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 H2O. 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 (H2O-NaCl), with low salinity, low density and total homogenization temperature between 243º-315º C, is compatible with final temperatures in the pegmatite pocket zone.摘要：绿柱石通常赋存于花岗伟晶岩（granite-pegmatite）体系中。在其晶质结构中引入钒（V）、铬（Cr）、锰（Mn）、铁（Fe）等致色元素（chromophore elements）可改变绿柱石的体色，进而造就了全球诸多名贵宝石品种，如祖母绿、摩根石、金绿柱石与海蓝宝石。Massangana多阶段岩基是巴西朗多尼亚州知名的锡石、黑钨矿及宝石矿床，该矿床中的金属矿物与蓝晶类宝石（黄玉与海蓝宝石）均赋存于富长石的花岗伟晶岩岩体中。海蓝宝石晶体的体色呈浅至中等蓝色，并发育同心生长环带。电子探针显微分析（electron-probe microanalyses）结果显示，铁是主要的致色元素，占据八面体配位的铝位点；而钠（Na）与水（H₂O）一同赋存于通道位点，在电荷平衡中发挥关键作用。海蓝宝石成核与生长过程中铁、钠供给的不稳定性，是导致其体色变化的核心成因。流体包裹体（fluid inclusion）研究表明，海蓝宝石形成于低盐度、低密度的H₂O-NaCl水溶液体系中，整体均一温度介于243℃至315℃之间，该条件与伟晶岩囊带的最终形成温度相吻合。