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Crystal Structure of Protoanthophyllite: a New Mineral from the Takase Ultramafic Complex, Japan

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Crystal Structure of Protoanthophyllite: a New Mineral from the Takase Ultramafic Complex, Japan American Mineralogist, Volume 88, pages 1718–1723, 2003 Crystal structure of protoanthophyllite: A new mineral from the Takase ultramafic complex, Japan HIROMI KONISHI,1,* THOMAS L. GROY,2 ISTVÁN DÓDONY,2,3 RITSURO MIYAWAKI,4 SATOSHI MATSUBARA,4 AND PETER R. BUSECK1,2 1Department of Geological Sciences, Arizona State University, Tempe, Arizona 85287-1404, U.S.A. 2Department of Chemistry/Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, U.S.A. 3Department of Mineralogy, Eötvös L. University, Budapest, H-1117, Pázmány Péter sétány 1/C, Hungary 4Department of Geology, National Science Museum, 3-23-1 Hyakunincho, Shinjuku, Tokyo 169-0073, Japan ABSTRACT Protoanthophyllite, (Mg, Fe)7Si8O22(OH)2, is a newly discovered amphibole species from the Takase ultramafic complex in Japan. It occurs as prismatic crystals up to 5 mm in length in a ther- mally altered serpentinite that experienced contact metamorphism. The protoanthophyllite is asso- ciated with forsterite, talc, serpentine minerals, chlorite, chromian spinel, magnetite, pentlandite, and calcite. Some protoanthophyllite crystals contain minute lamellae of anthophyllite, other pyriboles, or both. Protoanthophyllite is biaxial negative, with refractive indices na = 1.593(2), nb (calc.) = 1.609, ng = 1.615(2), and 2Vx = 64(5)∞. Electron microprobe analyses give an empirical formula of (Mg6.31Fe0.61Na0.06Mn0.01Ni0.01)S7.00(Si7.90Al0.14)S8.04O22(OH)2. It is orthorhombic with space group Pnmn. The unit-cell dimensions are a = 9.3553(8), b = 17.9308(15), and c = 5.3117(4) Å: with V = 891.0(3) Å3 and Z = 2. A single-crystal X-ray structure determination shows that, following the convention of Thompson (1981), protoanthophyllite has an (X) configuration. The topology of the silicate tetrahe- dral chains is similar to that of the anthophyllite A-chains. Silicate tetrahedral chains are O-rotated in protoanthophyllite, whereas those in protoferro-anthophyllite are S-rotated. Iron atoms are con- centrated in the 4-coordinated M4 sites. The unit-cell volume is ~1.5% larger than the equivalent volume of anthophyllite with Mg/(Mg + Fe) = 0.885, suggesting a high-temperature or low-pressure stability relative to anthophyllite, assuming that protoanthophyllite is not metastable. INTRODUCTION et al. (1960). Fe-species (protoferro-anthophyllite) occurs in The Pnmn amphibole structure was first identified in syn- pegmatites, and Fe-, Mn-species (protomangano-ferro- thetic Li-, F-, Mg-bearing and Li-, F-, Fe-, Mn-bearing speci- anthophyllite) occurs in both pegmatites and a metamorphosed mens, and the crystal structure was refined (Gibbs et al. 1960; manganese deposit. Subsequently, we discovered the Mg-, Fe- Gibbs 1969 and personal communication). It is structurally re- species of protoamphibole (protoanthophyllite) from three lated to orthoamphibole in the same way that protoenstatite metamorphosed serpentinites in Japan (Konishi et al. 2002). (Smith 1959) is related to enstatite, as predicted by Gibbs et al. Here, we present the results of the structural refinement of proto- (1960), who called it protoamphibole. Octahedral layers in anthophyllite from the Takase ultramafic complex. amphibole, like in other pyriboles, have two orientations re- The mineral and mineral name of protoanthophyllite have lated by a half-rotation about the b-axis. A cross (X) refers to a been approved by the Commission on New Minerals and Min- half-rotation between successive octahedral layers whereas a eral Names of the International Mineralogical Association (no. dot (•) refers to no rotation (Thompson 1970, 1981). Pnmn 2001-065). The mineral is named after the proto-type analogue amphibole, like protoenstatite, has an (X) configuration whereas of anthophyllite, following the report of amphibole nomencla- Pnma amphibole, like enstatite, has a (•X) configuration. Hence, ture by Leake et al. (1997). Type material is preserved at the the terms proto and ortho refer to (X) and (•X) configurations, National Science Museum, Tokyo, Japan, and the Smithsonian respectively. Institution, Washington, D.C. The first natural protoamphiboles (Sueno et al. 1998) were EXPERIMENTAL METHODS not reported until almost 40 years later than the paper by Gibbs Single-crystal X-ray intensity data were collected using a Bruker SMART APEX CCD diffractometer at Arizona State University. A crystal fragment, free *Present Address: Department of Earth and Planetary Sciences, of {100} and {010} lamellae, was selected and cut from a petrographic thin The University of New Mexico, Albuquerque, NM 87131-1116, section for the X-ray measurement. We used the SMART V5.622 system of U.S.A. E-mail: [email protected] programs for unit-cell determination and X-ray data collection, SAINT V6.02A 0003-004X/03/1112–1718$05.00 1718 KONISHI ET AL.: CRYSTAL STRUCTURE OF PROTOANTHOPHYLLITE 1719 for unit-cell refinement and data reduction, SHELXS97 (Sheldrick 1997b) for The optical properties were measured using a polarizing microscope