Alunites and REE rich APS minerals associated to hydrothermal clay deposits in the Barker area, Tandilia, Argentina

MARTÍNEZ, J.C.; DRISTAS, J.A.

Tokio, Japón

Congreso; XIII International Clay Conference; 2005

The Clay Science Society of Japan

Chemical bulk analyses of previously XRD determined intermediate alunite–natroalunite minerals indicate anomalous high contents of REE, Sr, Ba, Ca and P. The alunite in the collected samples occurs as veinlets or nodules cross–cutting different hydrothermal clay deposits, containing pyrophyllite, kaolinite and sericite, in the west Barker area (Dristas & Frisicale 1983, 1992). Petrographic studies reveal a pseudocubic habit for the alunite. No significant zoning has been detected in this mineral by EPMA backscattered images. Additionally, EPMA and EDX studies indicate that the tabular-like inclusions in cores of alunite contain significant amounts of P, Ba, Sr, REE, Ca and Fe. Alunite shows epitaxial growth on the inclusion mineral It suggested that the tabular-like inclusions correspond to APS (aluminium phosphate sulphate) minerals, such as florencite, formed at early stages before alunite crystallization from a solution with a high phosphorous/sulphur ratio. The crystal structure of APS minerals are suitable for REE, Sr and Ba substitution, since metal cations are situated in large cavities formed by 6-membered rings of M(O,OH)6 octahedra. A later increase of solution sulfurization would favor the formation of alunite that constrained the REE, P, Sr,Ca and Ba fixation.