Titre du document / Document title

Boralsilite, Al₁₆B₆Si₂O₃₇, and "boron-mullite:" : Compositional variations and associated phases in experiment and nature

Auteur(s) / Author(s)

GREW Edward S. ; GRAETSCH Heribert A. ; PÖTER Birgit ; YATES Martin G. ; BUICK Ian ; BERNHARDT Heinz-Jürgen ; SCHREYER Werner ; WERDING Günter ; CARSON Christopher J. ; CLARKE Geoffrey L. ;

Résumé / Abstract

Boralsilite, the only natural anhydrous ternary B₂O₃-Al₂O₃-SiO₂ (BAS) phase, has been synthesized from BASH gels with Al/Si ratios of 8:1 and 4:1 but variable B₂O₃ and H₂O contents at 700-800 °C, 1-4 kbar, close to the conditions estimated for natural boralsilite (600-700 °C, 3-4 kbar). Rietveld refinement gives monoclinic symmetry, C2/m, a = 14.797(1), b = 5.5800(3), c = 15.095(2) Å, β = 91.750(4)°, and V= 1245.8(2) ų. Boron replaces 14% of the Si at the Si site, and Si orAl replaces ca. 12% of the B at the tetrahedral B2 site. A relatively well-ordered boralsilite was also synthesized at 450 °C, 10 kbar with dumortierite and the OH analogue of jeremejevite. An orthorhombic phase ("boron-mullite") synthesized at 750 °C, 2 kbar has mullite-like cell parameters a = 7.505(1), b = 7.640(2), c = 2.8330(4) Å, and V = 162.44(6) ų. "Boron-mullite" also accompanied disordered boralsilite at 750-800 °C, 1-2 kbar. A possible natural analogue of "boron-mullite" is replacing the Fe-dominant analogue of werdingite in B-rich metapelites at Mount Stafford, central Australia; its composition extends from close to stoichiometric Al₂SiΟ₅ to Al_2.06B_0.26Si_0.76Ο₅, i.e., almost halfway to Al₅BO₉. Boralsilite is a minor constituent of pegmatites cutting granulite-facies rocks in the Larsemann Hills, Prydz Bay, East Antarctica, and at Almgotheii, Rogaland, Norway. Electron-microprobe analyses (including B) gave two distinct types: (1) a limited solid solution in which Si varies inversely with B over a narrow range, and (2) a more extensive solid solution containing up to 30% (Mg,Fe)₂Al₁₄B₄Sι₄Ο₃₇ (werdingite). Boralsilite in the Larsemann Hills is commonly associated with graphic tourmaline-quartz intergrowths, which could be the products of rapid growth due to oversaturation, leaving a residual melt thoroughly depleted in Fe and Mg, but not in Al and B. The combination of a B-rich source and relatively low water content, together with limited fractionation, resulted in an unusual buildup of B, but not of Li, Be, and other elements normally concentrated in pegmatites. The resulting conditions are favorable in the late stages of pegmatite crystallization for precipitation of boralsilite, werdingite, and grandidierite instead of elbaite and B minerals characteristic of the later stages in more fractionated pegmatites.

Revue / Journal Title

The American mineralogist   ISSN 0003-004X   CODEN AMMIAY 

Source / Source

2008, vol. 93, n^o2-3, pp. 283-299 [17 page(s) (article)]

Langue / Language

Anglais

Editeur / Publisher

Mineralogical Society of America, Washington, DC, ETATS-UNIS  (1916) (Revue)

Localisation / Location

INIST-CNRS, Cote INIST : 503, 35400017514648.0030