TY  - GEN
AU  - Colombo,Fernando
AU  - Lira,Raúl
AU  - Pannunzio Miner,Elisa V.
ED  - Asociación Geológica Argentina
TI  - Mineralogical characterization of topaz from miarolitic pegmatites and w-bearing greisen in the a-type El Portezuelo granite, Papachacra (Catamarca Province)
KW  - Granito tipo a
KW  - Pegmatita
KW  - Topacio
KW  - Catamarca
KW  - El Portezuelo
KW  - Artículo
N1  - Relevamiento: 2009; Artículo en pdf (RAGA 64_2_194.pdf)
N2  - The El Portezuelo Granite (Catamarca Province, NW Argentina), probably of Late Devonian or Early Carboniferous age, was emplaced at = 2 kbar and displays the chemical characteristics of an A-type granite. It hosts small miarolitic pegmatites and dikes that belong to the NYF petrogenetic family, miarolitic class, miarolitic-REE subclass, with features more similar to those reported for the gadolinite-fergusonite type. Topaz is widespread both in pegmatites and in a W-bearing greisen genetically related to the granite. In pegmatites it occurs as crystals up to over 15 cm long, showing the forms {120} and {041} (dominant), and also {100}, {010}, {001}, {110}, {130}, {230}, {410}, {150}, {340}, {201}, {203}, {011}, {021}, {031}, {111}, {112}, {221}, {223}, and {445}. Crystals are colorless to pale yellow, more rarely brownish or bluish. Electron-microprobe analyses of gemmy crystals from pegmatites and greisen show that topaz is F-rich (between 19.01 and 20.12 wt. por ciento F, most crystals being slightly zoned) and has very low MgO and TiO2 (= 0.02 wt. por ciento), low FeO (=; 0.03 wt. por ciento), and up to 0.10 wt. por ciento CaO. Refractive indices and unit-cell dimensions reflect the F-rich nature of the studied topaz. This occurrence produces the finest crystals yet known from western South America but probably has no economic potencial as gemstock producer
ER  -