MINERALOGICAL JOURNAL, VOL. 6, No. 4, pp. 203-215, MARCH, 1971

STRUCTURAL INVESTIGATION OF POLYMORPHIC TRANSITION BETWEEN 2M2-, 1M- LEPIDOLITE AND 2M1 MUSCOVITE

HIROSHI TAKEDA*, N. HAGA and R. SADANAGA

Mineralogical Institute, Faculty of Science University of Tokyo, Hongo, Tokyo, Japan

ABSTRACT

The crystal structure and cation distribution of a 2M2 lepidolite coexist ing with the 1M form, from Rozna , Moravia, Czechoslovakia has been deter mined by least squares refinement employing three-dimensional intensity data

collected by the precession method with MoKa radiation. The tetrahedral

rotation angle, a, is 5•‹, thus making the inner oxygen coordination around

potassium into trigonal prism. Determined site occupancies and bond lengths show that most of the aluminum is concentrated into position 8f(M2 site), as

was observed in polylithionite by Takeda and Burnham.

Structural changes that accompany polymorphic transitions from 1M or

2M2 lepidolite to 2M1•@ muscovite, are discussed in terms of the bond lengths,

tetrahedral rotation angle, tetrahedral collapses, OH-F substitution, K-F

configuration, and Madelung sums.

Introduction

The relation of the polymorphism of natural lepidolites to composi tion has received much attention in the literature. Levinson (1953),

Smith and Yoder (1956), Takeda and Donnay (1965) and Ross, Takeda and Wones (1966) carried out X-ray studies and reported 1M, 2M2 and

3T polymorphs. Foster (1960) and Radoslovich (1963) have considered the chemistry of Li substitution for Al in muscovite. Their results showed that a polymorphic transition exists at about 3.3% Li2O be-

* Present address: Geochemistry Branch, NASA-Manned Spacecraft Center, Houston, Texas 77058, U.S.A. 204 Structural Investigation of Polymorphic Transition tween the 2M1 form and the 1M and 2M2 forms in a continuous chem ical series between di-octahedral muscovite and tri-octahedral lepidolite. To understand this transition on the basis of crystal structures of these polymorphs, the crystal structure of a 2M2 lepidolite with a composi tion close to that of the apparent miscibility gap has been determined. The results were interpreted in the light of a recent structural work on polylithionite (Takeda & Burnham, 1969), and of a phase equilibria of the ternary system polylithionite-trilithionite-muscovite (Munoz, 1968). Since the lepidolite group is "transitional" between dioctahedral and trioctahedral micas, our results also present a data on what is the critical difference between the two mica groups.

Experimental

A single crystal of a 2M2 lepidolite suitable for structural studies has been obtained from the specimen from Rozna, Moravia, Czechoslovakia (U. S. Geol. Survey Record No. D-789) during a study of the polytypes of the lepidolites by the precession method (Ross, Takeda and Wones, 1966). The crystal data for this polytype obtained from precession photographs calibrated for shrinkage are listed in Table 1. Also found in this sample was a 1M mica whose cell dimensions are also given in the same table. It is to be noted that the beta angles of these micas are noticeably different from those of other trioctahedral micas (Takeda & Burnham, 1969).

Table 1. Crystal data for lepidolites from Rozna, Moravia, Czechoslovakia (Takeda & Bumham, 1969). H. TAKEDA, N. HAGA and R. SADANAGA 205

The chemical formula for a quarter cell (Z=4) calculated from the chemical analysis cited in Foster's paper (1960, No. 27, R. E . Stevens

written communication, 1938, U. S. Geol. Survey Lab. Record No . D-789)

is given in Table 2.

The size of the crystal used for the intensity data collection is 0 .35•~

0.30•~0.07mm. The crystal was mounted perpendicular to the cleavage

plane (001), and adjusted so that the c* axis is parallel to the dial

axis of the precession camera. Series of multi-exposure photographs

(36, 12, 3 hours) of the hOl, Okl, hhl, 3hhl nets (0th, 1st and 2nd

level) were taken by rotating dial and employing Zr-filtered MoKa

radiation. Computer program, ACACA, written by Dr. C. T. Prewitt.

