Structural and optical properties of schwazite from Dragodol (Serbia)
Karanović, Ljiljana; Cvetković, Ljubomir; Poleti, Dejan; Balićž
published: Nov 5, 2003
Crystal structure of schwazite from locality Dragodol (Serbia) has been refined using single-crystal data (Mo-Kα X-ray diffraction, CCD area detector) to the conventional R1 factor 0.0148. Schwazite crystallizes in space group I4¯3m with a = 10.4552 (9) Å and Z = 2. X-ray diffraction analysis gave the formula: Cu10.63Hg1.37 Sb3.33As0.67 S13 or IV(Cu4.63Hg1.37)IIICu6III(Sb3.33As0.67)IVS12VIS, where Roman numerals indicate coordination numbers, and 'Cu' in this formula includes additional substitution of Zn and Fe for Cu in tetrahedral site shown by chemical analysis. Structurally schwazite belongs to tetrahedrite-tennantite group of sulphosalts and consists of slightly distorted (Cu1,Hg)S4-tetrahedra, planar Cu2S3- triangles and (Sb,As)S3-trigonal pyramids with (Sb,As) on the apex. The effects of the substitution of Hg for Cu and As for Sb on unit cell parameter, bond distances and angles are discussed and compared with some other tetrahedrites. It is demonstrated that geometrical characteristics of schwazites change in the predictable manner with sample composition. Schwazite is isotropic and exhibits a light greyish creamy-yellow colour in reflected light. Optical reflection spectra of schwazites were also investigated quantitatively. Comparison of samples containing different amount of Hg showed that the position of maximum can be related to the Hg content in the sample.