Correlation of 

Dioctahedral Potassium 

Micas on the Basis of 

Their Charge Relations

GEOLOGICAL SURVEY BULLETIN 1036-D

Correlation of 

Dioctahedral Potassium 

Micas on the Basis of 

Their Charge Relations

By MARGARET D. FOSTER

A CONTRIBUTION TO GEOCHEMISTRY

GEOLOGICAL SURVEY BULLETIN 1036-D

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1956
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

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

Washington 25, D. G. - Price 15 cents (paper cover)
CONTENTS

Abstract-.--__________________

Introduction. ____________________

Pyrophyllite-muscovite relationship. 

Trisilicic-tetrasilicic series_______.

Hydrous micas___________________

References cited._________________

ILLUSTRATIONS

Page

Figure 13. Tetrahedral-octahedral charge relationship in selected dioctahedral potassium micas_______________________________________ 62

TABLES

Page 

Table 1. Analyses of dioctahedral potassium micas ____________________ 65

2. Formulas for dioctahedral potassium micas ___________________ 66

3. Generalized formulas for selected dioctahedral potassium micas__ 67

in

A CONTRIBUTION TO GEOCHEMISTRY

CORRELATION OF DIOGTAHEDRAL POTASSIUM MICAS 

ON THE BASIS OF THEIR CHARGE RELATIONS

By MAEGAEET D. FOSTEE

ABSTRACT

In formulas derived from analyses of many varieties of dioctahedral potassium 

micas the number of Si ions in the tetrahedral group lies between three and four, 

that is, the micas represented by the formulas are all members of a trisilicictetrasilicic series like that suggested by Schaller to interpret the composition of 

high silica sericites

KAlAl(Si8Al)010 (OH) 2 KAlMg(Si4)010 (OH)2

Generalization of these formulas by substitution of the symbols R+3 and R+2 

for Al and Mg permits extension of this series to interpret the composition of 

dioctahedral potassium micas containing other cations than Al and Mg, such as 

ferric and ferrous iron, vanadium, chromium and manganese,

-.00 -1.00 -1.00 -.00 

XR+3R+3(Si3Al)010(OH)2

In this series constancy of charge and potassium content require that increase 

in silicon ion content be accompanied by an equivalent increase in divalent ion 

content and by a decrease in aluminum ion content equivalent to the increase 

in silicon and divalent ion content. In the trisilicic end-member the charge on 

the structural layers responsible for holding potassium is due entirely to the 

proxying of trivalent aluminum for one-fourth of the tetravalent silicon in the 

tetrahedral group; in the tetrasilieic end-member it is due to the proxying of 

bivalent ions, ion for ion, for one-half the trivalent ions in the octahedral group. 

From the trisilicic to the tetrasilieic end-members there is, therefore, a shift in the 

seat of the charge from the tetrahedral to the octahedral group. In intermediate 

members of the series the charge is due partly to replacement of aluminum for 

silicon in the tetrahedral group and partly to replacement of bivalent for trivalent ions in the octahedral group. It is this tetrahedral-octahedral charge 

relationship that is herein suggested as a basis for the classification and correlation of the dioctahedral potassium micas.

INTRODUCTION

The dioctahedral potassium micas vary greatly in composition, 

particularly in their relative content of silicon, aluminum, ferric and 

ferrous iron, and magnesium. Some also contain chromium, manganese, and vanadium. The composition of some of the high-silica 

sericites and their relation to muscovite was explained by Schaller

363642-56 57
58 CONTRIBUTIONS TO GEOCHEMISTRY

(1950) but the composition and relationships of other dioctahedral 

potassium, micas have not been fully interpreted.

The great difference in composition found among these micas is the 

result of isomorphous replacement. The position that a cation takes 

in the mica structure is determined by its size, not by its valence. 

