Structural changes induced by cation ordering in ferrotapiolite

Structural modifications as a function of the degree of order (Q) in FeTa2O6 ferrotapiolite have been characterized by means of single-crystal X-ray diffraction (SC-XRD). A total of 26 datasets covering the range of Q between 0.154 and 1 have been obtained by thermal treatments followed by quenching of natural tapiolite crystals. Ordering of Fe2+ at the A sites and of Ta5+ at the B sites causes a linear increase in the a/c lattice constants ratio, as a consequence of a linear decrease of the c dimension and only slight modifications of the a parameter. Calibration of a/c vs. Q represents a very useful tool for a rapid determination of the degree of order of tapiolite samples. Polyhedral volumes of the two octahedral sites vary linearly with Q as a consequence of the different ionic radii of the two species. Both the sites remain almost regular at all Q values but the B site shows an increasing off-centre displacement of the cation with increasing Q. Observed structure factors of supercell reflections, characterized by 1 ≠ 3n, increase linearly as a function of Q, thus representing a further tool for a quick evaluation of the degree of order. © 2006 The Mineralogical Society

Thermal behaviour of libethenite from room temperature up to dehydration

The structural modifications with temperature of libethenite, Cu2(PO4)(OH), were determined by single-crystal X-ray diffraction up to dehydration and consequent decomposition of the crystal under investigation. In the temperature range 25-475 °C, libethenite shows positive and linear expansion. The axial thermal expansion coefficients, determined over this temperature range are: αa = 6.6(1)•10-6 K-1, αb = 1.21(2)•10-5 K-1, αc = 9.0(2)•10-6 K-1, αV = 2.78(3)•10-5 K-1. Axial expansion is then anisotropic with αa:αb:αc = 1:1.83:1.33. Structure refinements of X-ray diffraction data collected at different temperatures allowed to characterise the mechanisms by which the libethenite structure accommodates variations in temperature. Increasing temperature induces expansion of both Cu polyhedra and no significant variation of the P tetrahedron, which acts as a rigid unit. Cu(1) octahedra expand mostly as a consequence of the increase of the axial bonds, and become more distorted. Starting from T = 500 °C, precursor signs of incoming dehydration are visible: two adjacent OH groups approach each other and cause dramatic changes in the whole structure. Concomitantly, libethenite crystal begins to deteriorate and at T = 600°C broad and weak diffraction effects of polycrystalline material are observed