Mineralogy and geochemistry of the micas from Santa Elena and El Penon pegmatites, Pampean Pegmatite Province, northwestern Argentina.

This study describes the physical and optical properties, polytypism, chemi cal composition and paragenesis of a Li-Al-bearing mica suite from the nort hernmost pegmatite field of Pampean Pegmatite Province. The micas are from two possible cogenetic rare-element pegmatites: Santa Elena, a complex-type , petalite subtype pegmatite, and El Penon, a beryl-type, beryl-columbite-p hosphate subtype pegmatite. The micas are muscovite, lithian muscovite, mix ed forms and lepidolite. The muscovite s.l. has 2 M-1 structure and chemist ry with comparatively low contents of Mn, Li, Rb, Cs, TI and Y; one specime n could be classified as a 'rose muscovite'. The lepidolite and mixed forms have 2 M-2 dominant polytypism with some scarce X-ray reflections of 1 M o r 2 M, polytypes. They are higher in SiO2, MnO, CaO, Li2O, Rb2O, Cs, TI, Y and P. Li2O ranges between 3,71 and 4,73%, and Rb2O from 0,29 to 2,53%. The geochemical evolution of the micas, illustrated by K/Rb versus lithium, ma nganese, cesium, thallium and ytrium diagrams, shows a differentiation tren d similar to micas from other LCT pegmatite fields. The inferred crystalliz ation path has evolved approximately around 2-3 kbar and 350-400 degrees C, in the stability field of spodumene, in a H2O-saturated environment, under increasing HF, KF and LIF activities.