A RIETVELD AND INFRARED STUDY OF SYNTHETIC AMPHIBOLES ALONG THE POTASSIUM-RICHTERITE-TREMOLITE JOIN

Amphiboles were synthesized at 750 degrees C, 1 kbar (H2O) for composi tions at 20% intervals along the join potassium-richterite-tremolite. Structural variations, site occupancies, and modal analyses of the exp erimental products (amphibole + minor diopside, quartz, and enstatite) were characterized by Rietveld structure refinement, with final R-Bra gg indices in the range 4-6%, and by infrared spectroscopy in the prin cipal OH-stretching region. Amphibole compositions were determined by (1) site-scattering refinement fur the A and M4 sites that are occupie d by (K, square) (square = vacancy) and (Na,Ca), respectively; and (2) mass-balance calculations involving the modal analysis and the nomina l experimental product composition. These measurements agree within 1% absolute and show close agreement with electron-microprobe compositio ns for the two samples that we could analyze. Deviations from nominal amphibole composition are up to 19% absolute. The resulting relations between cell dimension and composition are linear. The major change in cell dimensions is a decrease of 0.25 Angstrom in Angstrom with incre asing tremolite component. The infrared spectra show two principal pea ks at 3735 and 3675 cm(-1), corresponding to the local arrangements Mg MgMS-OH-K-A (the Kr band) and MgMgMg-OH-(A) square (the Tr band), resp ectively. The relative variation in peak intensity as a function of am phibole composition shows that the molar absorptivities of the two ban ds are significantly different. The ratio of the molar absorptivities for the two bands is 2.2.