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.
Charge-transfer in ferromagnesian silicates: The polarized electronic spectra of trioctahedral micas
Summary Polarized spectra (2000 to 25000 Å) have been obtained for fifteen analysed trioctahedral micas covering a wide range of compositions, and including four phlogopites, nine biotites, and two lepidomelanes. Three main contributions to the absorption have been noted: charge transfer from oxygen to ferrous iron throughout the visible and near ultraviolet, charge transfer from ferrous to ferric ions at the red end of the visible spectrum and internal d-d transitions of ferrous ions in the near infrared. Substitutions in the brucite layer (especially Ti) cause the O → Fe2+ band to broaden, and thus to encroach on the visible region for vibration directions in the plane of the cleavage flake. The transmission window between these bands is further blocked by the Fe2+ → Fe3+ charge-transfer band, which is polarized in the plane of the flake. The overall effect is to produce very strong absorption throughout the visible region for (β, γ) vibration directions, and relatively weak absorption for α, i.e. the observed very strong pleochroism of biotite. Many features of the spectra of chlorite, amphiboles, and tourmaline, which contain octahedral ions in brucite-like sheets, strips, and fragments respectively, may be interpreted by analogy with biotite. Two types of solid solution effect are described: substitutional broadening, which is responsible for the extension of the O → Fe2+ band into the visible region in biotites and aluminous ortho-pyroxenes, and substitutional intensification, which permits in solid solutions transitions that are forbidden by the conventional selection rules.
SCIENCE AND EMPIRICISM IN PILE FOUNDATION DESIGN
Scientific approaches to pile design have advanced enormously in recent decades and yet, still, the most fundamental aspect of pile design—that of estimating the axial capacity—relies heavily upon ...