Experimental evidence on the substitution of Ti in biotite Jtincnr
Various Ti substitutions in biotite were tested by hydrothermal syntheses of Fe and Mg biotites over the range of 600-800 'C at I 000-bars pressure and at the oxidation conditions of the methane-graphite buffer. Yields of greater than 950/o biotite were obtained in a few experiments, but complete syntheses were generally not possible. Microprobe analyses of the run products indicate that (l) the Ti-Tschermak's substitution (Ti + 2Al: R'z* + 2Si) can be the dominant Ti substitution in both Fe and Mg biotites; (2) the Ti-vacancy substitution (Ti + 16r! : 2Rr*) can occur as the dominant Ti substitution in Mg biotite and in conjunction with other substitutions in Fe biotite; and (3) the experimental data do not support, but cannot rule out, the Ti-oxy substitution (Ti + ZO': R2+ + 2OH-). Specific substitution types appear to be governed by bulk composition and by the crystal-chemical constraints of the biotite structure.
Limitations on the interpretation of biotite substitutions from chemical analyses of natural samples
Biotites, as well as other phases, are sufficiently complex chemically that there are numerous sets of linearly independent exchange components with which to describe the composition of a naturally occurring crystal. Even after complete chemical analysis, this leads to ambiguities in the correlation of exchange components with valid substitution mechanisms in complex phases. The best means of identifuing valid substitutions are by the observation of simple endmember compositions, the investigation of compositional changes in zoned crystals or petrogenetic sequences, and by direct experimental synthesis.
Geological, mineralogical and chemical characterization of Devonian kaolinite-bearing sediments for further applications in the ceramic (tiles) industry in La Paz, Bolivia
Clay minerals are widely distributed in Bolivia; among them, illitic clays are most common in the Altiplano where they are mined for use in the ceramic industry that has been growing in the last few years. In addition to illitic sediments, kaolinitic sediments have been recently discovered in sedimentary units in the Bolivian Altiplano. Residual ball clay occurrences in Devonian sedimentary units were studied as part of this work. Geological mapping and geophysical studies (ERT and GPR) were done for better understanding the origin of the deposits and were part of a preliminary study of the mineral potential to define the relationship with the host rock. Chemical and mineralogical techniques such as X-Ray Diffraction, Scanning Electron Microscopy and Inductively Coupled Plasma analyses were performed in samples from the studied area to verify the presence of kaolinite. Atterberg limits and behaviour of the raw material in ceramic specimens supported by chemical analyses show that this material is suitable for manufacturing tile ceramics. This study provides fundamental knowledge for deposit exploitation and future generation of an alternative source of employment for the inhabitants of Micaya.