Resumo

CRAVERO, Fernanda; MARTINEZ, Gabriel Alexis  e  PESTALARD, Fernando. Halloysite deposits in Mamil Choique, provinceof Río Negro, Patagonia. Rev. Asoc. Geol. Argent. [online]. 2009, vol.65, n.3, pp.586-592. ISSN 0004-4822.

A white material in RíoNegro Province, Patagonia, has been mined as kaolin. Through mineralogicalstudies, the main mineral has been identified as halloysite-(10 Å) with minoramounts of smectite and cristobalite. Halloysite tubes and cylinders are about0.5 µm long and 0.2 µm in diameter. This is the first occurrence of anhalloysite deposit in Argentina and one of the few in the world. Positivedetermination of halloysite-(10 Å), was possible using the technique offormamide intercalation and also due to the extremecare taken during storageand transportation which avoided dehydratation. Once dehydrated, halloysite-(7Å) is formed and easily mistaken for kaolinite. The alteration is developed onrhyolitic pyroclastic rocks of the Huitrera Formation (Eocene) where alkalilixiviation and alumina concentration were the main processes. The origin ofthe alteration has not been unequivocally established but the absence ofminerals and minor and trace elements characteristics of epithermal depositsand a favourable humid temperate climate since the Eocene favours a meteoricorigin. Being halloysite the main mineral of the deposit, it makes theutilization of the mined material to be revised since the properties ofhalloysite are different to those of kaolinite.

Palavras-chave : Halloysite (10 Å); Huitrera Formation; Eocene; Patagonia.

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