Heat capacities of Tschermak substituted Fe-biotite

Six members of the annite-siderophyllite join were synthesized in a three s tep process - crystallization of biotite from gels, decomposition of the fi ne-grained biotite under oxidizing conditions and resynthesis of FeAl bioti te with planned compositions from these products - producing biotite crysta ls with thicknesses of up to 10 mu m. The biotite was characterized by micr oprobe, electron microscopy and X-ray diffraction. Heat capacities of these biotites were measured with a DSC (differential scanning calorimeter) over the temperature range 143 to 623 K. Using a least-squares technique, the d ata were fitted to a c(p)-polynomial, c(p) = k(0) + k(1)T(-0.5) + k(2)T(-2) + k(3)T(-3). In the temperature range 143 to 250 K, heat capacities of the different annite-siderophyllite members decrease linearly with increasing Al content. At higher temperatures, however, the c(p)-polynomial of biotite s with intermediate composition (except Ann(79)Sid(21)) exhibit a steeper s lope than those of other biotites. This produces positive excess heat capac ities in the annite-siderophyllite join at higher temperatures. The activit y-composition data of the same binary derived from phase equilibrium experi ments (Benisek et al. 1996) and the data of this study suggest two composit ional regions along this join, with different extent of deviation from idea lity. One at X-Sid < 0.3, characterized by a small deviation, one at X-Sid > 0.3 showing a higher nonideality, resulting in a discontinuity visible at this composition. Powder IR-spectra of these solid solutions were measured with a FTIR-spectrometer and used to calculate heat capacities according t o the vibrational model of Kieffer (1979). The comparison of the vibrationa l function with the c(p)-polynomials shows that the vibrational function re produces well the DSC-data of the siderophyllite-poor and -rich members, bu t deviates for intermediate compositions, where the excess heats of mixing occur. With increasing Tschermak vector, the tetrahedral rotation angle a i ncreases from 0 to 13 degrees for annite to siderophyllite, respectively. A t the composition of the discontinuity, this rotation angle alpha reaches a value of similar to 8 degrees. The processing of similar to 300 chemical d ata of natural biotites indicates that over 90% of them have a tetrahedral rotation angle that lies between 7 and 9 degrees. It would appear that biot ites with these structural characteristics are most stable.