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 (ƒ¢,) 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.