For this reason the composition of the outer or tetrahedral layers is 

usually comparatively simple, as only silicon and aluminum normally 

are small enough to take tetrahedral coordination. On the other 

hand, a number of cations, such as Al, Fe+3, Cr+3, Mn+3 , V+3, Mg, 

Fe+2, Mn+2, and Li, have the proper size to take octahedral coordination. As several of these may occur in the same specimen, the composition of the octahedral layer may be quite complex.

Although the size of a cation determines the position it can take in 

the structure, its valence in this position is of importance with respect 

to the charge in the structural layers. Aluminum proxying for silicon 

in tetrahedral layers gives rise to a charge equivalent to the amount 

of aluminum in the tetrahedral layers. Octahedral layers that contain 

some divalent ions in the two positions occupied in dioctahedral micas 

have a charge equivalent to the divalent ions present. The seeming 

complexity in the composition of dioctahedral potassium micas can, 

therefore, be simplified if the composition is considered in terms of the 

position and valence of the constituent ions. It is the purpose of this 

paper to show that these micas can be classified and correlated on the 

basis of the relation between the charges on their tetrahedral and 

octahedral layers.

PYROPHYLLITE-MUSCOVITE RELATIONSHIP

The general structural scheme of the micas and related minerals 

was formulated by Pauling (1930), who showed that these minerals 

consist of composite layers made up of a sequence of atomic sheets or 

layers along the pseudohexagonal axis. For pyrophyllite and muscovite the succession of atomic layers is as follows:

Pyrophyllite Muscovite 

Tetrahedral J6 O~- 6 O~

layer. [4 Si+* 3 Si+4+Al+3

Octahedral 

layer.

4O--+2(OH,F)-

4A1+3

4O--+2(OH,F)-

4 O-+2(OH,F)'

4A1+3

4 O-+2(OH,F)'

Tetrahedral J4 Si+4 

layer. [6 O~ 

6O--

4S1+4

3 6OCharge (-2)

2K+

60-

3 Si+4 +Al+8

Charge (+2)
CORRELATION OF DIOCTAHEDRAL POTASSIUM MICAS 59

In pyrophyllite the succession of atomic layers forms a neutral 

structure; the number of negative charges equals the number of 

positive charges. But in muscovite the composite layer has a negative 

charge due to the replacement of one-fourth of the tetravalent silicon 

ions in the tetrahedral layers by trivalent aluminum. To achieve a 

neutral structure, positive ions (such as K) must be introduced. 

There is room for such ions in. the pockets formed by the six oxygen 

ions on the top of the composite layer below and on the bottom of the 

composite layer above.

As the negative charge, which is neutralized by potassium, is due to 

the replacement of one-fourth of the silicon ions by alummum hi the 

tetrahedral layers, the seat of the charge in muscovite may be said to 

be in the tetrahedral layers.

The chemical composition of pyrophyllite is represented by the 

formula, Al2Si4Oio(OH) 2 and that of muscovite by the formula, 

K-Al2 (Si3Al)010 (OH)2, although these formulas do not represent the 

composition of the unit cells of these minerals.

TRISILICIC-TETRASILICIC SERIES

Winchell (1927, p. 267-268) suggested the following relationship 

between muscovite and phengite:

Muscovite K-AlAl(Si3 . 0Al1 .o)O1o(OH) 2 

Phengite K- (M-sFeJJ) (Si3 . 5Al0 . 5)O10 (OH) 2

and stated that "it is understood that the iron in the phengite formula 

is ferrous and that magnesium may proxy for it in any amount, hi fact, 

magnesium is more abundant than ferrous iron in nearly all modern 

analyses of phengitic micas." Thus the phengite formula could as 

weU be written, K(M.8Mg0 . 8) (Si3.8AV8)Oio(OH)2 . Muscovite and phengite have the same potassium content, indicating that replacements of higher valence cations by lower valence 

cations have induced the same amount of charge in the atomic layers. 

As phengite has only half as much aluminum proxying for silicon in 

the tetrahedral layers as muscovite, it has only half as much tetrahedral charge. The other half of the charge is provided by bivalent 

iron and magnesium proxying for aluminum in the octahedral layer. 

