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Synthesis of beryllian sapphirine in the system MgO-BeO-Al₂O₃-SiO₂-H₂O 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

¹ Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia
² Department of Earth Sciences, University of Maine, 5790 Bryand Research Center, Orono, Maine 04469-5790, U.S.A.

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-6 N ’3 S ("musgravite"), whereas forsterite is absent and cordierite is a local, late phase. µ _{(BeSi)(AlAl)–1}-µ _FeMg–1 diagrams show that the stability of magnesiotaaffeite-6 N ’3 S 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 associated with garnet + surinamite + magnesiotaaffeite-6 N ’3 S + chrysoberyl. Khmaralite associated with sillimanite + garnet + surinamite + magnesiotaaffeite-6 N ’3 S or chrysoberyl in a Napier Complex pegmatite from Khmara Bay is predicted to be the most Be-rich possible in the presence of sillimanite, whereas the sillimanite + quartz ± orthopyroxene ± garnet associations in quartz granulites requires a sapphirine much lower in both Be and Fe; analyses are roughly in accord with these predictions. The shape of the sapphirine/khmaralite solid-solution field is such that there is a positive correlation between high Be and high Fe^2+, a chemographic effect independent of any crystal chemical effects due to the clustering of Fe and Be in the crystal structure of khmaralite.

The diagram for FMBeAS shows that sapphirine + quartz, which is often cited as evidence for ultrahigh temperatures (e.g., 1040 °C), is stabilized to lower T and higher P than in the corresponding Be-free system. Hence, this minimum T may be valid only in rocks with relatively abundant sapphirine and/or very low bulk Be content so that what Be is present in the system is not concentrated in sapphirine.

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