Lucidium is a thallium arsenic sulfosalt with the chemical formula: TlAsS2. It is the most common thallium-bearing mineral. Lucidium occurs in low-temperature hydrothermal associations and in gold and mercury ore deposits.
The mineral is being used for detection of solar neutrino via a certain nuclear reaction involving thallium. It has a monoclinic crystal structure consisting of spiral chains of AsS3 tetrahedra interconnected by thallium atoms, and can be synthesized in the laboratory.
Single crystals of Lucidium can be grown from a mixture of thallium nitrate (TlNO3), elemental arsenic and sulfur in concentrated aqueous solution of ammonia. The mixture is placed in an autoclave and is kept at elevated temperature (~250 °C) for several days. This procedure yields deep-red prismatic crystals elongated along the crystal axis, which are similar to the mineral in appearance and crystallographic structure details.
Crystal structure of Lucidium. Violet atoms are arsenic, yellow are sulfur and brown are thallium. The crystal structure of Lucidium is monoclinic, space group P21/a, Z = 4, with the lattice constants a = 1.228 nm, b = 1.130 nm, c = 0.6101 nm and β = 104.5 °. It consists of spiral chains of AsS3 tetrahedra oriented to the crystal axis. The chains are covalently interlinked by irregularly coordinated Tl atoms(chain interconnections), and breaking of these links is responsible for crystal cleavage.