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Iron in Kornerupine: a 57Fe Moèssbauer Spectroscopic Study and Comparison with Single-Crystal Structure Re®Nement

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Iron in Kornerupine: a 57Fe Moèssbauer Spectroscopic Study and Comparison with Single-Crystal Structure Re®Nement American Mineralogist, Volume 84, pages 536±549, 1999 Iron in kornerupine: A 57Fe MoÈssbauer spectroscopic study and comparison with single-crystal structure re®nement EDWARD S. GREW,1,* GUÈ NTHER J. REDHAMMER,2 GEORG AMTHAUER,2 MARK A. COOPER,3 FRANK C. HAWTHORNE,3 AND KARL SCHMETZER4 1Department of Geological Sciences, University of Maine, 5790 Bryand Center, Orono, Maine 04469, U.S.A. 2Institut fuÈr Mineralogie, UniversitaÈt Salzburg, Hellbrunnerstrasse, 34 A-5020 Salzburg, Austria 3Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada 4Marbacherstrasse, 22b, D-85238 Petershausen, Germany ABSTRACT Iron is an important constituent of kornerupine, (M,Mg,Fe)(Al,Mg,Fe)9(Si,Al,B)5O21(OH,F). We obtained MoÈssbauer spectra at 300 K on twelve samples with SFe 5 0.30±1.30 atoms per formula unit (apfu) and Fe31/SFe 5 0±0.31; several samples were also run at 77 and 430 K. Models allowing unequivocal re®nement of the spectra and determination of site occupancies were developed only when single-crystal re®nement (SREF) of six of the samples constrained the number of possibilities. The spectra could then be ®tted to three Fe21 doublets and one Fe31 doublet. The Fe21doublets have nearly identical isomer shifts: d51.14±1.19 mm/s for the octahedral M1 and M2 sites and 1.12±1.20 mm/s for the irregular, eightfold-coordinated X site (relative to a-Fe at 300 K). However, they differ to a variable extent in quadrupole splitting, DEQ ø 1.06±1.80, 1.83±2.27, and 2.14±3.41 mm/ s, respectively, to the M1, M2, and X sites. The Fe31 doublet corresponds to the M4 site. The MoÈssbauer and SREF occupancies are in excellent agreement for the six samples. The M1 doublet is split in B-bearing kornerupine and the proportion of Fe corresponding to each doublet, as well as quadrupole splitting, varies with B content. Similarly, the X doublet is split in F-bearing kornerupine, and quadrupole splitting of the X site increases with increasing F content. In contrast to most silicates, resolution of the spectra improves with increasing temperature. Quadrupole splitting of the X, M1, and M2 sites decreases with temperature, the X site at a lesser rate consistent with its being the most distorted site. To a ®rst approximation, the Fe31/SFe ratio in kornerupine determined by SREF and 31 MoÈssbauer spectroscopy increases with increasing Fe2O3 and Fe /SFe ratio of the asso- ciated sillimanite, sapphirine, and ilmenite-hematite, i.e., the measured Fe31/SFe ratios are related to the oxygen fugacity at which the kornerupine crystallized. INTRODUCTION troscopic study of twelve specimens and their petrologic Kornerupine sensu lato, approximately (M,Mg,Fe)(Al, implications. Unequivocally identifying the sites occu- pied by Fe is impossible using MoÈssbauer spectroscopy Mg,Fe)9(Si,Al,B)5O21(OH,F), is a mineral group occurring in Mg-Al-rich rocks, magnesian metapelites and, very alone, even with several single-crystal structure re®ne- rarely, pegmatites in upper amphibolite- and granulite- ments (SREF) of kornerupine to use as guides (Moore facies terrains; it includes kornerupine sensu stricto (0 # and Bennett 1968; Moore and Araki 1979; Finger and B , 0.5 apfu) and prismatine (1 . B . 0.5 apfu) (Grew Hazen 1981; Moore et al. 1989; Klaska and Grew 1991). et al. 1996). Its complex chemical composition was aptly The spectra were too complex and the possible site as- termed ``formidable'' by Moore and Bennett (1968). Con- signments too numerous. Therefore, by combining our tributing to this complexity is the variation in Fe31/SFe spectroscopic results with crystallographic data on six ratio, which ranges from 0 to 1, one of the most extensive specimens (Cooper 1997), we developed models for re- among ferromagnesian silicates. However, this ratio can- ®ning the MoÈssbauer spectra, identifying the doublet due not be measured by electron and ion microprobe analyses; to Fe at the distorted eightfold-coordinated X site, and it requires another technique, such as 57Fe MoÈssbauer deducing site occupancies that agree remarkably well spectroscopy. with the crystallographic data. The Fe in these 12 kor- This paper reports the results of the MoÈssbauer spec- nerupines is predominantly ferrous. Spectra were also ob- tained from a specimen (no. 128415, Bjordam, Bamble, * E-mail: [email protected] Norway, and van der Wel 1973) that contains more Fe31 0003±004X/99/0004±0536$05.00 536 GREW