NOVEL BEHAVIOR IN FELDSPARS AT HIGH PRESSURE

Paul Ribbe’s interest in feldspars goes back to his PhD research at Cambridge where he showed that low albite had a fully-ordered Al/Si distribution [1]. He then edited “Feldspar Mineralogy” as the second volume in the series “Short Course Notes” (that subsequently evolved into the Reviews in Mineralogy) to which he also contributed five chapters as the primary author. The underlying theme of these chapters was the connection between the microstructural state and measurable macroscopic parameters of feldspars such as lattice parameters. This concept evolved into the identification of three major trends in the response of the feldspar structure to changes in temperature and composition. Since the publication of the RiM volume, numerous studies of feldspars at high pressure have revealed an even richer behavior than expected by extrapolation of the trends found with temperature and composition. They include structural phase transitions in anorthite [2] and sanidine [3], a change in the bonding in microcline [4], and complexities in the equations of state of plagioclase [5]. We have recently completed a high-pressure single-crystal diffraction study of albite which not only displays the complex EoS behavior previously found in plagioclases [5], but also exhibits a new framework distortion and partial collapse at pressures in excess of 8 GPa. These complexities reveal the role of polyhedral compressibility in the response of framework structures in general to pressure and indicate that feldspars will continue to remain the most stringent testing ground of our understanding of the behavior of not just minerals, but the crystalline state.

[1] Ribbe (1963) PhD Thesis; Ribbe et al. (1969) Acta Cryst. B25, 1503

[2] Angel (1988) Amer. Mineral. 73, 1114

[3] Work in progress

[4] Allan & Angel (1997) Euro. J. Mineral. 9, 263

[5] Angel(2004) Contribs. Min. Pet. 146,506.