• journal_title：European Journal of Mineralogy
• Contributor：Roberto BRAGA ; Athos CALLEGARI ; Bruno MESSIGA ; Luisa OTTOLINI ; Maria Rosaria RENNA ; Riccardo TRIBUZIO
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：2003-
• Format：text/html
• Language：en
• Identifier：10.1127/0935-1221/2003/0015-0393
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：15
• issue：2
• firstpage：393
• section：Articles

A granulite from the Sondalo femic complex, Italian Central Alps, contains prismatine, the boron-rich member of the kornerupine group. This is the first report of prismatine in the Alps. The granulite consists of albite-rich plagioclase + cordierite + sillimanite + rutile + hercynite + corundum + quartz and is interpreted as a restite formed after partial melting of amphibolite-facies tourmaline-bearing metasediments at about 900°C and 0.8 GPa. Prismatine grains, which form a coarse aggregate with tourmaline, biotite and albitic plagioclase, have inclusions of sapphirine, hercynite and corundum. Secondary-ion mass spectrometry (SIMS) analyses on prismatine gives B₂O₃ = 2.30–2.89 wt.%, Li₂O = 0.067–0.125 wt.%, BeO = 0.005–0.007 wt. %, F = 0.32–0.49 wt. % and H₂O = 0.90–1.02 wt.%. The cell parameters a and c and V of the Sondalo prismatine fit with the B₂O₃ co-variation reported in literature. We propose a mechanism of prismatine formation involving the breakdown of tourmaline during the anatexis of the amphibolite-facies metasediments and the development of prismatine as a refractory phase. No intervention of a metaso-matic boron-rich fluid is required. The possible tourmaline breakdown process is the reaction 7.82 tourmaline + 1.03 biotite + 2.41 sapphirine + 3.22 quartz = 8.00 prismatine + 5.93 melt + 1 B₂O₃.