Polimorfos del TiO2 de aplitas-pegmatitas en Papachacra, Provincia de Catamarca

COLOMBO, FERNANDO

San Salvador de Jujuy

Congreso; 9º Congreso de Mineralogía y Metalogenia; 2008

Asociación Mineralógica Argentina

Rutile, brookite and anatase occur in aplite-pegmatites and pegmatitic pods related to the epizonal A-type El Portezuelo granite. Anatase is found as amber to black striated crystals up to 1 mm long, displaying the forms {101} (prominent) and {001} (small). It is chemically rather homogeneous and contains up to 0.67% Ta2O5, 1.83% Nb2O5, 2.48% Fe2O3 and 0.32% MnO. Brookite occurs as mm-sized light yellow tabular crystals or as an alteration product of ilmenite, mixed with rutile. Rutile is the most abundant TiO2 polymorph; it occurs as: 1) single homogeneous grains, 2) as grains with exsolutions of tantalite-(Fe), and 3) as pseudomorphs after ilmenite. It displays chemical zoning, with rims enriched in Ta and Nb (up to 5.98 % Nb2O5 and 8.66 % Ta2O5); Fe/(Fe+Mn) ranges between 0.80 and 0.99, while Nb/(Nb+Ta) is between 0.51 and 0.83. Rutile associated with tantalite-(Fe) is very Ta-rich (37.29% Ta2O5, 8.54% Nb2O5). Most analyses show an excess of Fe over that required to balance M5+. Ilmenite occurs as euhedral crystals (now mostly replaced by TiO2) that are Mn-poor (up to 0.33% MnO); its Nb content (0.34% Nb2O5) is normal for ilmenite from pegmatitic environments, but it contains rather high Ta (0.10% Ta2O5). Rutile and ilmenite are the major Ta and Nb carriers in El Portezuelo granite, and they are also a sink for these elements in aplite-pegmatites, although in these last rocks probably most of the Ta and Nb are hosted by pyrochlore-group minerals and complex Y-Ta-Nb oxides. F played an important role in the concentration of these HFSE.