Thus, whereas in muscovite the charge is entirely tetrahedral, in 

phengite the charge is half tetrahedral and half octahedral.

The formula proposed by Winchell for phengite is a theoretical 

formula suggested to account for the high silica content and the presence of ferrous iron and magnesium in certain micas. In formulas 

derived from actual analyses the octahedral charge may be less than 

that required by the formula. For example, hi the formula derived
60 CONTRIBUTIONS TO GEOCHEMISTRY

from a phengite analysis reported by Jakob (1925), the octahedral 

charge amounts only to approximately one-third of the total charge: 

Xo.96 (Al1 .5oFe0+135Fe0t027Mg0 .28)(Si3 .4oAlo.6o)O1 o(OH) 2 . The formula also 

indicates that the specimen contains ferric as well as ferrous iron, 

but the dominant bivalent ion is magnesium.

From muscovite to phengite the trend is toward greater occupancy 

of the tetrahedral positions by silicon. If no compensatory change 

took place in the octahedral layers the end result of such a trend 

would be pyrophyllite. In order to maintain the charge and the 

potassium content constant, filling of the tetrahedral positions by 

silicon and the decrease of the charge in the tetrahedral layers must 

be accompanied by increasing proxying, ion-for-ion, of octahedral 

aluminum by bivalent ions with a consequently increasing charge in 

the octahedral layer. The limit of such a trend is complete occupancy 

of the tetrahedral positions by silicon and the filling of one of the two 

occupied octahedral positions by bivalent ions with the final location 

of the whole charge in the octahedral layer.

(Schaller (1950, p. 408) suggested that the composition of the highsilica sericites can be explained if they are considered members of 

such a trisilicic-tetrasilicic series,

K-AlAl(Si3Al)010 (OH) 2 K.AlMg(Si4)010 (OH) 2 .

In members of this series maintenance of a constant charge requires 

the increase in silicon content be accompanied by an equivalent 

increase in magnesium and by a decrease in aluminum content equivalent to the increase in silicon plus magnesium. In the end-members 

the charge is due, on the one hand, entirely to replacement in the tetrahedral layers, and on the other hand, entirely to replacement in the 

octahedral layer. In intermediate members of the series othe charge 

is partly tetrahedral and partly octahedral.

In interpreting the composition of high-silica sericites Schaller 

calculated all Fe2O3 , Cr2O3 , and Mn2O3 reported in the analyses to 

equivalent A12O3 , and all FeO and MnO as equivalent MgO. Hence 

A12O3 and MgO were used by Schaller as collective terms but as other 

trivalent and bivalent constituents were present in only minor 

amounts, A12O3 and MgO were the essential octahedral constituents in 

the specimens considered by him. This is also true of many other 

dioctahedral potassium micas but not of all, as is evident from the 

analyses and the derived formulas presented in tables 1 and 2. In 

some, constituents other than A12O3 and MgO are the important 

octahedral constituents as, for example, ferrous and ferric iron in 

glauconite and celadonite, and vanadium in roscoelite.

The formulas in table 2 indicate that all the specimens they represent 

are members of a trisilicic-tetrasilicic series. In order to correlate
CORRELATION OF DIOCTAHEDRAL POTASSIUM MICAS 61

them, however, more generalized formulas than those suggested by 

Schaller are desirable, such as

-.00 -1.00 -1.00 -.00 X(R+3R+3) (SUAl)010 (OH) a X(R+3R+2) (Si4 .o)010 (OH) 2

in which the general symbols R+3 and R+2 are used to represent trivalent and bivalent ions in the octahedral group. X is used to represent the large cation balancing the total charge for the reason that in 

