Interpretation of 

the Composition of 

Trioctahedral Micas

GEOLOGICAL SURVEY PROFESSIONAL PAPER 354-B
Interpretation of 

the Composition of 

Trioctahedral Micas

By MARGARET D. FOSTER

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 354-B

A study of the compositional and 

layer charge relations of 

phlogopiteS) biotiteS) siderophyllites 

and lepidomelanes

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1960
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documenst, U.S. Government Printing Office 

Washington 25, D.C. - Price 35 cents (paper cover)
CONTENTS

Page 

Abstract___________________________________________ 11

Introduction _______________________________________ 12

Layer charge relations of the trioctahedral micas, tlieoreticaL __________________________________________ 12

Accommodation of trivalent cations in natural trioctahedral micas_____________________________________ 13

Calculation of fornmlas__________________________ 13

Excluded analyses and formulas __________________ 15

Accommodation of extra positive octahedral charges

in micas studied______________________________ 16

R+2-R+3 relation in trioctahedral micas________________ 22

Mg-Fe+2 relation in trioctahedral micas _______________ 23

Magnesium replacement system. 

Phlogopites. __________________

Eastonite_ ____________________

Biotites ______________________

Annite _______________________

Siderophyllites and lepidomelanes_____________________

Chemical composition and geologic occurrence__________

Discussion of relation between mica composition and occurrence. _ _______________________________________

Summary and conclusions____________________________

References cited ____________________________________

Index _____________________________________________

Page 

24 

26

26

27

29

30

32

37

38

39

49

ILLUSTRATIONS

Page 

FIGURE 6. Relation between additional positive octahedral charges and negative charges available to neutralize them____ 17

7. Relation between additional positive octahedral charges and negative tetrahedral charges in excess of 1.00______ 18

8. Relation between octahedral occupancy and the number of additional octahedral positive charges due to trivalent and quadrivalent cations in trioctahedral micas_--_______-___________--_-__-_-___---_---_-_--- 20

9. Relation between R+2 cations displaced and proxying R+3 (and R+4) octahedral cations_____________________ 23

10. Distribution of trioctahedral micas with respect to the number of additional positive octahedral charges carried

by trivalent and quadrivalent octahedral cations___________________________--_______-___--___-__--_- 24

11. Relation between Mg, Fe+2 (Mn+2), and R+3 (A1, Fe+3 , and Ti) in trioctahedral micas______________--____-_ 25

12. Relation between chemical composition and geologic occurrence of trioctahedral micas.___________________ 33

13. Relation between the composition and the geologic occurrence of biotites________________________________ 38

TABLES

Page 

TABLE 1. Calculation of structural formula________________-__-__-___________________-_-___-_____-_-----_-_---_ 13

2. Average octahedral and tetrahedral cation content of formulas for phlogopites, Mg-dominant biotites, and Fe+2

dominant biotites in order of number of additional positive octahedral charges __________________________ 19

3. Selected data on phlogopites from Holzner (1936, table 2) with added data for octahedral and total charge____ 22

4. Selected data on biotites from Holzner (1936, table 3) with added data for octahedral and total charge______ 22

5. Range in octahedral composition of phlogopites (in order of decreasing Mg content in terms of octahedral

positions)______________________________________________________________________________________ 26

6. Range in octahedral composition of biotites (in order of decreasing Mg content in terms of octahedral positions

occupied)______________________________________________________________________________________ 28

7. Average octahedral composition of biotites in order of decreasing Mg content___________________________ 29

8. Formulas of representative Siderophyllites____________________-________-_-------_-________-.------------ 30

9. Formulas of representative lepidomelanes_____________-________-_--____________-_-_---_-___--------_--_ 31

10. Formulas of micas intermediate in composition between Siderophyllites and lepidomelanes ___________________ 32

11. Analyses and data for writing formulas of trioctahedral micas used in correlation study (in order of decreasing

MgO content)_________________________________________________________________ 41

12. Analyses and data for writing formulas of trioctahedral micas not used in correlation study (in order of decreasing

MgO content)__________________________________________________________ 46

ill
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

INTERPRETATION OF THE COMPOSITION OF TRIOCTAHEDRAL MICAS

By MARGARET D. FOSTER

ABSTRACT

Structural formulas calculated for more than 200 published 

analyses of phlogopites, biotites, siderophyllites, and lepidomelanes indicate that the additional positive charges carried by 

trivalent cations proxying for bivalent Mg in the octahedral 

group of trioctahedral micas are accommodated in two different 

ways, (1) by a positive charge on the octahedral layer, which is 

neutralized by an equivalent increase in the negative tetrahedral 

charge (increased replacement of Si by Al), and all the octahedral 

sites are occupied, or (2) are neutralized by negative charges 

associated with unoccupied octahedral sites.

In almost all the structural formulas the octahedral group had 

a positive charge and octahedral occupancy was less than 3.00, 

indicating some degree of accommodation by both methods. 

The degree to which accommodation is made by (1) or (2) varies 

greatly. In general, however, there is a greater accommodation 

by (2) than by (1) the greater the octahedral trivalent cation 

content. As a result, there is also a general decrease in octahedral occupancy with increase in octahedral trivalent cation 

content. In most biotites, siderophyllites, and lepidomelanes 

octahedral occupancy is significantly less than 3.00 sites hence 

they are not truly trioctahedral, nor are they octaphyllites. The 

few formulas in which trivalent cations occupy more than one 

octahedral site suggests that this is the essential limit of replacement of R+2 by R+3 in these micas. It is also strong evidence 

against the existence of a complete series between phlogopite 

and muscovite.

Coincident with replacement of Mg by E+3, there is also 

replacement of Mg by Fe+2, ion for ion. With few exceptions

the calculated formulas show both types of octahedral replacement. But the two types, although coincident, are independent; 

there is no relation between the amount of E+3 present and the 

amount of Fe+2 present. Neither type of replacement forms a 

separate series, as a series of Fe+2 only replacing Mg, of which 

Winchell's annite is the theoretical end member. No representative of this end member was found among the more than 

200 analyses collected. On the evidence of the analyses and their 

calculated formulas, an octrahedral occupancy of more than 

2.20 positions by Fe+2 is not to be expected.

The only trioctahedral micas in which more than 90 percent 

of the octahedral positions are occupied by one species of cation 

are some phlogopites. From pure phlogopite as the prototype, 

the composition of all other trioctahedral micas may be derived 

by replacement of Mg by, most commonly, Fe+2 and R+3 (Al and 

Fe+3). In phlogopites the proxying of such cations for Mg is 

minor, and Mg occupies more than 70 percent of the occupied 

positions. Progressively greater proxying of these other cations 

for Mg leads successively to Mg biotites, in which Mg is still the 

dominant octahedral cation but in which Fe+2 is present in 

significant amounts, Fe+2 biotites, in which Fe+2 is the dominant 

octahedral cation, with Mg present in subordinate but significant 

amounts, and siderophyllites and lepidomelanes, in which Mg is 

essentially absent, with Fe+2 the greatly dominant bivalent 

octahedral cation and with significant amounts of aluminum and 

(or) ferric iron. These relations are expressed in the following 

formulas, which show the range in composition of the different 

groups:

Phlogopite

Mg biotite 

Fe+2 biotite

<+0.30

3.00-2.85 

<+0.50

(-1.00)- (-1.30)

2.90-2.65

(-1.00)-(-1.50)

Siderophyllites

and 

lepidomelanes

<+0.50 (-1.00)-(1.50)

2t2Vi .5oMg<0 .15^

2.90-2.65

Thus the trioctahedral micas can be considered members of a 

complete system, at one end of which is phlogopite, with essentially complete octahedral occupancy by Mg, and at the other 

end of which is siderophyllite and lepidomelane, with essentially 

zero octahedral occupancy by Mg.

There is not a clear distinction in the composition of biotites 

from different kinds of rocks. Biotites from different igneous 

rocks, granites, diorites, granodiorites, nepheline syenites may

be very similar in composition; on the other hand, biotites from 

the same kind of rock may differ greatly in composition. Micas 

at the extreme ends of the trioctahedral replacement system, 

phlogopites and siderophyllites-lepidomelanes, which have 

extremely high or extremely low Mg content, occur in more 

extreme types of rocks, peridotites and other ultramafic rocks, 

metamorphosed limestone, and contact zones of metamorphism 

on the one hand, and pegmatites and greisen on the other.

11
12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

INTRODUCTION

This study of the compositional and layer charge relations of the trioctahedral micas is an extension of a 

similar study (Foster, 1956) of the composition and 

layer charge relations of the dioctahedral potassium 

micas. In that study it was shown that these micas 

may be classified and correlated on the basis of the relation between the charges on their tetrahedral and 

octahedral layers. It is the purpose of this paper to 

present a similar interpretation of the composition and 

layer charge relations of the trioctahedral micas.

LAYER CHARGE RELATIONS OF THE TRIOCTAHEDRAL 

MICAS, THEORETICAL

Ideally, the layer charge relations of the trioctahedral 

micas are the same as those of muscovite, as expressed 

in their theoretical or ideal formulas,

0.00 -1.00

0.00 -1.00

[Al2 .0o(Si3 .ooAl1 .0o)01 o(OH) 2 ]-i.ooK1+01600 .

In both of these formulas the octahedral group is neutral; all the inherent unit layer charge is on the tetrahedral group, owing to occupation of one-fourth of the 

tetrahedral cationic sites by Al.

Few natural micas, however, have the exact composition specified by these theoretical or ideal formulas. 

In the dioctahedral micas bivalent cations occupy some 

of the octahedral positions ideally presumed to be occupied by Al, and in the trioctahedral micas some of 

the octahedral positions, ideally presumed to be occupied by bivalent ions like Mg or Fe+2, are occupied by 

trivalent ions like Al or Fe+3. Proxying of a bivalent 

for a trivalent octahedral cation, ion for ion, in the 

dioctahedral micas induces a negative charge on the 

octahedral layer equivalent to the number of bivalent 

octahedral cations. Constancy of charge and K content is preserved by an equivalent decrease in the 

negative tetrahedral charge, that is, equivalent decrease in tetrahedral Al content and increase in Si content. These changes can be expressed by the equation, 

-\-nR+2 -\-nSi- 2riR+*. Replacement of one-half of 

the trivalent octahedral cations by bivalent cations 

produces the tetrasilicic end member of the trisilicictetrasilicic series,

-1.00 0.00

in which all the inherent unit layer charge is on the 

octahedral layer.

By analogy, proxying of a trivalent cation for a bivalent ion, ion for ion, in the octahedral group of a

trioctahedral mica should induce a positive charge on 

the octahedral layer, which, for constancy of inherent 

layer charge and K content, would require an equivalent increase in the negative tetrahedral charge and, 

consequently, equivalently increased proxying of Al for 

Si. This charge relation is assumed in Winchell and 

WinchelFs (1951, p. 373) formulas (recast in the notation used in this paper) for eastonite,

+0.5 -1.5

[(Alo.5Mg2 .5)(Si2 . 5Al1 .5)01 o(OH) 2 ]-'-<>o 

and siderophyllite,

+0.5 -1.5

i 'e£S)(Si3.5Ali.j)010 (OH) a ]-'-<»

In these formulas it is also assumed that all 3 octahedral 

positions are occupied, as they would be if the proxying 

of trivalent for bivalent cations were 1 to 1.

Holzner (1936, p. 435) noticed, however, that in formulas calculated from biotite analyses, the number of 

occupied octahedral positions is between 2.89 and 2.49; 

in none of his formulas were 3.00 positions occupied. 

To explain this low octahedral occupancy he postulated 

that biotite is intermediate between phlogopite and 

muscovite in composition and that the crystal structure 

is built up of the 2 mica types, most biotites approximating the composition of 2 of phlogopite to 1 of 

muscovite, which yields a formula for biotite of:

0.00 -1.00

[(Alp.67Mg2.oo) (Sia.ooAlj.oo) Ow (OH),]-»-» K^00 

2^67

In this formula the tetrahedral group has the same composition and charge as it has in the ideal formulas for 

phlogopite and muscovite, and the octahedral layer is 

neutral, as in the theoretical or ideal formulas. The 

0.67 additional positive charges carried by the 0.67 

trivalent ions are neutralized by the 0.67 negative 

charges associated with the 0.33 unoccupied octahedral 

positions.

Thus in WinchelPs formulas for eastonite and siderophyllite and in the formula suggested by Holzner for 

biotite, trivalent octahedral ions are accommodated in 

two quite different ways. In one the proxying of trivalent for bivalent octahedral ions is 1:1; all the octahedral cationic positions are occupied; and the additional 

positive charges carried by the trivalent octahedral 

ions form a positive charge on the octahedral layer 

and are neutralized by an equivalent increase in the 

negative tetrahedral charge owing to greater proxying 

of Al for Si. These relations are expressed in the 

equation

(+2nR+3 ) = (- (A)
INTERPRETATION OF TRIOCTAHEDRAL MICAS 13

In the other way of accommodating trivalent octahedral 

cations, 2 trivalent cations proxy for 3 bivalent cations. 

There is, consequently, no increase in the number of 

positive charges present, nor is there any change in the 

layer charge relations, between the octahedral and 

tetrahedral groups, but there is a decrease in the 

number of octahedral positions occupied equivalent to 

one-half of the number of trivalent octahedral cations. 

These relations are expressed in the equation

(B)

In the first type of accommodation for trivalent octahedral ions, the formula is characterized by full octahedral occupancy, a positive octahedral charge, and a 

negative tetrahedral charge greater than 1.00 by the 

number of trivalent octahedral cations; in the second 

type of accommodation the formula is characterized 

by deficient octahedral occupancy, a neutral octahedral 

layer, and a negative tetrahedral charge very close to 

1.00.

ACCOMMODATION OF TRIVALENT CATIONS IN 

NATURAL TRIOCTAHEDRAL MICAS

CALCULATION OF FORMULAS

For a study of the general characteristics of natural 

trioctahedral micas and, particularly, their accommodation of trivalent octahedral cations, structural 

formulas were calculated from more than 200 analyses 

of phlogopites, biotites, siderophyllites, and lepidomelanes. The formulas were calculated by the method 

devised by Marshall (1949, p. 58) and modified by the 

author (Foster, 1951, p. 728). The calculations involved are illustrated by example in table 1. The 

gram-equivalents of cationic constituents in the structure are obtained by dividing the percent of each constituent present by a factor obtained by dividing the 

molecular weight of that constituent by the number of 

cationic valencies in its molecular formula. Thus for 

Fe2O3 the factor to be used is the molecular weight of 

Fe2O3 , 159.70, divided by 6, which is 26.617; for FeO 

it is 71.85 divided by 2, or 35.925.

TABLE 1. Calculation of structural formula 

[For 49, table 11]

Gram- Cationic 

equivalents valencies Cations 

of cationic per per 

Percent constituents unit cell unit cell Charge

SiOj. __36. 32-^ (60. 04-f-4) = 2. 419-nO. 1109=21. 912-J-4= 5. 453"

2. 547 

/

tetrahedral -2.547

/ 8.00oJ gr°Up

AljOs 

TiO2 -__

FeaOi 

FeO

-_14. 66-K101. 96 6)-0. 863 0. 1109= 7. 781 3 = 2. 594-»0. 047]

__ 3. 42-s- (79.90 4)= .171 .1109= 1.542 4= .385

_ 4 8(H-(159. 70 6)= . 180 . 1109= 1. 623 3= . 541

14 74: C7\ Sfi 2^ = 410 1109 3 RQ7 2 1.848

MgO._ _ 12. 38-s- (40. 32 2)= . 614 . 1109= 5. 537 2= 2. 768 

MnO 0 24 : (70 Q3 9.\ 010 1109 090 2 .045

Sum (-Si and Al) 12. 489 5. 634

+ 12. 630 

-12. 000

octahedral 3Q 

group

(octahedral 1. 917 (composite 

occupancy) layer charge)

+ 0. 630 (octahedral

charge)

CaO--.__Trace

Na2O_.__. 0. 90-h- (61. 98-*-2) = 0. 029-*-0. 1109= 0. 261-f-l = 

K2O____ 8. 63-^ (94. 20-^2) = . 183-f- . 1109= 1. 650-^-1 =

0. 261

1. 650

4. 879 1. 911 

4. 879-4-44=0. 1109

1. 911 (interlayer 

===== cationic 

charge)

H,0. 2.72

+0.27

Structural formula (half-cell)

-1.27

\ +0.95

2.80
14 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

The second step in the calculation is to convert 

gram-equivalents of the cationic constituents to cationic 

valencies in the unit cell. As the number of cationic 

valencies must be the same as the number of anionic 

valencies present, the number of cationic valencies 

present is usually based on the number of anionic 

valencies furnished by the H20+ and F reported in the 

analysis. However, in many of the analyses insufficient 

data were given for this; in some F was not reported, in 

others plus and minus waters were not reported, only 

total water, and in still others theH^O+ and F together 

were much too low. . The formulas were, therefore, all 

calculated on the basis of the cationic and anionic 

valence content of the idealized unit cell, 44. The sum 

of the gram-equivalents of the cationic constituents 

was, therefore, divided by 44 to obtain the factor for 

converting them to cationic valencies per unit cell. 

After dividing the gram-equivalents of each cationic 

constituent by this factor, the number of valencies per 

unit cell thus obtained of each cation is then divided by 

its valence to obtain the number of cations per unit cell.

The aluminum cations are divided between the 

tetrahedral and octahedral groups; enough Al is allocated to the tetrahedral group to bring the total, with 

silicon, to 8.00, and the remainder is assigned to the 

octahedral group. The sum of the cations in the 

octahedral group should, theoretically, be 6.00, but in 

most of the calculated formulas for trioctahedral micas 

it is less than 6.00.

Throughout this paper the discussion is based on the 

half-cell formula. To write the half-cell formula, the 

values for the cations and charges per unit cell are 

merely halved, as in the formula at the bottom of table 

1. Attention is called to the order in which the groups 

and cations within each group in the formula are 

written. First the octahedral group (enclosed in 

parentheses) is noted, with the cations in that group 

written in order of decreasing valence, except for Ti, 

which is written after Al. Then the tetrahedral group 

(also enclosed in parentheses) is noted, with Si, the 

principle cation, written first. Above each group its 

charge is noted. After these two groups, which indicate the cationic composition, the anionic composition, 

Oi0 (OH) 2 , is written; and the whole is bracketed, as 

indicating the composition of the composite layers. 

After the upper part of the closing bracket, the total 

negative charge on the composite layer unit is written. 

This is followed by a notation of interlayer cation content, with the positive charge carried by these cations being written at the top of the closing parenthesis, and the 

number of positions occupied by these cations at the 

bottom. The interlayer cations are written last, 

rather than first, the more conventional position, 

because the amount and charge of the interlayer cations

is dependent on and must neutralize the charge on the 

composite layers.

Theoretically 32 of the 44 anionic or negative 

charges are allocated to the tetrahedral group and 12 

to the octahedral group. Thus if the tetrahedral group 

is entirely made up of Si cations, which are quadrivalent, the number of positive charges present is 

exactly equal to the number of negative charges, 

4X8=32, and the tetrahedral layer is neutral. But 

if the tetrahedral group contains some Al, which is 

trivalent, there are not enough positive charges to 

neutralize all the negative charges present and the 

tetrahedral layer has a negative charge. Thus in the 

example given in table 1, the tetrahedral group contains 

5.453 Si cations and 2.547 Al cations, which carry 

29.453 positive charges, and the tetrahedral layer has 

a negative charge of 2.547, exactly equal to the number 

of Al cations in that layer. If all the 6 octahedral 

positions in the unit cell of a mica are occupied by 

bivalent cations, as by Mg in phlogopite, the 12 positive 

charges they carry exactly equal the 12 negative charges 

allocated to that layer, and the layer is neutral; but if 

some of the positions are occupied by trivalent ions, 

the octahedral layer may have a positive charge, even 

though all the octahedral positions are not occupied, 

as in the example given in table 1. The number of 

positive octahedral charges present is calculated by 

multiplying the number of cations of each constituent 

by its valence and by adding these together. More 

simply, the same result can be obtained by taking the 

sum of the gram-equivalents per total of 44 of all the 

octahedral cations except Al cations and adding to this 

the number of octahedral Al cations (from the column 

headed cations per unit cell) multiplied by 3, as is 

shown in table 1. If this number, as in the example 

given, exceeds 12.000, the octahedral layer has a positive charge equal to the excess; if it is less than 12.000, 

the octahedral layer has a negative charge.

The algebraic sum of the tetrahedral and octahedral 

charges is the inherent charge on the unit layers and 

should closely agree with the charge on the interlayer 

cations, as it does in the example given. The negative inherent charge on the unit layers and the positive charge on the interlayer cations should be close 

to 2.00 (1.00 in the half-cell formula) for trioctahedral 

micas like the phlogopites and biotites. In evaluating 

the half-cell formulas used in this study, a variation 

of plus or minus 0.1 (0.2 in the unit cell) was permitted in these values. Formulas, in which either the 

negative inherent charge on the unit layers or the positive interlayer cation charge, or both, were greater or 

less than 1.00 by more than 0.1, were not used in the 

study. This was done in order to exclude from the 

study the formulas that may represent hydrous micas,
INTERPRETATION OF TRIOCTAHEDRAL MICAS 15

in which the interlayer cations are generally lower than 

in true micas, or, possible contamination of trisilicic 

trioctahedral micas with other materials.

The role of titanium in micas is ambiguous. Apparently no studies have been made to determine whether 

or not the titanium reported is actually in the mica 

structure. In some micas discrete crystals of rutile 

can be discerned, but what proportion of the total 

titanium content these represent is not known. In 

other micas discrete crystals of rutile are not discernible, and the color of the micas suggests that the 

titanium is hi the layer structure. For some analyses 

used hi this study, formulas were calculated as if all 

the titanium were in the layer structure and as if none 

of the titanium were in the layer structure. These 

calculations showed that for the amount of titanium 

usually reported, the inclusion or exclusion of titanium 

in calculating the formula has only a slight effect on 

the number of positions occupied by other octahedral 

cations. In accordance with general usage, therefore, 

titanium was included in the octahedral group in calculating the formulas herein used. The analyses (and 

data from which their calculated formulas may be 

written) that were used in the study are given in 

table 11; the excluded analyses (and data from which 

their calculated formulas may be written) are given 

in table 12.

EXCLUDED ANALYSES AND FORMULAS

Exclusion of certain analyses and formulas from the 

study does not necessarily indicate that they represent poor specimens or poor analyses. Many of the 

formulas in which the negative inherent unit layer 

charge and the positive interlayer cation charge is less 

than 0.90 represent very high magnesium trioctahedral 

micas, that is, phlogopites. In addition to their low 

inherent unit layer charge and low interlayer cation 

charge these formulas are, in general, characterized by a positive octahedral layer charge greater than 

the number of extra positive charges carried by their 

trivalent octahedral cations, and by a high octahedral 

occupancy, many of the formulas having an octahedral 

occupancy greater than 3.00. These characteristics 

indicate the allocation of too many cations to the 

octahedral layer, and too few to interlayer cation positions. Reallocation of some octahedral cations in these 

formulas to interlayer positions would produce more 

rational formulas.

For many years it was customary in calculating formulas from analyses of montmorillonite, which has the 

same structure as the micas, to allocate all the Mg to 

the octahedral group. The result was that in most 

formulas for montmorillonite the number of occupied 

octahedral positions was greater than 2.00, up to 2.24,

and Ross and Hendricks (1945, p. 44) concluded that 

in montmorillonite more than 2.00 octahedral positions 

were characteristically occupied. However, the writer 

(Foster, 1951, p. 725) determined the exchangeable 

cations in a number of montmorillonites and found 

that they all contained some exchangeable Mg, and 

that when only nonexchangeable Mg was allocated to 

the octahedral layer, the number of octahedral positions occupied was 2.00±0.02 in the same specimens 

that apparently had as many as 2.12 octahedral positions occupied when all the Mg was allocated to the 

octahedral group.

