Thermodynamics and lattice vibrations of minerals: 2. Vibrational characteristics of silicates

Abstract

General lattice vibrational properties of minerals, particularly those properties which strongly influence the thermodynamic behavior, are discussed in this paper (the second of a series relating thermodynamic and lattice vibrational properties of minerals). Infrared (IR), Raman (R), and inelastic neutron scattering (INS) data are summarized, if known, for the following minerals: halite (INS), periclase (INS), corundum (IR, R), brucite (IR, R, partial INS), quartz (R, IR, INS), cristobalite (IR, R), silica glass (IR, R, partial INS), stishovite (IR), rutile (R, IR, INS), albite (IR, R), microcline (IR), spinel (IR, R), muscovite (IR, R), jadeite (IR), diopside (IR, R), enstatite (IR, R), olivine (IR, R), zircon (IR, R), kyanite (IR), andalusite (IR), sillimanite (IR), pyrope (IR), andradite (IR), grossular (IR), and calcite (IR, R, INS). New high‐resolution infrared data are given for kyanite, andalusite, sillimanite, stishovite, coesite, jadeite, diopside, pyrope, andradite, and grossular. The data presented and summarized show that optic modes of vibration of silicates are spread over a range of frequencies, generally from wave numbers as low as 75 cm −1 to wave numbers as high as 1200 cm −1 . The far‐infrared (low frequency) optic modes are particularly important in this study because they strongly influence the low‐temperature heat capacity and therefore the entropy. The mid‐infrared (high frequency) optic modes generally arise from the vibrations of tightly bound clusters within the structures and can generally be recognized as isolated bands in the vibrational spectrum. A method is given for enumeration of the fraction of total vibrational modes which are internal stretching modes. Inelastic neutron scattering data are reviewed to show the magnitude of anisotropy and dispersion of acoustic modes and of dispersion of optic modes in the minerals. The combined infrared, Raman, and INS data show that, excluding the stretching modes of quasi‐molecular clusters within the minerals, the optic modes do not appear to follow a simple recognizable distribution, such as a Debye distribution, but are rather uniformly distributed across a broad range of frequencies.