Las micas blancas del manto del Veleta (cordillera Bética, España)

Resumo

We have studied the white micas of the lower formation of the Nevado-Filabride complex in Sierra Nevada (Spain). In the study with ESEM and chemical spectra (EDX) we have distinguished several types of micas: phengite, paragonite, Na-K mica and mica with interlaminar cations deficit. The XRD patterns indicate the presence of potassium mica (9.99 Å), paragonite (9.63 Å), ammonium mica (10.16-10.20 Å) and micas with intermediate compositions sodium-potassium (9.75 Å). The presence of N has been confirmed by elemental analysis and the presence of NH4 +, by FTIR. EMP study revealed that NH4-mica have high Si4+ content (up to 3.51 apfu), and the octaedrical cation content in dioctahedral micas is higher than the ideal. TEM study shows that the mineral assemblage is similar in S1 and S2, and that mica were formed during the initial stages of the diagenesis and metamorphism and persisted through several deformation and metamorphism events without major changes. The results call into question the assumed stability of ammonium micas at low temperature and the metastability of the sodium-potassium micas.