These facts indicate that Mg can occupy interlayer 

positions and suggest that in the highly magnesium 

micas mentioned some of the magnesium should be 

considered as occupying interlayer positions. For 

example, if all the MgO, 24.60 percent, analysis D15, 

table 12, is allocated to the octahedral group in the 

usual way, the positive octahedral layer charge is 0.44, 

whereas only 0.32 additional positive charges are carried 

by the trivalent octahedral cations present; an excessive 

number of octahedral positions are occupied, 3.06; the 

inherent unit layer charge is only 0.82; and the interlayer cation charge in only +0.83. If, however, 0.75 

percent of the MgO is allocated to the interlayer cations 

the positive octahedral layer charge is 0.28, 0.04 less 

than the number of positive charges carried by the 

trivalent octahedral cations present, 2.98 octahedral 

positions are occupied, the inherent unit layer charge 

is 0.98, and the interlayer cations charge is 0.99. A 

comparison of the two formulas,

+0.44 -1.26

[(Alo.18Tio.o8 Feo+o34 Feo+129 Mg2 .8! )(Si2 . 74Ai1 .2fi)01 o(OH) 2 J-''-82

3.06

+ .28

(K,Na,Ba/2)0+8°i8

-1 .26

[(A10

2.98

(K,Na,Ba/2,Mg/2)0+8°9' + 0.99

shows that the only change in layer occupancy is in 

the number of octahedral positions occupied by Mg; 

the number of positions occupied by all the other 

cations, in both the octahedral and tetrahedral layers, 

is the same in both formulas. Therefore, if Mg does 

occupy interlayer positions in micas, analyses like this 

group which contain considerable Mg and in which the 

negative inherent layer charge and the positive interlayer cation charge are low, may not be faulty, or 

represent poor materials, but may simply represent 

trioctahedral micas in which some of the Mg occupies 

interlayer positions.

527919 59-
16 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Another group of formulas was excluded from the 

study because their negative inherent unit layer charge 

and their positive interlayer ion charge was greater 

than 1.10. In formulas calculated from analyses in 

which lithium was not determined the high unit layer 

and interlayer charges may be due to the failure to 

separate lithium from sodium. Failure to separate 

lithium from sodium has a double effect on the formula 

calculation, as it is reported as sodium and is calculated 

as an interlayer cation rather than as an octahedral 

cation. Thus, in the formula the interlayer cations 

and their charge are high, the octahedral cations and 

the positive octahedral charge are low; and, consequently, the negative inherent unit layer charge is 

high. This double effect is shown in the formulas 

below for D58, table 12,

+0.31 -1.54

[(Alo, ooTio,

2.57

+0.56

(K,Na,Ca/2) +2

+4

2.87

-1.55

(Si2 .45M.34Feo+|2)010 (OH) 3]-<>-<» (K, Na,

In the first formula, which is calculated from the 

analysis as reported, the negative inherent layer charge 

and the positive interlayer cationic charge are very 

high, but the octahedral occupancy is quite low. The 

second formula is a recalculation from the same analysis 

on the assumption that the Na2O reported contained 

1 percent of Li2O. The formula content of other cations 

present is very little changed, but the octahedral 

occupancy is raised from 2.57 to 2.87, and the negative 

unit layer charge and the positive interlayer cation 

charge are reduced to 0.99 and 0.98, respectively. 

Thirteen of the eighteen formulas that were not used 

because of high unit layer charge and high interlayer 

cation charge were of the type most likely to contain 

lithium, high in Fe+2 and very low in Mg. Thus, 

although high unit layer and interlayer cation charges 

may be due to faulty analysis, particularly in determination of the alkalies, it is also possible that a calculated 

formula may show such high charges because of the 

analyst's failure to separate sodium and lithium. 

Failure to separate and determine Rb and Cs may also 

contribute to an apparently high alkali content.

In calculating the formulas for several analyses published as analyses of phlogopites or biotites, it was found 

necessary to allocate all the A12O3 , all the Fe2O3 , and

some of the TiO2 to the tetrahedral layer to complete 

the filling of that layer, and in one phlogopite analysis 

it was necessary to assign all the A12O3 , Fe2O3 , TiO2, and 

FeO to the tetrahedral layer, leaving only MgO and a 

little Li2O in the octahedral layer even so, the octahedral occupancy was high, 3.08 positions. All such 

analyses and formulas were excluded from the study 

as not representing the general chemical composition 

of trioctahedral micas, the subject of this study, whether 

or not the analyses themselves may be faulty or may 

represent contaminated materials.

Two other small groups of analyses with their derived 

formulas that were excluded from the study were not 

representative of the kind of micas under consideration. 

In one group the K content was very low, occupying 

fewer than 0.75 positions, with relatively high Ca 

and (or) Na content; in the other group the octahedral 

trivalent occupancy was very high, greater than 1.50, 

and total octahedral occupancy was very low, less than 

2.50 positions. In several of the latter group the trivalent octahedral cations made up niore than 65 percent of the octahedral cations. In one analysis, D61, 

table 12, which has 19.49 percent of Fe2O3 , and 14.10 

percent of FeO, Clarke (1903, p. 77) suspected alteration. A later analysis showed 24.22 percent of Fe2O3 

and 13.11 percent of FeO. Another analysis reports 

27.19 percent of Fe2O3 with only 0.64 per cent of FeO. 

In the formula for this analysis bivalent cations occupy 

less than 30 percent of the occupied octahedral positions, 

all the rest are occupied by trivalent cations. In 

analyses that yield formulas with unusually high octahedral Al content, contamination with muscovite may 

be suspected. For example, the material represented 

by one analysis was obtained from arborescent aggregates of biotites with muscovite occurring in the terminals of the aggregates.

ACCOMMODATION OP EXTBAPOSITIVB OCTAHEDRAL 

CHARGES IN MICA8 STUDIED

In a few of the formulas included in the study, the 

proxying of trivalent for bivalent octahedral cations is 

in accordance with equation A, (+2r&R+3)=( nR+2) 

+ ( ft,Si+4), as in the following formula for a mica from 

Russia (26, table 11),

+0.34 -1.33

3i».wAlui)Oio(OH),]-«J»

3.01

In this formula the proxying of trivalent for bivalent 

ions is 1:1, 0.26 Al, Ti, and Fe+3 ions proxy for 0.25 

R+2 ions, and all the octahedral positions are occupied. 

The additional positive charges carried by Al, Fe+3, and 

Ti form a positive charge of 0.34 on the octahedral
INTERPRETATION OF TRIOCTAHEDRAL MICAS 17

layer, which is neutralized by an equivalent increase in 

the negative tetrahedral charge. On the other hand, 

a few formulas exhibit the type of accommodation for 

trivalent octahedral ions expressed by equation B, 

(+27iR+3 ) = (-37fcR+2)-K-7fcOct. Pos.), as does this 

formula for a biotite from Laven Island (84 table 11),

+0.04 -1.07

2.79

In this formula the proxying of trivalent for bivalent 

octahedral cations is approximately 2:3; 0.42 Al, Ti, 

and Fe+3 cations carrying 0.46 additional positive 

charges proxy for 0.63 R+2 cations, and 0.21 unoccupied 

octahedral positions furnish 0.42 negative charges to 

neutralize 0.42 of the additional positive charges. 

The octahedral layer is, therefore, almost neutral, 

having a positive charge of only 0.04; and the tetrahedral charge is 1.07, close to the 1.00 of the theoretical 

formula. However, most of the formulas calculated 

for this study are characterized by both a positive octahedral charge and a deficiency in octahedral occupation; 

this indicates a dual adjustment, as in this formula for 

a phlogopite from Uganda, 33, table 11,

+ .22 -1.29

[ (Alo.ooTio.i5Feo4232Feo+321 Mg2 .i8Mno.02) (Si2 . Alt .28) 010 (OH) 2 1-i <*

2.85

In this formula the trivalent and quadrivalent octahedral cations carry 0.52 positive charges in excess of the 

number of positive charges carried by the same number 

of bivalent cations. Of these additional charges, 0.30 

are neutralized by 0.15 unoccupied octahedral positions 

and 0.22 form a positive charge on the octahedral layer, 

which is neutralized by an increase in the negative 

charge on the tetrahedral layers. The inherent layer 

charge is, consequently, very close to 1.00 (1.07), and 

is balanced by interlayer cations, K, Na, Ba, and Ca, 

carrying an equivalent positive charge.

As illustrated by this example, in dual accommodation for trivalent and quadrivalent octahedral cations 

in trioctahedral micas, the additional positive octahedral charges are neutralized by the negative 

tetrahedral charge in excess of 1.00, in accordance with 

equation A, and by the negative octahedral charges left 

available by unoccupied octahedral positions, in accordance with equation B. Theoretically, the sum of these 

negative charges should exactly equal the number of 

additional positive charges carried by the trivalent and 

quadrivalent octahedral actions. This relation in the 

trioctahedral micas studied is shown in figure 6, in 

which the number of additional positive charges carried

0.00 " 0.20 0.40 0.60 0.80 1.00 1.20 

ADDITIONAL POSITIVE OCTAHEDRAL CHARGES

FIOUEE 6. Relation between additional positive octahedral charges and negative 

charges available to neutralize them.

by trivalent and quadrivalent octahedral cations in 

each formula is plotted against the number of negative 

charges available to neutralize them; that is, the sum 

of the tetrahedral charge in excess of 1.00 plus the 

number of unoccupied octahedral positions doubled. 

Points for formulas in which the number of additional 

positive octahedral charges is exactly equal to the 

sum of negative charges available fall on the line 

bisecting the figure. Location of a point above or 

below the line indicates that in the formula represented 

by the point the negative charges available are greater 

or less, respectively, than the number of additional 

positive octahedral charges, by the distance of the 

point from the line. The close clustering of the points 

along the line indicates that there is a fairly close 

agreement in the formulas between the number of 

additional positive octahedral charges and the number 

of negative charges available to neutralize them The 

points on or close to the line represent formulas in 

which the inherent unit layer negative charge and the 

interlayer cation positive charge is almost exactly 1.00. 

Those farthest from the line represent formulas in 

which the inherent unit layer negative charge and the 

interlayer cation positive charge is near 0.90 or 1.10. 

In the formulas in which the inherent unit layer negative charge and the interlayer cation charge is less than 

1.00, the tetrahedral negative charge in excess of 1.00 

is less than the octahedral positive charge; in formulas
18 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

in which the inherent layer negative charge and the 

interlayer cation positive charge is greater than 1.00, 

the tetrahedral negative charge in excess of 1.00 is 

greater than the octahedral positive charge. As 

pointed out in the discussion on the calculation of 

formulas (p. 13), these discrepancies may be due to 

analytical error, particularly in the determination of 

Ca, Na, and K, to failure to determine Li or CO2, or to 

the use, in calculating the formulas, of the theoretical 

oxygen and hydroxide, fluorine values (O10 (OH,F) 2) 

because of lack or obvious inaccuracy of data on these 

constituents in the analyses.

The degree of accommodation for additional positive 

octahedral charges by each of the two methods in the 

trioctahedral micas under study is shown (fig. 7) by 

plotting, fpr each formula, the additional positive 

octahedral charges against the number of negative 

tetrahedral charges in excess of 1.00. Points for

0.00

I

"z

1 

m

jf 

m 

QC

CM

^** X

CQ

O

0°°

y

X 

X x

8

x

x

X

<

O

X

O

0

O

O 

O

O

O

o

x° 

O

." :""'

o

°0

o 

o

o ^

0 /

8 </

o <y

o 

o

X

XXX 

X

X X *.* (

oo**K x

O x O 

v x x

.9 i x

1 ox

o x

: " 6

X

X

/$/(

o 

E>

F

Mg

ox 

x

»

X

X

.

/

PLANATIC

O 

Mg dominant

X

e+2 dominan

andFe+ 2 at 

equivalent

x

\/

>N 

t

)OUt

0.10 0.20 0.30 0.40 0.50 

NEGATIVE TETRAHEDRAL CHARGES IN EXCESS OF 1.00

FIGURE 7. Relation between additional positive octahedral charges and negative 

tetrahedral charges in excess of 1.00.

formulas that exhibit accommodation for trivalent and 

quadrivalent octahedral cations exclusively in accordance with equation A fall on line A, those for formulas 

that exhibit accommodation for trivalent and quadrivalent octahedral cations exclusively in accordance with 

equation B fall on the ordinate, and those for formulas 

exhibiting both types of accommodation for trivalent 

octahedral cations fall between these two lines. The 

location of most of the points between .these two lines 

makes it quite apparent that in most of these trioctahedral micas both types of accommodation for 

trivalent and quadrivalent octahedral cations are represented. In formulas represented by points near the 

middle of the area between lines A and B, accommodation has been made both ways about equally; in formulas 

represented by points closer to line A or to the ordinate 

of the graph, line B, that type of accommodation represented by line A or line B was made to a greater degree 

than by the other type.

The average adjustment in trioctahedral micas for 

different amounts of additional positive octahedral 

charges due to trivalent cations is shown by the average 

formulas in table 2. To arrive at these average formulas 

the calculated formulas used in this study were grouped 

according to the number of additional positive charges 

carried by their trivalent (and quadrivalent) octahedral 

cations. If the formula contains no Ti, the number of 

additional octahedral positive charges is equivalent 

to the number of octahedral positions occupied by 

trivalent cations, usually Al and Fe+3. But if the 

formula contains Ti, the number of positions occupied 

by Ti must be multiplied by 2, as Ti+* carries 2 more 

charges than a bivalent cation, and this number is 

added to the number of octahedral positions occupied 

by trivalent cations. For example, in the formula for 

No. 33, table 11, trivalent cations occupy 0.22 octahedral 

positions and Ti occupies 0.15 octahedral positions. 

Thus the number of additional positive octahedral 

charges present is 0.22+(0.15X2)=0.52. From the 

formulas in each of these groups, separate average 

formulas were derived for the magnesium and for the 

bivalent iron-dominant micas when both are sufficiently 

represented in a group. However, sufficient numbers 

of formulas to permit derivation of average Fe^2- 

dominant formulas are found only in the groups in 

which the number of additional positive charges is 

greater than 0.50. In only three Fe+2-dominant formulas were the number of additional positive charges 

carried by trivalent and quadrivalent octahedral cations 

less than 0.50. On the other hand, Mg-dominant 

formulas are found in all the groups, and from these 

the general mode of adjustment of trioctahedral micas 

to the presence of trivalent and quadrivalent cations 

in the octahedral layer can be deduced.
INTERPRETATION OF TRIOCTAHEDRAL MICAS 19

TABLE 2. Average octahedral and tetrahedral cation content of formulas for phlogopites, Ms-dominant biotites, and Fe+2-dominant biotites

in order of number of additional positive octahedral charges

Group

A.......

B....._.

 .._.._.

D_._.__

E

F_______

G____-_.

H......

I.... 

Number of 

additional 

positive 

octahedral 

charges

0. 00-0. 10

0.05

0. 11-0. 20

0.15

0. 21-0. 30

0.25

0. 31-0. 40

0.35

0. 41-0. 50

0.45

0. 51-0. 60

0.55

0. 61-0. 70

0.65

0. 71-0. 80

0.75

0. 81-0. 90

0.85

Number of 

formulas

6

14

11

11

6

9

8

4

Mg-dominant formulas

+0.05 -1.05 

( Alo.ooFeo+os Feo+o26 Mg2 .9o) (Si, .,, Al, .«)

3.00

+0.15 -1.15 

(Al0 .1 oFeo+o36FeoH:i2oMg2 .75)(Si2 .86Al1 .,5)

3.00

+0.25 -1.25

(A]o.1«Feo+i3oFe0H:loMg2 .«)(Si2 . 76Al1 .45)

3.00

+0.25 -1.25

(Al<,.»Tio.o5Fe6f13oFeoH:loMg2 .JS)(Si2 .76Al1 .25)

2.95

+0.25 -1.25

(Alo.isTio.osFeo+loFeo+fsMg, .96) (Si2 .78Al, .26)

2.90

+0.25 -1.25

(A]o.2oTio.1 oFeo+i35Fe0H:625Mg1 .76)(Si2 .76Al1 .25)

2.85

+0.25 -1.25

(Al0 .i,Tio.wFe0+j|,FelJ5.Mg1 .«)(Si,.wAl1 .l,)

2.80

+0.25 -1.25

(Alo.wTio.1 5Feo+235Fe0+825Mg1 .30) (Si, .76A1, .25)

2.75

+0.25 -1.25

(Alo.«Tio.2oFeof23oFe0+!oMg1 .26) (Si, .7,A1, .,5)

2.70

Number of 

formulas

None

N^one

1

2

6

14

17

12

IV'-dominant formulas

+0.25 -1.25

(Alo.ioTio.isFeotfsFe^Mgo.M) (Si,.7 ,Al, . )

2.85

+0 .25 -1 .25

( A10 .uTio .isFeo+loFe^ Mgo .«) (Si, .« Al, . )

2.80

+0.25 -1.25

( A10 .20Ti0 .uFeo^Fe^s Mg0 . 80) (Si, ,, Al, .25)

2.75

+0 .25 -1 .25

(Alo.wTio.isF^oFe^Mgo.w) (Si,.78Al, .25)

2.70

The average Mg-dominant formulas indicate, in 

general, that if the number of additional positive charges 

carried by trivalent and quadrivalent octahedral cations 

is less than 0.25, all the additional positive charges form 

a positive charge on the octahedral layer, the negative 

tetrahedral charge is greater than 1.00 by an amount

equivalent to the positive octahedral charge, and all the 

octahedral cationic positions are occupied; that is, 

adjustment for the additional positive octahedral 

charges is predominantly in accordance with equation 

A. Thus the average formula for group A, in which the 

average number of additional octahedral positive
20 SHOETEE CONTEIBUTIONS TO GENEEAL GEOLOGY

charges is 0.05, lias a positive octahedral charge of 0.05, 

a negative tetrahedral charge of 1.05, and all the octahedral positions are occupied. Similarly in the average 

formulas for groups B and C, in which the average 

number of additional positive octahedral charges is 

0.15 and 0.25, respectively, the positive charge on the 

octahedral layer is equivalent to the number of additional positive charges carried by R+3 octahedral 

cations, the negative tetrahedral charge is greater than 

1.00 by an equivalent amount and all the octahedral 

positions are occupied. In the average formula for 

group D, however, in which the number of additional 

positive charges carried by R+3 (and R+4) octahedral 

cations averages 0.35, only 0.25 of these extra charges 

form a positive charge on the octahedral layer the 

remaining 0.10 charges are neutralized by the 0.10 

negative charges associated with the 0.05 unoccupied 

octahedral positions. In succeeding group formulas the 

increasingly greater number of additional positive 

charges is reflected not in greater positive charges on 

the octahedral layer and greater negative tetrahedral 

charges but in greater nonoccupancy of octahedral 

positions, that is, an increasingly greater proportion of 

the adjustment for trivalent and quadrivalent octahedral cations is made in accordance with equation B. 

Thus, in the average formulas for groups E, F, G, H, and 

I, the average positive octahedral charge is the same, 

0.25, but the average number of octahedral positions

occupied progressively decreases from 2.90, in formula 

E, to 2.70, in formula I.

These relations between positive octahedral charge 

(equation A) and octahedral occupancy (equation B) 

with increase in the number of additional positive octahedral charges suggest that the amount of positive 

charge on the octahedral layer that can be accommodated comfortably by the structure is somewhat limited, 

and that with a greater number of positive octahedral 

charges to be neutralized, neutralization by negative 

charges associated with unoccupied octahedral positions 

is generally more tolerable to the structure than neutralization by a greater negative tetrahedral charge and the 

necessarily greater proxying of Al for Si. Throughout 

the range there are, of course, some formulas that 

deviate greatly from the average in their mode of adjustment (fig. 7) but the average formulas illustrate the 

general way in which different amounts of additional 

positive charges carried by trivalent octahedral cations 

are accommodated in the trioctahedral micas.

The general decrease in octahedral occupancy with 

increase in the number of additional positive octahedral 

charges is shown graphically in figure 8. In this figure, 

for each formula, the number of occupied octahedral 

positions is plotted against the number of additional 

positive charges carried by trivalent (and quadrivalent) 

octahedral cations. If three octahedral positions were 

generally occupied in the trioctahedral micas and if

0.10 0.20 C 0.30 0 0.40 E 0.50 F 0.60 G , 0.70 H 0.80 I 0.90 J 1.00

ADDITIONAL OCTAHEDRAL POSITIVE CHARGES DUE TO TRIVALENT AND TETRAVALENT CATIONS

FIGTJEE 8. Eelation between octahedral occupancy and the number of additional octahedral positive charges due to trivalent and quadrivalent cations in trioctahedral micas.
INTERPRETATION OF TRIOCTAHEDRAL MICAS 21

accommodation for trivalent octahedral cations were 

generally made according to equation A, the points 

representing the extra R+3 charge-octahedral occupancy relation of the formulas would fall in a narrow 

horizontal band across the top of the figure. The 

general trend of the points diagonally downward from 

left to right indicates a general decrease in octahedral 

occupancy with increase in the number of additional 

octahedral positive charges due to some degree of 

adjustment for these additional charges in accordance 

with equation B. The width of the band indicates the 

variation in the degree to which adjustment for trivalent and quadrivalent octahedral cations is made in 

one way or the other. Formulas having quite different 

amounts of additional R+3 (and R+4) positive octahedral 

charges may have the same octahedral occupancy, and 

vice versa, depending on the degree to which accommodation for additional R+3 (and R+4) positive octahedral charges has been made in accordance with 

equation A or equation B. For example, the formulas 

for analysis 84, table 11,

and 96, table 11,

+0.34 -1.34

+0.04 -1.07

r) Oio(OH) 2 ]-'-«i

2.79

and for 109, table 11

+0.45 -1.42

[(Alo.o8Tio.22Fe0+|5Fe1+426 Mgo.68)(Si2 M Alt .«) Ow (OH) 2 ]~«

2.79

(K0 .7,,Nao.i3 ,Ca/2o.02)o+9049a

differ greatly hi the number of additional R+3 positive 

charges present, 0.46 and 0.87, respectively, and in 

positive octahedral charge but have the same octahedral 

occupancy. In the formula for 84 accommodation for 

R+3 (and R+4) cations has been made almost entirely 

in accordance with equation B, 0.42 R+3 (and R+4) ions 

proxying for 0.63 R+2 ions, whereas in the formula for 

109 accommodation has been made almost equally by 

both methods. Of the 0.87 additional positive charges 

carried by the R+3 (and R+4) octahedral cations present 

hi the formula for 109, 0.45 form a positive charge on 

the octahedral layer and are neutralized by an approximately equivalent increase hi the negative tetrahedral 

charge, and 0.42 are neutralized by the 0.42 negative 

charges associated with 0.21 unoccupied octahedral 

positions. The formulas for analyses 36,

+0.05

2.78

-1.13

i.8 4) 010 (OH) 2]-'-o°

2.91

(Si2 .87Al1 .1 3)01o(OH) 2]-i-08(Ko.74 > Nao.12 ,Ca/2o.1 2)o+9V1

have the same number of additional R+3 (and R+4) 

charges, 0.51 and 0.52, respectively, but differ greatly 

in octahedral occupancy, in positive octahedral charge, 

and in negative tetrahedral charge. In the formula for 

36, accommodation for R+3 and R+4 octahedral cations 

has been made predominantly in accordance with 

equation B, with 0.45 R+3 and( R+4) cations proxying 

for 0.68 R+2 cations; in the formula for 96 about twothirds of the accommodation for R+3 (and R+4) octahedral cations is in accordance with equation A, and 

one-third in accordance with equation B.

Thus the vertical location of a point in figure 8 

depends on the manner in which octahedral R+3 (and 

R+4) cations have been accommodated in the structure. 

Points near the top of the diagonal band represent 

formulas of individuals in which the accommodation 

was predominantly in accordance with equation A; 

points near the bottom of the diagonal band represent 

formulas of individuals in which accommodation was 

predominantly in accordance with equation B. The 

width of the diagonal band indicates, therefore, the 

degree of variation in the manner in which the trioctahedral micas represented by the points have accommodated themselves to the presence of trivalent 

octahedral cations. However, despite such variations 

in manner of accommodation, it is quite obvious, from 

the average formulas in table 2, and from the downward trend of the band in figure 8, that there is, in 

the trioctahedral micas, a general decrease in octahedral 

occupancy with increase in octahedral trivalent cations. 

The trioctahedral micas are, in general, therefore, not 

strictly trioctahedral, nor are they strictly octaphyllites, as they contain fewer than eight cations per 

half cell.

Although Holzner (1936, p. 435) noted deficiency 

in octahedral occupancy in his biotite formulas and 

attempted to account for it by postulating that the 

biotite structure is made up of 2 layers of phlogopite 

to 1 of muscovite, he apparently failed to notice that 

in his formulas tetrahedral aluminum and consequently, tetrahedral charge, is, with but 2 exceptions 

(21 and 30, table 4) greater than 1.00, ranging from 

1.01 to 1.40. On the other hand, the K(Na,Ca) 

values are, with but five exceptions, lower than the 

corresponding tetrahedral Al values. As the number 

of interlayer cations present is dependent on, and is 

usually equivalent to, the inherent layer charge, this 

discrepancy between Holzner's Al[4] (which represents 

the tetrahedral charge) and his K(Na,Ca) values
22 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

suggests that the octahedral layer in his formulas 

must carry a positive charge approximately equivalent 

to the difference between his Al[4] and K(Na,Ca) 

values. As Holzner included in his tables II and III 

the octahedral bivalent (Y") and trivalent (Y'") 

ion content, it is possible to calculate from them the 

octahedral charge and the inherent layer charge of 

his formulas. The results of these calculations are 

given in tables 3 and 4, together with Holzner's values 

for Y", Y'", Y"+Y'" (equivalent to octahedral 

occupancy), Al[4] (equivalent to tetrahedral charge), 

and K(Na,Ca). All but five (2, table 3, and 13, 18,

TABLE 3. Selected data on phlogopites from Holzner (1936, table 2} 

with added data for octahedral and total charge

No. of 

analysis

1. 

