THERMAL EXPANSION COEFFICIENTS OF PLAGIOCLASE FELDSPARS

Studies of the in-situ high temperature behaviour of plagioclase feldspars are scarce, in spite of their major relevance as rock forming minerals. For instance, thermodynamic data bases commonly use thermal expansion coefficients for plagioclase based upon experimental data collected more than 3 decades ago. The advent of modern instrumentation on synchrotron X-ray sources now allows precise cell parameters to be obtained from powder diffraction patterns collected on a timescale of a few minutes, providing much denser datasets of higher precision than were previously attainable.

We have therefore undertaken a systematic study of nine well-characterized plagioclase samples that were previously used for calorimetry (Carpenter et al., 1985). The compositions are, in terms of albite (NaAlSi₃O₈, Ab) and anorthite (CaAl₂Si₂O₈, An): Ab100, An27Ab73, An35Ab65, An46Ab54, An60Ab40, An78Ab22, An89Ab11, An96Ab4, An100. Powder X-ray diffraction patterns of each sample were collected on ESRF beamline ID31 over the temperature range 90 to 950 K at steps of 3-5 K at lower temperatures and 10 K at higher temperatures. Cell parameters were obtained from sequential Rietveld analysis of the diffraction patterns. The Ab100 sample was also measured by single crystal X-ray diffraction to cross-calibrate the powder diffraction results. At lower temperatures the thermal expansion exhibits partial saturation, and the anorthite-rich samples undergo the I-1 to P-1 phase transition below 515K. Therefore only the higher-temperature data were used to determine the volume thermal expansion coefficient a in the equation V=V₀exp[a(T-T₀)], which assumes that at higher temperatures the thermal expansion coefficient does not change with temperature. The thermal expansion coefficient varies from 3.03(1)·10^-5 K^-1 in albite to 1.53(1)·10^-5 K^-1 in anorthite, and decreases continuously between the two end members. A linear inverse relationship between the thermal expansion and the volume, similar to that found in alkali feldspars (Hovis et al., 2008), is therefore also observed for plagioclase feldspars.

Carpenter, Navrotsky, McConnell (1985) GCA 49, 947.

Hovis et al. (2008) Am Min 93, 1586.