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Polysynthetically-Twinned Structures of Enstatite and Wollastonite

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Polysynthetically-Twinned Structures of Enstatite and Wollastonite Phys Chem Minerals (1984) 10:217-229 PHYSICS CHEMISTRY [[MIHERALS © Springer-Verlag 1984 Polysynthetically-Twinned Structures of Enstatite and Wollastonite Yoshikazu Ohashi * Department of Geology, University of Pennsylvania, Philadelphia, PA 19104, USA Abstract. Crystal structures of clinoenstatite, orthoenstatite, wollastonite polytypes established the structural relation- wollastonite-lT and wollastonite-2M (parawollastonite) ships between polytypes (Morimoto et al. 1960; Burnham were refined to an R factor 3-4 percent level. Molar vol- 1967; Morimoto and Koto 1969; Trojer 1968), and empha- umes at room temperature are 31.270(15), 31.315(8), sized the close relationship of the structure to the model 39.842(5) and 39.901(10) cm3/MSiO3, in the above-men- produced by applying mathematical operations to the basic tioned order, indicating that one-layer polytypes (clinoen- slab. Stacking sequences of new 3T, 4T, 5T, and 7T wollas- statite and wollastonite-lT) are stable at higher pressures tonite polytypes were determined (Henmi etal. 1978; than two-layer polytypes (orthoenstatite and wollastonite- Henmi et al. 1983), although refinements of atomic coordi- 2M). The polytypic relation of the enstatite polytypes can nates of these longer repeat polytypes were impossible. be described by four twinning operations - b glide [1 to Because of the similarities in chemistries and silicate (110), a glide I[ to (001), twofold screw axis [I to a (of linkages, and the existence of polytypes, enstatite and wol- orthoenstatite) and a twofold screw axis I[ to c. For the lastonite are often compared in parallel. Polytypes of ensta- wollastonite polytypes, twinning operations are twofold tite and wollastonite are, however, different in the way twin- screw axis II to b and a glide [I to (010). Structural adjust- ning is formed with respect to tetrahedral and octahedral ments after twinning are not necessarily the largest at the layers. A twinning plane of the wollastonite polytypes is twin boundary (true in enstatite but not so in wollastonite). not parallel to oxygen closest-packed layers. Thus the wol- In both cases octahedral sites that involve bridging oxygens lastonite polytypes cannot be regarded as unique stacking tend to show relatively large changes. Lattice strain ellip- of tetrahedral and octahedral layers, which is the case for soids associated with twinning are also different for ensta- enstatite. Enstatite transforms rapidly as temperature chan- tite and wollastonite, which implies that wollastonite may ges (Smyth 1974), whereas the direct transformation be- react differently from enstatite to non-hydrostatic pressure. tween wollastonite 1T and 2M has not been observed. De- terminations of phase relationships of polytypes are often complicated by small differences in free energy. Two major factors of the crystal structure that may make two polytypes thermodynamically different are (1)difference in stacking Introduction and (2) distortion of the layer modules. A special case of polymorphs is polytypes that have a nearly The purpose of this study is to analyze (1) this local identical unit layer but are different in the stacking order structural relaxation, and (2) twinning relationships to of layers. Polytypes of magnesium metasilicates, MgSiO3 close-packed oxygen layers. The crystal structures of all enstatite, and calcium metasilicates, CaSiO3 wollastonite, four polytypes have been determined previously but the have been described (e.g. Ito 1935, 1950) as unit cell scale precision of refined atomic coordinates varies from one re- twinning on the plane (100). For example, twolayer poly- finement to another. The first part of this study was acquisi- types (orthoenstatite and wollastonite-2M 1) may be derived tion of structural data with a comparable precision for poly- from an untwinned single layer polytype (clinoenstatite and type pairs of both enstatite and wollastonite. wollastonite-lT, respectively) by displacing a "slab" on the (100) plane. Early crystal structural work on pyroxene and Experimental * Present address. ARCO Oil and Gas, Exploration and Produc- tion Research Center, Plano Texas 75075, USA Crystals Used in This Study In order to obtain single and untwinned crystals of identical 1 The form wollastonite-2M notation is used in this paper in place chemistries, crystals of ortho and clinoenstatite were synthe- of the Gard notation, wollastonite-M2abc (Bailey 1977), and the common name parawollastonite. Crystallographic descrip- sized by Dr. J. Ito using flux method (Ito 1975). Among tion of wollastonite is given for the conventional unit cell setting various clinoenstatites examined, these crystals were the with the tetrahedral chain parallel to b