.

Neptunite, 97,106.

Newtonite, 65,66,103.

Nosite, 22,103.
INDEX TO SPECIES. 109

0.

(Ellacherite, 46.

OHretite,43,44,102.

Okenite, 81,84,105.

Oligoclase, 28.

Olivine, 22,68,71,72,91,94.

Orangito, 77,78.

Orthoclaae, 13,28,30,37,39,43,44,101.

Ottrelite, 48,54,103.

Ouvarovite, 21.

P.

Paragonite, 16,19,46,101.

Partschinite, 21,24.

Pectolite, 85,86,106.

Pennine, 53.

Petalite, 13,97,106.

Phenakite, 68,69,104.

Phengite, 46.

Phillipaite, 33,41,42,44,102.

Phlogopite, 16,45,46,47,48,49,52,53,56,103.

Phlogopite-chlorite, 53.

Pholidolite, 84.

Picroamine, 73,105.

Piedmontite, 21.

Pilinite, 97.

Plombierito, 82,105.

Pollucite, 31,103,106.

Polylithionite, 48.

Polymignite, 79,80.

Prehnite, 16,20,22,28,30,31,44,45,102.

Prismatine, 65.

Prochlorite, 54,55,104.

Protovenniculite, 50.

Pseudobrookite, 67.

Ptilolite,13,32,97,106.

Pyrope, 21.

Pyrophyllite.95,106.

Pyrosmalite, 71,105.

Pyroxene, 71,72,85,87,88,89,90,93,94,106.

K.

Kectorite, 66,101. 

Rhodonite, 86. 

Eichterite, 90. 

Biebeckite, 92. 

Kinkite, 78,105. 

Eoscoelite, 51. 

Eoseite, 50.

liosenbuschite, 77,89,105. 

Eowlandite, 84,106. 

Eumpflte, 54,55,66.

S.

Sahlite, 86. 

Samoite, 66,101. 

Saponite, 74,83,105. 

Sapphirine, 65,104. 

Sarcolite. 21,104. 

Schefferite, 86. 

 Schorlonrite, 21. 

Schrotterite, 66,104. 

Scolecite, 33,35,36,44,45,102.

Serpentine, 71,72,73,94,105. 

Seybertite,47. 

Siderop hyllite, 47,102. 

Sillimanite, 15,20,66,95,101. 

Sodalite, 22,23,25.30,33,102. 

Spadaite, 83,105. 

Spessartite,21,24,26. 

Sphene, 79,105. 

Sphenoclaae,84,106. 

Spinel, 47,56.

Spodumene, 67,87,88,91,94,104. 

Staurolite, 63,64,65,103. 

Steenstrupine, 78. 

Stilbite, 33,39,40,41,44,102. 

Stilpnomelane, 54,103. 

Strigovite, 55,103. 

Syntagmatite, 91.

T.

Talc, 94,95,106. 

Tephroite, 68,104.

Thaumasite, 100.

Thomsonite, 33,34,35,44,102. 

Thorite, 77,78,105. 

Thuringite, 55. 

Topaz, 16,19,45,101. 

Tourmaline, 56,57,58,59,104. 

Tremolite, 90,91. 

Trimerite,68,69,104. 

Tritomite, 59,60,104. 

Trooatite, 69. 

Tsoherfkinite, 79,105.

Uranophane, 100.

V.

Vaalite, 50.

Venaaquite, 48.

Vermiculite, 49,50,51,55,102,103.

Vesuvianite, 25,26,27,55,103.

Villarsite, 72,105.

* W.

Waldheimite, 91,106. 

"Weatanite, 65,66,103. 

"Willcoxite, 51. 

Willemite,68,69,104. 

Woehlerite, 77,89,195. 

"Woerthite, 65,66. 

Wolchonskoite, 66. 

Wollaatonite, 85,86,106.

X.

Xanthitane, 79. 

Xanthophyllite, 47,56. 

Xenolite, 16,101.

Yttrialite, 78,105.

Zinnwaldite, 48. 

Zircon, 75,76,78,105. 

Zoisite, 21. 

Zunyite, 20,25,102.

Y. 

Z.