EFFECT OF DISORDER ON THE BULK MODULI OF PLAGIOCLASE FELDSPARS

Feldspars comprise 60 percent of the Earth's crust and are one of the archetypical framework silicates with the basic chemical formula of AT₄O₈. The A cation can be Ca, Ba, K or Na. The T- cation sites are occupied by Al and Si. Feldspars are subdivided based on the A site occupancies; the plagioclase feldspars contain Na and Ca, alkali feldspars contain Na and K, and celsian contains Ba. Bulk moduli and pressure-volume relationships are important thermodynamic and elastic properties of materials, because they provide critical information for extracting thermodynamic data from petrological experiments and for the interpretation of seismic wave velocities.

The bulk modulus of end-member albite (NaAlSi₃O₈) was determined to be 52.3(9)GPa by single-crystal diffraction by Benusa et al. (2005). The bulk modulus increases as Ca is added across the plagioclase join (Angel, (2004) and Johnson, (2007)). There was no measurable affect of Al/Si disorder on the bulk modulus a plagioclase of An77 composition (Angel,2004). By contrast, Tenner et al. (2007) collected P-V data on a high-albite (i.e. with Al/Si disorder) that suggested that it was 2% stiffer than low albite, suggesting that Al/Si disorder stiffens the feldspar structure.

The data of Tenner et al. (2008) were not very precise, so we have re-determined the EoS of high albite by single-crystal X-ray diffraction. Single-crystal structure determination to X-ray diffraction data collected at room conditions showed that the crystal used was essentially completely disordered (t1o= 0.29, t1m=t2m= 0.24, t2o= 0.23). A fit of the 21 P-V data points collected to 8.710(8) GPa with a Birch-Murnaghan EoS yields a value for the bulk modulus of high albite of 50.1(6) GPa, which is 4½ percent lower than that of low albite. This result suggests that Al/Si disorder softens the structure of albite. Further work is in progress to determine how Al/Si disorder and Ca/Na substitution combine to determine the bulk moduli of plagioclase feldspars.

Angel (2004) Contrib Min Petrol, vol. 146: pg 506-512.

Benusa et al. (2005) Am Min, vol. 90: pg 1115-1120.

Johnson (2007) MS thesis, Virginia Tech.

Tenner et al. (2008) Am Min, vol. 92: pg 1573-1585.

Research supported by NSF EAR-0738692