Original paper
The optical absorption spectrum of lorandite, TlAsS2
Bernroider, M.; Amthauer, G.
Neues Jahrbuch für Mineralogie - Abhandlungen Band 167 Heft 2-3 (1994), p. 339 - 343
16 references
published: Nov 11, 1994
DOI: 10.1127/njma/167/1994/339
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Abstract
The crystal structure of lorandite, TlAsS2, has been determined by Zemann & Zemann (1959) and later on refined by Fleet (1973). According to these authors lorandite is monoclinic and has the space group P21/a. The structure consists of spiral chains of AsS3-pyramides which are oriented parallel to [010] (cf. Fig. 1). These chains are connected by Tl-atoms. The bonding between the chains is very weak, but very strong within the chains, respectively. This explains the cleavage of lorandite, which is excellent parallel to (100), very good parallel to (201) and still good parallel to (001). All these faces are parallel to the b-axis or [010] respectively. Small crystals of lorandite are transparent and have a deep red color. The colour is purple red with light polarized parallel to Y and orange red with light polarized parallel to Z. However, this pleochroism is not very strong. Refractive index (nli <2.72) and birefringence are extremely high (palache et al. 1944). according to these authors z = b, x is nearly perpendicular to [100] and therefore y must be nearly parallel to (001). the present study has been undertaken in order to obtain informations on the nature of the color and the crystal chemistry of lorandite by optical absorption spectroscopy. lorandite is the most important mineral in the lorex project, which deals with the solar neutrino detection with 2O5Tl (Pavićević 1988).
Keywords
Tl-atoms • crystal chemistry • lorandite • solar neutrinos • refractive index