Thermal expansion of plagioclase feldspars.

Abstract: The volume thermal expansion coefficient and the anisotropy of thermal expansion were determined for nine natural feldspars with compositions, in terms of albite (NaAlSi3O8, Ab) and anorthite (CaAl2Si2O8, An), of Ab100, An27Ab73, An35Ab65, An46Ab54, An60Ab40, An78Ab22, An89Ab11, An96Ab4 and An100 by high resolution powder diffraction with a synchrotron radiation source. Unit-cell parameters were determined from 124 powder patterns of each sample, collected over the temperature range 298K – 935K. The volume thermal expansion coefficient of the samples determined by a linear fit of V/V0 = α(T-T0) varies with composition (XAn in mol %) as: αV = 2.90(4)⋅10-5 - 3.0(2)⋅10-7*XAn + 1.8(2)⋅10-9*XAn2 Two empirical models for the non-linear behaviour of volume with temperature give a better fit to the experimental data. The change with composition in the a° parameter of the non-linear Holland-Powell model V/V0= 1+a°(T-T0)+20a° (√T-√T0) is: a° = 4.96(5)⋅10-5- 4.7(2)⋅10-7*XAn + 2.2(2)⋅10-9*XAn2 For the Berman model, V/V0= a1(T-T0)+a2*(T-T0)2, the parameters change with composition as: a1= 2.44(15)⋅10-5 - 3.1(6)⋅10-7*XAn + 1.8(5)⋅10-9*XAn2 a2= 9(1) ⋅10-9 - 4(2)⋅10-11*XAn The thermal expansion of all plagioclases is very anisotropic, with more than 70% of the volume expansion being accommodated by a direction fairly close to the (100) plane normal, whereas perpendicular directions exhibit smaller, and in some cases slightly negative or zero, thermal expansion.