Anatase crystals of unusual habit in quartz veins from the Diamantina region (Espinhaço Range, Minas Gerais)

Abstract The Espinhaço Range, in the Diamantina region (central part of the State of Minas Gerais), is known worldwide as a famous source of associated occurrence of quartz, rutile and/or anatase crystals. The latter is also a classic accompanying mineral of diamonds in the alluvial placers. Primary anatase is related to hydrothermal quartz veins cutting geological units of the Espinhaço Range. In the last decade, outstanding large sized anatase crystals (up to 4 cm) in such veins have been reported from two areas, showing unusual habits and partial intergrowth with rutile. They are colorless, ocher brown, or golden yellowish, often double terminated, and iron rich. Due to their strong adamantine luster and pseudo-octahedral habit, some resemble diamonds. Two different morphological types have been studied: a simple crystal habit solely with {011} faces, and double terminated complex anatase crystals with one or more of the following forms: {011}, {012}, {013}, {017} and {112}. The geological environments as well as general mineralogical characteristics are also described.

摘要 位于米纳斯吉拉斯州（Minas Gerais）中部迪亚曼蒂纳地区（Diamantina region）的埃斯皮尼亚苏山脉（Espinhaço Range），是全球知名的石英（quartz）、金红石（rutile）与锐钛矿（anatase）伴生产出的经典产地。锐钛矿也是冲积砂矿（alluvial placers）中金刚石（diamond）的典型伴生矿物。原生锐钛矿赋存于切割埃斯皮尼亚苏山脉各地质单元的热液石英脉（hydrothermal quartz veins）中。近十年来，已有研究在两处该类脉体中报道了尺寸可达4厘米的大型锐钛矿晶体，其晶形罕见且与金红石存在部分连生。这些晶体呈无色、赭褐色或金黄调，常为双终端晶形且富铁。凭借强烈的金刚光泽与假八面体晶形，部分晶体外观酷似金刚石。本次研究对两种不同的形貌类型展开了分析：一种为仅发育{011}晶面的简单晶形，另一种为带有以下一种或多种晶形的双终端复杂锐钛矿晶体：{011}、{012}、{013}、{017}及{112}。本文同时阐述了该类晶体产出的地质环境与基本矿物学特征。