most of these micas, as in others, K is not entirely equivalent to the 

total charge; small amounts of Na or Ca are generally present. No 

generalization is needed for the tetrahedral group, made up as it is of 

varying proportions of Si and Al. From such generalized formulas 

any dioctahedral potassium mica can be placed in the series and thus 

can be correlated with other micas in this family, regardless of the 

constituent ions making up the octahedral group. Generalized forms 

of certain formulas selected from table 2 are listed in table 3. In 

these formulas the charge due to lower valence cations proxying for 

higher in the tetrahedral and octahedral groups are noted above these 

groups. The formulas in table 3 represent a complete and continuous 

series from the trisilicic to tetrasilicic end members. In the pink 

muscovite (formula 1) the entire charge is due to replacement of onefourth of the tetravalent Si ions by trivalent aluminum ions in the 

tetrahedral group. In succeeding formulas there is a gradual shift in 

the seat of the charge, due to less and less replacement of Si by Al in the 

tetrahedral group and more and more replacement of trivalent by 

bivalent ions in the octahedral group, until in celadonite (formula 17) 

the tetrahedral group is entirely occupied by Si ions and bivalent ions 

occupy half of the two occupied octahedral positions.

It must be emphasized that in this series, proxying of octahedral 

trivalent ions by bivalent ions is ion-for-ion with no tendency toward 

filling the third unoccupied octahedral position. In the formulas 

shown in table 2 the maximum number of occupied octahedral positions is 2.03. In the tetrasilicic end-member, celadonite (formula 17), 

2.01 octahedral positions are occupied.

The tetrahedral-octahedral charge relationships of the general 

formulas in table 3 are shown graphically in figure 13. The four 

columns of these histograms represent, from left to right, total charge, 

tetrahedral charge, octahedral charge, and potassium, in terms of 

equivalents of charge. The histograms show clearly that all proportions of tetrahedral charge to octahedral charge may be found among 

these micas. Progress from trisilicic to tetrasilicic is not stepwise but 

continuous, although formulas representing stages in the shift are 

shown above the histograms at appropriate places.

Alurgite was referred to by Stevens (1946, p. 106) as the tetrasilicic 

end-member of the K.AlAl(Si3Al)O10 (OH) 2 K.AlMg(Si4)0,0 (OH) 2
EXPLANATION

Total charge Tetrahedral Octahedral Potassium 

charge charge

Numbers refer to generalized formulas in table 3

in + 10

11 12 

Muscovite

Embudo,

New Mexico

vv v - 555 -^

S^lss

Muscovite 

Sultan Basin, 

Washington

Fuchsite

W. Otago,

New Zealand

Roscoelite 

Placerville, 

California

Manposite

Mariposa Co.,

California

Metasencite 

Wildschapbach 

Thai, Germany

FIGURE 13. Tetrahedral-octahedral charge relationship in selected dioct
CORRELATION OF DIOCTAHEDRAL POTASSIUM MICAS 63

series. However, with aluminum occupying 0.4 positions in the 

tetrahedral group it is very little beyond the middle of the series. On 

the other hand, it is the most tetrasilicic dioctahedral potassium mica 

known in which aluminum is the predominant trivalent octahedral ion. 

Leucophyllite, referred to by Schaller (1950, p. 407) as the tetrasilicic 

end-member of the above series, cannot be considered from available 

analyses a potassium mica as it has only 3.39 percent K2O, only onethird of the required amount.

The tetrasilicic end of the general series is represented, therefore, 

only by the high-iron dioctahedral potassium micas glauconite and 

celadonite. These materials are commonly considered hydrous micas, 

rather than true micas. Hendricks and Ross (1941) did not clearly 

differentiate between micas and hydrous micas among the glauconites 

and celadonites, as they were considering the origin and chemical 

composition of these materials. However, among the analyses cited 

by them are some whose potassium-water relation is that of true 

micas, not hydrous micas. One of these, a celadonite (table 2, 

formula 17) is the tetrasilicic end-member of the general series. It 

has no tetrahedral aluminum; the tetrahedral group is completely 

occupied by silicon. This specimen has been examined by Dr. H. S. 

Yoder of the Geophysical Laboratory (oral communication) and found 

to be a 1M mica.