Table 2. Chemical data for lepidolites given in Table 1, derived from the chemical analysis by Stevens (Foster, 1960)

E. I. du pont de Nemours and Co., Wilmington, Delaware, and modified by H. Takeda was used to reduce the recorded data, including correc tions for Lorentz, polarization and absorption factors, with a linear absorption coefficient of 17.26. The averaged Fo values and their standard deviations were obtained by a program used for the previous mica studies (Takeda & Donnay, 1966). 206 Structural Investigation of Polymorphic Transition

Structures derivation and refinement

The trial structure model of the 2M2 form was derived from the atomic coordinates of a synthetic 1M polylithionite refined by Takeda and Burnham (1969) employing a program TWMC written for the

HITAC 5020E computer (Takeda, 1968). The refinements at the prelimi nary stages were carried out only with the zero level data of the four different orientations (Haga, 1967). In the final run, 471 observable

3-dimensional data were included, and the full-matrix least-squares refinements were carried out with the aid of the program ORFLS

(Busing, Martin & Levy, 1962), adapted by Y. Iitaka for the HITAC

5020E, and modified by H. Takeda to do site occupancy refinements of

the two octahedral position. A method used is similar to that used in

the previous mica structure refinement (Takeda & Donnay, 1966). The

total amount of aluminum and lithium and the neutrality of charge

were kept constant after each cycle. The refinements converged to occu

pancies corresponding to Li0.35 Al0.10• 0.55 for position 4c(Ml) and Al0.65

Li0.35 for position 8f(M2), showing that most of the aluminum is concen

trated in position 8f. A part of the vacancy could also be introduced

to position 8f. These site assignments are in agreement with the sizes

of the octahedra and with the occupancies of polylithionite (Takeda &

Burnham, 1969). The atomic coordinates obtained by the refinement

with isotropic temperature factors are given in Table 3. The final

residual is 0.072 for 471 observed reflections. Bond lengths and angles

and their standard deviations computed by the ORFFE program are

given in Table 4.

Discussion

Trigonal prism coordination of oxygens around potassium.

This 2M2 lepidolite structure reveals structural features similar to

those of polylithionite previously determined by Takeda and Burnham

(1969). The surface oxygen ring of this mica is not as strictly hex- H. TAKEDA, N. HAGA and R. SADANAGA 207

Table 3. Atomic coordinates and isotropic temperature factors (in A2) of 2M2 lepidolite.

* The symbol Tj , Mi or Oij stands for the ith atom in the 1M asymmetric unit derived from the jth layer. The standard deviations given between parentheses are expressed in units of the last digit stated.

agonal as that of polylithionite. The deformation of the ring from hex

agonal to ditrigonal expressed by the "tetrahedral rotation angle" a is 5.3•‹ which compares with 3•‹ for polylithionite. Comparing

the ring to that of other micas, this ring is nearly hexagonal (Fig. 1).

Because of the 60•‹ rotation between adjacent layers, the K-O coordina

tion polyhedron is a trigonal prism for the inner oxygens. The bond

lengths are compared with those of polylithionite and muscovite in

Fig. 2. The structure determination of polylithionite (Takeda & Burham,

1969) resolves the apparent contradiction between the predicted high a

angle and Radoslovich's hypothesis that lepidolites should have nearly hexagonal rings that allow polymorphs with 60•‹ stacking rotation angles.

Recently, Franzini and Sartori (1969) again predicted a structure

with the mean tetrahedral rotation of about 11•‹for their 2M2 poly

morph on the basis of their one-dimensional Fourier projection on c*, and

proposed a structure of the 2M2 polymorph with octahedral coordina- 208 Structural Investigation of Polymorphic Transition tion around the potassium ion. This hypothesis now shown to be not warranted by our structure determination of the 2M2 form. The struc tural features of lepidolite discovered by Takeda and Burnham (1969), may not be elucidated by means of a one-dimensional Fourier projec tionsTable 4. Bond-lengths for 2M2 lepidolite.

* apical oxygens The standard deviations given between parentheses are expressed in units of the last digit stated. H. TAKEDA, N, HAGA and R. SADANAGA 209

Fig. 1. Stereoscopic drawings of the 2M2 lepidolite structure showing a lower half of the cell. Views along c* by Carroll K . Johnson's program, ORTEP, (Oak Ridge National Laboratory) . The rectangle represents the base of the cell.

