HIGH-RESOLUTION TRANSMISSION ELECTRON MICROSCOPY (HRTEM) STUDY OF STACKING IRREGULARITY IN Fe-RICH CHLORITE FROM SELECTED HYDROTHERMAL ORE DEPOSITS

The structures of Fe-rich chlorite and berthierine and the formation mechanisms of 7 angstrom-14 angstrom interstratified minerals were not previously fully understood owing to the difficulties in analyzing them by X-ray diffraction (XRD). The present study characterizes Fe-rich chlorites in quartz veins of epithermal to xenothermal vein-type ore deposits without later structural modifications, based on high-resolution transmission electron microscopy (HRTEM) along with XRD examination and chemical analysis. Samples have a wide range of Fe/(Fe+Mg) ratios from 0.38 to 0.98 and tetrahedral Al substitution for Si from 0.94 to 1.44 atoms per formula unit (apfu). The variation in Fe content nearly parallels the tetrahedral Al content. The formation temperatures estimated by chlorite geothermometry range. from 190 degrees C to 320 degrees C. In HRTEM, most of the samples showed interstratification between 7 angstrom, 14 angstrom, and/or (in some samples) smectite layers. Chlorites with relatively low Fe contents (Fe/(Fe+Mg) approximate to 0.4) were characterized by mostly 14 angstrom periodicity with the polytype IIbb. In contrast, interstratification of 7 angstrom and 14 angstrom layers predominated with increasing Fe content and the proportion of 7 angstrom layers exceeds 80% in Fe-rich samples with Fe/(Fe+Mg) > 0.9. The 7 angstrom component layer approximated Fe-rich berthierine based on the chemical composition. Layer stacking structures in the Fe-rich samples were complex, and characterized by disorder of 7 angstrom and 14 angstrom layers, differences in the polarity of the tetrahedral sheets, variations of the slant of the octahedral sheets, and positional disorder between octahedral and tetrahedral sheets involving the hydrogen bonding, as indicated from HRTEM observations along the Y, directions of the phyllosilicates. The complex stacking structures observed in Fe-rich samples suggest that irregularity was controlled by neither the Fe/(Fe+Mg) ratio nor the formation temperature; stacking was controlled by kinetic factors in the process of mineral precipitation under disequilibrium conditions.