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Discovery of a new high-temperature borosilicate mineral: boromullite

Ian Buick1, Ed Grew2, Thomas Armbruster3, Olaf Medenbach4, Martin Yates2, Gray Bebout5 and Geoff Clarke6.

1Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2Dept. of Earth Sciences, University of Maine, Maine 04469-5790 USA
3 Institut für Geologie, Mineralogie-Kristallographie, Universität Bern, Bern, Switzerland
4Institut für Geowissenschaften / Mineralogie, Ruhr-Universität Bochum, D-44780 Bochum, Germany
5Dept. of Earth & Environmental Sciences, Lehigh University, Bethelem, PA 18015 USA
6School of Geosciences, University of Sydney, NSW 2006, Australia

New research this year has focussed on the characterisation of a new borosilicate mineral, named boromullite. The sole occurrence of this mineral is in a sequence of unusually boron-rich, low-pressure (~3kbar) granulite-faciesmetapelitesfrom Mt. Stafford, central Australia. Boromullite isa prismatic, colourless mineralthat occursas bundles of prisms up to 0.4 mm long, typically as fringes or overgrowths on aggregates of sillimanite or as narrow overgrowths around embayed prisms ofwerdingite, anotherrareborosilicatemineral (known from only 6 localities worldwide). Boromullite and sillimanite are locally intergrown on a fine (1-100µm) scale (Fig. 1). The boron-rich granulites additionally contain Fe-cordierite, potassium feldspar, biotite, hercynite and ilmenite, with other B-rich  minerals  (grandidierite and tourmaline) present locally.

Bormullite is compositionally heterogeneouson evena1-5 micron scale (Fig. 1) and in the type specimen ranges in composition from Mg0.00Fe0.01Al4.36Si1.22BO9.60 to Mg0.00Fe0.02Al4.60Si0.68B0.70O9.47 (Fig. 1). Structural refinements of the holotype crystal shows that it corresponds to a 1:1 polysome of Al2SiO5 and Al5BO9 modules (Fig. 2). 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 holotype crystal has a measured composition of Mg0.01Fe0.02Al4.44Si0.97B0.57O9.47. Boromullite is the first recorded natural analogue of a group of B2O3-Al2O3-SiO2 phases ("boron-mullites") that have previously been synthesised experimentally and that are of relevance to the ceramics industry.

Boromullite at Mt. Stafford formed during anatexis of B-rich metapelitic rocks under granulite facies conditions (T 3 ~790°C, P ~ 3.6 kbar), mostly from the reaction of werdingite with siilimanite/andalusite, but possibly in some cases from the incongruent melting of werdingite. It also occurs as feathery intergrowths withincoarse-grained blockyprismaticsillimanite (Fig.1), raisingthe possibilitythatitmay alsobea previouslyunrecognised prograde mineral in boron-richrocks.

Figure 1: a) compositionalvariationin typesample 2006-MST22 (circles) and "boron-mullite" synthesized in the presence of melt (triangles); b) and c) BSE images of boromullite