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Unusual anatase crystal from the Diamantina region (Minas Gerais, Brazil)

Unusual doubly terminated anatase crystals were found in aluvial deposits near Diamantina, Minas Gerais, Brazil. They are colourless to ochre brown and black in colour with frequently octahedral habit resembling to diamonds. The crystals are usually small with the largest individuals not exceeding 2 cm. They are rarely rounded indicating short transport. Two types of anatase crystals were investigated. One type has simple crystal habit only with faces of {;011};present. Another type occurs in a form of complex dipyramidal crystals in parallel growth resembling to small swards. The central crystal is the biggest one and every further crystal is smaller. This formation ends up with {;013}; and {;017}; form. It frequently happens for the smaller crystals to obey parallel growth both on the top and the bottom of the central crystal. Sometimes they are arranged at the corners of the central crystal like satellites containing pinacoid beside dipyramids. Also, these complex formations can vary in respect to the size of the smaller crystal, but also can contain additional forms, for instance {;112};. X-ray diffraction experiments on powdered samples confirm the presence of anatase and rutile. The calculated unit cell parameters for anatase are a=3.773(2) &Aring ; , c=9.475(9) &Aring ; and V=134.9(2) &Aring ; 3, suggesting no important substitutions in its crystal structure. Along rutile, two types of mica, zircon, monazite-(Ce) and Mg-Fe carbonate were identified using SEM equipped with EDS. The relationship between anatase and rutile seems to be complex since rutile was found in a form of needle-like inclusions in anatase, but also some complete psudomorphs of rutile after anatase with preserved anatase morphology were collected. These different types of anatase-rutile occurrence impart contradictory interpretations of their formation, as rutile included in anatase can be considered to have formed first, but the one found as a pseudomorph on anatase obviously grew later.

anatase; parallel growth; Diamantina region; Brazil

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