Epidote Characterisation in a Mexican Geothermal Field

Abstract

Epidote is a common mineral in geothermal fields. A dating process using epidote has been established. However, it requires determining degree of crystallinity, elemental composition and impurities in the epidote. Epidote characterization was performed in eleven drill m e drillings from three production areas in the Lo? Azufres geothermal field in M6xico. Epidote minerals were separated from samples at depths between 250m and 2500m. X-ray diffraction analysis allowed us to identified three kinds of crystallographic groups: epidote, piemontite and clinozoisite. Quartz, albite, clinochlore, calcite and anorthite were also identified, No epidotes were present before the first IlOOm, aRer that depth all samples contained epidotes. Morphology of all epidote minerals, observed under a low vacuum scanning electron microscope (SEM) showed well shaped crystals, and an X-ray microprobe attached io the SEMI permitted to identify elemental composition of single crystals in areas, as small as 1 pm2. The aluminium concentration is indicative of both epidote and clinozoisite, whereas manganese indicative of piemontite could not be identified for being in concentrations below the detection limit (0.7 %). Manganese and other metal impurities, in the range of a few ppb and ppm, were assessed in same samples by means of neutron activation analysis.