2 

3 

4.........

5.........

6. .....

7 

8... ......

9 

10 

11 

12 .

13 

14........

15........

16. 

17.. _ ...

18 .

19 

20........

Y"

3.031

2.906

2.931

2.843

2.S15

2.891

2.808

2.695

2.809

2.677

2.657

2.755

2.697

2.675

2.656

2.608

2.651

2.549

2.515

2.420

Y'"

0.020

.053

.082

.092

.122

.197

.206

.223

.268

.270

.280

.287

.303

.337

.338

.350

.359

.458

.496

Y"+Y'"

3.031

2.926

2.984

2.925

3.007

3.013

3.005

2.901

3.032

2.945

2.927

3.035

2.984

2.978

2.993

2.946

3.001

2.908

2.973

2.916

Octahedral 

charge

+0.062

-.128

+.021

+.068

+.106

+.148

+.207

+.008

+.287

+.158

+.124

+.350

+.235

+.259

+.323

+.230

+.352

+.175

+.404

+.328

Al[4]= 

Tetrahedral 

charge

-1.127

-1.049

-1.134

-1.158

-1.086

-1. 148

-1. 155

-1.223

-1.091

-1.217

-1.190

-1.323

-1.134

-1.177

-1. 110

-1. 170

-1.290

-1.180

-1.288

-1.332

Total 

charge

-1.065

-1. 177

-1.113

-1.090

-.980

-1.000

-.948

-1.215

-.804

-1.059

-1.066

-.973

-.899

-.918

-.787

-.940

-.938

-1.005

-.884

-1.004

K(Na, 

Ca)

1.077

1.180

1.081

1.077

.985

.993

.950

1.210

.794

1.066

1.064

.993

.878

.921

.783

.942

.938

1.002

.888

1.004

TABLE 4. Selected data on biotites from Holzner (1936, table 3) 

with added data for octahedral and total charge

No. of 

analysis

1 _

0

3...... ...

4.........

5.........

6.. ..

7. . .

8.. ...

9 ..

10 

11 

12

13 

14........

15........

16... ... ..

17 

18........

19........

20 

91

22 

23........

24........

25 

26........

27... .

28........

29........

30. 

31 

32 

33........

34........

35 

Y"

2.402 

2.358 

2.162 

2.307 

2.015 

2.169 

2.190 

2.040 

2.508 

2.315 

2.299 

2.161 

2.360 

2.040 

2.096 

2.061 

1.966 

2.080 

1.820 

2.029 

2.135 

1.684 

2.413 

2.049 

2.150 

2.027 

1.990 

2.228 

2.563 

1.805 

1.924 

1.877 

2.139 

1.950 

2.119

Y»/

0.418 

.499 

.654 

.533 

.791 

.614 

.614 

.681 

.356 

.538 

.595 

.574 

.413 

.680 

.653 

.644 

.755 

.596 

.798 

.707 

.461 

1.085 

.385 

.661 

.607 

.745 

.706 

.525 

.302 

.842 

.821 

.873 

.582 

.812 

.678

Y"+Y'"

2.820 

2.857 

2.816 

2.840 

2.806 

2.783 

2.804 

2.721 

2.864 

2.853 

2.894 

2.735 

2.773 

2.720 

2.749 

2.705 

2.721 

2.676 

2.618 

2.736 

2.596 

2.769 

2.798 

2.710 

2.757 

2.772 

2.692 

2.753 

2.865 

2.647 

2.745 

2.750 

2.721 

2.762 

2.797

Octahedral 

charge

+

-

:

-

-

0.058 

K213 

-.286 

-.213 

-.403 

-.180 

-.222 

-.123 

-.084 

-.244 

-.383 

-.044 

-.041 

K120 

-.151 

-.054 

-.197 

-.052 

K034 

-.179

-.347

+.623 

-.019 

+.081 

+.121 

+.289 

+.098 

+.031 

+.030 

+.136 

+.311 

+.373 

+.024 

+.336 

+.272

Al[4]= 

Tetrahedral 

charge

-1.186

1 939

-1.203 

-1. 169 

-1.330 

-1. 251 

-1.254 

-1.140 

-1. 114 

-1.242 

-1.396 

-1.063 

-1.040 

-1.185 

-1.150 

-1. 181 

-1.210 

-1.042 

-1.202 

-1.221 

-.666 

-1.277 

-1.010 

-1.187 

-1. 169 

-1.136 

-1. 194 

-1. 117 

-1.050 

-.958 

-1.224 

-1. 187 

-1.144 

-1.290 

-1.027

K(Na, 

Ca)

1.078 

1.016 

.922 

.951 

.881 

.997 

.969 

.992 

!940 

.967 

1.013 

1.016 

1.080 

1.035 

.993 

1.071 

.993 

1.023 

1.097 

.894 

.974 

.633 

1.027 

1.093 

1.052 

.845 

1.098 

1.118 

.991 

.893 

.952 

.948 

1.054 

.983 

.952

Total 

charge

-1.128 

-1. 019 

-.917 

-.956 

-.927 

-1.071 

-1. 032 

-1.017 

-1.030 

-.998 

-1.013 

-1.019 

-1.081 

-1.065 

-.999 

-1.127 

-1.013 

-1.094 

-1.168 

-1.042 

-1.013 

-.654 

-1.029 

-1.106 

-1.048 

-.847 

-1.096 

-1.086 

-1.020 

-.822 

-.913 

-.814 

-1. 120 

-.954 

-.855

21, and 23, table 4) have a positive octahedral charge, 

and the algebraic sum of these positive octahedral 

charges and the corresponding negative tetrahedral 

charges yields values for inherent layer charge consistent with the corresponding values for K(Na,Ca). 

Thus Holzner's formulas also exhibit the 2 types of 

adjustment for R+3 cations that were found in the 

formulas used in this study, 1 of which, adjustment 

in accordance with equation A, is not explained by his 

hypothesis, which is based entirely on accommodation 

in accordance with equation B.

R+2-R+* RELATION IN TRIOCTAHEDRAL MICAS

In the accommodation of trivalent octahedral cations 

according to equation A, (+2nR+3) = ( r&R+2) + 

( nSi*4), the proxying of trivalent for bivalent octahedral ions is 1 to 1. In accommodation according to 

equation B, (+2rcR+3) = (-3rcR+2) + (-rcOct.Pos.), the 

proxying of trivalent for bivalent octahedral cations is 

2 to 3 or 0.67 to 1. As both methods are exhibited to 

varying degrees in the natural trioctahedral micas 

studied, it follows that the proxying of trivalent for 

bivalent octahedral cations in these micas must vary 

between 1:1 and 0.67:1, depending on the relative 

degrees to which accommodation was made by one 

way or the other. This is illustrated in figure 9, in 

which the number of R+2 cations displaced is plotted 

against the number of proxying octahedral R+3 and 

R+4 cations. With few exceptions all the points fall 

along or between line A, which represents the proxying 

ratio of equation A, R+3 :R+2=1:1, and line B, which 

represents the proxying ratio of equation B, R+3 :R+2= 

0.67:1. Most of the points representing formulas 

having a trivalent octahedral cation content of less than 

0.40 positions, as in most trioctahedral micas very high 

in Mg, that is, phlogopites, lie on or close to line A, 

whereas points representing formulas having a trivalent 

octahedral ion content of more than 0.40 positions lie 

closer to line B. These relations agree with the type of 

adjustment found for different amounts of additional 

trivalent octahedral positive charges in the average 

formulas (table 2). In trioctahedral micas in which 

the trivalent ion content is low, accommodation for 

their extra charges is predominantly in accordance with 

equation A, and the proxying ratio is 1 to 1. With 

increase in trivalent octahedral ion content an increasingly greater degree of accommodation is made 

according to equation B, and the proxying ratio 

approaches more nearly 0.67 to 1.0. As the trioctahedral micas generally exhibit both types of accommodation for octahedral trivalent cations, an equation 

representing the change in composition involved, as 

compared with the ideal, must be a combination of 

equations A and B. However, as the relative degree
INTERPRETATION OF TRIOCTAHEDRAL MICAS 23

i i

EXPLANATION

O 

Mg dominant

x 

Fe+ 2 dominant

0.20 0.40 ' 0.60 0.80 1.00 1.20 

OCTAHEDRAL POSITIONS OCCUPIED BY R+ 3 AND R + 4 CATIONS

FIOUEE 9. Eelation between E+3 cations displaced and proxying 

octahedral cations.

(and R+4)

to which the two types are used differs with each 

individual, the equation would, consequently, differ 

with each individual. The difficulties are further complicated by Ti which carries twice as many extra charges 

as octahedral positions occupied.

The scarcity of formulas having an octahedral 

R+3 (+R+4) occupancy of more than 0.90 positions 

suggests that this R+3 (+R+4) occupancy (about onethird of the occupied octahedral positions) is about the 

upper limit of the R+2 by R+3 (+R+4) replacement in 

the trioctahedral micas. This is shown another way 

in figure 10, in which histograms represent the type 

and number of formulas in each additional positive 

octahedral charge group. Group H, in which the 

number of additional octahedral positive charges is 

between 0.71 and 0.80, includes the greatest number of 

formulas (25). From this high the number of formulas 

in each successive group drops to 16 in group I and to 

2 in group J. This sharp drop in the number of formulas in each succeeding group from the high in group H, 

and the scarcity of formulas having an R+3 occupancy 

greater than 1.00, supports the suggestion that an

R+3 (+R+4) octahedral occupancy of one-third of the 

octahedral positions is the essential limit of replacement of octahedral R+2 (Mg) by R+3 (and R+4) in the 

trioctahedral micas.

The small number of formulas having more than 1.00 

positions occupied by R+3 and R+4 is also strong evidence against the existence of a complete series between phlogopite and muscovite and supports Deer 

(1937, p. 449) and Hutton (1947, p. 482) in their conclusions that it is not likely that the number of octahedral positions occupied varies between 2 and 3, as 

postulated by Pauling (1930, p. 128). Deer believed 

that any value between 2.5 and 3.0 is possible, but 

Hutton set the lower limit for octahedral occupancy at 

not less than 2.75. In six formulas used in the present 

study in which R+3 and R+4 occupied 1.00 ±0.10 octahedral positions, total octahedral occupancy was between 2.45 and 2.75 positions, and averaged 2.60. In 

the two formulas having the lowest octahedral occupancy, 2.45 and 2.51 positions, accommodation for 

trivalent octahedral cations is almost entirely in 

accordance with equation B.

Mg-Fe+2 RELATION IN TRIOCTAHEDRAL MICAS

In addition to replacement of Mg by R+3 cations in 

the octahedral layer of trioctahedral micas, there is also 

replacement of Mg by Fe+2 cations. With but very 

few exceptions, all the formulas calculated from analyses 

of trioctahedral micas contain both Fe+2 and R+3 

cations in the octahedral group. The exceptional 

analyses are of two kinds almost pure phlogopites 

that contain only a very small amount of Fe+2 cations 

but no octahedral R+3 cations, or vice versa, and 

biotites in which Fe+2 was not determined but was included in total Fe as Fe2O3 in the analysis. Thus, coincident with replacement of Mg by R+3 cations, there 

is also replacement of Mg by Fe+2 cations. A very 

general relation between the amounts of Fe+2 and octahedral R+3 is suggested by the histograms in figure 

10; (1) in formulas having very little octahedral R+3 , 

Mg is greatly predominant, (2) in formulas having 

moderate amounts of octahedral R+3 , Mg is still dominant but less so than in (1), and (3) in formulas having more than 0.60 octahedral positions occupied by 

R+3 , Fe+2 is the dominant octahedral cation in most of 

the formulas. However, the histograms also show the 

presence of some Mg-dominant biotites in the higher 

R+3 groups. This fact, and the great variability in the 

Fe+2-octahedral R+3 ratio in the formulas indicate that 

no real relation exists, and that the two series, although 

concurrent, are independent. Also, the amount of 

octahedral R+3 present has no relation to the amount 

of Fe+2 present, and vice versa. On the other hand, 

the presence in all but a very few of the formulas of

527919 5S
24 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

0.10 0.20 0.30 D 0.40 E 0.50 F 0.60 G 

ADDITIONAL POSITIVE OCTAHEDRAL CHARGES

0.70 0.80 0.90 1.00

FlOUEE 10. I

both R+3 and Fe+2 in the octahedral group indicates 

that neither type of replacement forms a separate series. 

This statement includes Winchell's annite, the Fe+2 

analog of phlogopite, and the theoretical end member of 

a Mg-Fe+2 replacement series. Such a mica was not 

found among the more than 200 analyses collected for 

this study.

Substitution of Fe+2 for Mg poses no problem of adjustment as does the substitution of R+3 for Mg. Both 

cations are bivalent, no excess nor deficiency of charge 

is involved, and substitution is ion for ion with no other 

change in the octahedral or tetrahedral layers.

On the evidence of the analyses and their derived 

formulas, an octahedral occupancy of more than 2.20 

positions by Fe+2 is not to be expected. The highest 

Fe+2 in the formulas studied was 2.17, and only 4 

formulas had an Fe+2 octahedral occupancy of more 

than 2.00.

MAGNESIUM REPLACEMENT SYSTEM

It has been shown that the only trioctahedral micas 

in which all, or almost all, the octahedral positions are

occupied by one kind of cation are some phlogopites in 

which more than 90 percent of the octahedral positions 

are occupied by Mg. The Fe+2 analog of phlogopite, 

Winchell's annite, apparently does not exist in nature, 

or is very rare. With pure phlogopite as the prototype, 

all the other trioctahedral micas may be derived by the 

replacement of Mg by, most commonly, Fe+2 and R+3. 

Thus, the trioctahedral micas may be considered as 

members of a system in which two principal types of 

replacement proceed concurrently but independently. 

The relation between Mg, (Fe+2 and Mn), and (Al, 

Fe+3, and Ti) in this system is shown in figure 11, in 

which, for each formula, the percent of occupied 

octahedral positions occupied by Mg is plotted against 

the percent of occupied octahedral positions occupied 

by Fe+2 and R+3 on a triangular diagram. The points 

fall into 2 large groups and 1 small group. The first 

large group, at the top of the triangle, is made up of 

points representing trioctahedral micas in which Mg 

occupies more than 70 percent of the occupied octahedral 

positions; in the second large group are the points that 

represent trioctahedral micas in which Mg occupies
INTERPRETATION OF TRIOCTAHEDRAL MICAS

Mg

25

EXPLANATION

 

Phlogopites

o 

Mg biotites

x 

Fe+2 biotites

 

Mg=Fe+2 biotites

Siderpphyllites and 

lepidomelanes

* Siderophyllites and 

. lepidomelanes

'*

* *

IK)70 60 50 40 30 20 10 Fe+2(+Mn +2) 

FIOUBE 11. Relation between Mg, Fe+2 (Mn+2), and R+' (Al, Fc+*, and Ti) in trioctahedral micas.

between 63 and 20 percent of the occupied octahedral 

positions; in the small group, at the bottom of the 

triangle, are points representing trioctahedral micas in 

which Mg occupies fewer than 10 percent of the occupied 

octahedral positions.

Three-fourths of the analyses from which the formulas represented by points in the first group were 

derived were designated as analyses of phlogopite in 

the literature. A few were termed "biotites", with 

biotite being used in a general sense, simply meaning a 

trioctahedral mica. Consequently, the first group is 

considered as being made up of phlogopites and the first 

hiatus in the grouping of the points is considered as 

differentiating phlogopites from Mg biotites.

The second group of points represent biotites. Points 

representing Mg biotites occupy the upper half of this 

area, and, about halfway down the area, mingle with 

the points representing Fe+2 biotites, which occupy the 

lower half of the biotite area.

The third group of points represent Fe+2 dominant 

formulas having a very low Mg content, less than 0.15 

octahedral positions, and high amounts of Al and (or) 

Fe+3, that is, siderophyllites and lepidomelanes.

The trioctahedral micas may, therefore, be thought 

of as members of a system characterized by concurrent 

but independent replacement of Mg by Fe+2 and R+3 

ions. At the beginning of the system, Mg occupies 

more than 98 percent of the octahedral positions; at its
26 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

end, Mg occupies fewer than 1 percent of the occupied 

positions. Thus pKLogopite, Mg3.oo(Si3.ooAl)O10 (OH) 2 K, 

is the prototype of the trioctahedral system, from which 

by displacement of Mg all other trioctahedral micas 

may be considered to be derived.

It is obvious that the character and composition of 

the members of a system like this cannot be adequately 

expressed by the kind of formulas usually given for 

these minerals. Such formulas suggest minerals of 

definite composition, not members of a system that are 

variable, within limits, in composition and that merge 

into one another. For members of such a system the 

formulas should indicate permissible limits of composition. The failure to define compositional limits has led 

in the past to considerable confusion in differentiating 

the various members of the system.

PHLOGOPITES

The name "phlogopite" refers to a trioctahedral 

mica in which almost all the octahedral positions are 

occupied by Mg. The formula usually given for 

phlogopite, Mg3 (Si3Al)Oio(OH) 2K, specifies a complete 

occupancy of the octahedral group by Mg. Naturallyoccurring phlogopites, however, contain other octahedral cations, most commonly Al, Fe+3 , and Fe4"2, and 

so do not conform perfectly to the ideal formula, 

although some, like the one below (1, table 11), approach the ideal \ery closely

TABLE 5. Range in octahedral composition of phlogopites (in order 

of decreasing Mg content in terms of octahedral positions)

+0.04

[(Alo.o1Tio.0!!Fe0H:o31 Feo+o21 Mg2 .94)

-1.08

This formula has the highest Mg occupancy of any 

of the formulas studied, Mg occupying 98 percent 

of the octahedral positions. Even in this formula, 

however, there are determinable amounts of Al, Ti, 

Fe+3, and Fe4"2. Other formulas of phlogopites, arranged in order of decreasing Mg content, exhibit 

increasingly greater occupancy of octahedral positions 

by other cations, Fe+2, Fe+3, and (or) Al. However, 

all the formulas represented by points in the phlogopite 

area, figure 11, have more than 70 percent of their 

occupied octahedral positions occupied by Mg.

The average octahedral composition of phlogopites, 

based on the 42 formulas at hand, are shown in table 

5. Only 4 of these formulas had a Fe+2 occupancy 

of more than 0.45 octahedral positions; in 2 of these 

Fe4"2 occupied 0.50 positions, in the other 2, Fe+2 occupied 0.54 positions. The number of octahedral 

positions occupied by R+3 ions varies up to 0.54, with 

Al occupying between 0.00 and 0.35 positions, Ti

Number 

of 

formulas

15.. 

5-._ 

15-. 

8- 

Mg

3. 00-2. 75

2. 75-2. 50 

2. 50-2. 25

2. 25-2. 00

R« extra 

charge

0.00-0.25

.25- .35 

.20- .45

.25- .50

Al

0. 00-0. 15

.10- .20 

.00-. 25

.00-. 35

Fe«

0. 00-0. 10

.05- .20 

.00- .30

. 05- . 25

Fe«

0. 00-0. 15

.00- .20 

.15-. 50

.30- .50

Octahedral 

positions

3.00-2.95

3.00-2.95 

3.00-2.85

3.00-2.85

Formula indicating range in composition of phlogopites 

<+0.30 (-1.00)-(-1.30)

(R<0.5oFe<0.50 Mgs .00-2.00) (Slj .00-2.70All .00-1.3o) 

3.00-2.85

between 0.00 and 0.10 positions, and Fe+3 between 

0.00 and 0.30. However, in 30 of the 43 phlogopite 

formulas Al occupies 0.15 or fewer octahedral positions, 

and in 7 of these there is not only no octahedral Al, 

but also insufficient Al to complete the filling of the 

tetrahedral group, necessitating allocation of some 

Fe4"3 to this group. Although Fe+3 occupies up to 

0.30 positions, most of the phlogopite formulas had 

less than half that many positions occupied by Fe4"3 . 

Titanium is generally very low, occupying an average 

of 0.10 positions.

From the ranges for Mg, Fe4"2, and R4"3 in these 

formulas, general limits for these constituents in 

phlogopites may be formulated as shown at the bottom 

of table 5. The low of 2.00 octahedral positions 

occupied by Mg is equivalent to approximately 19 

percent of MgO, and the high of 0.50 positions occupied 

by Fe4"2 is equivalent to approximately 8.5 percent 

of FeO.

A number of phlogopite analyses were discarded 

because the formulas derived from them were low in 

interlayer ion content, that is, deficient in the amount 

of K, or other large cations present, and (or) were 

excessively high in octahedral occupancy. The implication of these features is discussed earlier in this 

paper.

EASTONITE

The name "eastonite" for pure dialuminum magnesium mica, H4K2Mg5Al4Si5O24, or (Alo.5Mg2.5) 

(Si2 .5A] 1 .5)Oio(OH)2K, in the notation used in this study, 

was derived from Easton, Pa., where, quoting Winchell 

(1925, p. 322, footnote 30) "Eyerman found a sample 

which approaches this composition very closely." 

Eyerman (1904, p. 46) published three analyses of 

biotite from Easton. One of these, analysis B, has 

only 6.30 percent K2O, which is very low for a potassium 

mica. In the formulas derived from his analyses A 

and C, 16 and 15, table 11,
INTERPRETATION OF TRIOCTAHEDRAL MICAS 27

+0.14 -1.16

and

2.87

+0.30 -1.28

[(Al0 . 15Feg9Mg2 . 4i))(Sl2 .72Al1 .28)01 o(OH) 2]-''-''8(Ko.»o) Ca/2o.o3) 0+90396 ,

2.93

Al occupies only 0.24 and 0.15 octahedral positions, 

respectively, not 0.50 as specified in Winchell's formula. 

These formulas can be said to approach the composition 

specified by Winchell's eastonite formula only if Al is 

interpreted broadly as meaning R+3 ions and to include 

Fe+3 . Furthermore, it is very doubtful whether the 

values given for Fe2O3 represent the actual Fe+3 content 

of the samples. No FeO is reported, and it is very 

probable that FeO was not determined and that the 

value given for Fe2O3 represents the total iron content 

of the sample, not Fe+3 only. In most samples of 

similar Mg and total iron content, in which both Fe2O3 

and FeO were determined, FeO makes up one-half or 

more of the total iron content. It is very likely, therefore, that the Fe2O3 reported in these analyses includes 

FeO and that the actual trivalent ion content of these 

biotites is considerably less than shown in the formulas.

Aside from the Al and the total trivalent ion content, 

there are other points of disagreement between these 

formulas and Winchell's eastonite formula. In the 

eastonite formula it is presumed that all the octahedral 

positions are occupied, and, because tetrahedral Al, 

and consequently, tetrahedral charge, is 1.5, that all 

the extra charges (+0.50) carried by octahedral Al 

induce a positive charge on the octahedral layer. In 

the Easton formulas only 2.87 and 2.93 octahedral positions, respectively, are occupied, and only part of the 

additional charges carried by trivalent ions form a 

positive charge on the octahedral group, the rest being 

neutralized by negative charges associated with unoccupied positions. Consequently, the positive octahedral charge, the negative tetrahedral charge, and 

octahedral occupancy of the Easton formulas are all 

less than in the eastonite formula. X-ray study of the 

Easton, Pa., samples show them to be mixtures of 

serpentine and phlogopite. (Hatton S. Yoder, oral 

communication, 1957.)

Because in trioctahedral micas the extra charges 

carried by trivalent octahedral cations are accommodated in two different ways, in accordance with equation 

A or equation B, because decrease in Mg content is 

accompanied by increase in Fe+2 and R+3 , and because 

increase in R+3 is made up of increase in Fe+3 as well as 

increase in octahedral Al, none of the Mg dominant 

formulas studied closely approach Winchell's eastonite

527919 60 4

formula, in which accommodation for extra trivalent 

charges is assumed to be made only in accordance with 

equation A, and in which replacement of Mg is by Al 

only. All the formulas with a Mg content comparable 

to that in the eastonite formula contain Fe+3 and Fe+2 

and have some unoccupied octahedral positions.

Among the analyses of trioctahedral micas collected 

for this study, only one had been called that of an 

eastonite (Simpson, 1932, p. 63), (D27, table 12). 

