Synthesis of beryllian sapphirine in the system MgO-BeO-Al 2 O 3 -SiO 2 -H 2 O and comparison with naturally occurring beryllian sapphirine and khmaralite, part 2: A chemographic study of Be content as a function of P , T , assemblage and FeMg -1 exchange

• Main Authors: Christy, Andrew, Grew, ES
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Mineralogical Society of America
• Subjects: Keywords: beryllium; crystal structure; mineral synthesis; P-T conditions; sapphirine; East Antarctica; Napier; Antarc*; Antarctica; Enderby Land
• Online Access: http://hdl.handle.net/1885/86823

Beryllium is a significant constituent in sapphirine in some metamorphic and pegmatitic rocks, and thus could have a major effect on its stability relationships. Using the stoichiometries of reactions involving sapphirine and associated phases in the MgO-BeO-Al2O3-SiO2 (MBeAS) system in conjunction with molar volume data, we have plotted maps of the sapphirine solid-solution field in both μ-μ and μ-P space, where μ is the chemical potential of an exchange component such as (BeSi)(AlAl)-1. These maps give a pressure sequence of stable MBeAS univariant reactions and divariant assemblages that are consistent with experimental data, e.g., they show how Be stabilizes sapphirine + forsterite, which is rare in nature but readily synthesized over a wide P-T range in the presence of Be. We generate a MBeAS petrogenetic grid for sapphirine-bearing assemblages over the approximate range T = 700-900 °C, P = 0-2.5 GPa, identify divariant and univariant assemblages containing sapphirine with maximum Be, and determine the sense of variation of maximum Be content with P. At lower T, maximum Be occurs at the low-P limit of surinamite stability, ca. 0.5 GPa. At higher T, maximum Be increases with P, following the MBeAS univariant reactions involving (sapphirine + surinamite + orthopyroxene + chrysoberyl + forsterite or spinel). Natural assemblages containing sapphirine and its Be-rich near-analog khmaralite from the Napier Complex, Enderby Land, East Antarctica formed at higher T (900-1100 °C) than the experiments and in bulk compositions containing substantial Fe. Associated minerals include garnet, sillimanite, quartz, and magnesiotaaffeite-6N′3S ("musgravite"), whereas forsterite is absent and cordierite is a local, late phase. μ(BeSi)(AlAl)-1-μFeMg-1 diagrams show that the stability of magnesiotaaffeite-6N′3S causes the maximally beryllian khmaralite to shift from a magnesian composition in equilibrium with orthopyroxene + surinamite + forsterite + chrysoberyl, as in the MBeAS subsystem, to a more Fe-rich composition ...