T135 Frontiers in Experimental Petrology: In Honor of Robert C. Newton

Monday, 1 November 2010
1:30 PM - 5:30 PM
Room 203

STRUCTURE-BASED THERMODYNAMIC PROPERTIES OF FELDSPARS

Paper number: T135-8
Program: Technical Sessions
Time: 3:15 PM – 3:30 PM

Authors

Abstract

The complex and often non-linear structural response of framework minerals to changes in pressure, temperature, and especially composition, invalidates the naïve use of simple solution models to define the thermodynamic properties and elasticity of real minerals; for example, the bulk solid solution properties do not apply to the dilute limit (the plateau effect). The solution is to develop models to predict the thermodynamic and elastic properties of mineral solid solutions based directly upon crystal chemistry and that implicitly incorporate the atomic-scale mechanisms of the structural response of the mineral to the intensive thermodynamic variables. For example, in our previous work we have shown that the bulk modulus and shear modulus of perovskites scales linearly with the ratio of compressibilities of the two cation sites within the structure, because this ratio controls the tilt rates of the octahedra within the structure.

The rigid-unit deformations of the tetrahedral framework of feldspars can be decomposed in to four tilts of the four tetrahedra that comprise the 4-rings that lie parallel to (010). Analysis of the high-pressure and high-temperature data now available, from both experiment and DFT calculations, shows that two tilt mechanisms are dominant in alkali feldspars. In particular, we have found that changes in the wrinkle tilt are responsible for changes in the length of the feldspar crankshaft, and are thus responsible for 70% of the volume change of alkali feldspars with P, T, or X. This provides the structural mechanism to explain the observation (Hovis et al. 2008) that the thermal expansion of alkali feldspar scales with the room-pressure volume. This shows that further development and calibration of this model with experimental data will allow a complete description of the thermodynamic properties of feldpspars from structure alone.

Hovis et al. (2008) Am. Min. 93,1568.

This work was supported in part by NSF grant EAR0738692 to NL Ross and RJ Angel