GAGARINITA-(Y) COM POLIMORFO DE FLUOCERITA: PROVÁVEL CASO DE FORMAÇÃO DE UM NOVO MINERAL POR EXSOLUÇÃO DE ETRL A PARTIR DE FLUORETO DE ETRLP (MINA PITINGA - AM)

Abstract

In Pitinga mine, the primary ore (Sn, Nb, Ta and cryolite) is associated to albite granite facies of Madeira granite. The cryolite ore occurs disseminated and in a massive deposit. The (Y)-gagarinite occurs in the low leveis of the cryolite massive deposit. The crystals show anhedral, up to 7mm, which are either interstitial or included in the criolite. Its composition (Na 0,24 Ca 0,58 Y 1,01 ETR 0,39 F 5,81 ) is characterized by enrichment in HREE compared with LREE. ln gagarinite occur LREE fluoride crystals distributed with several patterns, varying from irregular and without orientation until to strings and stringlets forms, interpretated as result of exsolution processo The composition ofthe exsolved phase (Ce 0,53-066 La 0,09-0,26 Nd 0,08-0,26 Sm 0,01-0,04 Eu 0,01 Y 0-0,03 F 3,3-4,14 ) is similar to that off luocerite, which peaks, however, don't appear in the XR15. The REE pattern of this phase shows fractionation with continuous HREE depletion. The initial gagarinite (Na 0,23 Ca 0,53 ETRa 0,99 Y 0,48 F 7,69 ), established according to modal proportions and densities of host and exsolved phases, has LREE content much higher than that found in natural gagarinites. The mineral system before exsolution may be described as a solid solution, forming a meta-stable gagarinite with LREE content exceptionally high, whose presence was compensated by vacancies in the VI sites. Decreasing in temperature caused LREE exsolution. The residual fase kept Y, HREE and Na contents, establishing a structure with less vacancies and better charge balance. The exsolved phase, with a cation/fluorineratio=1/3, is unlike to have a structure similar to that of the gagarinite. Further investigation will be necessary to define its nature and its relationship with fluocerite.