This analysis yielded the following formula,

+0.42 -1.29

[(Alo.2oTi0 .o2Fe0+|4Fe0+923Mg1 . 56Mn0+022 )(Si2 . 71 Al1 . 2a )010 (OH) 2 ]-o."

2.97

(K0 .82 ) Na0 . 07 )o+80989

In this formula Mg occupies only three-fifths and Al 

occupies only two-fifths of the number of octahedral 

positions designated in the eastonite formula, and the 

octahedral and tetrahedral charges are both considerably lower than in the eastonite formula. Only if Mg 

and Al in the eastonite formula are interpreted broadly 

as representing R+2 and R+3 ions generally, does the 

formula for Simpson's eastonite,

+0.42 -1.29

[(Al, Ti, Fe+3) 0 . 46 (Fe+2, Mg, Mn+») 2 . 81 (Si2 .nAl1 .2»)Oio(OH),]-<i.87

2.97

(K,

approach Winchell's formula for eastonite. However, 

Winchell's formula represents the hypothetical dialuminum magnesium mica which was used by him as 

one of the components of biotite and as an analog of 

siderophyllite. A broader interpretation, as above, 

would tend to engender confusion with biotites proper. 

In view of the confusion a broader interpretation 

would tend to engender, the apparent lack of natural 

representatives and the fact that the name had been 

previously given by Hamilton (1899, p.19) to a vermiculite occurring in the same locality as an alteration 

product of biotite, it is recommended that the name 

"eastonite" be discarded as referring to a natural 

trioctahedral mica and retained only as a hypothetical 

end-member.

BIOTITES

The term "biotite," although also used as a group 

name for the trioctahedral micas, is here specifically 

applied to trioctahedral micas that contain significant 

amounts of both Mg and Fe+2. Biotites in which Mg 

is the dominant octahedral cation are termed Mg 

biotites; those in which Fe+2 is the dominant octahedral 

cation are termed Fe+2 biotites. Occurring midway in 

the Mg Fe+2-R+3 system (fig. 11) between a Mg octahedral percentage of 50 and 20, the biotites occupy the
28 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

greatest area and constitute the greatest number of 

samples. Magnesium biotites dominate the upper 

part of the biotite area, with points representing 

formulas having the highest Mg content occupying 

the uppermost part of the area. With decreasing Mg 

content and increasing Fe+2 content Mg is gradually 

superseded by Fe+2 as the dominant octahedral cation, 

and Fe+2 biotites dominate the lower part of the biotite area. In the transition zone, points representing 

Mg biotites, Fe+2 biotites, and biotites having about 

equivalent amounts of Mg and Fe+2 intermingle.

The highest Mg content in any of the biotite formulas 

studied is 1.77 octahedral positions, the lowest is 0.48 

octahedral positions. The highest Fe+2 content in any 

biotite formula is 1.78 octahedral positions; the lowest 

is 0.47. Thus Mg and Fe+2 in these biotite formulas 

have the same range, 1.75 to 0.50 octahedral positions 

(in rounded figures), but they are present in a reciprocal 

relation. This relation is illustrated in the following 

formulas, 36 and 116, table 11,

+0.05

[(Al0 .21Tio.o8Feo+138Fe0+& Mgl .7

-1.13 

io.o3)(Si2 . 87Al1 . 13)010 (OH)2 ]-i-»8

2.78

+0.26

(K0 .75,Na<) .I2 ,Ca/2o.12)o+919

-1.25

2.82

(Ko.88,

which belong, respectively, very near the high Mg-low 

Fe+2 and the low Mg-high Fe+2 ends of the Mg-Fe+2 

range in biotites. In other biotite formulas, the 

Mg-Fe+2 relation varies between these extremes. 

However, because R+3 ions are present in different 

amounts, the relation between Mg and Fe+2 is quite 

variable. Formulas with similar Mg contents may 

have quite different Fe+2 contents and formulas with 

quite different Mg contents may have the same Fe+2 

contents. Similarly, the relation between Mg and 

R+3 content is quite variable, as is also the relation 

between Mg and any of the ions making up R+3 . 

The relation between Mg and (Fe2 +R+3) is however, 

quite regular.

Fe+3 generally occupies fewer than 0.35 octahedral 

positions in the biotite formulas. Only 6 formulas 

had a greater Fe+3 content, and of these 6 only 1 had 

a Fe+3 content greater than 0.45. There is no significant difference in the Fe+3 content of Mg dominant 

and Fe+2 dominant biotites. Indeed, the average 

Fe+3 content of these two types of biotites is essentially 

the same, 0.23 octahedral positions in Mg biotites, 0.25 

octahedral positions in Fe+2 biotites.

With respect to Al content the biotite formulas fall 

into two groups; in about one-half the formulas there

is little or no octahedral Al (fewer than 0.10 octahedral 

positions), in the other half, Al occupies between 0.20 

and 0.55 octahedral positions. In about one-third of 

the low Al group, Al is so deficient that there is not only 

no octahedral Al, but there is not even sufficient Al 

present to complete the filling of the tetrahedral group; 

this requires the allocation of some Fe+3 to this group 

to complete it. Lack of sufficient Al to fill the tetrahedral group may be due simply to low Al content, or 

it may be due to low Si content, which would require 

proportionately more Al to complete the tetrahedral 

group. A comparison of the amounts of SiO2 and A12O3 

reported in the analyses from which the formulas were 

derived shows, however, that low A12O3 content, rather 

than low SiO2 content, is the reason for the insufficiency 

of Al in formulas containing little or no octahedral Al. 

The SiO2 content in analyses for the 2 groups is about 

the same, varying between 32 and 38 percent in analyses 

for both groups, with an average SiO2 content of 35.5 

percent for the low Al group, and an average of 34.8 

percent for the higher Al group. The range in A12O3 

content in analyses of the low Al group, 11.0 to 16.3 

percent, is, however, considerably lower than the range 

in A12O3 content of the higher Al group, 15.7 and 21.3 

percent, and the average A12O3 content in analyses for 

the low Al group is almost 4 percent lower than the 

average A12O3 content in analyses for the higher Al 

group, 14.6 percent as compared with 18.4 percent.

The compositional characteristics of each of the 

groups for different ranges of Mg content are summarized in table 6, and the average octahedral composition of each group for the same ranges of Mg content 

are given in table 7. For comparable Mg contents the 

low octahedral Al biotites are, in general, lower in R+3 

but higher in Ti and Fe+3 content than the higher

TABLE 6. Range in octahedral composition of biotites 

[In order of decreasing Mg content In terms of octahedral positions occupied]

Mg

1.76-1.50 1.50-1.25 1.25-1.00 1.00-0.75 0.75-0.50

Al<0.15

Number of formulas _ 

R« . - 

A!...- 

TI... . 

Fe« ~ - 

Fe« - - -

(Fe«+R«) -- --- 

7 

0.35-0.70

.00-. 05

.20- .25

. 10- . 45

.60- .90

1. 05-1. 30

8 

0. 60-0. 75

.00- .15

.10- .20

.20-. 35

.80- .95

1. 25-1. 50

8 

0.50-0. 80

.00- .15

.20- .30

. 10- . 45

. 90-1. 35

1.45-1.85

11 

0. 45-0. 80

.00- .15

. 05- . 30

. 10- . 35

1. 25-1. 55

1. 75-2. 05

9 

0. 55-0. 85

.00- .15

.10-. 30

.05-. 45

1. 45-1. 75

2. 05-2. 20

Al>0.15

Number of formulas 

R«. . 

AL..- .... ... ...

Ti. . ._. 

Fe« 

Fe«~ - .. .

(Fe«+R+*)..~. . 

6 

0. 45-0. 70

.20- .55

.00- .15

.00- .25

.50- .95

1. 00-1. 35

4 

0.60-0.80

.20- .40

.10- .20

. 10- . 25

.65- .95

1. 30-1. 55

6 

0.60-0.90

.30- .50

.00- .15

.00- .40

.70-1.25

1.50-1.80

12 

0.70-1.00

.20- .40

.10- .30

. 05- . 35

1. 00-1. 35

1.70-2.00

11 

0.65-0.90

.25- .50

.05- .20

.05- .30

1. 15-1. 70

1.90-2.20
INTERPRETATION OF TRIOCTAHEDRAL MICAS 29

TABLE 7. Average octahedral composition of biotites in order of decreasing Mg content 

[The figures preceding the parentheses enclosing the octahedral-group notation refer to the extra E+' charge of the octahedral groups they precede]

Mg

1. 75-1. 50

1. 50-1. 25

1. 25-1. 00

1. 00-0. 75

0. 75-0. 50

Al <0.15

+0.25

2.85 

+0.25

2.80

+0.30

2.80 

+0.25

o .es (Alo.o5Ti0 .i5Feot33oFe1+42oMgQ .9o)

2.80 

+0.25

0.75 (A10 .05 Tip .aoFeptifo 275' Fe^ Mg0 .M)

Al >0.15

+0.25

o .55 (Alp .3oTi0 .05Feoti35 Fe^o Mgt .

2.85

+0.25

2.75 

+0.30

o.8o(Alo.35Tio.ioFe0+235Feo+925Mgi.io)

2.75 

+0.25

2.70 

+0.25

2.70

octahedral Al biotites, although formulas with low 

octahedral Al are found throughout the entire R+3 

range. Fe+2 is consistently higher, for comparable Mg 

contents, in low Al biotites than in higher Al biotites. 

Because of this difference in Fe+2 content with the 

same Mg content, the average formula for low Al 

biotites in the Mg range 1.25-1.00 has Fe+2 as the 

dominant octahedral cation, whereas the average formula for the higher Al biotites has Mg as the dominant 

octahedral cation. In an average of all formulas in 

this Mg range Mg and Fe+2 are present in approximately 

equivalent amounts,

+0.30 -1.30 

.i,) (Si, . 7oAl, .3o)010 (OH) 2]-'.<>°

280

K(Na, Ca/2) 1+016°±0 10

Thus the Mg range of 1.25 to 1.00 octahedral positions 

is the transition zone between Mg and Fe+2 dominant 

biotites. All biotites having more than 1.25 octahedral 

positions occupied by Mg are Mg dominant; all biotites 

having fewer than 1.00 positions occupied by Mg are 

Fe+2 dominant. When the Mg content is between 1.25 

and 1.00 octahedral positions, Mg is dominant in some 

biotites, Fe+2 is dominant in others, and Mg and Fe+2 

are present in approximately equivalent amounts in 

still others.

On the basis of the ranges in content of Mg, Fe+2, and 

R+3 shown in table 6, the ranges in content of these

constituents to be expected in biotites may be formulated as

<+0.50

- 1.0oFe0.60 - 1.75

(-1,00)- (-1.50)

i .75-0 .50) (Sis .00-2 .50 All .00-1 .5fl) Ojo (OH) 2

2.90-2.65

K(Na, Ca/2)j.00*6.10.

ANNITE

Originally the name "annite" was applied by Dana 

(1868, p. 308) to a high Fe+3 biotite that occurs at Cape 

Ann, Mass., which was analyzed and described by Cooke 

(1867, p. 222). This material contained 12.07 percent 

Fe2 03 . Subsequently Winchell (1925, p. 323) applied 

the name to a hypothetical Fe+2 analog of phlogopite, 

Fe^2 (Si3Al)Oio(OH) 2K, which was used by him as 1 of 

the 4 components of biotite. Thus the name which 

Dana gave to a high trivalent iron mica was applied by 

Winchell to a hypothetical bivalent iron mica which was 

presumed to contain no trivalent iron.

One of the most significant features brought out by 

this study is the complete absence of Fe+2 dominant 

micas analogous to phlogopite. Only two formulas 

have as many as 2.00 octahedral positions occupied by 

Fe+2 , the lower limit for Mg in phlogopite, and in these, 

although Mg is very low, R+3 ions occupy about 0.50 

octahedral positions.

Considering that the name "annite" had been used 

previously by Dana for a high Fe+3 biotite and the name 

and formula was used by Winchell for a hypothetical
30 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

ferrous trioctahedral mica, which does not, on the evidence of the formulas gathered, occur in nature, it is 

recommended that annite be discarded as referring to a 

natural trioctahedral mica, and retained only as a 

hypothetical end member.

SIDEROPHYLLITES AND LEPIDOMELANES

A group of Fe+2 dominant trioctahedral mica formulas 

in which Mg occupies less than 0.15 octahedral positions 

represents the end of the Mg replacement system. 

Points representing these formulas fall close to the base 

of the triangular diagram shown in figure 11, and are 

separated from the biotite area by narrow, very sparsely 

populated area. Some of these formulas are characterized by high Al content; others, by high Fe+3 content; and still others, by significant amounts of both 

Al and Fe+3 .

Siderophyllite, according to Winchell and Winchell 

(1951, p. 373) has the formula, HJ^FesAliSisOsM, or, in 

the notation herein used, and assuming Fe to represent 

Fe+2

(Alo.sFe2tfXSi2.5oAl!.50) OioCOH^K! .00

The full octahedral occupancy and the composition of 

the tetrahedral in this formula indicate that all the 

accommodation for extra positive charges (+0.50) 

carried by octahedral Al is assumed to have been made 

in accordance with equation A. Nockolds and Richey 

(1939, p. 38) suggested the formula, H4K2Fe3Al4SieO24 , 

or (Al1>00Fe1+|>)(Si3 > ooAl1 ,oo)O1o(OH)2K, for siderophyllite, 

as this formula was closer to the composition of a 

specimen studied by them (129; table 8). In this 

formula all the additional positive charges carried by 

octahedral aluminum (+1.00) are accommodated in 

accordance with equation B. The octahedral layer is 

neutral, and the tetrahedral layers have the same 

charge as in the classic formulas for phlogopite and 

muscovite. These two formulas, therefore, represent 

the two extremes in the accommodation of extra positive octahedral charges by the biotite structure. Octahedral Al in this formula occupies twice as many positions as in the Winchell formula and Fe+2 occupies only 

three-fifths as many positions.

In the formulas shown in table 8, octahedral Al 

varies between 0.47 and 0.99 positions. These relations suggest an isomorphous series in which Fe+2 is 

replaced by Al. Hypothetically, the beginning of the 

series would be a mineral similar to Winchell's annite, 

which would yield on increasing replacement of Fe+2 by 

Al, minerals whose compositions, except for lack of 

Li, approximate those of the siderophyllites shown in 

table 8:

0.00 -1 .00

[Fe£20 (Si3.ooAli.oo)O10 (OH) 2 ]-i-o° Kf<Jj00 Not represented.

i

0.00 -1.00

[(Alo.5oFe2+225)(Si3.ooAli.M)Oio(OH) 2 ]-i-<>°

2.75

0.00 -1.00

[(Ah.ooFe^) (Si3 .ooAl1 .oo)Oio(OH),]-i-<>o

2.50

^o00 Approximates 

132, table 8.

Approximates 

129, table 8.

The first half of the series, up to an Al content of 0.50 

octahedral positions, is not represented among the formulas studied. The second half is represented by for 

mulas in table 8. Variations in octahedral occupancy, 

octahedral charges and tetrahedral charge are due to 

the varying degrees to which the structure has accommodated the extra positive charges carried by Al in 

accordance with equation A or equation B. In the formula for 132, table 8, 0.33 of the extra 0.47 charges 

carried by Al are balanced by the deficiency in the 

number of charges carried by Li, as one Li and one 

Al together carry the same number of charges as two 

bivalent cations. The 0.16 negative charges associated 

with 0.08 unoccupied octahedral positions are more 

than sufficient to neutralize the rest of the additional 

positive charges, and the octahedral layer has a slight 

negative charge. In the formula for 135, table 8, 

part of the additional positive octahedral charges are 

neutralized by negative charges associated with unocNumber in table 11

TABLE 8. Formulas of representative siderophyllites

Formula

-0.02

2.92 

+0.35

135

2.82

-1.03

-1.35

-0.06 -1.03

129-... _-__ ___-___ [(Alo.99Tio.o1 reo+o3sFe1+|2 Mgo.05 MDo+o22)(Si2 .97Al1 .o3 )01 o(OH) 2 ]-i-''« (Ko.g

2.44
INTERPRETATION OF TRIOCTAHEDRAL MICAS 31

cupied octahedral positions. In 129 all the extra positive R+3 charges are neutralized by negative charges 

associated with unoccupied octahedral positions, as in 

Nockolds and Richey's formula for siderophyllite. 

Because of these various degrees to which the extra 

positive charges carried by R+3 ions have been accommodated in accordance with equations A and B in 

these siderophyllite formulas, the formulas for the 

siderophyllite series are written to represent similar 

mixed accommodation of extra positive charges.

"Lepidomelane" is a term that has not been definitely defined and that has different meanings for 

different mineralogists. Dana (1892, p. 634) defines 

lepidomelane as a variety of biotite characterized by 

a large amount of ferric iron. Winchell and Winchell 

(1951, p. 376) states that "lepidomelane is any kind of 

biotite rich in iron." The state of valence of the iron 

is, apparently, unimportant, as this definition would 

include his annite. Kunitz (1924, p. 388) suggests 

that all the iron was originally in the ferrous state, 

and gives a formula in which all the iron is present in 

the ferrous state, KH2AlFeV(SiO4 ). However, Hallimond (1926, p. 30) considers that there is no reason 

"* * * to believe that the natural melts are so deficient 

in oxygen as this assumption would imply, and it is 

much more probable that the average fresh biotite is 

substantially in the condition in which it crystallized." 

The formula given by Hey (1955, p. *193), 2[K2 

(Fe+3Fe+2Mg)4_e,(Si,Al,Fe+3 ) 8O2o(OHU which includes 

both trivalent and bivalent iron, is qualified by the 

remark "* * * with high Fe+3 and low Mg (usually 

Fe+2>Mg)." Hey includes Fe+3 in the tetrahedral 

group, as does also Grigoriev (1935, p. 79) in his formula 

for fluor (or hydroxyl) lepidomelane FK(MgFe+2) 3 

[(Al,Fe+3)Si3]O10 , although Grigoriev includes only Fe+2 

in the octahedral group. These examples illustrate 

the lack of agreement among mineralogists as to the 

meaning of the term "lepidomelane."

Among the Fe+2 dominant formulas studied was a 

group characterized by very low Mg content and 

analogous to the siderophyllite formulas but containing 

Fe+3 in place of Al. These formulas, of which typical 

examples are presented in table 9, bear the same relation 

to each other as do the siderophyllite formulas; that is, 

they represent part of an isomorphous series in which 

there is proxying of Fe+2 by Fe+3, and their composition 

can be expressed by the same general formulas as 

given for the siderophyllite half series, by substituting 

Fe+3 for Al.

0.00 _i.oo

[Fe3+o2o(Si3 .o0Al1 .oo)O10 (OH) 2 ]-i-°<> K&' Not represented.

I

0.00 _i.oo

[(Fep+ye&a) (Si3.00 Al 1 .oo)O,o(OH) 2 ]-i-» Kj+

I

0.00 _ 100

[(Fe^Fe+l,)(Si3 .00 Al,.oo)O10 (OH) 2 ]-»-«»

0 Approximates 

124, table 9.

Approximates 

123, table 9.

As in the siderophyllite series, only the latter half of 

the series is represented.

It is suggested here that the term "lepidomelane" be 

applied to minerals similar in composition to the formulas above and to those in table 9 minerals characterized by high Fe+2 and Fe+3 and by very low 

octahedral Al and Mg content, and analogous to siderophyllite (as herein defined) but containing Fe+3 in place 

of Al.

Another group of formulas characterized by high 

Fe+2 and very low Mg content contain significant 

amounts of both octahedral Al and Fe+3 (table 10), 

These formulas are intermediate in composition beNumber 

(table 11)

TABLE 9. Formulas of representative lepidomelanes

Formula 

+0.17 -1.23

126.

125.

124.

123.

.MM

2.73

+0.22

2.63 

+0.39

[ ( A10 .oeTio.o

2.89 

+0.39

-l.ll

-1.42

Mgo .n Mn^022) (Si2 .58 Alt . 42) 010 (OH) 2]-i -

-1.36

2.75
32 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Number 

(tables 11 and IX)

130.

TABLE 10. Formulas of micas intermediate in composition between siderophyllites and lepidomelanes

+0.36

2.85 

+0.25

127.

Formula 

-1.31

-1.32

io.13) (Si2".68Ali .32

D63.

2.61 

+0.01 -1.02

[(Alo.45Feo+63sFe1+029 Mgo.o7 MD0+034 Li0 . 18) (Sig.wAl

2.51

tween those given in tables 8 and 9. Other formulas, of an isomorphous series between siderophyllite and

which could not be used because of their high alkali lepidomelane as they are herein defined. The intercontent (see discussion, p. 16), contain varying relations possible between siderophyllites and lepidproportions of Al and Fe+3 and suggest the existence omelanes are expressed in the following diagram.

.is) (Si,.»Al, ., ) O,0 (OH) ,K, . (Fe0+|,Fe2+25Lio.i5)(Si2 . 85Al1 . 15)01 o(OH) 2K1 .o

(Al1 .ooFe1+52oLio.]5)(Si2 .85Al1 . 16)01o(OH) 2K1 .o (Fe1+JS,Fe1+lJ,Lio. 18)(Si1 .B«Al1 . 18)01 o(OH) 1KI .o

Micas intermediate in composition between siderophyllites and lepidomelanes can be referred to as ferrian 

siderophyllites or aluminian lepidomelanes, depending 

on whether aluminum or trivalent iron is the dominant 

trivalent octahedral cation. By this terminology all 

the formulas in table 10 would be called aluminian 

lepidomelanes, as the number of octahedral positions 

occupied by trivalent iron is greater than the number 

of positions occupied by aluminum in all of them.

Only one formula in tables 8, 9, and 10 contains 

tetrahedral trivalent iron, as specified in Hey's and 

Grigoriev's formulas for lepidomelane. The presence 

or absence of tetrahedral Fe+3 in a formula is indicative, 

not of high Fe+3, but of insufficient Si or Al to fill the 

tetrahedral group, as Fe+3 is assigned to the tetrahedral 

group only when there is insufficient Si and Al to fill 

that group. Some of the phlogopite formulas calculated 

for this study contain tetrahedral Fe+3 because the Al 

content is very low and is not sufficient to complete 

the filling of the tetrahedral group, although the Fe+3 

content is also very low. Thus tetrahedral Fe+3 is not 

a necessary characteristic of high Fe+3 biotites.

CHEMICAL COMPOSITION AND GEOLOGIC 

OCCURRENCE

The trioctahedral micas form under a great variety of 

conditions and occur in many kinds of rocks in igneous 

rocks, both intrusive and extrusive, ranging in composition from felsic to ultramafic; in metamorphic rocks, 

such as gneiss, schist, marble, and "serpentine" (serpentinite); in pegmatites; and in hydro thermal veins. 

As the chemical composition of trioctahedral micas 

depends not only on kinds and relative proportions of 

elements present at the time of their formation but 

also on other environmental factors, micas from different 

kinds of rocks would be expected to differ in composition.

Information on geologic occurrence was available 

for about two-thirds of the analyses used in this study. 

Most of the analyses for which such information was 

available were those of biotites, siderophyllites, and 

lepidomelanes. Information on the phlogopites was 

sparse, and that available indicated quite diverse occurrences. The kinds of rocks from which there were 

enough micas to furnish an adequate conception of the 

composition of the micas in them were divided into the 

following groups: Granite, monzonite and quartz monzonite granodiorite, diorite and quartz dioritejnepheline syenite, gneiss and schist, and pegmatite. Micas 

that formed under apparently atypical conditions such 

as a biotite from a large zenolith (plagioclase) in biotite 

granite, or a biotite from a dark veinlet in granite, are 

not included in this discussion.

The relation between the chemical composition of 

the micas and their geologic occurrence is shown in 

figure 12. In this figure the Mg-(Fe+2,Mn)-(Al,Fe+3 ,
INTERPRETATION OF TRIOCTAHEDRAL MICAS 33

EXPLANATION

Trioctahedral micas from granite, gr

Trioctahedral micas from monzonite 

and quartz monzonite

O 

Biotites from granodiorite, gd

A

Biotites from quartz diorite, qd

 

Biotites from nepheline syenite, ns

A 

Biotites from gneiss and schist, gs

 

Trioctahedral micas from pegmatite, peg

Trioctahedral micas from greisen

AI+3, Fe +3,+Ti+4 Fe+2 +Mn+2

FIGURE 12. Relation between chemical composition and geologic occurrence of trioctahedral micas.

Ti"1"4 ) relation in the octahedral group of each mica is 

plotted on a triangular diagram, as in figure 11, and 

the area occupied by points representing micas from a 

given kind of rock is enclosed. The occupied areas 

indicate that the micas from the first six groups of 

rocks are all biotites; those from the seventh group, 

with a few exceptions, are siderophyllites or lepidomelanes.

