VERY LOW-GRADE CHLORITE WITH ANOMALOUS CHEMISTRY AND OPTICAL-PROPERTIES FROM THE MALAGUIDE COMPLEX, BETIC CORDILLERAS, SPAIN

Two generations of chlorite, in very low grade metaclastites from the Malaguide Complex (Betic Cordilleras, southern Spain) ascribed to Vari scan and to Alpine metamorphic events, have been characterized by micr oscopy, chemical analysis, XRD, IR and DTA-TG. All of these chlorites show abnormal optical properties and low Al-IV/Al-VI ratios if all iro n is considered as ferrous iron. Nevertheless, results of electron-mic roprobe analyses show a slight excess of H2O, and a chemical determina tion of ferrous iron indicates Fe3+/Fe2+ ratios in the order of 0.25. On the basis of the octahedral occupancy, these chlorites may be consi dered as a transition from tri-to di,trioctahedral minerals. Although these chlorites differ widely from Al-rich di, trioctahedral chlorites (sudoite or cookeite) in Al content, their di,trioctahedral nature is also supported by the IR spectra and the thermal behavior. Optical pr operties also agree and correlate with an appreciable Fe3+ content. Th e chlorites evolve during the Variscan metamorphism by showing a sligh t increase in Fe content. Prograde Alpine metamorphism produced a simu ltaneous increase in tetrahedral Al and Fe/(Fe + Mg).