The high temperature behavior of trace hydrous components in silicate minerals

Abstract

We have studied the high temperature behavior of water and hydroxide in quartz, feldspar, topaz, zircon, muscovite, cordierite, and beryl using high temperature infrared spectroscopy. We have directly observed processes such as dehydration and changes in hydrogen speciation. In some minerals trace hydroxyl and water speciation and properties at temperatures of geologic interest can be dramatically different from those at 25°C. In muscovite, no changes in speciation occur prior to dehydration at 750°C, whereas in topaz hydroxyl sites interconvert at 500 °C. In metamict zircon strongly hydrogen-bonded hydroxyl is preferentially lost from a continuum of sites during continuous dehydration occurring from 400° to 900°C. There are only minor changes in the O-H region spectrum of natural quartz at the ɑ-β transition point. In feldspar one type of molecular water is lost at ~200°C, and at 600° to 800° a second water type converts irreversibly to a new hydrous species. Changes at high temperature common to the infrared absorption bands of all minerals studied are: broadening, a shift to lower wave numbers, and a slight decrease in integral intensity. Temperature coefficients for 0-H stretching peak shifts range from 0 to -0.045 cm^(-1)/°C. Lattice modes also broaden and shift to lower wave numbers, typically with temperature coefficients of about -0.03 cm-^(-1)/°C.

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