Biotites from granite. Biotites from granite comprise 

the largest group (23). Points representing these 

biotites occupy an area (gr) in figure 12 which embraces 

almost the entire biotite area as shown in figure 11. 

Thus biotites from granite vary widely in character, 

from those in which Mg is greatly predominant, as in

these formulas calculated from analyses 41 and 42, 

table 11.

+0.26 -1.34

2.84

+1.08

+ 0.21 -1.28

[(Alo.o3Tio.2 oFe0+130Fe0t920Mg1 . 6oMn0+021)(Si2 .72Al1 .28)01o(OH) 2]-i^

2.84

N-fl.08 (Ko .si, Na<> .14, Ca/20 .01)1.07

to those in which Fe+2 is similarly dominant, as in these 

formulas calculated from analyses 116 and 120, table 11.
34 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

+0.26 -1.25

2.82

+0.37 -1.34 

I(Alo.25Tio.1 4Fe0+134Fe1+6|Mgo.52Mnot121)(Si2 .»aAl1 .34)Q1 o(QH) 2 ]-o^

2.85

o .74, Nan .07 > Ca/2o .07) o.ss

+0.95

In terms of percent composition, biotites from granite 

varied between 14.92 and 4.70 in MgO and between 

12.69 and 26.80 in FeO.

In about one-half the biotites from granite, octahedral aluminum occupies fewer than 0.15 positions, as 

in 42 and 116, table 11 above. In some of these there 

is no octahedral aluminum, the aluminum present being 

insufficient to complete the filling of the tetrahedral 

group, and it was necessary to allocate some Fe+3 to 

that group. On the other hand, in other biotites from 

granite, octahedral aluminum is quite high, occupying 

0.50 or more positions, as in this formula calculated 

from analysis 110, table 11,

+0.36 -1.37

2.77

Q, Nao .06 j Ca/2o .03) o.98

In these biotites from granite, A12O3 varies between 

13.64 and 20.55 percent.

Fe2O3 content is also quite variable in these biotites 

between 0.35 and 8.0 percent. These amounts of 

Fe2O3 are equivalent to 0.02 and 0.44 octahedral 

positions, respectively, occupied by Fe+3. In the 

biotite in which Fe+3 occupies 0.44 octahedral positions, 

as well as in one other in which Fe+3 is fairly high and 

octahedral Al is fairly low, Fe+3 is the dominant trivalent octahedral cation. However, in most of the 

biotites from granite in which there is little or no octahedral aluminum, there is less Fe+3 than there is in 

those that have considerable octahedral aluminum. 

There is no apparent relation between FeO and Fe2O3 , 

individuals of moderate FeO content may contain more 

Fe2O3 than others with twice as much FeO and vice 

versa.

The TiO2 content of the granite biotites is quite 

uniform, varying between only 2.0 and 3.75 percent in 

18 of the 22 analyses of biotites from granite.

In their accommodation of the additional charges 

carried by trivalent or quadrivalent octahedral cations 

the granite biotites apparently do not greatly tend to 

favor one method over the other, but exhibit a fairly 

equal degree of adjustment by both methods. Thus 

they are characterized by a moderate octahedral positive charge, +0.20 to +0.35, with an equivalent

excess over 1 .00 in tetrahedral negative charge, and by 

a moderate decrease (below 3.00) in octahedral occupancy, the number of octahedral positions occupied 

being generally between 2.85 and 2.70.

In summary, therefore, biotites from granite differ 

greatly in character and composition, varying from 

those in which Mg is the considerably predominant 

octahedral cation, through those in which Mg and 

Fe+2 are approximately equal, to those in which Fe+2 is 

considerably predominant. In any of these, octahedral 

Al may be very low or absent, moderate or high, and 

Fe+3 may be insignificant or an important constituent. 

Fuller data on the kinds of granite from which these 

biotites are derived might permit correlation between 

a particular type of granite and a particular type of 

biotite, but with the data at hand this is not possible.

Biotites from monzonite and quartz monzonite. Only 

three of the biotites for which information as to geologic occurrence is available were from monzonite or 

quartz monzonite. Of these, the 2 from quartz monzonite (1 from Colorado and 1 from California) were 

very similar in composition, as shown by these formulas 

calculated from their analyses (48 and 50, respectively, 

table 11).

+0.31 -1.30

2.85

+0.31

o ,75Nan .07, Ca/20 .og) 0.96

1.28

2.82

( KO .go, Nao .05, Ca/2o .01) 0.95 +0.97

Both are Mg dominant biotites with very low octahedral Al and moderate Fe+3 content. Both have a 

moderate octahedral positive charge, with an equivalent increase in the negative tetrahedral charge; and in 

both the deficiency in octahedral occupancy is moderate, 

which indicates about equal adjustments by methods 

A and B for the additional positive charges carried by 

trivalent and quadrivalent octahedral cations. In the 

third member of the group, a mica from monzonite in 

Thuringia, Mg and Fe+2 occupy 1.01 and 1.02 octahedral positions, respectively. This biotite is also 

somewhat higher in Fe+3 content than the other two 

from quartz monzonite, but it is similar to them in 

octahedral and tetrahedral Al content and in TiO2 and 

MnO content. In this biotite one-third of the adjustment for additional positive octahedral cations is by 

method A and two-thirds is by method B.

The points representing these three biotites fall on 

the left boundary of the area occupied by biotites from 

granodiorite (gd), figure 12.
INTERPRETATION OF TRIOCTAHEDRAL MICAS 35

Biotites from granodiorite. The eight biotites from 

granodiorite occupy a large area (gd), in the upper 

part of the granite area (fig. 12). It also overlaps the 

line (Mg:Fe+2=l:l) between the Mg and Fe+2 dominant biotites; thus both types of biotites are obtained 

from granodiorite. In these biotites, Mg and Fe+2 

together generally occupy from approximately 2.30 to 

2.45 octahedral positions, but the relative proportions 

of the two cations present varies greatly, as illustrated 

in the following formulas calculated from analyses 44, 

67, and 111, table 11,

+0.33 -1.26

[(Alo.ooTio.21Feo+137Feo+921 Mgl .g6Mn0+022) (Si2 .74Al1 .21Fe0+035) Oin (OH) 2]-<>-«

2.87

+0.18 -1.25

2.77

+0.28

+1.06

-1.26

i2 . 74^.26)

2.83

o .jo , Nao .o2 , Ca/2o .03)0.96

These formulas also serve to illustrate the characteristics of biotites from granodiorite: their generally low 

octahedral Al content, their fairly low Fe+3 content, 

and their moderately high TiC>2 content. In these 

biotites, as in those from granite and quartz monzonite, 

adjustment for the additional charges carried by these 

trivalent and quadrivalent octahedral cations is made 

almost equally by method A and method B, except for 

two, as in the second formula above, in which adjustment is predominately by method B.

Biotites from diorite and quartz diorite. The points 

representing biotites from diorite and quartz diorite 

occupy an elongated area (qd) on the middle right side 

of the granite area in figure 12, most of the area being 

below the Mg:Fe+2= 1 :1 line. This group also contains 

both Mg and Fe+2 dominant individuals, but 5 of the 7 

in the group are Fe+2 dominant. However, in neither 

the Mg or Fe+2 dominant individuals is the dominance 

extreme, as is illustrated by these formulas of analyses 

(53 and 101, table 11) which represent the Mg and Fe+2 

extremes of the group

+0.30 -1.26

2.85

+0.51 -1.54

[(Alo.3gTio.1 gFeo+23oFe1+326Mgo.75Mn0+022 )(gi2 .46Al1 . 54)010 (QH) 2]-'-'«

2.83

The Mg dominant members of the group contain considerable Fe+2, and the Fe+2 dominant members contain 

considerable Mg although Fe+2 may be more dominant 

in some Fe+2 dominant members than Mg is in any of 

the Mg dominant members. These two formulas also 

illustrate the extremes of octahedral aluminum content 

in members of the group. The Fe2O3 content of 5 of 

the 7 biotites in this group was less than 1 percent. The 

biotite represented by the formula for 101 above has 

the highest Fe2O3 content, 3.49 percent. Adjustment 

for the additional positive charges carried by trivalent 

octahedral cations in these biotites is, in general, fairly 

evenly distributed between method A and method B.

The one point representing biotite from diorite falls 

not in the quartz diorite area (qd) but near the top of the 

granite area. This biotite is more dominantly magnesian than other biotites from diorite and quartz diorite, 

but is not unlike them in other compositional characteristics.

Biotites from nepheline syenite. The biotites from 

nepheline syenite are definitely more ferroan than the 

biotites heretofore considered. The almost round area 

(ns) in figure 12 enclosing points representing biotites 

in this group lies well below the Mg-Fe+2=l:l line, 

with the upper edge just touching the lower edge of 

the granodiorite area (gd). The formula calculated 

for analysis 109, table 11

+0.45 -1.42 

[(Alo.o8Tio.22Fe0+335Fe1+^Mgo.68)(Si2 .58Al1 .42)01 o(OH) 2 ]-o-«

2.79

(Ko.79 ,Nao.i3,Ca/2o.o2) +g:!t

fairly well represents the general characteristics of the 

members of this group, which are quite uniform in 

composition. Octahedral Al is very low. In some 

members there is not enough to complete the filling of 

the tetrahedral group, and some Fe+3 must be allocated 

to this group for its completion. The Fe2O3 content 

is generally fairly high, varying between 4 and 10 percent in the analyses at hand. Mn is considerably 

higher than in the biotites heretofore studied, several 

of the nepheline syenite biotites having between 2 and 

3 percent MnO. TiO2 content is quite variable. Although in a few biotites in this group adjustment for 

additional positive octahedral charges is made equally 

by the two methods, in most the adjustment is dominantly by method B. Thus, many of the formulas for 

this group are characterized by a low positive octahedral charge, with fewer octahedral positions occupied 

than in the biotites heretofore considered.

Biotites from gneiss and schist. Points representing 

biotites from gneiss and schist occupy an area in figure 

12 which lies athwart the lower part of the granodiorite
36 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

(gd) area, the middle of the quartz diorite (qd) area, and 

the upper part of the nepheline syenite (ns) area. This 

location indicates that the biotites from gneiss and 

schist are somewhat higher in Fe+2 content than the 

biotites from granodiorite, but generally lower in Fe+2 

content than biotites from nepheline syenite. The formula calculated for analysis 86, table 11, represents 

the general character of the biotites in this group :

+0.33 -1.36 

i2 .«4Al1 .36)010 (OH) 2]-i-<B

2.76

These biotites are all Fe+2 dominant, with moderate 

octahedral Al content and fairly low Fe+3 content. Adjustment for additional positive octahedral cations is 

made about equally by methods A and B, with a moderate octahedral positive charge (0.30-.40), a moderate 

negative tetrahedral charge in excess of 1.00, and with 

an octahedral occupancy deficiency of about 0.2 

position.

Biotites from pegmatite. Of 14 analyses of micas from 

pegmatite, only 4 represented biotites; all the others 

represented Mg deficient trioctahedral micas, siderophyllites, lepidomelanes, and intermediates of these. 

Three of the four biotites from pegmatite are highly 

Mg dominant and are lower in Fe+2 than any of the 

biotites from granite or granodiorite. Thus, the points 

representing them in figure 12 fall outside the upper 

boundary of the granite area. The fourth biotite from 

pegmatite is highly Fe+2 dominant. The point representing it falls just outside the granite area on the lower 

right side of figure 12.

Trioctahedral micas from pegmatite. Ten of the fourteen analyses of micas from pegmatites are very low in 

Mg «1.5 percent MgO) and high in Fe+2 content. 

They therefore represent the Mg deficient end of the 

Mg replacement system and occupy an area at the bottom of the triangular diagram shown in figure 12. Two 

of these analyses represent siderophyllites, with Al the 

greatly dominant trivalent octahedral cation, and with 

Fe+3 very low or absent; five represent lepidomelanes, 

with Fe+3 the greatly dominant trivalent octahedral 

cation, and with Al very low or absent. The other 

three analyses represent aluminian lepidomelanes, with 

significant amounts of both Fe+3 and octahedral Al, 

but with Fe+3 present in somewhat greater amounts 

than octahedral Al. The three micas from nepheline 

syenite pegmatite are, however, quite similar in composition, like biotites from nepheline syenite, and like

them are characterized by iron. The Mg content in the 

micas from nepheline syenite pegmatite is, however, 

very much lower. All three of these micas (124, 125, 

and 126) are lepidomelanes, with octahedral Al very 

low or absent. In one the Al is so low that there is not 

sufficient to complete the filling of the tetrahedral group, 

making it necessary to assign some Fe+3 to that group.

Li is reported in 7 of the 10 analyses of Mg deficient 

trioctahedral micas from pegmatites, the amount reported varying from 0.08 to 1.01 percent Li2O. No Li 

is reported in the three analyses from nepheline syenite 

pegmatite. This failure to report Li does not mean, 

necessarily, that Li is not present. Li, if present, but 

not determined, is reported as Na. As Li is considered 

an octahedral cation and calculated as such, failure to 

determine Li if present causes a double error in the 

formula calculation, too few octahedral cations and too 

many interlayer cations, the amount of error depending 

on the amount of Li present. In one of the formulas 

calculated from the three analyses in which Li is not 

reported, 126, table 11, the higher interlayer cation 

content, 1.11, suggests the possible presence of Li. In 

the other two formulas calculated from analyses in 

which Li is not reported, the interlayer cation content 

is not high, but this does not rule out the possible presence of small amounts of Li.

Other Mg-deficient trioctahedral micas. Of the 3 other 

analyses of Mg deficient trioctahedral micas at hand, 2 

are of micas from greisen, and 1 is of mica from offshoot 

veins of trap dike. One of the two micas from greisen 

is a siderophyllite, with less than 1 percent of Fe2O3, the 

other is about halfway between siderophyllite and lepidomelane. The latter is reported to contain 0.39 percent Li2O. No Li is reported in the siderophyllite, 

but the high interlayer ion content is suggestive of its 

possible presence. The mica from offshoot veins from 

trap dike is an aluminian lepidomelane containing 0.59 

percent Li2O.

The Mg-deficient trioctahedral micas show a greater 

tendency toward one-sided adjustment for the extra 

charges carried by trivalent and quadrivalent octahedral 

cations than do the biotites. Of the 13 Mg deficient 

trioctahedral micas here considered, 8 showed preferential adjustment, 75 percent or more, by method B. 

In three the adjustment is entirely by method B, except 

for compensation due to Li.

Li, with its single positive charge, compensates for as 

many extra charges carried by trivalent octahedral 

cations as the number of octahedral positions it occupies. This may be illustrated by referring to the formula 

for D63, table 12,
INTERPRETATION OF TRIOCTAHEDRAL MICAS 37

-0.02

-».oo

2.51 -1.02

In this formula Al and Fe+3 carry 1.14 positive charges 

in excess of the number of positive charges that would 

be carried by the same number of bivalent cations. 

On the other hand, 0.18 Li carries only half as many 

positive charges as would be carried by the same number 

of bivalent cations. Thus the xIA cations compensate 

for x R+3 cations, the number of extra positive charges 

carried by the trivalent cations is reduced by the number 

of positions occupied by Li, and the layer structure 

must adjust for 0.96 extra charges, not 1.14. This 

adjustment is made entirely by method B, that is, 

unoccupancy of octahedral positions, with the anionic 

charges associated with these unoccupied positions 

available for the neutralization of the extra charges. 

Octahedral occupancy is only 2.51, the 0.49 unoccupied 

positions contributing 0.98 negative charges for the 

neutralization of the 0.96 extra positive charges, and 

the octahedral layer is left with a residual charge of 

-0.02.

DISCUSSION OF RELATION BETWEEN MICA COMPOSITION AND OCCURRENCE

This study of the relation between the composition 

and character of trioctahedral micas and their geologic 

occurrence indicates that igneous rocks such as granites, 

diorites, and nephelme syenites, and metamorphic 

rocks, such as gneiss and schist, yield trioctahedral 

micas in which both Mg and Fe+2 are present in significant amounts, that is, biotites, but do not yield 

trioctahedral micas, like phlogopites, with very high 

Mg content and low Fe+2 content, or micas like 

siderophyllites and lepidomelanes, with very low Mg 

content and high Fe+2 content. Heinrich (1946, p. 842, 

844) found that phlogopites occur in peridotites and 

other ultramafic rocks and in metamorphosed limestones. The data that are available on phlogopites 

for the present study indicate that some phlogopites 

are from contact zones of metamorphism. All the 

siderophyllites and lepidomelanes are from pegmatites 

or greisen. Thus the extreme types of trioctahedral 

micas, those with extremely high or extremely low Mg 

content, are from extreme types of rocks.

There is not a clear distinction in the composition 

of biotites from different kinds of rocks. This is illustrated by these three formulas, calculated from analyses 

49, 50, and 51, table 11:

+0.27 -1.27

[(Alo.ozTio^^eo+^Feo+JaMgLssMno+o2,) (Si^Al^OjoCOH),] _g_

+0.31 -1.28 

[(Alo.o4Tio.i8Feo+237Fe0+920Mg1 .40Mn0+023)(Si2 .72Al1 .28)01o(OH) 2]-o^

2.82

+0.26

(K,Na,Ca/2)0:9V

-1.24

2.78

(K,Na,Ca/2)0+9°395.

These formulas are essentially identical, not only in 

the kinds and amounts of ions present, but in octahedral charge and occupancy, tetrahedral charge, and 

total charge. They are much more alike than many 

calculated formulas for biotites not only from the 

same kind of rock, but than some from the same rock 

at the same location. Yet 49 represents a biotite from 

granodiorite, 50 a biotite from monzonite, and 51 a 

biotite from granite. Thus biotites from different rocks 

may be very similar in composition and character. 

That they are not differentiated clearly in character 

is attested by the superimposing and overlapping of 

areas in figure 12. At 1 place at the middle right-hand 

side of the biotite area, the areas for 4 different rocks 

are superimposed.

Conversely, biotites from the same kind of rock 

may differ greatly in character and composition. This 

is particularly true of biotites from granites and to a 

lesser extent of biotites from granodiorite, diorite, 

and quartz diorite. The diversity in composition and 

character of biotites from granite is shown by the 

large area occupied by these biotites, which embraces 

practically the entire biotite area as shown in figure 11. 

This picture, however, is probably distorted to some 

extent by the loose usage of the term "granite" to 

cover monzonite, quartz monzonite, granodiorite, 

diorite, and quartz diorite.

The figures in Heinrich's study (1946, p. 836-848) 

of the relation between composition and geologic 

occurrence in the biotite-phlogopite series seem to 

indicate a more clear-cut differentiation between the 

composition of biotites and their parent rocks than was 

found in this study. However, he used a separate 

diagram for each of his rock groups. If the diagrams 

for his group 2 (granites, quartz monzonites, granodiorites), group 3 (tonalites, diorites), group 6 (syenites, 

nephelme syenites, but excluding nepheline syenite
38 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

EXPLANATION 

Gneisses and schists 

Tonalites and diorites

Granites, quartz rnonzonites, 

and granodiorites

Syenites and nepheline syenites 

Fe203 +Ti02 FeO+MnO

FIGURE 13. Relation between the composition and the geologic occurrence of biotites. 

(After Heinrich, 1946, figs. 2,3, 6, and 7 superimposed and reoriented.)

pegmatites), and group 7 (gneisses and schists), which 

include all the groups shown in figure 12 except pegmatites and greisen, are superimposed on each other, as 

in figure 13, it is found that the areas occupied by 

biotites from these different rocks overlap considerably. 

The tonalite-diorite area lies almost wholly within the 

granite, quartz monzonite, granodiorite area, and half 

of the gneiss-schist area overlies the same part of the 

same area. Thus at this 1 place the areas for 3 groups 

overlie each other. In figure 13 the triangular diagram 

of Heinrich has also been reoriented, so that MgO, 

FeO-j-MnO, and Fe2O3 -f-TiO2 occupy the same corners 

with respect to the viewer as do Mg, Fe+2-|-Mn, and 

R+3+Ti in figure 12. The occupied areas in the two 

figures occur in the same general part of triangular 

diagrams a very good agreement in view of the 

different methods used in plotting the biotite compositions; Heinrich's biotites being plotted on the basis 

of percentage by analysis, in which octahedral Al and 

the great equivalent difference between Mg and Fe"1"2

is ignored, and the biotites in figure 12 being plotted 

on the basis of octahedral positions occupied, including 

octahedral Al.

However, correlations of this sort can show only the 

broader aspects of the relation between biotite composition and the parent rocks; details of the relation are 

possible only in more localized studies.

SUMMARY AND CONCLUSIONS

The composition of the trioctahedral micas can be 

interpreted as derived from phlogopite, Mg3 .oo(Si3 .ooAl) 

O10 (OH)2K1 .oo, by the proxying of other cations, particularly Fe+2, Fe+3, and Al, for Mg. Phlogopite, in its 

ideal form may, therefore, be considered the prototype 

of the trioctahedral micas, just as muscovite, in its 

ideal form, is the prototype of the dioctahedral micas. 

In natural phlogopites the proxying of other cations 

for Mg is minor, and Mg is greatly predominant, 

occupying more than 70 percent of the octahedral 

positions. Progressively greater proxying of other 

cations, particularly Fe+2, for Mg leads successively to 

Mg biotites, in which Mg is still the dominant octahedral cation, but in which Fe+2 is present in significant 

amounts, Fe+2 biotites, in which Fe+2 is the dominant 

octahedral cation, with Mg present in subordinate but 

significant amounts, and siderophyllites and lepidomelanes, in which Mg is essentially absent, occupying 

less than 5 percent of the octahedral positions. Thus, 

in order of decreasing Mg content, the trioctahedral 

micas, phlogopites, biotites, siderophyllites, and lepidomelanes form a complete system, from full octahedral 

occupancy by Mg at one end to nil Mg occupancy at 

the other end. These relations are expressed in the 

following equations, which show the range in composition of three principal groups.

<+0.30

Phlogopite

Mg biotite 

Fe+2 biotite

<+0.60

(-1.00)- (-1.30)

.^

(-1.00)- (-1.50)

W^^

2.90-2.65

Siderophyllites

and 

lepidomelanes

<+o.60 (-i.oo)-(-i,60) 

^

Considered as derivatives of phlogopite as the prototype, all trioctahedral micas exhibit multiple proxying 

of other cations for Mg. The phlogopites are the only 

trioctahedral micas in which one kind of cation is 

greatly predominant in the octahedral group. Consequently, the low Mg end of the system is represented

by siderophyllite and lepidomelane, not by annite, the 

Fe+2 analog of phlogopite.

The proxying of bivalent ions, like Fe+2 or Mn+2, for 

Mg is ion for ion, and no layer charge adjustments are 

necessary, but the proxying of trivalent ions, like Al, 

or Fe+3, for Mg, because of their greater valence, re-
INTERPRETATION OP TRIOCTAHEDRAL MICAS 39

quires layer charge adjustments. The two possible 

types of adjustment may be expressed by the following 

equations:

2nR+3=nR+*+n$i (A)

(B)

In the (A) type of adjustment nR+3 ions proxy for 

nMg ions in the octahedral layer, and an additional 

nAl ion proxies for nSi ions in the tetrahedral layers. 

Formulas for trioctahedral micas that have accommodated trivalent octahedral cations in this way are characterized by a positive octahedral charge equivalent to 

the number of trivalent octahedral cations present, a 

negative tetrahedral charge greater than 1.00 by an 

amount equivalent to the positive octahedral charge, 

that is, to the number of trivalent octahedral cations, 

and by full octahedral occupancy. In the (B) type of 

adjustment 2nR+3 cations proxy for 3nMg ions. Formulas for trioctahedral micas that have accommodated 

trivalent octahedral cations in this way are characterized by a neutral octahedral layer, and a negative tetrahedral charge of 1.00 as in the ideal formula for 

phlogopite, but octahedral occupancy is deficient by 

an amount equivalent to one-half the number of trivalent octahedral cations. A few formulas for trioctahedral micas have the octahedral positive charge, the 

equivalently greater negative tetrahedral charge, and 

the full octahedral occupancy indicative of the (A) 

type of adjustment, and a few have the neutral octahedral group and deficient octahedral occupancy of the 

(B) type of adjustment, but most formulas for trioctahedral micas have characteristics of both types of 

adjustment a positive octahedral charge, a negative 

tetrahedral charge greater than 1.00, and octahedral 

deficiency indicating a combination of the two types 

of adjustment. The relative degree of adjustment by

(A) or (B), and, consequently, the proxying ratio of 

trivalent for bivalent octahedral cations varies. However, the average proxying ratio in the 250 formulas 

included in this study is 0.73 Al for 1.00 Mg, a proxying ratio which is considerably closer to that of the

(B) than to that of the (A) type of adjustment, indicating generally greater adjustment by (B) than by (A).

