Boromullite, Al₂BSi₂O₁₉, a new mineral from granulite-facies metapelites, Mount Stafford, central Australia: A natural analogue of a synthetic "boron-mullite"

Boromullite is a new mineral corresponding to a 1:1 polysome composed of Al₅BO₉ and Al₂SiO₅ modules. Electron-microprobe analysis of the holotype prism is SiO₂ 19.01(l.12), TiO₂ 0.01(0.02), B₂O₃ 6.52(0.75), Al₂O₃ 74.10(0.95), MgO 0.07(0.03), CaO 0.00(0.02), MnO 0.01(0.04), FeO 0.40(0.08), Sum 100.12 wt.%, which gives Mg_0.01 Fe_0.03 Al_8.88 Si_1.93 B_1.14 O_18.94 (normalised to 12 cations), ideally Al₉BSi₂O₁₉. Overall, in the type specimen, it ranges in composition from Mg_0.01 Fe_0.03 Al_8.72 Si_2.44 B_0.80 O_19.20 to Mg_0.01 Fe_0.03 Al_9.22 Si_1.38 B_1.35 O_18.67. Single-crystal X-ray diffraction gives orthorhombic symmetry, Cmc2₁, a 5.7168(19) Å, b 15.023(5) Å, c 7.675(3) Å, V 659.2(7) ų, calculated density 3.081 g/cm³, Z = 2. The refined structure model indicates two superimposed modules present in equal proportions in the holotype prism. Module 1 has the topology and stoichiometry of sillimanite and carries all the Si, whereas module 2 is a type of mullite defect structure in which Si is replaced by B in triangular coordination and by Al in tetrahedral coordination, i.e., Al₅BO₉. The strongest lines in the powder pattern [d in Å, I_meas.), (hkl)] are 5.37(50) (021), 3.38(100) (022, 041), 2.67 (60) (042), 2.51(60) (221, 023), 2.19(80) (222), 2.11(50) (043), 1.512(90) (263). Boromullite is colourless and transparent, biaxial (+), n_x 1.627(1), n_y 1.634(1), n_z 1.649(1) (589 nm). 2V_z (meas) = 57(2)°, 2V_z (calc) = 69(12)°. In the type specimen boromullite tends to form prisms or bundles of prisms up to 0.4 mm long, typically as fringes or overgrowths on aggregates of sillimanite or as narrow overgrowths around embayed werdingite prisms. In other samples boromullite and sillimanite are intergrown on a fine scale (from < 1 μm to > 10 μm). Sekaninaite-cordierite, potassium feldspar, biotite, werdingite and its Fe-dominant analogue, hercynite, and ilmenite are other commonly associated minerals, whereas ominelite-grandidierite, plagioclase, andalusite, and tourmaline are much subordinate. The most widespread accessories are monazite-(Ce), an apatite-group mineral and zircon. Boromullite formed during anatexis of B-rich pelitic rocks under granulite facies conditions (810 °C ≈ T ≥ 775-785 °C, P = 3.3-4 kbar), possibly due to a shift in bulk composition to lower SiO₂ and B₂O₃ contents associated with melt extraction. The assemblage boromullite + cordierite + sillimanite lies at lower SiO₂ and B₂O₃ contents than the assemblage werdingite + cordierite + sillimanite and thus a decrease in SiO₂ and B₂O₃ leads to the replacement of werdingite by boromullite, consistent with textural relations.