Taaffeite

Abstract: 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-Al 2 O 3 -SiO 2 (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.

Abstract: The atomic scale structure and chemistry of (111) twins in MgAl2O4 spinel crystals from the Pinpyit locality near Mogok (Myanmar, formerly Burma) were analysed using complementary methods of transmission electron microscopy (TEM). To obtain a three-dimensional information on the atomic structure, the twin boundaries were investigated in crystallographic projections $$ [\ifmmode\expandafter\bar\else\expandafter\=\fi{1}10] $$ and $$ [11\ifmmode\expandafter\bar\else\expandafter\=\fi{2}]. $$ Using conventional electron diffraction and high-resolution TEM (HRTEM) analysis we have shown that (111) twins in spinel can be crystallographically described by 180° rotation of the oxygen sublattice normal to the twin composition plane. This operation generates a local hcp stacking in otherwise ccp lattice and maintains a regular sequence of kagome and mixed layers. In addition to rotation, no other translations are present in (111) twins in these spinel crystals. Chemical analysis of the twin boundary was performed by energy-dispersive X-ray spectroscopy (EDS) using a variable beam diameter (VBD) technique, which is perfectly suited for analysing chemical composition of twin boundaries on a sub-nm scale. The VBD/EDS measurements indicated that (111) twin boundary in spinel is Mg-deficient. Quantitative analyses of HRTEM (phase contrast) and HAADF-STEM (Z-contrast) images of (111) twin boundary have confirmed that Mg2+ ions are replaced with Be2+ ions in boundary tetrahedral sites. The Be-rich twin boundary structure is closely related to BeAl2O4 (chrysoberyl) and BeMg3Al8O16 (taaffeite) group of intermediate polysomatic minerals. Based on these results, we conclude that the formation of (111) twins in spinel is a preparatory stage of polytype/polysome formation (taaffeite) and is a result of thermodynamically favourable formation of hcp stacking due to Be incorporation on the {111} planes of the spinel structure in the nucleation stage of crystal growth. The twin structure grows as long as the surrounding geochemical conditions allow its formation. The incorporation of Be induces a 2D-anisotropy and exaggerated growth of the crystal along the (111) twin boundary.

Abstract: A spinel-phlogopite schist from the Mount Painter Province 0fSouth Australia forms part of a highly magnesiumand aluminium-rich unit and contains the minerals h6gbomite and taaffeite. Taaffeite nucleated in spinel during upper amphibolite facies metamorphism. During a subsequent upper amphibolite facies metamorphic event both spinel and taaffeite were partially replaced by h6gbomite. Chemical analyses of hSgbomite, taaffeite, and spinel are presented.