Boromullite, Al (sub 9) BSi (sub 2) O (sub 19) , 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 Al (sub 5) BO (sub 9) and Al (sub 2) SiO (sub 5) modules. Electron-microprobe analysis of the holotype prism is SiO (sub 2) 19.01(1.12), TiO (sub 2) 0.01(0.02), B (sub 2) O (sub 3) 6.52(0.75), Al (sub 2) O (sub 3) 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 (sub 0.01) Fe (sub 0.03) Al (sub 8.88) Si (sub 1.93) B (sub 1.14) O (sub 18.94) (normalized to 12 cations), ideally Al (sub 9) BSi (sub 2) O (sub 19) . Overall, in the type specimen, it ranges in composition from Mg (sub 0.01) Fe (sub 0.03) Al (sub 8.72) Si (sub 2.44) B (sub 0.80) O (sub 19.20) to Mg (sub 0.01) Fe (sub 0.03) Al (sub 9.22) Si (sub 1.38) B (sub 1.35) O (sub 18.67) . Single-crystal X-ray diffraction gives orthorhombic symmetry, Cmc2 (sub 1) , a 5.7168(19) Aa, b 15.023(5) Aa, c 7.675(3) Aa, V 659.2(7) Aa (super 3) , calculated density 3.081 g/cm (super 3) , 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., Al (sub 5) BO (sub 9) . The strongest lines in the powder pattern [d in Aa, (I (sub 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(80) (263). Boromullite is colorless and transparent, biaxial (+), n (sub x) 1.627(1), n (sub y) 1.634(1), n (sub z) 1.649(1) (589 nm). 2V (sub z) (meas) = 57(2) (super -) , 2V (sub z) (calc) = 69(12) (super -) . 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 mu m to >10 mu m). Sekaninaite-cordierite, potassium feldspar, biotite, werdingite and its Fe-dominant analog, 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 degrees C nearly equal T > or = 775-785 degrees C, P = 3.3-4 kbar), possibly due to a shift in bulk composition to lower SiO (sub 2) and B (sub 2) O (sub 3) contents associated with melt extraction. The assemblage boromullite + cordierite + sillimanite lies at lower SiO (sub 2) and B (sub 2) O (sub 3) contents than the assemblage werdingite + cordierite + sillimanite and thus a decrease in SiO (sub 2) and B (sub 2) O (sub 3) leads to the replacement of werdingite by boromullite, consistent with textural relations.