In general, the degree of adjustment by (B) increases 

with the octahedral R+3 content. This is reflected in 

a general decrease in the number of octahedral positions occupied with increase in octahedral R+3 content, 

with the average octahedral positive charge remaining 

about the same throughout the octahedral R+3 range. 

Thus most trioctahedral micas are not truly trioctahedral. Nor are they true octaphyllites, as the number 

of cations in the half cell is usually less than eight.

The trend of R+3 proxying is, therefore, toward muscovite or other dioctahedral mica. However, the distribution of the formulas studied over the R+3 range, 

and the scarcity of formulas having more 0.90 octahedral positions occupied by R+3, strongly indicates that 

the normal extent of R+3 proxying for Mg is approximately one-third of the occupied octahedral positions 

and that there is not a continuous series between the 

trioctahedral and dioctahedral micas.

REFERENCES CITED

Clarke, F. W., 1903, Mineral analyses from the laboratories of

the United States Geological Survey 1880 to 1903: U. S.

Geol. Survey Bull. 220, p. 77. 

Cooke, J. P., Jr., 1867, On cryophyllite, a new mineral species of

the mica family, with some associated minerals in the granite

of Rockport, Mass.: Am. Jour. Sci., 2d ser., v. 43, p.

217-230. 

Dana, E. S., 1892, The system of mineralogy, 6th ed.: New York,

John Wiley and Sons. 

Dana, J. D., 1868, A system of mineralogy, 5th ed.: New York,

John Wiley and Sons. 

Deer, W. A., 1937, The composition and paragenesis of the

biotites of the Carsphairn igneous complex: Mineralog.

Mag., v. 24, p. 495-502. 

Eyerman, John, 1904, Contributions to mineralogy: Am.

Geologist, v. 34, p. 42-48.

Foster, M. D., 1951, The importance of the exchangeable magnesium and cation-exchange capacity in the study of montmorillonitic clays: Am. Mineralogist, v. 36, p. 717-730. 

 1956, Correlation of dioctahedral potassium micas on the

basis of their charge relations: U. S. Geol. Survey Bull.

1036-D, p. 57-67. 

Grigoriev, D. P., 1935, Study of magnesium-iron micas: Soc.

Russe mineralogie M6m., ser. 2, v. 64, p. 21-79. [Russian

with English summary.] 

Hallimond, A. F., 1926, On the chemical classification of the

mica group II. The basic micas: Mineralog. Mag., v. 21,

p. 25-33. 

Hamilton, S. Harbert, 1899, Exploration of the Delaware Valley:

The Mineral Collector, v. 6, p. 117-122. 

Heinrich, E. W., 1946, Studies in the mica group; the biotitephlogopite series: Am. Jour. Sci., v. 244, p. 836-848. 

Hey, Max H., 1955, An index of mineral species and varieties,

2d rev. ed.,: London, British Mus. (Nat. History), 728 p. 

Holzner, Julius, 1936, Uber den "anomalen" Kristallbau der

Biotite: Zeitschr. Kristallographie, v. 95, p. 435-450. 

Hutton, C. O., 1947, Contributions to the mineralogy of New

Zealand, part 3: Royal Soc. New Zealand Trans., v. 76, p.

481-491. 

Kunitz, W., 1924, Die Beziehungen zwischen der chemischen

Zusammensetzung und der physikalisch-optischen Eigenschaften innerhalb der Glimmergruppe: Neues Jahrb.

Mineralogie, Geologie u. Palaontologie, Beilage-Band 50,

p. 365-413.

Marshall, C. E., 1949, The colloid chemistry of the silicate minerals: New York, Academic Press, 195 p. 

Nockolds, S. R., 1947, The relation between chemical composition

and paragenesis in the biotite micas of igneous rocks: Am.

Jour. Sci., v. 245, p. 401-420. 

Nockolds, S. R., and Richey, J. E., 1939, Replacement veins in

the Mourne mountains granites, Northern Ireland: Am.

Jour. Sci., v. 237, p. 27-47.
40 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Pauling, Linus, 1930, The structure of mica and related minerals: Australia Series VIII: Royal Soc. W. Australia Jour., v.

Natl. Acad. Sci. Proc., v. 16, p. 123-129. 18, p. 61-74.

Ross, C. S, and Hendricks, S. B., 1945, Minerals of the mont- Winchell, A. N 1925 Studies in the mica group: Am. Jour. Sci.,

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mineralogy, part 2, Description of minerals, 4th ed.: New 

Simpson, E. S., 1932, Contributions to the mineralogy of Western York, John Wiley and Sons, 551 p.
TABLE 11. Analyses and data for writing formulas of trioctahedral micas used in correlation study

[In order of decreasing MgO content]

Analysis 

(Seep 

44-45)

I.....

2.....

3 

4 

5_ _

6- 

7.. .

7a _ 

8_ 

9_ 

9a_.._ 

10 

11 

12... 

13-___ 

14 

15..-

16 .

17 

18 

19.-.. 

20 

21..- 

22 

23 

24 

25... 

26 

27 . 

28...- 

29.,-

30. 

31 

32 

33 

34 

35.- 

35a. 

36-... 

37 

38 

39 

40 

41 

42 

43 

44 ... 

45 

46...

47. 

48 __ 

49 __ . 

50. 

51.... 

52. 

53- 

54. .. 

55 

56 

57 

58 

59. 

60.... 

61.... 

62 

63.-.. 

64 

65. 

66.... 

67 

68 

Percent

Si08

41.68

4ft QP;

40.64 

43.00 

42.55 

40.42 

41 97

40.56

on oo

40 ftl

40.80 

39.66 

39.68 

42.19 

38.91 

36.96 

40.32

41 19

40.00 

38.37 

40.95 

39.62 

40.16 

42.56 

36.81 

37.53 

43.02 

37.80 

40.63 

40.81 

37.10

38.89 

39.82 

38.06 

37.05 

38.67 

38.04 

38.20 

39.70 

40.65 

38.56 

35.67 

35.68 

36.85 

37.12 

37.71 

37.12 

38.22 

37.88

35.51 

35.62 

36.32 

35.75 

35.79 

36.88 

36.40 

36.34 

36.60 

36.22 

37.37 

35.12 

34.96 

35.88 

36.02 

37.39 

35.21 

37. 07 

35.64 

36.62 

36. 13 

35.51

TiOa

0.38

.34

.10

.05 

.56 

1.14 

.61 

.22 

.52

".~54~ 

.82 

.35 

.45 

2.09 

1.00 

1.51 

.86 

1.20 

1.16

~1.~24~

~~.~i{T 

2.81 

3.45 

.39

1.11 

0.33 

4.95 

7.42 

1.29 

3.15 

3.74 

1.64 

3.84 

2.96 

2.06

3.57 

2.61 

3.42 

3.16 

3.51 

1.88 

4.68 

3.29 

3.30 

3.02 

5.75 

2.77 

3.99 

4.17 

1.13 

2.48 

2.65

irijr

3.03 

3.15 

3.07

AljOs

13.21

13.14 

14.11 

13.27 

12.74 

14.76 

11.41 

14.63 

15.92

17.03

13.16 

17.00 

14.28 

12.60 

16.53 

18.45 

18.03

17.23

17.28 

17.01 

17.28 

16.02 

12.65 

12.18 

17.00 

14.74 

13.54 

17.46 

13.04 

16.47 

17.20

14.53 

19.25 

14.74 

15.04 

13.00 

15.77 

15.45 

15.92 

19.88 

14.12 

15.52 

19.60 

18.24 

15.30 

19.29 

13.86 

14.71 

19.23

17.57 

15.24 

14.66 

14.70 

13.70 

16.79 

14.60 

13.88 

14.98 

16.39 

14.78 

18.65 

15.29 

14.96 

18.81 

20.17 

20.07 

18.79 

18.62 

14.37 

15. 23 

15. 80

Fe2O

0.16

.85 

2.28 

1.71 

1.31 

.73 

1.13 

1.14 

.71

1.67

1.84 

.27 

.77 

.33 

2.01 

.70 

5.80

3.14

.72 

.86 

.43 

2.21 

5.52 

2.73 

2.32 

4.93 

.39 

1.56 

1.12 

2.16 

1.74

4.58 

2.62 

2.01 

4.12 

2.18 

2.62 

8.63 

3.31 

Tr. 

5.51 

2.78 

1.87 

.36 

1.72 

2.24 

3.92 

3.83 

4.09

3.52 

4.69 

4 80 

4.65 

5.22 

3.01 

.76 

4.49 

1.00 

3.92 

2.07 

.37 

2.90 

2.33 

5.60 

2.08 

.68 

6.48 

5.54 

4.04 

3.00 

2.95

FeO

0.11

1.71 

.69

.49 

.81 

.68 

.41 

.80

.65

2.16 

.20 

2.15 

2.91 

1.46 

1.37

4.88 

3.76 

2.38 

5.37 

4.39 

.90 

7.70 

3.02 

5.18 

7.72 

7.62 

5.92 

8.30

8.92 

5.73 

8.76 

5.10 

7.68 

11.13 

9.59 

8.96 

7.89 

8.25 

10.31 

11.45 

13.44 

14.68 

9.72 

14.64 

13.44 

10.47

12.69 

13.67 

14.74 

14.08 

13.72 

15.57 

18.39 

15.00 

19.23 

14.41 

14.89 

19.91 

18.30 

20.88 

14.75 

14.32 

16.88 

15.2S 

14.60 

17.09 

18.83 

18. 87

MgO

28.16

28.03 

27.97 

27.70

27.62 

27.58 

27.42 

27.37 

26.85

26.84

26.72 

26.49 

26.28 

25.61 

25.45 

25.23 

24.79 

24.00

23.91 

23.20 

22.95 

22.86 

22.68 

22.40 

22.00 

21.83 

21.85 

21.80 

21.47 

21.08 

21.00

20.28 

20.00 

19.74 

19.74 

19.37 

18.35 

16.58 

16.15 

15.66 

15.59 

15.38 

15.06 

14.92 

14.65 

14.22 

14.14 

13.45 

13.19

12.96 

12.70 

12.38 

12.37 

12.13 

12.10 

11.90 

11.80 

11.15 

11.11 

10.98 

10.48 

10.42 

10.04 

9.82 

9.80 

9.78 

9 72 

9.72 

9.68 

9.61 

9.52

MnO

......

0.03

.08

"."65" 

.02 

.05 

.01

.07

.09

.17 

.33

.05

~~.~13~

.19 

.28

".16"

~".~24~ 

.12 

.20

""."2l" 

.17 

.18 

.31 

.35 

.52 

.30

.30 

.74 

.34 

.45 

.19 

.11 

.20 

.52 

.13 

.16

."27" 

.39 

.18 

.80

.35

.79 

.40 

.34 

.35

CaO

0.40

.07 

.28

.46 

nil

.06 

.19 

.21 

.46

.89

.06 

.24 

.20

.61 

.15

.64

" U)9~

.33

.12 

.91 

1.50 

1.60 

.14 

1.06 

.58 

.97 

.30 

.16 

.00 

Tr. 

1.46 

.00

.42 

.95 

Tr. 

.17 

.05 

.09 

.11 

.28 

.18

1.08 

.14 

1.11 

.12 

.65 

.51 

Tr.

.90 

1.48 

.63 

.35

NazO

1.04 

.96 

1.16 

.30 

.83 

.71 

1.27 

.20 

.62 

.66 

2.44 

.60 

.56 

.31 

.28 

.58

.42 

1.47 

.43

.16 

1.55 

nil 

.44

1.00 

.92

.84

1.00 

.57 

1.55

.78

.40 

.66 

.42

.23 

.73 

.27 

.18 

.86 

.83 

.81 

1.07 

1.04 

1.00 

.98 

1.33 

.63 

.50 

1.40

.67 

.50 

.90 

.32 

.15 

.18 

.30 

.57 

.37 

.37 

1.06 

.48 

.41 

.36 

.38 

.87 

.37 

1.92 

.38 

.45 

.52 

.26

Kj,O

8.78 

10.25 

8.16 

10.32 

8.92 

8.58 

9.98 

10.73 

9.84 

9.58 

7.95 

9.97 

8.64 

9.91 

9.84 

9.45 

10.50 

9.50 

8.57

9.80 

8.87 

9.62 

10.70

8.78 

10.35 

10.75

8.20 

10.14 

9.01 

8.60

10.08 

8.33

9.55 

10.00 

9.23 

9.17 

8.08 

9.46 

8.68 

8.77 

8.12 

9.66 

9.81 

9.33 

8.84 

7.90 

9.85

9.42 

7.72 

8.63 

9.19 

9.09 

9.47 

8.97 

8.80 

9.06 

8.97 

8.22 

9.42 

7.99 

9.20 

9.32 

8.56 

8.94 

8.93 

9.22 

8.20 

9.05 

9.40

HjO+

3. 

2. 

3.

1.

4.03 

4. 

4.68 

4.69 

2. 

2. 

2.33 

3.90 

2.84 

3.04

3!

1.

4.23 

.60 

3. 

2.35

1.65 

4. 

1.52

1.60 

2. 

2. 

1.67

l!

4.15 

4. 

2.24 

1. 

3. 

2. 

1.03 

2. 

2.92 

1.88 

1.59 

4.29 

1. 

1.89 

1.56

2.74 

4.36 

2. 

3.64 

3.64 

3.70 

2.90 

3.60 

3.20 

4.00 

3.24 

2.08 

3.07 

. 

2. 

3.03 

3.80 

2. 

2.54 

3.26 

2.75 

3.13

H2 0-

10 

06 

21 

38 

18 

0.15 

06

.38 

42 

95 

.66 

.23 

.33 

.67 

.82 

25 

56 

37 

.15 

.48 

.04 

00

.43

78

.27 

47 

19 

.35

94 

69 

.12 

.30 

28 

.20 

94 

21 

90

59

.56

.......

.00 

54 

.60 

.00

""."94" 

?2 

1.03 

1.21 

.00 

.08 

.70 

.20 

.75

.41 

?0 

57

""53" 

!3 

.48 

.90 

.20 

.10

F

3.07 

3.15 

0.82 

5.67 

4.59 

1.17 

3.67 

.05 

.62 

2.78 

2.89 

2.24 

1.16 

2.66

......

1.57 

2.23 

.62 

.76

2.46

1.26 

.30

4.00

1.49

2.21 

.40

.26 

1.15 

1.40 

2.57 

1.25

.47 

.41 

.43

1.05

.17 

.76

......

.08

"."67" 

1.17 

1.58 

.26 

1.67

"."26" 

.10

Total

' 101. 77 

101. 50 

3 101. 58 

102. 35 

100.23 

« 100. 44 

101. 09 

100. 02 

« 100. 39 

102. 44 

101. 42 

« 100. 60 

' 99. 94 

* 100. 51

»99.91 

100. 15

99.86

99.77 

10 100. 83 

11 100. 13 

12 100. 22

is 100. 80 

99.47

101. 35 

» 99. 91 

99.30

is 102. 26 

99.19

99.20

100. 11 

i« 99. 51

17 100. 03 

18 100. 36

i9 100. 15 

102. 39 

20 100. 73 

21 101. 08 

22 100. 44 

99.89

28 100. 00 

100. 38 

100. 36 

100. 08

24 99. 56

100.03

100.43 

25 100. 00

26 99. 88 

2' 99. 59 

" 99. 85

99.29 

» 99. 34

so 99. 53

si 100. 19 

99.44 

99.76 

2 100. 74 

100. 50 

100.11 

' 100. 90 

99. 20

101. 12 

3*101.01 

« 99. 95 

" 99. 65

99.31

O=F

-1.29 

-1.33 

-.35 

-2.39 

-1.93 

-.49 

-1.55 

-.02 

-.26 

-1.18 

-1.22 

-.94 

-.70 

-1.12

.........

-.66 

-.93 

-.26 

-.32

-1.03

-.53 

-.13

-1.68

-.63

-.93 

-.17

-.11 

-.48 

-.59 

-1.08 

-.53

-.20 

-.17 

-.18

-.44

-.07 

-.37

 ...

-.03

"~-~03~ 

-.49 

-.66 

 .11 

-.70

-.11 

-.04

Adjusted 

total

100.48 

100. 17 

101.23 

99.96 

98.30 

99.95 

99.94 

100. 00 

100. 13 

101.26 

100.20 

99.66 

99.24 

99.39

99.11 

99.90 

99.87 

99.90

99.77

100. 82 

99.78

100. 58

99.48

99.10 

100. 19

100. 04 

101. 91 

100. 14 

100. 00 

99.91

99.80 

100. 21 

100. 18

99.99

99.81 

99.22

- ....

100. 16

"99773" 

100. 25 

99.84 

100.00 

100.20

99.90 

99.91

Octahedral positions occupied by 

Al

0.01 

.00 

.00 

.04 

.03 

.06 

.00 

.06 

.12 

.15 

.00 

.17 

.02 

.04 

.13

.20 

.15

.24

.20 

.20 

.30 

.10 

.00

.02 

.05 

.00 

.10 

.12

.04 

.23

.09 

.02 

.36

.11 

.00 

.00

.11 

.03 

.21 

.56 

.00 

.00

.31 

.22 

.03 

.39 

.00 

.08 

.33 

.15 

.07 

.02 

.04 

.00 

.24 

.04 

.02 

.12 

.20 

.07 

.27 

.03 

.03 

.32 

.49 

.44 

.37 

.32 

.09 

.11 

.14

Ti

0.02

.02

.01

.03 

.06 

.03 

.01

.03

.03

.04 

.02 

.02

.11 

.05 

.08 

.04 

.06

.06

.07

.01 

.15 

.19

.02

.06

.02 

.26 

.40

.07 

.17 

.20 

.09 

.21 

.16 

.11 

.20 

.15 

.19 

.18 

.20 

.11 

.26 

.19 

.19 

.17 

.32 

.15 

.23 

.24 

.07 

.14 

.15

~~.~6e"

.17 

.18 

.18

Fe«

0.01 

.00

.10

.09

.07

.04 

.01

.06 

.04

.09

.06

.01 

.04 

.02 

.11

.04 

.29

.16

.04

.05 

.02 

.12 

.24

.15

.12 

.24 

.02 

.08

.06 

.11

.09 

.25

.14

.11 

.22 

.07

.14 

.46

.18

"."is"

.06

.10 

.02 

.10 

.12 

.17 

.21 

.22 

.20 

.27 

.27 

.27 

.30 

.17 

.04 

.26 

.06 

.23 

.12 

.02 

.17 

.13 

.31 

.11 

.04 

.36 

.31 

.23 

.17 

.17

Fe«

0.01 

.10

.04

.03

.05 

.02

.02 

.05

.04

.13

.01 

.13

.17 

.09

.08

.28

.22 

.14 

.31

.05 

.46 

.18 

.30 

.46

.46 

.35

.50

.33

.54 

.31

.47

.68 

.57

.54 

.47 

.66 

.63

.70 

.81 

.90 

.59 

.91 

.82 

.63 

.78 

.87 

.92 

.90 

.88 

.97 

1.16 

.96 

1.22 

.91 

.92 

1.25 

1.16 

1.31 

.92 

.88 

1.06 

.93 

.91 

1.09 

1.20 

1.21

Mn

......

------

......

. .

------

"."61"

"."61"

.02

.01

.01 

.02

.01

.02 

.01 

.01

.01 

.01 

.01 

.02 

.02 

.03 

.02 

.02 

.05 

.02 

.03 

.01 

.01 

.01 

.03 

.01 

.01

""."62" 

.02 

.01 

.05

.02

.05 

.02 

.02 

.02

Mg

2.94 

2.92 

2.90 

2.85 

2.88 

2.89 

2.91 

2.86 

2.83 

2.74 

2.80 

2.76 

2.79 

2.70 

2.66 

2.69 

2.49 

2.47 

2.48 

2.48 

2.40 

2.39 

2.42 

2.36 

2.34 

2.36 

2.26 

2.29 

2.30 

2.20 

2.23 

2.17 

2.08 

2.18 

2.15 

2.11 

1.98 

1.77 

1.73 

1.66 

1.64 

1.66 

1.64 

1.61 

1.60 

1.54 

1.56 

1.47 

1.41 

1.43 

1.44 

1.38 

1.40 

1.39 

1.35 

1.34 

1.34 

1.26 

1.25 

1.22 

1.17 

1.18 

1.13 

1.08 

1.07 

1.10 

1.06 

1.09 

1.10 

1.09 

1.08

Total

2.99 

3.02 

3.04

2.98 

3.01 

3.06 

2.94 

3.01 

3.04 

3.02 

2.99 

2.98 

3.04 

2.96 

3.00 

3.04 

2.93 

2.87 

3.00 

2.98 

2.90 

2.94 

2.94 

13 2. 73 

3.03 

2.88 

« 2. 87 

3.01 

2.92 

2.89 

2.99 

2.98 

2.91 

2.96 

2.85 

2.84 

19 2. 94 

2.83 

'» 2. 78 

212.80 

2.75 

2.75 

23 2. 90 

2.84 

2.84 

2.75 

2.87 

2.77 

2.72 

2.78 

2.85 

2.80 

2.82 

2.78 

2.85 

2.85 

2.80 

2.86 

2.77 

2.65 

2.88 

32 2. 89 

2.85 

2.75 

2.69 

2.81 

2.72 

2,74 

2.70 

2.77 

2.80

h

c

+

See footnotes at end of table, p. 43.
42 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

11

__28s^ 

777777777777777777777777777777777777777777777777777777777777777777

I

(3 -3

§o

I1

<=>.§ § T++I+++

SSSSS 5SSSSS

:g :§s

I^SSSS^

eoci

tHO'

l ^1 i-H »-l i-H IN O O N t-H O O O

rH>Oi-i ia»

O lO

I

<3 

jj 

^

25S

82

=5 '

i^ tH ^t< CO t^ 

COW i-iu5r-l

O

t-i f4 IN CO CO CO tO

I I

t^ Ol-^ Oi CD CD ^

 *IMOO itO i

t-H055 ItH I

5 3 S

.a_6.

g§g§

O »H IV k

COr-(COC

»H N »H i-i 60 OS

>O CR»O>OOO»Oco « « *' co to co ^}i^}it-Jc<icoc<3 <H ei N co

ow

i C*5 <M Oi CD O> CO .C^eOvHOOOCOCO 

i^H^t^t iOCOtjQDT^OOft JJNO r''' '* * 'e ''"' 'I '^

fe ^ fc

I 00 I OS CO U3 I

iei i«4i-i^ i

COtH N Oft t^ O »O CO CP ^ ^ I CO 00 00 00 t^ >O O C3 CO US CD * U3 t^ 00 ^f O> »^ ^f 5* *-t <3> iM OS CO «

 * S K co K ,=< £ Si eo ab 3< 19 B S * S a o o 83 * 35 S * o Si » t><N S. £5 1-- oo o S5 t-i eii

398

<N CO ?3 CO CO N IV * »"M " rH CO N "CO^CCSN * C4 tH ^9* * "lONCO^tHWOiOSCOCdN "^'rHCOCfl^CO»HOdcOtUlONOdcO'ON^cdNOdl>IodOIV * I> 06 U> *H tH

«S5Se?feg

OrHC^W^^Ot^OOOsOrHNCO^iO^t^OOOSOtHC^C*5^W5CDl>»QOOSOtH(?QCO^iO ooSo5o5SSsS3a33SS »^
The following numbers are in terms of percentages.

i Includes 2.08 BaO.

8 A little Fe+' present in tetrahedral group.

»Includes 2.54 BaO.

* Includes 1.02 BaO, 0.05 Cl, and 0.06 S.

»Includes 0.30 COs, which is equivalent to all CaO and to 0.07 

MgO reported.

«Includes 0.62 BaO.

1 1ncludes 0.007 CnOs, 0.97 BaO, 0.05 Cl, and 0.07 S.

«Includes 0.13 P2 05.

«Includes 0.50 BaO, 0.08 Cl, 0.13 S, 0.13 P2O6, and 4.77ignition loss. 

i» Includes 0.69 BaO, 0.04 Cl, 0.09 S, and 3.76 ignition loss. 

"Includes 0.03 BaO.

»»Includes 1.51 BaO, 0.07 Craps, 0.08 PaOs, 0.14 Cl, and 0.02 S. 

"Includes 0.73 Cr2 0s (equivalent to 0.04 octahedral formula 

positions), 1.00 BaO, and 0.06 P2 O4 .

" Includes 0.74 CrjOs (0.04 octahedral positions), 0.52 NiO (0.03 

octahedral positions), 0.28 LiaO (0.08 octahedral positions), and 0.12 

BaO.

IB Includes 0.04 BaO. 

««Includes 1.41 BaO.

if Includes0.79 BaO, 0.03 Cl, 0.10 S, 0.01 P205, and 3.24ignition loss. 

« 8 Includes 1.26 BaO.

