Abstract
The crystal chemistry of synthetic amphibole solid solutions with nominal bulk compositions between Mn0.3Mg6.7[Si8O22] (OH)2 and Mn2.3 Mg4.7[Si8O22] (OH)2 has been investigated. Synthesis runs carried out at 750°C/2000 bar (Mn0.3 to Mn1.8) and 650°C/6000 bar (Mn2.0 to Mn2 3) led to essentially single-phase products. In contrast to natural amphiboles of similar composition, X-ray and optical properties that reflect long-range symmetry are consistent with space group Pnma. The cell volume and mean refractive index generally increase with increasing Mn-substitution, but a and b lattice parameter variation is not a simple function of Mn-content. hrtem studies reveal abundant chain multiplicity (chain-width) and chain arrangement (stacking) faults. The Mn-bearing analogs of chesterite and jimthompsonite, as well as other statistically significant multiple chain combinations, have been noted. The observed consistent and reproducible nature of structural and compositional inhomogeneity suggests that many synthetic amphiboles are poor experimental analogs of their natural counterparts.
