Aquamarine from Massangana batholith, Rondônia State: mineral chemistry and fluid inclusion data

Abstract

Beryl is usually found in granite-pegmatite systems. The addition of chromophore elements (V, Cr, Mn, Fe) into the crystalline structure favors color changes in beryl and thus generates some of the world's expensive gems such as emerald, morganite, heliodor and aquamarine. The Massangana polyphasic batholith is a well-known cassiterite, wolframite and gems deposit in the Rondônia state. These metals and blue-gems (topaz and aquamarine) are located in feldspar-rich pegmatite granite bodies. The aquamarine crystals show color ranging from light-to medium-blue and display concentric growth zones. Electron-probe microanalyses revealed that the Fe is the main chromophore element, occupying the octahedral Al-site, while Na had an important role in the charge balance, inserted in the channel sites together with H 2 O. The irregular supply of Fe and Na during the nucleation and growth of aquamarine was the main cause for the color change. A fluid inclusion study indicated that the aquamarine growth under an aqueous fluid system (H 2 O-NaCl), with low salinity, low density and total homogenization temperature between 243 o -315 o C, is compatible with final temperatures in the pegmatite pocket zone.