" Includes 0.09 CuO, 0.02 Cr2 03, 0.03 NiO, 0.01 V2 03 , 0.01 SrO, 

0.15 BaO, and 0.33 P208 .

M Includes 0.08 Cr2 0s (0.01 octahedral positions), and 0.11 Li2O 

(0.03 octahedral positions).

21 Includes 0.28 LijO (0.08 octahedral positions), 0.37 RbaO, and 

trace Cs2 0.

22 Includes 0.43 BaO.

23 Includes 0.53 BaO, and 0.23 LijO (0.07 octahedral positions).

24 Includes 0.08 PsOs. 

2« Includes 0.26 BaO. 

2« Includes 0.05 V2 03, 0.12 BaO, and 0.03 PaOs.

27 Includes 0.13 BaO, 0.20 Cl, and 0.10 PzOs.

28 Includes 0.07 C Os, which is equivalent to the CaO (0.09) reported.

29 Includes 0.07 P2 05. 

3° Includes 0.13 P2 06.

31 Includes 0.05 V2 0s, and 0.74 BaO.

32 Includes 0.33 Li2 0 (0.10 octahedral positions).

33 Includes 0.02 Cr2 03 . 

a* Includes 0.20 PzOs. 

35 Includes 0.33 BaO. 

38 Includes 0.21 P2 06 . 

« Includes 0.05 Zr02, and 0.03 Cr2 03, 

38 Includes 0.12 P2 O6 . 

as Includes 0.30 P2 0j.

"Includes 0.65 Li2O (0.20 octahedral positions), 1.46 Rb2O, and 

1.14 Cs2 0. 

41 Includes 0.25 Li2 0 (0.08 octahedral positions).

«Includes 0.12 LisO (0.04 oc

«Includes 0.35 PS 05.

*4 Includes 0.09 Bao.

«Includes 0.07 BaO.

«Includes 1.65 Zr02 .

47 Includes 0.04 V2 0s, and 0.1

4? Includes 0.19 Li2 0 (0.06 oc

49 Includes 0.32 LiaO (0.10 oc

««Includes 0.24 P2 05, 0.15 SO

51 Includes 0.37 Li2 0 (0.11 oct

V2 0s.

M Includes 0.23 P2 06 . M Includes 0.21 LisO (0.07 oc

V2 03 . M Includes 0.50 Li2 0 (0.16 oc

«5 Includes 0.39 Li2O (0.13 oc

69 Includes 0.50 Li2 0 (0.16 oc

« Includes 0.59 LiaO (0.20 oc

« Includes 0.48 Li2 0 (0.15 oc

S9 Includes 1.01 LisO (0.33 

Cs2 0, and 0.24 Cl.

«° Includes 0=C1.

ei Includes 1.48 Li2 0 (0.47 oc

62 Includes 0.08 Li2O (0.02 oc

63 Includes 0.32 Li2 0 (0.10 oc
44 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Locality and reference for analyses in table 11

1. DeKalb, N.Y., Dana, E S., 1892, The system of mineralogy, 6th ed., New York, 

John Wiley and Sons, p. 633, No. 11.

2. Mansjo Mountain, Sweden, Eckermann, H. v., 1925, Tschermaks mincvalog. 

petrog. Mitt., v. 38, p. 281, No. 2.

3. Edwards, N.Y., Dana, E. S., 1892, The system of mineralogy, 6th ed., New 

York, John Wiley and Sons, p. 633, No. 4.

4. Gouverneur, N.Y., Dana, E. S., 1892, The system of mineralogy, 6th ed., New 

York, John Wiley and Sons, p. 633, No. 7.

5. Pargas, Finland, Dana, E. S., 1892, The system of mineralogy, 6th ed., New 

York, John Wiley and Sons, p. 633, No. 5.

6. Slyudianka, Baikalia, U.S.S.R., Grigoriev, D. P., 1935, Soc. Russe mineralogie Mem., 2d ser., v. 64, p. 31, No. 1.

7. Pargas, Finland, Jakob, J., 1932, Zeitschr. Kristallographie, v. 82, p. 273, No. 71. 

7a. Snake Creek, Salt Lake City, Utah, Orcel, J., 1925, Soc. Franchise mineralogie 

Bull., v. 48, p. 364. From phyllite.

8. Montville, N.J., Clarke, F. W., 1903, U.S. Geol. Survey Bull. 220, p. 76, D. C. 

Cattell, analyst. From serpentine.

9. Jefferson, Wrooman Lake, State not given, Kunitz, W., 1924, Neues Jahrb. 

Mineralogie, Geologie, u. Palaontologie, Beilage-Band 50, p. 412, table 2, No. 

2.

9a. Mansjo Mountain, Sweden, Eckermann, H. v., 1925, Tschermaks mineralog. 

petrog. Mitt., v. 38, p. 281.

10. Burgess, Ontario, Clarke, F. W., and Schneider, E. A., 1890, Am. Jour. Sci., 

3d ser., v. 40, p. 411. E. A. Schneider, analyst.

11. Slyudianka, Baikalia, U.S.S.R., Grigoriev, D. P., 1935, Soc. Russe mineralogie 

Mem., 2d ser., v. 64, p.31, No. 5.

12. Dent, Idaho, Hietanen, A., U.S. Geol. Survey Prof. Paper 344, in press.

13. U.S.S.R. (exact locality not given), Berkhin, S. I., 1954, Akad. Nauk SSSR 

Doklady, v. 95, p. 146, No. 260.

14. Slyudianka, Baikalia, U.S.S.R., Grigoriev, D. P., 1935, Soc. Russe. miueralogie 

Mem. 2d ser., v. 64, p. 31, No. 3.

15. Easton, Pa., Eyerman, J., 1904, Am. Geologist v. 34, p. 46, C.

16. Easton, Pa., Eyerman, J., 1904, Am. Geologist v. 34, p. 46, A.

17. Lake Baikal, U.S.S.R., Dana, E. S., 1892, The system of mineralogy, 6th ed., 

New York, John Wiley and Sons, p. 630, No. 1.

18. Slyudianka, Baikalia, U.S.S.R., Grigoriev, D. P., 1935, Soc. Russe mineralogie 

Mem. 2d ser., v. 64, p. 31, No. 8.

19. Nancy Sound, New Zealand, Hutton, C. O., 1947, Royal Soc. New Zealand 

Trans., v. 76, p. 486. In marble associated with gneisses.

20. Southern Yakutia, U.S.S.R., Serdyuchenko, D. P., 1954, Akad. Nauk SSSR 

Doklady, v. 97, p. 152.

21. Northwest of Barkly West, Cape Province Africa, Nockolds, S. R., 1947, Am. 

Jour. Sci., v. 245, p. 413, No. 60. Associated with olivine and diopside augite.

22. Lucite Hills, Wyo., Hillebrand, W. F., 1903, U.S. Geol. Survey Bull. 220, p. 76, 

C.

23. U.S.S.R. (exact locality not given), Berkhin, S. I., 1954, Akad. Nauk SSSR 

Doklady, v. 95, p. 146, No. 1.

24. Alban Hills, near Rome, Italy, Washington, H. S., 1927, Am. Jour. Sci., 5th ser. 

v. 14, p. 189. Associated with nephilite-mellitite rock.

25. Northern Caucasus, U.S.S.R., Serdyuchenko, D. P., 1951, Soc. Russe. minSralogie MSm., v. 80, p. 175. Associated with diopside, actinolite, and almandite.

26. U.S.S.R. (exact locality not given), Berkhin, S. I., 1954, Akad. Nauk SSSR 

Doklady, v. 95, p. 146, No. 3.

27. Rossie, N.Y., Dana, E. S., 1892, The system of mineralogy, 6th ed., New York, 

N.Y., John Wiley and Sons, p. 633, No. 12.

28. Greenwood Furnace, N.J., Dana, E. S., The system of mineralogy, 6th ed., 

New York, N.Y., John Wiley and Sons, p. 630, No. 2.

29. U.S.S.R. (exact locality not given), Berkhin, S. I., 1954, Akad. Nauk SSSR 

Doklady, v. 95, p. 146, No. 2.

30. Arendal, Norway, Dana, E. S., 1892, The system of mineralogy, 6th ed., New 

York, N.Y., John Wiley and Sons, p. 630, No. 10.

31. Sterzing. Tyrol, Dana, E. S., 1892, The system of mineralogy, 6th ed., New 

York, N.Y., John Wiley and Sons,p. 630, No. 14.

32. Slyudianka, Baikalia, U.S.S.R., Grigoriev, D.P., 1935, Soc. Russe minerologie 

Mem. 2d ser., v. 64, p. 31, No. 9.

33. Bufumbira, S. W. Uganda, Combe, A. D., and Holmes, A., 1948, Royal Soc. 

Edinburgh Trans. v. 61, p. 377.

34. Weinheim, Germany, Weyberg, Z., 1912, Neues, Jahrb. Min. B.B.I., p. 398, No. 

6 a. Associated with augite and minette.

35. Malvern, England, Tha Hla, 1945, Mineralog. Mag. v. 27, p. 139. From granite

pegmatite.

35a. Fillipstad, Sweden, Dana, E. S., 1892, The system of mineralogy, 6th ed., New 

York, N.Y., John Wiley and Sons, p. 630, No. 13.

36. Ridgway, Va., Stevens, R. E., 1945, U.S. Geol. Survey Bull. 950, p. 118.

37. Monte Somma, Tuscany, Pieruccini, R., 1950, Soc. Toscana Sci. Nat. Atti 

Mem., ser. A, vol. 57, p. 152. Associated with plagioclase and sanidine.

38. San Juan district, Colo., Larsen, E. S., Gonyer, F. A., and Irving, J., Am, 

Mineralogist, v. 22, p. 902. Associated with pyroxene.

39. Radautal. Germany, Kunitz, W., 1936, Neues Jahrb. Mineralogie, Geologie u. 

Palaontologie Beilage-Band 70 A, p. 401.

40. Northern Caucasas, U.S.S.R., Serdyuchenko, D. P., 1951, Soc. Russe Mineralogie M6m., v. 80, p. 176.

41. Kojatin, near TrebliS, Moravia, Czechoslovakia, Dudek, A., 1954, Ceskoslovenska Akad. V6d Rozpravy, v. 64, p. 31. From amphibole-biotite granite.

42. Kosov, near Jihlava, Moravia, Czechoslovakia, Dudek, A., 1954, Ceskoslovenska Akad. V6d, Rozpravy, v. 64, p. 33. From pyroxene-biotite granite.

43. Monti di Daro, Italy, Jakob, J., 1931, Zeitschr. Kristallographie, v. 79, p. 373, 

no. 58.

44. Morven-Strontian complex, northern Scotland, Nockolds, S. R., and Mitchell, 

R. L., 1947, Royal Soc. Edinburgh Trans., v. 61, p. 562, no. 5. Associated 

with tonalite-granodiorite.

45. Chocholowska Valley, Tatra Mountains, Zastawniak, F., 1951, Soc. Geol. 

Pologne Annales, v. 20, p. 128. In diorite.

46. Monti di Daro, Italy, Jakob, J., 1931, Zeitschr. Kristallographie, v. 79, p. 373, 

no. 57.

47. Zernovka, near KiCany, Bohemia, Czechoslovakia, Dudek, A., 1954. Ceskoslovenska Akad. Ved, Rozpravy, v. 64, p. 27. From biotite granite.

48. Bloods Station, Alpine County, Calif., Clarke, F. W., 1903, U.S. Geol. Survey 

Bull. 220, p. 75, E. Associated with quartz-monzonite.

49. Garabal Hill-Glen Fyne complex, Scotland, Nockolds, S. R., and Mitchell, 

R. L., 1947, Royal Soc. Edinburgh Trans., v. 61, p. 562, no. 14. From porphyritic granodiorite contaminated with sedimentary material.

50. Mt. Hoffman, Mariposa County, Calif., Clarke, F. W., 1903. U.S. Geol. 

Survey Bull. 220, p. 75, D. From quartz-monzonite.

51. Butte, Mont., Clarke, F. W., 1903, U.S. Geol. Survey Bull. 220, p. 75, G. 

From granite.

52. Bethel, Vt., unpub. analysis by Charles Milton, U.S. Geol. Survey, lab. no. 

_ D-953.

53. Obatake, Kazro-Mura, Kuga-gori, Yamaguti Pref. Japan, Tsuboi, S., 1938, 

Japanese Jour. Geology Geography, v. 15, p. 126, no. 24. In hypersthenebiotite-quartz diorite.

54. Senmaya, Rikutyu, Japan, Tsuboi, S., 1935, Japanese Jour. Geology Geography, 

v. 12, p. 112, no. 6. From hornblende-bearing biotite granodiorite.

55. Yokodake, Euasan district, Mino, Tsuboi, S., 1935, Japanese Jour. Geology 

Geography, v. 12, p. 110, no. 9. In gneissose granite.

56. Old Point, Charles Sound, New Zealand, Hutton, C. O., 1947, Royal Soc. New 

Zealand Trans. and Proe., v. 76, p. 482, no. 1. In pegmatitelike lenses in 

oligoclase-quartz-biotite gneiss.

57. River Tschut-sou-su, Altai, Mongolia, Timofeev, K., 1927, Neues, Jahrb. 

Mineralogie, Geologie u. Palaontologie, Rcf. Bd. II, 81. From granodiorite.

58. MraS, near Benesov, Bohemia, Czechoslovakia, Dudek, A., 1954, Ceskoslovenska Akad. Ved Rozpravy, v. 64, p. 36. From amphibole-biotite granodiorite.

59. Carsphairn complex, Southern Scotland, Deer, W. A., 1937, Mineralog. Mag,, 

v. 24, p. 496, no. 3. From hornblende hybrid.

60. Ames Station, Belknap Mountains, N. H., Chapman, R. W., and Williams, 

C. R., 1935, Am. Mineralogist, v. 20, p. 512, no. 2. From monzodiorite.

61. El Capitan. Yosemite Valley, Calif., Iddings, J. P., 1911, Rock minerals, 2d ed., 

New York, John Wiley and Sons, p. 451.

62. Clove Valley, Dutchess County, N.Y., Earth F. W., 1936, Geol. Soc. America 

Bull., v. 47, p. 783, no. 2. From contact between sheared quartzitic schist 

and pegmatite.

63. Tenryukyo, Simoina-gori, Nagano Pref., Tsuboi, S., 1938, Japanese Jour. Geology 

Geography, v. 15, p. 128, no. 28. In cordierite-biotite hornfels.

64. Renchthal, Germany, Dana, J. D., 1892, The system of mineralogy, 6th ed., 

New York, John Wiley and Sons, p. 630, no. 16.

65. Yosemite Valley, Calif., Clarke, F. W., 1903, U.S. Geol. Survey Bull. 220, p. 75, 

C. From granite.

66. North fork of Mokelumne River, above mouth of Bear River, Amador County, 

Calif., Clarke, F. W., 1903, U.S. Geol. Survey Bull. 220, p. 75, F. From 

pyroxene gneiss.

67. Isikawa-mati, Hukusima Pref., Tsuboi, S., 1938, Japanese Jour. Geology Geography, v. 15, p. 129, no. 31. In gneissose hornblende-biotite granodiorite.

68. Akasaka, Samekawa-mura, Higasisirakawa-gori, Hukusima Pref., Tsuobi, S., 

1938, Japanese Jour. Geology Geography, v. 15, p. 125, no. 19. In gneissose 

tonalite.

69. Freiburg, Germany, Becker, A., 1890, Zeitschr. Kristallographie, v. 17, p. 129. 

From gneiss.

70. Mora, Minnesota, Grout, F. F., 1924, Am. Mineralogist, v. 9, p. 160, no. 3. In 

granite.

71. U.S.S.R. (exact locality not given), Berkhin, S. I., 1954, Akad. Nauk SSSR 

Doklady, v. 95, p. 146, no. 106.

72. Monterey Bay, Calif., Galliher, E. W., 1935, Geol. Soc. America Bull., v. 46, 

p. 1359. From granite.

73. Minedera, near Tokyo, Japan, Kawano, Y., 1933, Imp. Acad. Tokyo Proc., 

v. 9, p. 610. In porphyritic biotite granite.

74. Glen Buchat, Aberdeenshire, Scotland, Walker, G. F., 1949, Mineralog. Mag., 

v. 28, p. 698, no. 1. From hypersthene gabbro.

75. Ehrenberg, Germany, Kunitz, W., 1929, Zeitschr. Kristallographie, v. 70, p. 512. 

From monzonite.

76. Hirasawa, Oda Mura, Tukuba-gori, Ibaraki Pref., Tsuboi, S., 1938, Japanese 

Jour. Geology Geography, v. 15, p. 126, no. 21. Large zenolith enclosed in 

biotite granite.

77. Takato, Sinano, Japan, Tsuboi, S., 1935, Japanese Jour. Geology Geography, 

v. 12, p. 110, no. 8. From biotite granodiorite.

78. Tenryukyo, Simoina-gori, Nagano Pref., Tsuboi, S., 1938, Japan Jour. Geology 

Geography, v. 15, p. 126, no. 23. In biotite gneiss (injection gneiss).

79. Chestnut Ridge, Dutchess County, N.Y., Earth, F. W., 1936, Geol. Soc. 

America Bull., v. 47, p. 783, no. 1. From black phyllite.

80. Southwestern Finland, Hietanen, A., 1943, Acad. Sci. Fenn. Annales, ser. A. 

III. Geologica-Geographica., no. 6, p. 48. From diorite-trondhjemite.

81. Eight miles northwest of Ouster, S. Dak., Wells, R. C., 1937, U.S. Geol. Survey 

Bull. 878, p. 99. From pegmatite.

82. Upington, Cape Province, South Africa, Mathias, M., 1952, Mineralog. Mag., 

v. 29, p. 939. From cordierite rock.

83. Carsphairn complex, southern Scotland, Deer, W. A., 1937, Mineralog. Mag., 

v. 24, p. 496. From granite of a complex intrusive.

84. Laven Island, Kunitz, W., 1929, Zeitschr. Kristallographie, v. 70, p. 512. From 

nepheline syenite.

85. Kamenz, Germany, Kunitz, W., 1929, Zeitschr. Kristallographie, v. 70, p. 512. 

From granodiorite.

86. Ornak, Tatra Mountains, Zastawniak, F., 1951, Rocznik Polsk. Towarz. Qeol., 

v. 20 (1950), p. 129. From injection gneiss.

87. Mukuno, Kamano-mura, Osima-gori, Yamaguti Pref., Tsuboi, S., 1938, Japanese 

Jour. Geology Geography, v. 15, p. 127, no. 25. In garnet-bearing schistose 

biotite-quartz diorite.

88. Mansjo Mountain, Sweden, Eckermann, H. v., 1925, Tschermaks mineralog. 

petrog. Mitt., v. 38, p. 277. From contact zone of pegmatite intrusive into 

eulysite and limestone.

89. Usugi, Takanukl district, Iwaki, Tsuboi, S., 1935, Japanese Jour. Geology 

Geography, v. 12, p. 112, no. 5. In garnet-bearing gneissose biotite granite 

(contaminated rock).

90. Altai, Mongolia, Timofeev, K., 1927, Neues Jahrb. Mineralogie, Geologie u. 

Palaontologiej_Ref., v. 11, p. 81. From granodiorite.

91. Kamatu-mati, Osima-g5ri, Yamaguti Pref., Tsuboi, S., Japanese Jour. Geology 

Geography, v. 15, p. 127, no. 26. In garnet-bearing biotite-quartz diorite.

92. Kaziyabara, Hidumi-mura, Kuga-gori,Yamaguti Pref., Tsuboi, S.,1938, Japanese 

Jour. Geology Geography, v. 15, p. 127, no. 27. In biotite gneiss (injection 

gneiss).

93. Ruma Island, Los Archipelago, Kunitz, W., 1929, Zeitschr. Kristallographie, v. 

70, p. 512. From nepheline syenite.

94. Takanuki district, Iwaki, Tsuboi, S., 1935, Japanese Jour. Geology Geography, 

v. 12, p. 112, no. 4. From injection biotite gneiss.

95. Stewart Island, New Zealand, Williams, G. J., 1934, Geol. Soc. London Quart. 

Jour., v. 90, p. 336. From granite.

96. Faraday Township, Hastings County, Ontario, Canada, Walker, T. L., and 

Parsons, A. L., 1926, Toronto Univ. Studies, Geol. ser., no. 22, p. 22. From 

nepheline syenite.

97. Belhelvie, Aberdeenshire, Scotland, Stewart, F. H., 1942, Mineralog. Mag., 

v. 26, p. 263. From SiOj poor hornfels.
INTERPRETATION OF TRIOCTAHEDRAL MICAS 45

Locality and reference for analyses in table 11 Continued

98. Usugi, Takanuki district, Iwaki, Tsuboi, S., 1935, Japanese Jour, Geology 

Geography, v. 12, p. 112. From gneissose biotite-granite.

99. Risiok, Kola Penmsula, U.S.S.R., Ivanov, B. V., 1937, Akad. Nauk SSSR 

Lomonosovskii Institut Geokhimii, Kristallographii i Mineralogii Trudy, 

no. 10, p. 43. From pegmatite and mica-nepheline syenite.

100. Brevik, Norway, Kunitz, W., 1924, Neues. Jahrb. Mineralogie, Geologie u. 

Palaontologie, Beil.-Band 50, p. 413. Probably associated with aegirine, 

nepheline syenite pegmatite.

101. Prosecnice near Praha, Bohemia, Czechoslovakia, Dudek, A., 1954, Veskoslovenska Akad. Ved Rozpravy, v. 64, p. 29. From biotitic granodiorite.

102. Mariupol, Sea of Azov, U.S.S.R., Morozewicz ,J., 1930, Tschermaks Mineralog. 

Petrog. Mitt., v. 40, p. 374. From mariupolite.

103. Dermone, Halland, Sweden, Wiman, E., 1930, Upsala Univ., Geol. Inst., Bull., 

22, p. 56. From pegmatite containing microcline, quartz, and albite.

104. Hirasawa, Oda-muri, Tukuba-gori, Ibaraki Pref., Tsuboi, S., 1938, Japanese 

Jour. Geology Geography, v. 15, p. 125, no. 20. From biotite granite.

105. Fukushinzan district, central Korea, Inoue, T., 1950, Geol. Soc. Japan Jour., 

v. 56, p. 76. From nepheline syenite.

106. Rockville, Minnesota, Grout, F. F., 1924, Am. Mineralogist, v. 9, p. 161. From 

granite.

107. Sanage-mura, Nisi-Kamo-gori, Aiti Pref., Tsuboi, S., 1936, Japanese Jour. 

Geology Geography, v. 13, p. 335, no. 15. From biotite granite.

108. Widecombe, Dartmoor, England, Brammall, A., and Harwood, H. F., 1932, 

Geol. Soc. London Quart. Jour., v. 88, p. 234. From dark veinlet in granite.

109. Miask, Ural Mountains, U.S.S.R., Kunitz, W., 1924, Neues Jahrb. Mineralogie, 

Geologie u. PalSontologie Beil.-Band 50, p. 413. From nepheline syenite.

110. Saddle Tor, Dartmoor, England, Brammall, A., and Harwood, H. F., 1932, 

Geol. Soc. London Quart. Jour., v. 88, p. 234. From granite; associated with 

muscovite.

111. Hata, Tuba-mora, Kuga-gori, Yamaguti Pref., Tsuboi, S., 1938, Japanese Jour. 

Geology Geography, v. 15, p. 128, no. 29. From granodiorite.

112. Fukushinzan District, central Korea, Inoue, T., 1950, Geol. Soc. Japan Jour., 

v. 56, p. 76. From nepheline syenite.

113. Haytor quarry, Dartmoor, England, Brammall, A., and Harwood, H. F., 1923, 

Mineralog. Mag., v. 20, p. 23. From granite.

114. Tenryukyo, Simoina-gori, Nagano Pref., Tsuboi, S., 1938, Japanese Jour. Geology Geography, v. 15, p. 126, no. 22. From porphyrite schistose hornblendebiotite granite.

115. Miask, Ural Mountains, U.S.S.R., Dana, E. S., 1892, The system of mineralogy, 

6th ed., New York, John Wiley and Sons, p. 630, no. 22.

116. Tutiyane, Nada-mura, Kuga-gori, Kamaguti Pref., Tsuboi, S., 1936, Japanese 

Jour. Geology Geography, v. 13, p. 335, no. 14. From hornblende-bearing 

biotite granite.

117. Near Beech Hill, Percy quadrangle, N.H., Chapman, R. W., and Williams, 

C. R., 1935. Am. Mineralogist, v. 20, p. 512. From granite.

118. Unter-Wald-Michelliaeh, Odenwald, Germany, Klemm, G., 1926, Vereins fur 

Erdkunde und der Hessischen Geologischen Landesanstalt zu Darmstadt 

Notizblatt, 5th ser., No. 8, p. 166. Associated with muscovite, adamellite.

