Crystal chemistry of a Mg-vesuvianite and implications of phase equilibria in the system CaO-MgO-Al2O3-SiO2-H2O-CO21

Chemical analysis (including H,, F,, FeO, Fe,O,) of a Mg-vesuvianite from Georgetown, Calif., USA, yields a formula, R . 9 ? M g l . B B F e ~ . ~ 0 A ' 1 0 . ~ 7 S i 1 ~ . 8 1 06,.,,(OH)8.*4~0.,4~ in good agreement on a cation basis with the analysis reported by Pabst (1936). X-ray and electron diffraction reveal sharp reflections violating the space group P4/nnc as consistent with domains having space groups P4/n and P4nc. Refinement of the average crystal structure in space group P4/nnc is consistent with occupancy of the A site with AI, of the half-occupied B site by 0.8 Mg and 0.2 Fe, of the half-occupied C site by Ca, of the Ca ( I , 2,3) sites by Ca, and the O H and O( 10) sites by OH and 0. We infer an idealized formula for Mg-vesuvianite to be CaI9Mg(MgA1,)AI4Si, 8069(OH)9, which is related to Fe3 ' -vesuvianite by the substitutions Mg + OH = Fe3+ + 0 in the B and O(10) sites and Fe'+ = A1 in the AlFe site. Thermodynamic calculations using this formula for Mg-vesuvianite are consistent with the phase equilibria of Hochella, Liou, Keskinen & Kim (1982) but inconsistent with those of Olesch (1978). Further work is needed in determining the composition and entropy of synthetic us natural vesuvianite before quantitative phase equilibria can be dependably generated. A qualitative analysis of reactions in the system CaO-MgOAl,O,S i02-H,0-C0, shows that assemblages with Mg-vesuvianite are stable to high T in the ' Contribution No. 399 from the Mineralogical Laboratory, Department of Geological Sciences, University of Michigan. absence of quartz and require water-rich conditions (XH,O > 0.8). In the presence of wollastonite, Mg-vesuvianite requires very water-rich conditions (XH,O > 0.97). Key-words: petrogenetic grid; vesuvianite