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 298–935 K. The volume thermal expansion coefficient of the samples determined by a linear fit of V/V ₀ = α(T − T ₀) varies with composition (X _An in mol %) as:

$$ \alpha_{V} = 2.90\left( 4 \right) \times 10^{ - 5} - 3.0\left( 2 \right) \times 10^{ - 7} *X_{\text{An}} + 1.8\left( 2 \right) \times 10^{ - 9} *X_{\text{An}}^{2} $$

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/V ₀ = 1 + a°(T − T ₀) + 20a° (√T − √T ₀) is:

$$ a^\circ = 4.96\left( 5 \right) \times 10^{ - 5} - 4.7\left( 2 \right) \times 10^{ - 7} *X_{\text{An}} + 2.2\left( 2 \right) \times 10^{ - 9} *X_{\text{An}}^{2} $$

For the Berman model, V/V ₀ = a ₁(T − T ₀) + a ₂*(T−T ₀)², the parameters change with composition as:

$$ \begin{aligned}& a_{1} = 2.44\left( {15} \right) \times 10^{ - 5} - 3.1\left( 6 \right) \times 10^{ - 7} *X_{\text{An}}\\& \quad + 1.8\left( 5 \right) \times 10^{ - 9} *X_{\text{An}}^{2} \\& a_{2} = 9\left( 1 \right) \times 10^{ - 9} - 4\left( 2 \right) \times 10^{ - 11} *X_{\text{An}} \\ \end{aligned} $$

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.