X-Ray diffraction investigation of coloured zoisite (Tanzania)

Abstract

The practice of heated gemstones in order to improve their colour and to enhance their value is very common. Usually, the heating process causes systematic changes in the structures of minerals, such as changes in bond lengths and bond angles, which determine the degree of distortion of coordination polyhedra. Zoisite is a sorosilicate with idealized formula Ca2Al3[Si2O7][SiO4]O(OH), which is orthorhombic, space group Pnma. The crystal structure was determined by Ito [1] and Fesenko et al. [2], and later refined by Dollase [3]. It consists of one type of endless octahedral chains parallel to b with two distinct octahedral sites M1,2 and M3 (Figure 1a). These chains are cross by isolated tetrahedral SiO4 (T3) and Si2O7 groups (T1 and T2) in the a and c directions (Figure 1b). The Commission on New Minerals and Mineral Names (CNMMN) of the International Mineralogical Association (IMA) established at the beginning of 2003 the Subcommittee on Epidote-Group Mineral Nomenclature and decided to limit the epidote group to closely related species having monoclinic symmetry[4]. For this reason zoisite, a polymorph of clinozoisite which was the one exception having orthorhombic symmetry, is not included in the Epidote-Group. Alternatively, zoisite and clinozoisite may even be interpreted to have polytypic relations (Ito, 1950; Merlino, 1990). When zoisite is heated between 370-650°, it becomes an intense sapphire-blue colour (variety tanzanite) [4]; this behaviour was explained in terms of change of the oxidation state of transition metal ions, such as V [5-6].. In this work uncoloured, yellow, green and blue varieties of zoisite were structurally characterized by single crystal X-ray diffraction in order to compare their structural features before and after heating. Zoisite crystals from Merelani Hill, in the Arusha Region, were preliminarily characterized by XRF analysis in order to verify their chemical composition. TG and DTA measurements (heating rate 5°C/min) carried out under a constant flux of air using a STA 409 PC LUXX® - Netzch reveals that in all cases the weight loss is very low (~ 0.2%) and no deprotonation occurs. Single-crystal X-ray data were collected on a Nonius Kappa CCD diffractometer (MoKα radiation) at room temperature, and after heating in situ at 500°C. At room temperature structure refinements of our selected zoisites reveal differences in the unit cell parameters (Table 1), as well as in the orientation of coordination polyhedra. Table 1. Refined unit cell parameters of coloured and uncoloured zoisite. a (Å) b(Å) c(Å) V(Å3) Uncolour zoisite 16.2152(3) 5.5575(1) 10.0500(2) 905.67(3) Yellow zoisite 16.2068(2) 5.5577(1) 10.0536(2) 905.55(3) blue zoisite 16.2140(3) 5.5546(1) 10.0422(2) 904.42(3) Green zoisite 16.1963(3) 5.5558(1) 10.0395(2) 903.40(3) After heating, structural refinements of natural and treated crystals show no significant structural variation. A slight decrease of the coordination polyhedra volumes is detected. In yellow zoisite a rotation of M3 octahedron parallel to a is accompanied by a deformation of T1 and T2 tetrahedra parallel to c. The hydrogen bond O10 – H – O4 is strong. Increasing temperature does not cause any proton movement or switching between two adjacent H positions, as revealed by the H-O10, H-O4, O10-O4 distances refined at 120 K. References. [1] Ito, T., Morimoto, N. and Sadanga R.(1954): Acta Crystallogr., 7, 53-59; [2] Fesenko, E.G., Rumanova, I.M. and Belov N.V.(1955): Structure Reports, 19, 464-465; [3] Dollase, W.A. (1968): American Mineralogist, 53, 1882-1898; [4] Burns R. G., (1970) Cambridge University Press, 114-115; [5] Hutton, D. R. (1971) Phys. C: Solid St. Phys, 4, 1251-1257; [6] Faye G.H. and Nickel, E.H. (1971) Can. Mineral. 10, 812-821