axis (not c axis) and only clinoenstatite that did not show apparent twinning the tetrahedral and octahedral layers parallel to (101) of wollas- on h01 precession photographs (Fig. 1). Apparently, un- tonite-1T and to (201) of wollastonite-2M twinned clinoenstatite can only be obtained from direct 218 Ca) CLEN1 Fig. l. Comparison of (h0:1) rows of clinoenstatite, twinned clinoenstatite, heated clinoenstatite, and orthoenstatite. Synthetic clinoenstatite, (c), often shows (b) CLEN2 twinning of equal amount of clinoenstatite in two orientations, (a) and (b). The apparent diffraction symmetry of this twinned clinoenstatite is Twinned CLEN (c) orthorhombic with a = 27 A. When untwinned clinoensatite is heated to over 500° C and quenched, the result, (d), (d) ~~ .... Heated CLEN shows a partial transformation to orthoenstatite, (e). An (h0:1) precession photograph of untwinned clinoenstatite is shown at the bottom. All photographs (e) OREN were taken with Mok~ radiation of the orthoenstatite are 0.09 wt percent A1203, 0.08 wt per- cent TiO2, and 0.16 wt percent FeO (Sasaki et al. 1982). The two wollastonite crystals used in this study are from Willsboro, New York (wollastonite-lT) and Esashi mine, Iwata, Japan (wollastonite-2M) and are nearly identical in composition based on microprobe data (Table 1). Unit Cell Constants and Molar Volume Extreme care was taken to minimize the experimental errors d so that a precise molar volume calculation could be made. Reflections were measured for 50 to 100 reflections using Buerger's ll4mm-diameter back-reflection Weissenberg camera in a high angle region, ranging from 120 to 170 de- grees of 20 for FeK radiation. The computer program, LCLSQ (Burnham 1962), was used to make corrections Table 1. Electron microprobe analyses of wollastonite Oxide weight % SiOz FeO a MnO MgO CaO AIzO 3 total WO:1T b 51.52 0.32 0.13 0.03 48.47 0.03 100.50 :1 C ~ WO2M 51.30 0.:10 0.06 0.07 47.99 0.21 99.73 crystallization from melt, but not through solid-solid phase Cations based on 3 oxygens transition from protoenstatite, as is the case of the system Si Fe Mn Mg Ca A1 total MgSiO 3 without flux (Dr. Jun Ito private comm. 1975). The orthoenstatite sample was reported to have impurities WO1T 0.994 0.005 0.002 0.000 1.002 0.000 2.003 of 0.27 wt percent V205 and 0.17 wt percent Li20 (Haw- WO2M 0.994 0.001 0.000 0.00:1 0.996 0.005 1.997 thorne and Ito 1977), which correspond to approximately one vanadium atom for 300 magnesium atoms and one lith- a All iron as FeO ium atom for 100 magnesium atoms. Additional impurities b Wollastonite 1T and 2M Table 2. Refined unit cell parameters ~ of enstatite and wollastonite polytypes a(A) b c c~ (deg.) ~ ? Cell V (A 3) Mole V (cm3) b CLEN 9.606(1) 8.8131(7) 5.170(2) 90 108.35(1) 90 4:15.5(2) 31.270(15) OREN 18.225(2) 8.8128(6) 5.180(:1) 90 90 90 832.0(2) 31.315(8) WO1T 7.9258(4) 7.3202(4) 7.0653(4) 90.055(3) 95.217(3) 103.426(3) 396.96(5) 39.842(5) WO2M 15.424(1) 7.324(:1) 7.0692(7) 90 95.371 (4) 90 795.1 (2) 39.901 (:10) a Back-reflection Weissenberg method with FeK radiation. Reflections with 20 ranging 120 to 170 degrees were used in a least-squares refinement. Wave lengths used are 1.93597, 1.93991, and 1.75653 A for FeK~2, c~1 and fl respectively (International Tables for X-ray Crystallography, vol. III) b Based on one formula unit of MSiO~ 219 due to camera eccentricity, film shrinkage, and crystal ab- Table 3. Crystal and refinement data for enstatite and wollastonite sorption. The refined parameters are given in Table 2. The difference in molar volume is 0.045 cm 3 with an Enstatite MgSiO 3 Wollastonite CaSiO 3 estimated standard deviation of 0.015 cm 3 for the enstatite CLEN a OREN a WO1T a WO2M a polytypes, and 0.059 with e.s.d. 0.010 cm 3 for the wollas- tonite polytypes. In both cases unit-repeat polytypes have Symmetry mono- ortho- triclinic mono- smaller molar volumes than the longer repeat polytypes, clinic rhombic clinic which is consistent with clinoenstatite (and wollastonite-1 T) being stable at higher pressures. Space group P21/e Pbca P1 P21/a Source of synthetic natural crystal flux-grown Willsboro, Esashi Diffraction Data Collection and Refinements by J. Ito New York Mine, Iwate, X-ray diffraction intensity data were measured on an au- Japan tomated fourcircle diffractometer using an omega-two theta Size of crystal 0.11 0.14 0.13 0.09 variable scanning technique (Finger et al. 1973) with MoK~ (mm) x0.14 x0.21 x0.17 x0.11 radiation. Integrated intensities were corrected for Lorentz x 0.34 x 0.21 x 0.30 x 0.30 and polarization effects, and absorption corrections were Number of 1660 1525 2065 1824 computed by the numerical integration technique (Burnham reflections 1966). R factor (%)b Starting atomic coordinates used in the RFINE2 least- weighted 3.3 4.0 4.3 4.3 squares program (Finger and Prince 1975) were those of unweighted 3.1 3.3 3.3 6.8 hypersthene (Burnham et al.
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