with a the structure solution, and SHELXL97 (Sheldrick 1997a) for refinement. The universal stage. The indices of refraction (na and ng) were measured by the im- populations of the coordination polyhedra containing the M cations were re- mersion method in white light. From these data, the index nb was calculated. fined allowing for Mg and Fe summing to full occupancy. We tried to refine the structure assuming that Al occupies both T and M sites, but the resulting popu- SAMPLE DESCRIPTIONS lations of Al on M sites were very small or negative numbers. Therefore, we assumed that the Al occupies only T sites, and the populations on the T sites A drill-core sample was collected from a waste dump at the were refined allowing for Si and Al summing to full occupancy. We constrained closed Takase chromite mine in the Takase ultramafic com- the number of Na atoms to equal that of Al. Table 1 gives the details of the plex, Okayama Prefecture, Japan (35∞ N, 133∞20' E). The collection and refinement. sample consists of protoanthophyllite, forsterite, talc, serpen- The compositions of individual crystals were determined using electron tine minerals, chlorite, chromian spinel, magnetite, pentland- microprobe analyses with a JEOL JXA-8600SX instrument at Arizona State University using wavelength-dispersive spectroscopy at an accelerating volt- ite, and calcite. The protoanthophyllite occurs as prismatic age of 15 kV, a 10 nA beam current, and a ~1 mm beam diameter. Data reduction crystals less than 5 mm in length along the c-axis. Some was done using standard ZAF matrix correction procedures. The following natu- protoanthophyllite crystals include minute lamellae of ral and synthetic standards were used: hypersthene for Si, rutile for Ti, anorthite anthophyllite on {100}, other pyriboles on {010}, or both. for Al, chromite for Cr, fayalite for Fe, rhodonite for Mn, metallic Ni for Ni, forsterite for Mg, wollastonite for Ca, albite for Na, and orthoclase for K. The Takase ultramafic complex experienced contact meta- morphism through the intrusion of Cretaceous or Paleogene TABLE 1. Crystal data and results of structure refinement of granitic rocks, and has crystallized to critical mineral assem- protoanthophyllite blages of (1) forsterite + talc ± tremolite, (2) forsterite + Crystal Data anthophyllite ± tremolite, or (3) forsterite + enstatite ± tremo- Crystal system Orthorhombic lite (Matsumoto et al. 1995). The assemblages are similar to Space group Pnmn Cell parameters a = 9.3553(8), b = 17.9308(15), c = those described by Evans and Trommsdorff (1970) in the Cen- 5.3117(4) Å tral Alps. Based on the minerals in the drill-core sample, it is Cell parameters from 2459 reflections most likely from zone 2. Protoanthophyllite may have formed Volume 891.0(3) Å3 Z2 by either the reaction: 4 forsterite + 9 talc = 5 protoanthophyllite + Dx 2.982 g/cm3 4 H2O, or by inversion from anthophyllite produced by 4 forsterite 3 Crystal 0.07 ¥ 0.07 ¥ 0.04 mm + 9 talc = 5 anthophyllite + 4 H O (Konishi et al. 2002). Cleaved fragment, colorless 2 Cell measurement temperature 293(2) K The physical and optical properties of protoanthophyllite Cell measurement qmin 2.27 are summarized in Table 2. Protoanthophyllite is biaxial nega- Cell measurement qmax 29.99 tive and the refractive indices na = 1.593(2), nb (calc.) = 1.609, Data Collection ng = 1.615(2), and 2Vx = 64(5)∞. Most properties are similar to Radiation type MoKa those of anthophyllite. The formula, based on the data in Table Radiation source Fine-focus sealed tube Data collection wavelength (Å) 0.71073 3, is (Mg6.31Fe0.61Na0.06Mn0.01Ni0.01)S7.00(Si7.90Al0.14)S8.04O22(OH)2. Monochromator Graphite No F and Cl were detected using wavelength-dispersive X-ray Measurement method w scan spectroscopy. Absorption correction type Empirical Absorption correction details Bruker SADABS Minimum transmission Tmin 0.7844 Maximum transmission Tmax 0.9231 Measured reflections 9680 Independent reflections 1348 Reflections with I >2s(I ) 961 qmin 2.27 qmax 30 TABLE 2. Physical and optical properties of protoanthophyllite Miller index limits h = –13 to +13 k = –25 to +25 Morphology Prismatic, elongated along c-axis l = –7 to +7 Color Colorless in thin section Luster Vitreous Refinement Streak White Refinement on F2 Cleavage Perfect {110} Structure factors for least-squares Fsqd Tenacity Brittle Least-squares derivatives matrix Full Fracture Uneven Weighting scheme for least-squares Calc Twining Not observed Hardness Mohs approximately 6 Weighting scheme details 3 calc w = 1/[ 2(F 2)+(0.0437P)2+0.0000P] where P = (F 2+2F 2)/3 Density (calc.) 2.98 g/cm s o o c Optical characters Biaxial 2 Optical sign X = a, Y = b, and Z = c R [F >2s(I )] 0.0471 wR(F 2) 0.1013 Indices of refraction a = 1.593(2) Goodness of Fit (S) 0.985 b (calc.) = 1.609 Refl. used in L.S. derivatives 1348 g = 1.615(2) Parameters refined 111 2V 64(5) Number of restraints 12 Orientation Negative Primary structure solution Direct Elongation Positive Secondary structure solution Difmap Pleochroism Not observed Hydrogen sites solution Geom Notes: Luster, streak, tenacity, fracture, hardness, indices of refraction, Hydrogen sites refinement Mixed and elongation were obtained by S.M.
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