Theuerdankite, Ag₃AsO₄, a new mineral from the Alter Theuerdank Mine, St. Andreasberg, Germany

Abstract

The new mineral theuerdankite, ideally Ag3AsO4, was found in the Alter Theuerdank Mine, Beerberg, St. Andreasberg, Goslar District, Lower Saxony, Germany. Theuerdankite occurs as aggregates of anhedral grains up to 3 mm in size, growing in cavities of strongly supergene-weathered material consisting of native silver and chlorargyrite (but with calcite present). It is dark violet, changing to reddish and black when exposed to the air and light. It has a grey to violet grey streak; when readily fresh, its streak is brownish-red. The Mohs hardness is about 2. It is brittle without any observable cleavage or parting and with a conchoidal fracture. The calculated density is 6.622 g•cm -3 . In reflected light, theuerdankite is dark grey with a pinkish tint, without any observable bireflectance, pleochroism, or anisotropy. It shows dark red internal reflections. The reflectance values for wavelengths recommended by the Commission on Ore Mineralogy of the IMA are (R %): 13.3 (470 nm), 12.8 (546 nm), 12.7 (589 nm), 12.5 (650 nm). The empirical formula (based on 4 apfu) is Ag3.00As1.00O4. Theuerdankite is cubic, space group P-43n, a = 6.144(2) Å, V = 231.93(13) Å 3 and Z = 2. The six strongest X-ray powder diffraction lines are [dobs in Å, (I), hkl]: