Fundamental difference between synthetic powder and natural or synthetic single-crystal 1M micas: Geometric homo-octahedral vs. geometric meso-octahedral sheets

Abstract

Based on powder X-ray diffraction (pXRD) measurements of many mica solid solution series of synthetic powder samples (between Mg, Co, Ni, and Fe end-members, with different degrees of oxidation, vacancy, and Al/Si contents, and including an OH/F substitution series; 75 samples indexed as 1M polytype, space group C2/m) and on 175 previously published structural refi nements of 1M micas (refi ned in space group C2/m) comprising 37 synthetic and 138 natural single-crystal samples, we show that the relation (c/a) cos β* = 1/3 *

Reversed phase equilibrium constraints on the stability of Mg-Fe-Al biotite

“…Although detailed investigation has revealed differences in octahedral cation ordering between synthetic and natural Fe-bearing biotite (Mercier et al 2005), we expect that the magnitude of these differences does not signiÞ cantly affect application of our experimental data to naturally occurring biotite. To compare the Al contents determined in this study with natural data, assumptions must be made regarding minor element octahedral and tetrahedral site substitutions in natural biotites, particularly Ti and Fe…”

“…bdl = below detection level; nd = not determined. * Samples described by Mercier et al (2005) were indexed on 33-35 peaks.…”

“…Unit-cell parameters of most Fe-biotites used in this study (Table 1) were reported by Mercier et al (2005) on the basis of 33-35 reß ections. Previous experimental work (Rutherford 1973;Hewitt and Wones 1975;Partin 1984;Rebbert et al 1995;Benisek et al 1996) has shown that the molar volume decreases with both increasing Al and increasing Fe 3+ in biotite (Fig.…”

“…An alternative approach is calculation of activity coefÞ cient corrections on the basis of cross-site terms applied to the energetics of internal equilibria among end-members of a solid solution (e.g., Wood and Nicholls 1978;Aranovich et al 1988;Aranovich 1991). Although recent analyses of structural reÞ nement data for single crystals of biotite indicate size differences between M1 and M2 that likely reß ect cation ordering (Mercier et al 2005), available data for Fe-Mg biotite demonstrate very limited octahedral site partitioning (Redhammer and Roth 2002). Assuming equal FeMg partitioning on the Ann-Phl binary (no octahedral Al), the cross-site corrections are given by: The garnet-biotite exchange thermometer 8 can be calibrated by combining the thermodynamic properties of biotite extracted here (Table 5) with those of garnet derived by Berman and Aranovich (1996).…”

Upper limit of the tetrahedral rotation angle and factors affecting octahedral flattening in synthetic and natural 1M polytype C2/m space group micas

“…In the present paper, we follow an approach based on the geometric crystal chemical models of Mercier et al (2005a), applied to a large suite of synthetic powder samples previously studied by powder X-ray diffraction (Mercier et al 2005b) and using previously published single-crystal reÞ nement data. We Þ nd that such models provide a powerful framework for analyzing lattice and structural reÞ nement parameters, by allowing clear distinctions to be made between local effects owing to chemical disorder vs. long-range geometric effects that either appear as average network adjustments (e.g., average polyhedral ß attening and rotation) or as true geometric limits (i.e., that would require excessive bond stretching or bending).…”