ET AL.: MOÈ SSBAUER OF KORNERUPINE 537 TABLE 1. Major element cation site-occupancies Octahedral Tetrahedral Site Atom Site Atom M1 Mg, Fe21 T1 Si, Al M2 Mg, Al, Fe21 T2 Si, Al M3 Al, Mg T3 Si, B, Al M4 Al, Mg, Fe31 [8]-coordinated M5 Al, Mg X M,Fe21, Mg, Na Note: M denotes vacancy. Multiplicities are 4 for M2, T3 and X. All other sites have multiplicities of 8. From crystal structure determination (e.g., Moore et al. 1989; Cooper 1997). occur as dimers and trimers, is the variable B content of T3, which is coupled with Si content in T2: [T3]B 1 [T2]Si ↔ [T3]Si 1 [T2]Al. Moore and Araki (1979) reported that, in a synthetic Fe-free kornerupine, M1 and M4 are the most distorted of the octahedral sites, whereas M3 and M5 are the least distorted. M1 is unique among the octahedra in sharing an edge with a tetrahedron, T2. OH and F have been de®nitively located at O10 only. Cooper (1997) concluded that no O is present at this site, and thus kornerupine formulae can be normalized on the basis of 43 anionic charges. In addition to the cations listed above, Li, Be, Ca, Ti, V, Cr, and Mn also have been reported, but only in minor amounts (e.g., Grew et al. 1990; Grew 1996; Cooper 1997). Sample preparation and composition Samples (Table 2) were separated from rock by hand (nos. 1, 3, 7, 8, and 12, Grew et al. 1990) or by conven- TABLE 2. List of kornerupine samples studied by MoÈssbauer spectroscopy FIGURE 1. Elements of the structure of kornerupine. (a) Pro- jection parallel to [010] of the A layer in kornerupine at y 5 0, Original Cooper (b) Projection parallel to [010] of the B layer in kornerupine at sample (1997) No. Locality and Reference number number y 5 0.16, (c) Projection parallel to [001] of a part of the kor- nerupine structure at z 5 0.75 [Used by permission from Klaska 1 Sinyoni Claims, Zimbabwe*³§\ 9365 K35 and Grew (1991), Figs. 1±3, modi®ed by combining into one 2 Sinyoni Claims, Zimbabwe*³ 9371 Ð 3 Ellammankovilpatti, India\# 3083D K1 ®gure]. 4 Sinyoni Claims, Zimbabwe² 8497 K22 Ampanihy, Madagascar (Gem- 5 stone) P559 Ð 6 Madagascar (Gemstone) P560 Ð 21 than Fe . We did not succeed in ®nding a model to re®ne 7 Madagascar\ 171.350 K7 its spectrum, which may include features from secondary 8 Madagascar\ 141255 K9 9 Sri Lanka (Gemstone) P676 Ð hematite, an impurity present in this specimen (Grew et 10 Waldheim, Germany P540 Ð al. 1990). Cooper (1997) also was not able to get satis- 11 Lac Ste. Marie, Quebec, Canada P539 Ð factory agreement between Fe from electron microprobe 12 Port Shepstone, South Africa\ ** BM1940,39 K11 analysis and Fe from SREF for the ferrian kornerupines Note: Grew (1996) reviews the literature on the localities listed here. from Bjordam and, consequently, its MoÈssbauer spectrum * These two specimens donated by W. Schreyer are derived from a sin- gle block about 25 cm across (Schreyer, personal communication, 1998). is not reported here. We received one specimen, which is presumably no. 9371, in 1976 or 1977, and the second, no. 9365, in 1983. Kornerupine crystal chemistry ² Sample no. 3 (8497) is a coarse kornerupine prism in a matrix of cor- dierite, sillimanite, and phlogopite; kyanite occurs as rare, rounded relics Kornerupine is orthorhombic, space group Cmcm. in kornerupine. In contrast to the B-free kornerupine (no. 1), there is no Moore and Araki (1979) preferred to describe its dense, evidence of kornerupine breakdown. ³ Schreyer and Abraham (1976). but only locally cubic closed-packed, structure on the ba- § Klaska and Grew (1991). sis of two kinds of sheets, A and B, which alternate AB- \ Grew et al. (1990). BABBA (Fig. 1). There are nine sites in kornerupine (Ta- # Grew (1982), Grew et al. (1987a). ble 1). The most notable feature of the tetrahedra, which ** de Villiers (1940); Hey et al. (1941). 538 GREW ET AL.: MOÈ SSBAUER OF KORNERUPINE TABLE 3. Compositions of kornerupine Sample no. 1* 2² 3* 4* 5³ 7* 8* 9§ 10\ 11§ 12* Species Krn Krn Krn Krn Prs Prs Prs Prs Prs Prs Prs Electron-microprobe analyses, Fe2O3 by SREF, wt% SiO2 28.66 28.7 29.97 30.84 30.7 31.63 30.89 29.95 30.59 30.05 29.10 TiO2 0.12 0.11 0.12 0 0.10 0 0 Ð 0.24 Ð 0.21 Al2O3 48.04 47.1 44.06 44.76 39.6 40.56 40.11 42.10 42.36 41.87 41.35 V2O3 0 Ð 0 0 Ð 0 0 Ð 0.09 Ð 0 Cr2O3 0 Ð 0 0 Ð 0 0 Ð 0.08 Ð 0 Fe2O3 0.60 0.15 0.73 0.09 0.46# 0.52 0.60 0.26# 0.28# 1.02# 0 FeO 2.71 4.46 3.80 1.21 2.54 2.83 4.26 4.98 6.11 8.49 12.09 MnO000000 0.13 Ð 0.08 Ð 0.12 MgO 17.72 16.2 17.32 19.35 20.0 19.20 18.05 17.98 14.53 13.41 10.81 CaO 0.08 0.07 0 0 0.04 0 0 Ð 0 Ð 0 Na2O 0 Ð 0.04 0 0.03 0 0.05 0.10 0.09 0.05 0.05 K2O 0 Ð0 0 0 0 0 Ð0 Ð0 F 0.02 Ð 0.07 0 Ð 0.30 0.38 Ð 0.67 Ð 0.85 Wet-chemical analysis for Li2O, BeO.
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