It is well known that the practice of heated gemstones in order to improve their colour and to enhance their value is very common. 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, are common during the heating process. 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. These chains are cross by isolated tetrahedral SiO4 (T3) and Si2O7 groups (T1 and T2) in the a and c directions. 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 and [5-6]. The aim of this work is to study the structural modifications induced by heating in uncoloured, yellow and blue varieties of zoisite when heated. Crystals of zoisite from Merelani Hill, in the Arusha Region, were preliminarily characterized by XRF analysis 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 (~ 0.3%) and no deprotonation occurs. The UV-VIS spectra, recorded by using a Xenon lamp and an integrating sphere, indicated that the change colour in the yellow zoisite is related to the disappareance of the 22000 cm-1 band. Single-crystal X-ray data were collected on a Nonius Kappa CCD diffractometer (MoKα radiation) at room temperature, and after heating at 500°C. Structural refinements of natural and treated crystals show no significant structural variation (Table 1). Natural Heated a (Å) b(Å) c(Å) V(Å3) a (Å) b(Å) c(Å) V(Å3) Uncolour zoisite 16.2152(3) 5.5575(1) 10.0500(2) 905.67 16.2067(4) 5,5902(1) 10.1130(5) 916.22 Yellow zoisite 16.2068(2) 5.5577(1) 10.0536(2) 905.55 16.2037(2) 5.55330(5) 10.0428(1) 903.69 blu zoisite 16.2140(3) 5.5546(1) 10.0422(2) 904.42 16.2099(2) 5.5543(1) 10.0380(1) 903.77 Increasing temperature enhances proton movement in the structure of yellow and blue varieties and consequently switching between two adjacent H positions. References. [1] T. Ito, N. Morimoto, and R. Sadanga , Acta Crystallogr., 7, 53-59, 1954; [2] E.G. Fesenko, I.M. Rumanova, and N.V. Belov, Structure Reports, 19, 464-465, 1955; [3] W.A. Dollase, American Mineralogist, 53, 1882-1898, 1968; [4] R. G. Burns, Cambridge University Press, 114-115, 1970; [5] D. R. Hutton, Phys. C: Solid St. Phys, 4, 1251-1257, 1971. [6] G.H. Faye and E.H. Nickel, Can. Mineral. 10, 812-821, 1971.
SINGLE CRYSTAL X-RAY DIFFRACTION STUDY OF STRUCTURAL MODIFICATIONS INDUCED BY HEATING IN UNCOLOUR, BLUE AND YELLOW ZOISITE FROM MERELANI ARUSHA (TANZANIA)
SACERDOTI, Michele;RODEGHERO, Elisa;MARTUCCI, Annalisa;CRUCIANI, Giuseppe;
2010
Abstract
It is well known that the practice of heated gemstones in order to improve their colour and to enhance their value is very common. 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, are common during the heating process. 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. These chains are cross by isolated tetrahedral SiO4 (T3) and Si2O7 groups (T1 and T2) in the a and c directions. 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 and [5-6]. The aim of this work is to study the structural modifications induced by heating in uncoloured, yellow and blue varieties of zoisite when heated. Crystals of zoisite from Merelani Hill, in the Arusha Region, were preliminarily characterized by XRF analysis 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 (~ 0.3%) and no deprotonation occurs. The UV-VIS spectra, recorded by using a Xenon lamp and an integrating sphere, indicated that the change colour in the yellow zoisite is related to the disappareance of the 22000 cm-1 band. Single-crystal X-ray data were collected on a Nonius Kappa CCD diffractometer (MoKα radiation) at room temperature, and after heating at 500°C. Structural refinements of natural and treated crystals show no significant structural variation (Table 1). Natural Heated a (Å) b(Å) c(Å) V(Å3) a (Å) b(Å) c(Å) V(Å3) Uncolour zoisite 16.2152(3) 5.5575(1) 10.0500(2) 905.67 16.2067(4) 5,5902(1) 10.1130(5) 916.22 Yellow zoisite 16.2068(2) 5.5577(1) 10.0536(2) 905.55 16.2037(2) 5.55330(5) 10.0428(1) 903.69 blu zoisite 16.2140(3) 5.5546(1) 10.0422(2) 904.42 16.2099(2) 5.5543(1) 10.0380(1) 903.77 Increasing temperature enhances proton movement in the structure of yellow and blue varieties and consequently switching between two adjacent H positions. References. [1] T. Ito, N. Morimoto, and R. Sadanga , Acta Crystallogr., 7, 53-59, 1954; [2] E.G. Fesenko, I.M. Rumanova, and N.V. Belov, Structure Reports, 19, 464-465, 1955; [3] W.A. Dollase, American Mineralogist, 53, 1882-1898, 1968; [4] R. G. Burns, Cambridge University Press, 114-115, 1970; [5] D. R. Hutton, Phys. C: Solid St. Phys, 4, 1251-1257, 1971. [6] G.H. Faye and E.H. Nickel, Can. Mineral. 10, 812-821, 1971.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
