RAMAN-SPECTROSCOPIC STUDIES OF CARBONATES .1. HIGH-PRESSURE AND HIGH-TEMPERATURE BEHAVIOR OF CALCITE, MAGNESITE, DOLOMITE AND ARAGONITE

The room-temperature Raman spectra of aragonite, magnesite and dolomit e have been recorded up to 30 GPa and 25 GPa, respectively and no phas e changes were observed during compression, unlike calcite. The effect of temperature on the room-pressure Raman spectra of calcite, aragoni te, magnesite and dolomite is reported up to 800-11 00 K. The measured relative pressure.and temperature-shifts of the Raman lines are great er for the lattice modes than for the internal modes of the CO3 groups . These shifts are used to calculate the mode anharmonic parameters of the observed Raman modes; they are negative and their absolute values are smaller (close to 0) for the internal CO3 modes than for the latt ice modes (4-17 10(-5) K-1). The temperature shifts of the lattice mod es in calcite are considerably larger than those for dolomite and magn esite, and a marked non-linear increase in linewidth is observed above 400-degrees-C for calcite. This is consistent with an increasing rela xational component to the libration of the CO3 groups about their thre efold axes, premonitory to the rotational order-disorder transition at higher temperature. This behaviour is not observed for the other calc ite structured minerals in this study. We examine systematic variation s in the lattice mode frequencies and linewidths with composition, to begin to understand these differences in their anharmonic behaviour. F inally, we have used a simple Debye-Waller model to calculate atomic d isplacements in calcite, magnesite and dolomite with increasing temper ature from the vibrational frequency data, to provide a direct compari son with atomic positional data from high-temperature structure refine ments.