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.

A new model of bauxitization in quartzitic landscapes: A case study from the Southern Espinhaço Range (Brazil)

“…In places, relict muscovite and quartz may also occur. The significant TiO 2 contents associated with the presence of rutile and anatase in the profiles have two likely origins: (a) hematite‐phyllite, which hosts titaniferous hematite with abundant rutile lamellae (Cabral et al, 2012); and (b) hydrothermal quartz veins containing rutile, anatase and hematite inclusions that frequently crosscut metapelites and metavolcanic units (Cabral et al, 2015; Chaves et al, 2018; Chaves, Brandão, & Buhn, 2010; Chaves & Menezes Filho, 2017; Santos et al, 2020). Titanium is a relatively insoluble element, and it is generally residually enriched towards the surface of weathering profiles.…”