In the 2M2 lepidolite structures, the coordination around potassium is actually a twelve-fold or nearly hexagonal prism if we consider the second nearest oxygens. The trigonal prism coordination of this sort may not be an unstable configuration as long as the charge of the surface oxygens is kept lower, that is less aluminum ions substitute for silicon, and enough fluorine ions substitute for hydroxyl ions, such con ditions being generally satisfied by the lepidolites.

Structural changes and polymorphic transitions

According to Foster (1960) and Munoz (1968) components of lepidolite can be plotted in the ternary system, polylithionite •kKLi2A1Si4O10F2•l

(Pl)-trilithionite •kKLi3/2 A13/2 Si3AlO10 (F, OH)2•l (Tl)-muscovite •kKA12 210 Structural Investigation of Polymorphic Transition

Fig. 2. Bond lengths and tetrahedral collapses (Ģ,)

of polylithionite (P1), 2M2 lepidolite (Lp) and muscovite

(Ms) plotted against octahedral vacancy. -stands for the basal oxygen and •É for the apical one.

• 1 Si3 A1O10 (OH)2•l(Ms). In the lepidolite series the following substitu

tion of ions may take place:

(1) Replacement of one octahedral Al by 2-3 Li, with a decrease of

octahedral vacancy.

(2) Substitution of Si for Al in the tetrahedral layer.

(3) Replacement of OH by F.

(4) Minor concentration of Rb in the interlayer.

In this paper, for convenience, we choose octahedral vacancy as an

index of substitution. There is an apparent discontinuity of polymorphs H. TAKEDA, N. HAGA and R. SADANAGA 211

at about 0.55 vacancy per 12 (O, OH, F). The octahedral vacancy of

our 2M2 lepidolite is 0.40, being very close to the above discontinuity

(A minor compositional change between the coexisting 1M and 2M2

forms is still open to question). The polymorph beyond this point up

to Ms is the 2M1 polymorph, so called lithian muscovite.

The bond lengths of our 2M2 lepidolite (intermediate composition)

together with those of the two end members, polylithionite (Takeda &

Burnham, 1969) and muscovite (Burnham & Radoslovich, 1964; Birle &

Tettenhorst, 1968) are plotted against the octahedral vacancy in Fig . 2.

In this figure, the tetrahedral collapse 4 is also given which is express

ed as:

ƒ¢= •bZ0(1)-z0(2)•b•EcsinƒÀ,

where O (1) and O (2) are the two basal oxygens. In the figure the straight

lines are drawn wherever three points lie on the same line. For the others, the lines are drawn from P1 up to the discontinuity.

From these data, the structural changes from polylithionite to mus covite may be summarized as follows:

(1) The very short non-bridging Si-O bond in polylithionite pointed out

by Takeda and Burnham (1969) becomes longer with increasing Al

contents. Tetrahedron T(2) seems to obey this rule and the aver

aged T-O distance is longer than that of Si-O, 1.620A. However,

the T(1) tetrahedron has longer non-bridging T-O distance, and yet

gives shorter averaged distance. This difference makes the conclu

sion on tetrahedral Si-Al ordering uncertain, especially if we take

into account the standard deviations of the bond lengths.

(2) The basal oxygen ring becomes more markedly ditrigonal and cor

rugated with increasing aluminum contents. These changes, as ex

pressed by the K-O distance and d are more pronounced near the

muscovite end member.

(3) This trend is also encountered in the Al-rich octahedral site. This

site corresponds to the Al site in the 2M1 muscovite structure, and

although it accommodates more Al than that in polylithionite, it 212 Structural Investigation of Polymorphic Transition

still contains considerable amounts of Li. In the 2M1 lithian mus covite, the structural features are the same as those of muscovite (Radoslovich, 1963; Burnham & Radoslovich, 1964). (4) The above octahedral substitutions are coupled to substitution of fluorine ion for hydroxyl ion.