The high-iron dioctahedral micas dominate the tetrasilicic end of 

the general series, but they do not (insofar as is known) extend 

beyond the middle of the series toward the trisilicic end. The failureof the aluminum members to extend far beyond the middle of the 

series toward the tetrasilicic end has already been noted. Thus, 

neither aluminum nor iron as the dominant trivalent octahedral ion 

forms a complete trisilicic-tetrasilicic series.

The trisilicic-tetrasilicic general series,

X(R+3R+3) (Si3 .0Al)010 (OH)2 X(R+3R+2) (Si4 .0)O10 (OH) 2 ,

affords a system for classifying and correlating the dioctahedral potassium micas on the basis of their tetrahedral-octahedral charge relationship. By this sytem the relationship between commonly knowo 

micas of this family, muscovite and phengite, and those having 

unusual or complex composition (table 2) the roscoelites (formulas 

4 and 8), fuchsite chromium muscovite (formula 5), mariposite 

(formula 11), or alurgite (formula 14) is clarified and the reversecharge relationships between muscovites and phengites and glauconites and celadonites is apparent. Thus the great variety of composition found in this family of minerals, the result of isomorphous 

replacements, is simplified and reduced to the single factor of the 

charge induced on the structural layers by the replacement.
 64 CONTRIBUTIONS TO GEOCHEMISTRY

HYDROUS MICAS

The same method can be used for the classification and correlation 

of hydrous micas. However, hydrous micas commonly occur in 

nature very ultimately mixed or even interlayered with other minerals 

and are very difficult, often impossible, to purify. Other materials 

called hydrous micas may in reality be mixtures of others minerals 

as of muscovite and montmorillonite. Such a mixture could be very 

similar in composition to some materials that have been called illite. 

It would yield a rational formula and have a comparable tetrahedraloctahedral charge relationship. The effect of montmorillonite in a 

mixture with a mica or hydrous mica is to cause a decrease in the 

total charge, the tetrahedral charge, and the potassium content, as 

montmorillonite is characterized by low total charge (less than half 

that of muscovite), little or no tetrahedral charge, and very little 

potassium. It also tends to increase the octahedral charge, as most 

of the charge in montmorillonite is octahedral. The greater the proportion of montmorillonite, the greater these effects would be. Other 

minerals, such as chlorite, kaolinite, and montmorillonite, often 

found intermixed or interlayered with floe-grained micas or hydrous 

micas, may or may not yield a mixture the composition of which can 

be expressed by rational formulas. The fact that a rational formula 

can be derived from an analysis does not guarantee that there is only 

one mineral present. This, however, does not invalidate the use of 

this method for classifying and correlating materials that are called 

hydrous micas.

REFERENCES CITED

Hendricks, S. B., and Ross, C. S., 1941, Chemical composition and genesis of

glauconite and celadonite: Am. Mineralogist, p. 26, 683-708. 

Jakob, J., 1925, Beitrage zur chemischen Konstitution der Glimmer, II Mitteilung Die Muskovite der Pegmatite, I Teil.: Zeitschr. Kristallographie

Band 62, p. 443-453. 

Pauling, L., 1930, The structure of micas and related minerals: Natl. Acad. Sci.

Proc. v. 16, p. 123-129. 

Schaller, W. T., 1950, An interpretation of the composition of high silica sericites:

Mineralog. Mag., v. 29,. p. 407-415. 

Stevens, R. E., 1946, A system for calculating analyses of micas and related

minerals to end members: U. S. Geol. Survey Bull. 950, p. 101-119. 

Winchell, A. N., 1927, Further studies in the mica group: Am. Mineralogist, v.

12, p. 267-279.
CORRELATION OF DIOCTAHEDRAL POTASSIUM MICAS

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66 CONTRIBUTIONS TO GEOCHEMISTRY

TABLE 2. Formulas for dioctahedral potassium micas.

