Genetic and chemical relationships between berthierine, chlorite and cordierite in nodules associated to granitic pegmatites of Sierra Albarrana (Iberian Massif, Spain)

Abstract

Chlorite and berthierine occur through alteration of cordierite within enclaves of metamorphic rocks transformed by the Sierra Albarrana pegmatites. The coexistence of both phyllosilicates allows us to study their stability relationships and to compare their chemical compositions. Samples showing incipient replacement of cordierite by small cryptocrystalline aggregates can be identified by X-ray diffraction (XRD) as berthierine with small quantities of chlorite. Electron Microprobe (EMP) analyses give mixed compositions of berthierine and cordierite. Samples with extensive replacement of cordierite by aggregates show similar characteristics to those with incipient replacement, but some small crystals are present. The last type of sample shows complete replacement of cordierite by crystals showing optical properties of chlorite and EMPA compositions coherent with chlorite or berthierine. Their XRD pattern corresponds to chlorite and their high resolution transmission electron microscopy (HRTEM) images only show perfect sequences of 14 A lattice fringes. The cryptocrystalline aggregates of the samples with incipient and extensive replacement present coexistent areas of 14 A and 7 A lattice fringes that are intergrown at different levels: (1) large areas (> 1 μm) of 7 A layers; (2) packets of 7 A layers between 14 A layer areas, with visible 7 A to 14 A lateral changes; (3) random mixed-layers 7 A/14 A. Chlorite is the final stable product of alteration of cordierite, with berthierine as an intermediate metastable phase. Energy dispersive X-ray spectrometry microanalyses of 14 A, 7 A and (14+7) A areas show lack of systematic differences in chemical compositions between both phyllosilicates which may be considered as true polymorphs.