One of the critical differences of the deformation of this octahedron is the following: In the lepidolite series, the shared edges between the Al-rich octahedral cations and the Li-rich ones (2.802A) are shorter than the unshared edges of the Al-rich octahedron (2.891A). In muscovite the shared edges between the aluminum ion and the vacant site (2.96A) are much longer than the unshared edges of the Al-octahedron (2.77A). This fact implies that, in lepidolites, there may be considerable amounts of cations present in both sites to produce appreciable cation-cation repulsion, a feature not found in dioctahedral 2M1 lithian muscovite. The presence of cations in both sites also affects the orientation of the OH bond. Hydrogen tends to be repelled from the three neighbouring octahedral cations, making the OH bond direction nearly parallel to c*. This place the hydrogen ion too close to potassium. If the OH ion is replaced by fluorine, this unfavorable configuration could be avoided. Especially in polylithionite end member, where the ring is hexagonal, the inner oxygen octahedron around potassium is flattened, and the K-F distance is very short (Takeda & Burnham, 1969). This would tend to make hydroxyl polylithionite unstable. This hypothesis is supported by the phase equilibria study of Munoz (1968) in which a hydroxyl-polylithionite end member could not be synthesized. In order to understand ionic substitution in this mica series, some characteristics of the 2M1 muscovite should be reviewed. In the dioctahedral end member, muscovite, hydrogen in the hydroxyl ion is located away from the aluminum ions and contributes to the deforma tion of the tetrahedral layer. This configuration (Takeuchi , 1965) has been considered to be responsible for stacking the 2M1 sequence . Note H. TAKEDA, N. HAGA and R. SADANAGA 213

that this proton is close to the basal oxygens which have a residual

negative charge, giving rise to favorable interaction between them .

If this OH is replaced by F, considerable repulsion between F and the

negatively charged basal oxygens will result so that such a configura

tion would be unfavoured. In polylithionite, since the surface oxygen

charge is neutral, such an unstable situation will not be encountered .

In the intermediate structure, there may be mixtures of the above

two extreme configurations. We focus our attention to three octahedra

which share one oxygen with a proton in the center. In muscovite,

two of them are always Al. Now if we replace one Al by Li and fill

the vacant site with Li, we have a local configuration (Al-Li-Li-F)

similar to that of the 1M trioctahedral end member, thus OH must be

replaced by F as explained above, and the repulsion of the three cations

will cause a deformation similar to that observed in the lepidolite

series, where all the shared edges are shorter than the unshared edges.

To sum up, when the Al-Li-Li-F assemblage becomes predominant over

the Al-Al-• -OH assemblage, the 2M1 structure would be no longer

favoured over the 1M or 2M2 forms.

These theoretical considerations are in agreement with the high

correlation between Li and F, observed in natural lepidolites (Foster,

1960) and in the phase equilibria study of Munoz (1968) and polymorphic

abundances (Levinson, 1953).

It is interesting to note that the abundances of the four polymor

phic forms might be related to the linearity of F-K-F configuration as shown in Fig. 3. The 1M and 2M2 forms are the most abundant in lepidolites. These offsets of fluorines from potassium positions by look ing down along c* are caused by the shortening of the F...F shared edges (Takeda & Donnay, 1966).

Other structural differences between these polymorphs lies in the relative positions of octahedral cations of two adjacent layers. In the

2M2 and 2O forms, those cations are superimposed when viewed along the c* axis, whereas in the 1M and 2M1 forms their positions are offset 214 Structural Investigation of Polymorphic Transition

Fig. 3. F-K-F configurations for four possible forms viewed along c*.

and most of them come on the top of the tetrahedral cations. In the 2M2 structure, only two out of six pairs in the cell are the pair of Al rich sites, but in the 20 structure, the same octahedral ordering being assumed, all the pairs are the same kind. Those differences in cation and anion assemblages can be evaluated in terms of electrostatic energies. Madelung sums for the determined 2M2 structures and the ideal 2M1, 2M2 and 2O structures derived from the parameters of a 1M synthetic polylithionite were computed by Dr. Q. Johnson's program adapted to the HITAC 5020E computer. The computed Madelung sums per formula unit for ideal forms range from 81.24 to 81.09. They can be arranged in the decreasing order as: 2O>1M>2M2>2M1. This sequence may be changed largely by small rearrangements of co ordinates from the ideal structure, which is derived from the 1M co ordinates by polytype operations as was observed in the present 2M2 structure and also by the readjustment of the site occupancies. Since there is no precise way of estimating entropy differences of these poly morphs, it may not be justified to discuss relative stability of these forms at present. The authors are indebted to Dr. Malcolm Ross for specimens of H. TAKEDA, N. HAGA and R. SADANAGA 215

Rozna lepidolite and for interest and valuable discussions , and to Prof. D. R. Wones for critical reading of the manuscript . The computations were performed at the Computer Center of the University of Tokyo .

References

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Manuscript received 4 August 1970.