+0.01 -1.02

1. Muscovite (pink)... Xi.oi(Ali.93Fe^iMn^iLi.o5)(Si2.98Ali.o2)Oio(OH)2

K.85

-.10 -.89

2. Muscovite.._-_._-_ X.99(Ali.84Fe^lFe^lMg.09)(Si3.iiAl.89)Oio(OH)2

K.90

.3. Sericite..__________ X.M(Ali. 93Fe:oiMg.o6)(Si3 .i2Al.88)Oio(OH)2

K.85

-.11 -.S2

4. Roscoelite.._.... X.93(Al. 73Vil:iaFe^|Mg.o2)(Si3 .i8Al.82)Oio(OH)2

K.75

5. Fuchsite (Chromian

muscovite)....... Xi o2 (AU i

K.91

6. Gilbertite .______ Xi.oi(Ali.76Fe+i8Mg.oa)(Si3 . 22Al. 78 )Oio(OH)2

K.85

- .29 - .72

7. Sericite.__________ Xi. 0 i(Ali.7oFe^lMg^l)(Si8 .28A1.72)Oio(OH)2

K.93

- .29 - .69

8. Roscoelite.________ X.98 (A1.3iVi+«Fe+ioMg.i8)(Si3 .3iAl.69)Oio(OH)2

K.97

-.36 -.71

9. Sericite (phengite).. Xi.o7(Ali.63Fe^?Fe^Mg.24)(Si3 .29A1.7i)Oi0 (OH)2

K.82

10. Phengite.......... X.96 (Ali. 50Fe+i!Fe^Mg. 28)(Si3.4oAl. 6o)Oio(OH)2

K.83

-.36 -.59

11. Mariposite (green).. X 9s(Ali s

K.85

12. Metasericite._._... X.w(Ali.4_Fe:__Mg.M)(Si_.«7A1.48)O_o(OH)2

K.82

-.50 -.38

13. Glauconite. _____._ X.88(Al. 29Feif; 22Fe[ifMg.35)(Si3 .62Al. 38)Oio(OH)2

K.84

-.60 -.41

14. Alurgite ._ . Xi.oi(Ali.28Fe^6Mn^!Fe^Mg. 6i)(Si3 .59A1.4i)Oio(OH)2

K.98

-.70 -.33

15. Glauconite..____ Xi.03 (Al.i8Fei;i2Fe^7Mg.32)(Si3.67A1.33)Oio(OH)2

K.88

-.85 -.12

16. Celadonite... .__ . X.97(A1.78Fe^7FeioMg.65)(Si3 .88Al.i2)Oio(OH)2

K.84

-.98 -.00

17. Celadonite........_ K. 98(Al.o7Fe+9iFei!Mg.77)(Si4.oo)Oio(OH) 2

K.92
CORRELATION OF DIOCTAHEDRAL POTASSIUM MICAS 67

TABLE 3. Generalized formulas for selected dioctahedral potassium micas

+ .02 -1.02 

1. Muscovite (pink)_____.___________ Xi.oo(R*97Rlot)(S

-.13 -.89

2. Muscovite_______________ X.99(R*fsR:is)(Sis.iiAl.89)Oio(OH)s

-.17 -.85

5. Fuchsite (Chromian muscovite)_._ Xi.o2(Ri".7sRi24)(Si3.i5Al. 85)Oio(OH) 2

- .29 - .69

8. Roscoelite__________________ XosfRi^raR

11. Mariposite...._______________ X.M(Ri:^:M)(Sia._iAl.»)0_o(OH)2

- .49 - .43

12. Metaserieite.._____________ X.g_(R£ttR:«)(3is.57A1.4

- .60 - .41

14. Alurgite. ...-__.___________ Xi.oiCR+tsR^HSis.sgA

-.33

15. Glaueonite_______________ Xi.03 (Ri;ioRi69)(Si3.67A1.33)Oio(OH) 2

-.85 -.12

16. Celadonite.._____________________ X. 97 (Ri:i6R^l)(Si3.88Al.i2)Oio(OH) 2

 .98

17. Celadonite._________________ X. 98 Ri+o\)Ri