119. Sweltor quarry, East Dartmoor, England, Brammall, A., and Harwood, H. F., 

1932, Geol. Soc. London Quart. Jour., v. 88, p. 234. From granite.

120. Near Iwakura Railway Station, Og5ri, Yamaguti Pref., Tsuboi, S., 1936, Japanese Jour. Geology Geography, v. 13, p. 334, no. 13. From biotite granite.

121. Prison quarry, Princeton, Dartmoor, England, Brammall, A., and Harwood, 

H. F., 1932, Geol. Soc. London Quart. Jour., v. 88, p. 234. From granite; 

associated with muscovite.

122. Laitila, Finland, Eskola, P., 1949, Comm. Geol. Finlande Bull., v. 23, p. 114. 

From fresh rapakivi ("moro").

123. lisaka, Fukushima Pref., Shibata.H., 1952, Tokyo Bunrika Daigaku Sci. repts., 

v. 2, sec. C, no. 12, p. 162, no. 8. From graphic granite pegmatite.

124. French River, Sudbury district, Ontario, Canada, Walker, T. L., and Parsons, 

A. L., 1926, Toronto Univ. Studies, Geol. ser. no. 22, p. 8. From nepheline 

syenite pegmatite.

125. Wausau, Wisconsin, Weidman, S., 1907, Wisconsin Geol. and Nat. History 

Survey Bull. 16, p. 295. From quartz syenite pegmatite.

126. Mount Royal, Quebec, Canada, Finley, F. L., 1930, Canadian Jour. Research, 

v. 2, p. 236. Associated with aegirine, nepheline syenite pegmatite.

127. Ebisu Mine, Naegi district, Japan. Shibata, H., 1952, Tokyo Bunrika Daigaku 

Sci. Repts., v. 2, sec. C, no. 12, p. 162, no. 6. In greisen.

128. Hachiman, Naegi district, Japan, Shibata, H., 1952, Tokyo Bunrika Daigaku 

Sci. Repts., v. 2, sec. C, no. 12, p. 161, no. 2. In pegmatite.

129. Mourne Mountains, New Castle County, Ireland, Nockolds, S. R., and Richey, 

J. E., 1939, Am. Jour. Sci., v. 237, p. 38. In greisen veins cutting aplite veins 

in granite.

130. Hachiman, Naegi district, Japan, Shibata, H., 1952, Tokyo Bunrika Daigaku 

Sci. Repts., v. 2, sec. C, no. 12, p. 161, no. 3. In pegmatite.

131. Hachiman, Naegi district, Japan, Shibata, H., 1952, Tokyo Bunrika Daigaku 

Sci. Repts., v. 2, sec. C, no. 10, p. 114. In granite pegmatite.

132. Brooks Mountain, Alaska, Coates, R. R., and Fahey, J. J., 1944, Am. Mineralogist, v. 29, p. 373. From pegmatite sill intrusive into granite.

133. Volhynia, U.S.S.R., Buryanova, E. Z., 1940, Soc. Russe Min6ralogie, Mem., 

v. 69, p. 532.

134. lisaka, Fukushima Pref., Japan, Shibata, H., 1952, Tokyo Bunrika Daigaku 

Sci. Repts., v. 2, sec. C, no. 12, p. 162, no. 9. In greisen.

135. Yagenyama, Japan, Shibata, H., 1952, Tokyo Bunrika Daigaku Sci. Repts., 

v. 2., sec. C, no. 12, p. 162, no. 7. In greisen.
46 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

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i^co^c^^o^

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106 i ICO-* no lOO ir-lOO lOOOi-iScO i53t-t-WCO«5(MF-lF-ieO«5 KM1OO iTHTHt-«5rHt-i-iO i |C

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3co^N^oo^c2^^^

OCOM
The following numbers are in terms of percentage.

i Includes 0.07 Li2O (0.02 octahedral positions).

a Includes 0.20 Li2O (0.05 octahedral positions), 1.28 BaO, and 

0.21 01.

»All the Al, Fe+a and Ti and some Fe+2 are necessary to fill the 

tetrahedral group.

*Includes 0.08 Li2 O (0.02 octahedral positions).

' All the Al and Fe+8, and some Ti are necessary to fill the tetrahedral group.

8Includes 0.53 Li2O (0.14 octahedral positions).

' Includes 0.03 CnOs, 0.30 percent BaO, 0.05 percent 01, 0.14 S, 

0.12 PsOs, and 4.23 ignition loss.

«Includes 0.11 LijO (0.03 octahedral positions).

9 Some tetrahedral Fe+3.

10 Includes 0.04 CnOs, 1.00 BaO, 0.07 01, and 0.09 S. 

i°A Includes 0.69 BaO, 0.06 01, and 0.05 S.

» Includes 0.79 CnOs (0.04 octahedral positions), and 0.19 BaO.

12 Includes 0.16 LiaO (0.05 octahedral positions), and 0.03 NiO.

13 Includes 0.34 NiO (0.02 octahedral positions) and 0.09 BaO. 

ISA Includes 1.06 BaO. 

w Includes 3.88 ignition loss, and 0.05 P2Os. 

« Includes 0.36 LijO (0.11 octahedral positions). 

« H2O by difference.

» Includes 0.27 CO2, and 0.01 01, and 0.58 BaO. 

is Includes 0.06 V2O3 ,0.05 ZrO2,0.08 BaO, 0.18 Li2O (0.06 octahedral 

positions), and 0.36 PaOs.

i» Includes 0.30 NiO (0.02 octahedral positions), and 0.04 LizO. 

20Includes 0.30 BaO. 

21 Includes 3.90 ignition loss. 

33 Includes 4.30 ignition loss.

23 Includes 0.18 Li2O (0.06 octahedral positions).

24 Includes 0.28 LiaO (0.09 octahedral positions). 

2« Includes 0.16 LizO (0.05 octahedral positions).

28 Includes 0.37 Li2 O (0.11 oc

2' Includes 0.17 LiaO (0.06 oc

28 Includes 0.60 Mn2Os (0.03 

(0.18 octahedral positions), 

so Includes 0.28 Li2O (0.10 oc

so Includes 0.18 P2Os. 

si Includes 0.27 01.

32 Includes 0.77 LizO (0.25 oc

33 MgO+CaO=0.26. The fo

assumption that the value giv

dominantly CaO. Assuming t

makes no change in the tet

positions occupied by Fe+z, b

occupied by Fe+3 to 1.52, adds 0

the octahedral charge to +0.16, 

This analysis represents the co
48 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Locality and reference for analyses in table 12

Dl. Edwards, St. Lawrence County, N.Y., Clarke, F. W., 1910, U.S. Geol. Survey D33.

Bull. 419, p. 289, A. 

D2. Mansjo Mountains, Sweden, Eckermann, H. v., 1925, Tschermaks mineralog., D34.

petrog Mitt., v. 38, p. 281. From contact zone of pegmatite intrusive into

eulysite and limestone. D35. 

D3. Edwards, St. Lawrence County, N.Y., Penfield, S. L., and Sperry, E. S.,

1888, Am. Jour. Sci., 3d ser., v. 36, p. 330. D36. 

D4. PennsviUe, Pa., Dana, E. S., 1892, The system of mineralogy, 6th ed., New D37.

York, John Wiley and Sons, p. 633, no. 2. 

D5. Monte Braccio, Val Malenco, Italy, Pagliani, G., 1940, Soc. Ital. Sci. Nat.

Milano, Atti., v. 79, p. 21. In limestone. D38. 

D6. Rossie, St. Lawrence County, N.Y., Dana, E. S., 1892, The system of miner- D39.

alogy, 6th ed.. New York, John Wiley and Sons, p. 633, no. 6. 

D7. Pargas, Finland, Dana, E. S., 1892, The system of mineralogy, 6th ed., New D40

York, John Wiley and Sons, p. 633, no. 1. 

D8. Baikalia, U.S.S.R., Grigoriev, D. P., 1935, Soc. Busse Mineralogie Mem., D41.

v. 64, ser. 2, p. 31, no. 2. 

D9. Mansjo Mountains, Sweden, Eckermann, H. v., 1925, Tschermaks mineralog. D42.

petrog Mitt., v. 38, p. 281. From contact zone of pegmatite intrusive into

eulysite and limestone. D43. 

D10. Morawitza, Hungary, Dana, E. S., 1892, The system of mineralogy, 6th ed.,

New York, John Wiley and Sons, p. 630, no. 5. In magnetite. D44. 

Dll. Sliudianka, Lake Baikal, U.S.S.B., Grigoriev, D. P., 1935, Soc. Busse Mine'ralogie M&n. v. 64, ser. 2, p. 31, no. 6. D45. 

D12. U.S.S.B. (exact locality not given), Berkhin, S. I., 1954, Akad. Nauk SSSB

Doklady, v. 95, p. 146, no. 48. D46. 

D13. Ambatoabo, Madagascar, Jakob, J., and Parga-Pondal, I., 1932, Zeitschr.

Kristallographie, v. 82, p. 273, no. 62. D47. 

D14. Monzoni, South Tyrol, Italy, Dana, E. S., 1892, The system of mineralogy,

6th ed., New York, John Wiley and Sons, p. 630, no. 4. D48. 

D15. Sliudianka, Lake Baikal, U.S.S.B., Grigoriev, D. P., 1935, Soc. Busse. Min6ralogie M6m. v. 64, p. 31, no. 7. D49. 

D16. Fortuna, Mureia, Spain, Osann, A., 1906, Bosenbusch Festschrift, p. 271.

Associated with olivine and augite, verite. D50. 

D17. Vesuvius, Italy, Berwerth, F., 1877, Mineralogische Mitteilungen, p. 112, no. 9. 

D18. Bolschii Laby, North Caucasus, U.S.S.B., Serdiuchenko, D. P., 1951, Soc. D51.

Busse Mineralogie M6m., v. 80, ser. 2, p. 177. In granite-aplite vein cutting

through serpentine. D52. 

D19. Muturezima, Japan, Kozu, S., and Tsurumi, S., 1931, Japanese Assoc. Mineralogists, Petrologists and Econ. Geologists Jour., v. 5, p. 160. D53. 

D20. Laurel Creek mine, Babun County, Ga., Clarke, F. W., 1912, U.S. Geol.

Survey Bull. 419, p. 289, B. From corundum mine. D54. 

D21. Smith Bidge, Boehls Butte quadrangle, Idaho, Hietanen, A., 1956, Am.

Mineralogist, v. 41, p. 7. From kyanite-andalusite-sillimanite-cordierite D55.

gneiss. 

D22. Tatra Mountains, Czechoslovakia, Weyberg, Z., 1912, Neues Jahrb. Miner- D56.

alogie, Geologic u. Palaontalogie, pt. l, p. 398. Probably associated with

augite, kersantite. D57. 

D23. Chebarkul, southern Urals, U.S.S.B., Dana, E. S., 1892, The system of

mineralogy, 6th ed., New York, John Wiley and Sons, p. 630, no. 11. D58. 

D23A. Morven-Strontian complex, Scotland, Nockolds, S. B., and Mitchell, B. L.,

1947, Boyal Soc. Edinburgh Trans., v. 61, p. 562. From granodiorite. D59. 

D24. Odenwald, Germany, Kunitz, W., 1936, Neues Jahrb. Mineralogie, Geologic,

u. Pal&ontologie, Beilage-band, v. 70, Abt. A., p. 401. Associated with D60.

augite and aegirine. 

D25. Hill of Strone, Forfarshire, Scotland, Phillips, F. C., 1930, Mineralog. Mag., D61.

V. 22, p. 255, no. Viii. D62. 

D26. Black Cascade, Tripyramid Mountain, N.H., Chapman, B. W., and Williams,

C. B., 1935, Am. Mineralogist, v. 20, p. 512. From gabbro. D63. 

D27. Dowerin, Western Australia, Simpson, E. S., 1931-32, Boyal Soc. Western D64.

Australia Jour., v. 18, p. 63. In pegmatite, associated with cummingtonite,

actinolite, almandine, andesine feldspar, and chrysoberyl. D65. 

D28. Katzenbuckel, Odenwald, Germany, Freudenberg, W., 1920, Badischen Geol. _ 

Landesanstalt, Mitt, v. 8, p. 321. From mica-nepheline porphyry. uoo. 

D29. San Juan district, Colo., Larsen, E. S., Gonyer, F. A., and Irving, J., 1937, T^-

Am. Mineralogist, v. 22, p. 902. From quartz-latite monzonite. 

D30. Bound Lake, near Kingston, Ontario, Love, W. T., 1940, Boyal Soc. Canada j)6g

Trans., 3d ser., v. 34, sec. 4, p. 58. From gneiss. 

D31. Persberg, Sweden, Dana, E. S., 1892, The system of mineralogy, 6th ed., New D69.

York, John Wiley and Sons, p. 630, no. 15. 

D32. Cairnsmore, southern Scotland, Deer, W. A., 1937, Mineralog. Mag., v. 24,

p. 496. From tonalite.

Ross-shire, northwestern Scotland, Harker, R. I., 1954, Geol. Mag., v. 91,

p. 452. From "cordierite"-biotite-plagioclase hornfels. 

Hittero, Norway, Dana, E. S., 1892, The system of mineralogy, 6th ed., New

York, John Wiley and Sons, p. 630, no. 17. 

Cowall, Argyllshire, Scotland, Phillips, F. C., 1930, Mineralog. Mag., v. 22,

p. 254.

Ketila, Impilahti, Finland, Mehmel, M., 1937, Chemie der Erde, v. 11, p. 307. 

Kukisvumchorr, Kola Peninsula, U.S.S.R., Ivanov, B. V., 1937, Akad. Nauk

SSSR Lomonosovskii Institut Geokhimii, Kristallografli i Mineralogii

Trudy, no. 10, p. 38.

Baltimore, Md., Clarke, F. W., 1903, U.S. Geol. Survey Bull. 220, p. 77, E. 

Claro, Val del Molino, Tessin, Italy, Jakob, J., 1931, Zeitschr. Kristallographie,

v. 79, p. 373, no. 59. 

Val Peder, Italy, Tomba, A.M., 1952, Accad. Naz. Lincei Atti., rend., Cl.

Sci. fls. mat. nat, ser 8, v. 13, p. 80. Inclusions in staurolite in mica schist. 

Claro, Val del Molino, Tessin, Italy, Jakob, J., 1931, Zeitschr. Kristallographie,

v. 79, p. 373, no. 60. 

Skiddaw, Cumberland, England, Hitchen, C. S., 1934, Geol. Soc., London,

Quart. Jour. v. 90, p. 173. From granite. 

Tomanowa Valley, Tatra Mountains, Poland, Zastawniak, F., 1951, Rocznik

Polsk. Towarz. Geol., v. 20 (1950), p. 128. In gneiss. 

Port Henry, New York, Clarke, F. W., 1903, U.S. Geol. Survey Bull. 220, p.

77, D. 

Kosista, East Tatra Mountains, Czechoslovakia, Weyberg, Z., 1912, Neues

Jahrb. Mineralogie, Geologic u. Palaontologie, pt. 1, p. 399. In granite. 

Lake Ilmen, Miask, U.S.S.B., Dana, E.S., 1892, The system of mineralogy,

6th ed., New York, John Wiley and Sons, p. 630, no. 12. 

Ballygihem, County Donegal, Ireland, Haughton, S., 1859, Geol. Soc. London

Quart. Jour., v. 15, p. 131. In granite. 

U.S.S.B. (exact locality not given), Berkhin, S. I., 1954, Akad. Nauk. SSSR

Doklady, v. 95, p. 146. 

Langesund Fiord, Norway, Dana, E. S., 1892, The system of mineralogy, 6th

ed., New York, John Wiley and Sons, p. 634, no. 7. 

Ishikawa, Fukushima, Pref. Japan, Yoshika, B., 1933, Imp. Acad. Tokyo Proc.,

v. 9, p. 321. In pegmatite dikes traversing granite. 

Ballyelin, County Carlow, Ireland, Haughton, S., 1859, Geol. Soc. London

Quart. Jour., v. 15, p. 130. In granite. 

Baveno, Italy, GalliteUi, P., 1936, Periodico Mineralogia, Roma, v. 7, p. 64.

In granite. 

Baveno, Italy, GalliteUi, P., 1936, Periodico Mineralogia Roma, v. 7, p. 64.

In granite. 

Itaka, Japan, Shibata, H., 1952, Tokyo Bunrika Daigaku Sci. Repts., v. 2,

sec. C., no. 12, p. 163, no. 16. 

Makabe Town, Ibarakil, Pref. Japan, Shibata, H., 1952, Tokyo Bunrika

Daigaku Sci. Repts., v. 2, sec. C., no. 12, p. 163, no. 16. From pegmatite. 

Merrow Ledge, Auburn, Maine, Clarke, F. W., 1903, U. S. Geol. Survey Bull.

220, p. 75, A. From quartz monzonite. 

Yamanoo, Tsukuba, Japan, Shibata, H., 1952, Tokyo Bunrika Daigaku Sci.

Bepts., v. 2, sec. C., no. 10, p. 143, no. 15. From pegmatite. 

Heiko-gun, Kogen-do, central Korea, Inoue, T, 1950, Geol. Soc. Japan Jour.,

v. 56, p. 76. 

Pikes Peak, Colo., Lewis, H. C., 1880, Philadelphia Acad. Nat. Sci., Proc.,

v. 32 p. 254. 

Brevick, Norway, Dana, E. S., 1892, The system of mineralogy, 6th ed., New

York, John Wiley and Sons, p. 634, no. 6.

Litchfleld, Maine, Clarke, F. W., 1903, U.S. Geol. Survey Bull. 220, p. 77, A. 

lisaka, Fukushima, Pref. Japan, Shibata, H., 1952, Tokyo Bunrika Daigaku

Sci. Bept., v. 2, sec. C., no. 10, p. 121. From granite pegmatite. 

Bockport, Mass., Cooke, J. P., Jr., 1867, Am. Jour. Sci. 2d ser., v. 43, p. 225. 

lisaka, Fukushima, Pref. Japan, Shibata, H., 1952, Tokyo Bunrika Daigaku

Sci. Bept., v. 2, sec. C., no. 10, p. 121. From granite pegmatite. 

Mangualde, Province Beira, Portugal, Mario de Jesus, A., 1933, Portugal,

Servicas Geol., Comun., v. 19, p. 128. 

Kudaru Town, Nigeria, Bain, A. D. N., 1933, Geol. Soc. London, Quart. Jour.,

v. 90, p. 227. From granite. 

Volhynia, U.S.S.B., Buryanova, E. Z., 1940, Soc. Busse Mineralogie Mem.,

v. 69, p. 532. From pegmatite in ravakivilike granite. 

Rockport, Mass., Clarke, F. W., 1903, U. S. Geol. Survey Bull. 220, p. 77, C.

In feldspathic vein in granite. 

Wermland, Sweden, Dana, E. S., 1892, The system of mineralogy, 6th ed.,

New York, John Wiley and Sons, p. 634, no. 1.
INDEX

Page 

Analyses and formulas, excluded from study_______________. __ 15-16, 46

used In study_____.______________________..___, 15,41-42 

Anionie composition of micas___________________ ____._ 14 

Annite, Dana's... _..__._____.____________....__.. 29-30

Winehell's-______.____-__.____-___..___.. ... 24,30,31

Biotites, average formula. __________________________ 29 

average octahedral composition..._____________________ 29 

composition of, Holzner's hypotheses. __________________ 12 

Fe+2 dominant-._____________________________.. 27-29 

from diorite__________________________,__________ 35 

from gneiss and schist... _______________________. ____ 35-36 

from granite___________________________________ 33-34 

from granodiorite_______________________________ 35 

from monzonite _________________________....__. 34 

from nepheline syenite_____________________________ 35 

from pegmatite________________________________ 36 

from quartz diorite________________________________ 35 

from quartz monzonite...___________________________ 34 

Mg-dominant_ _______________________________ 23, 28-29, 34 

range of composition_ ________________ _ _____ _.. 28 

selected data on, from Holzner-__________________.__... 22

Bivalent octahedral cations._________________________ 16,17,22

Cape Ann, Mass...._________________________.______ 29

Cationie composition of micas.. ________________________ 14

Cations, bivalent octahedral-_____________________........ 16,17,22

trivalent, accommodation of___________________________ 12

in natural trioctahedral micas___ __________________ __ 13,22

Charge, calculation of...________.____________________ 14

Charges, accommodation of extra positive octahedral______- ______ 10-22

Clarke, F. W.._._......._.____...__._..__._...._....... 16

Cooke, J. P., Jr _____.--..--_______..._____._____..._... 29

Dana, E. 8. ______.____.____.......____....___... ___.. 31

Dana, J. D___________.______._.______.___._____ 29

Deer, W. A ............. .. . . 23

Dioetahedral micas_________________________________ 12

Eastonite, Simpson's formula.____________________________ 27

Winehell's formula. _______________________________ 12,27

Extra positive octahedral charges_ ___. ________ _____ _____ 16-22

Eyerman, John__ ______________________ __ ________ 26

Formulas, calculation of, from analyses of montmorillonite--_________ 15 

Foster, M. D.........................._................................. 12,13,15

Fe+2 biotites ....__.._.....__.. . .. .. . 25,27, 28 

Fe+3, tetrahedral _____________._________.._.___-___ 32 

Formula, notation________________________________ 14 

Formulas, calculation_______________________________ 13-15

Grigoriev, D. P...._________________....______________ 31,32

Half-cell formula_________________________________ 14 

Hallimond, A. F________________...._____.__.___... 31

Hamilton, S. Harbert _________.______________.___ 27 

Heinrieh, E. W............................. 37

Hendrieks, S. B____________._____...________..._____ 15 

Hey, Max H................................................................. 31,32

Holzner, Julius____________________________________ 12, 21,22 

Hutton, C. O-.__..___.___.___i___._.________...._ 23

Kunitz, W.

Lepidomelaiie, aluminian.__._..-_..

Lepidomelanes, representative formulas--

Lithium, effect of, in formula calculation.

in siderophyllites and lepidomelanes..

Magnesian biotites-- ------- _ ... - 

Magnesium, exchangeable in montniorillonites... 

interlayer positions--- . - -- 

Magnesium replacement system- .....__. _ __ _ _ ..-----.--.-..-..-. .

Marshall, C. E. ......... __ - - -- 

Mg-deflcient trioctahedral micas. _ . _____ .-___- _____ ._.- 

Mg-doniinant biotites-------------------- _____ .. . _ . ____ 23,

Micas, anionic composition- . 

composition and occurrence _ - . 

dialuniinuni magnesium. ______ ........ .. 

dioctahedral potassium __ _ .. __ .,... _ __ --------- ..... 

intermediate between siderophyllites and leipidonielanus.-. __ ...._.

Fe+2 dominant-. ___ - _______________ - _ 

layer charge relations, theoretical.. __ _ -- - - 

octahedral group.. 

Montmorillonite- ... _ - __ ____ . __ . _ .- 

Muscovite, ideal formula.. _ __ __ ________ __. _.- 

Page 

32 

31 

16 

30

. 25,27 

15 

15

. 24-26 

13 

36

25,28 

14

32-38 

26,27 

12 

32 

18

12-13 

12-15 

15 

12

Nockolds, S. E . 30,31

Pauling, Linus.-- ------ , 23

Phlogopites, average formula.----___.. -- 19

geologic occurrence- ____________..... 32,37

interlayer Mg. __ -..- 15 

range in composition..___ _ 26

selected data from Holzner________.__ 22

Quadrivalent octahedral cations.----- -- 17,18,19

Eichey, J. E__ 30,31

Eoss, C. S 15

Siderophyllite, ferrian__ ______...._ 32

Nockold's and Eichey's formula____... 30

WincheU's formula.-__. .._.___-----______ 12, 30

Siderophyllite-lepidonielane series---. 32

Siderophyllites, representative formulas--.----.-- 30

Tetrahedralgroup 12,13,14,28,34

Titanium, role of, in micas__________. 15 

tetrahedral--_..______----- 32

Trioctahedral micas, accommodation of trivalent cation 13-22 

analyses not used in study________.. 15-16, 46 

analyses used in study...__________ 15, 41-42 

chemical composition 32-37 

from pegmatite 36 

geologic occurrence-. . 32-37 

layer Charge relations 12-13 

Mg-deflcient-_-____... -___.... - 36

Mg-Fe« relation .. ..... . - 23-24

Trisilicic-tetrasilicic series____._..... 12

Trivalent octahedral cations 12,15,16,17,18,19,20,21, 22,34

Winchell, A. N 12,26,29,30,31

WineheU, H. . . ._.... ... - 12,30,31

31 Yoder, Hatton- 27

49

O