Estudio mineralógico y cristaloquímico de la casiterita del yacimiento de Barquilla (Salamanca, España)

Abstract

The mineralogical and crystallochemical characteristics of cassiterite from the Barquilla deposit (Salamanca, Spain) are described in this paper. The analytical procedures have been microscopy, electron microprobe and electron paramagnetic resonance. Two generations of this mineral have been recognized: *Cassiterite I, corresponding to a pegmatitic episode, characterized by a high content of Fe (maximum 5% cationic), Nb and Ta (maximum 6% cationic) and lower of Ti (maximum 2% cationic). The analysis of zones and patches of different shades show the possible existence of coupling chemicaloptical zoning. *And cassiterite II, disseminated at sericite, mineral that corrodes the other minerals, in which Fe, Nb and Ta contents undergo an important decreased (maximum 1.8%, 3.6% and 0.88%, respectively) about cassiterite I. Colour and morphology of both cassiterite crystals indicate high temperature environments (>500°C), probably lower for cassiterite II because its lesser content in rare elements. The interpretation of interelement relationships, the cationic ratio Fe+Mn/Nb+Ta and the spectrometric results allow to propose and discusse the following sustitutions for the incorporation of these elements at the cassiterite structure: 3Sn4* ___ Fe2* + 2(Nb,Ta)5\ 2Sn4* __ Fe3+ + (Nb,Ta)5+, Sn4+ + O2' ___ Fe3* + OH , Sn4* ___ Fe3* + H*, Sn4* ___ (Nb,Ta)4* y 5Sn4* ___ 4(Nb,Ta)5* + cationic vacancy.