Anomalous thermal expansion behavior of zoisite

The mechanisms for the transport and storage of H2O in the Earth is of major importance. Although a great deal of research has been focussed recently on nominally anhydrous minerals the role of hydrous minerals should not be disregarded. They may be involved in metamorphic and melting reactions throughout the crust and the upper mantle. Thermal expansion and compressibility data of zoisite has attracted recently interest due to the proposal of the existence of two different phases, zoisite-I and zoisite-II (Liebscher and Gottschalk, 2004) with different stabilities with respect to Fe3+ content and temperature. All of the experiments at high-T conducted so far on zoisite have been done using powder X-ray diffraction on synthetic materials. Disagreement exists between the reported value for thermal expansion by Pawley et al. (1996) and Grevel et al. (2000): V/Vo = 1 +3.86(5) x 10-5 (T - 298) and 3.096 ± 0.386 x 10-5, respectively. We have performed a diffraction study of an almost pure (Fe3+ = 0.025 apfu) natural yellow zoisite coming from Merelani Hill, in the Arusha Region (Tanzania), the same locality studied previously by Rodeghero et al. (2008). The study was performed using X-ray powder diffraction heating in situ at high temperature and synchrotron radiation at ESRF (ID31 beamline). Each sample was measured between T = 450 and 950 K at steps of 10 K. Cell parameters were obtained after sequential Rietveld analysis of the diffraction patterns. The a parameter slightly increases with temperature and at T > 620 K starts decreasing. Volume expansion in both T-ranges is very similar (α T < 613 30.2(2)x10-6; α T < 613 30.6(1) x10-6; V(T) = VTr exp[α0(T - Tr)]). However, at T < 620 K thermal expansion of zoisite from Merelani is highly anisotropic (1:64:67) and becomes even more at T > 620 K due to contraction of a-axis. This means that thermal expansion induces a strong volume shear in the (100) plane. It is worth to remark that clinozoisite and zoisite differ by a shift of ¼ along [001] in the (100) plane producing the two different stacking sequences along [100]. Pawley et al. (1996) reported that the zoisite a parameter remains essentially constant, although a subtle decrease is also evident from their data. These authors claimed that expansion of the Al octahedra on heating would force rotation of cross-linkages on the Si tetrahedral pairs, preventing the enlargement of the a axis. We have performed an in-situ high-T single crystal diffraction study of the same Merelani Hill zoisite used for powder diffraction. Complete intensity data have been collected at T =300, 373, 473, 573, 773, 873, 973, and 1073 K in the theta range 2-28°. Preliminary data obtained from structure (Rall =3.4-5.3%) shows that the M3 polyhedron (which hosts the Fe3+ in solid solution) shows a shrinkage of the O4-O8 distance (almost parallel to the a axis; from 3.60 to 3.55 Å) that starts at T > 573 K, while the O4-M3-O8 angle shrinks from 177.9 to 173°. This flattened octahedron at room T becomes more deformed at the onset of the a-lattice shrinkage, while expands in its equatorial plane (TAV increases from 51.2 to 62.7 at T > 573 K). The O4 anion is involved in a strong hydrogen bond with an H atom attached to the O10 anion site. TG and DTA measurements performed in air up to 900°C (heating rate 5°C/min) by Rodeghero et al. (2008) reveals that in both cases the weight loss is very low (~ 0.3%) and no deprotonation occurs. We therefore hypothesise that the particular thermal behaviour of zosite may be related to the stability of the hydrogen bonding in this mineral phase. References: Grevel et al. (2000) Am. Mineral., 85, 206-216. Liebscher and Gottschalk (2004) Am. Mineral., 89, 31–38. Pawley et al. (1996) Am. Mineral., 81, 335–340. Rodeghero et al. (2008) 1st SIMP-AIC congress, Sestri Levante, Italy.