Uranospathite and Arsenuranospathite

Kurt Walenta
Institut für Mineralogie und Kristallchemie der Universität, Stuttgart, West Germany

Summary: A reinvestigation of uranospathite from the type locality, Redruth, Cornwall, shows that it is an aluminium uranyl phosphate belonging to the torbernite series but more highly hydrated than other members thereof. The composition (HAl)0·5(UO2)2(PO4)2.20H2O was derived by indirect methods and by making use of analysis by the electron microprobe, which showed an Al2O3 content 2·6% in the partially dehydrated state. Unit-cell dimensions: a 7·00, c 30·02 Å, Z = 2. Probable space group P42/n. Strongest lines of the powder pattern: 15·22 (10) 002, 7·60 (10) 004, 4·93 (10) 110, 111, 3·50 (8) 116, 200, 201.

The mineral is unstable and unless preserved at low temperature or in a humid atmosphere converts into a new phase with a loss of 10- 12H2O. This phase (HAl)0.5 (UO2)2(PO4)2.8–10H2O is identical with the mineral sabugalite. Experimental work led to the synthesis of a hydrate with the composition (HAl)o·5(UO2)2(PO4)2. 16H2O, but not to fully hydrated uranospathite. This synthetic compound also dehydrates rapidly under normal conditions, converting into the same phase as does natural uranospathite. Arsenian uranospathite occurs at Menzenschwand in the Southern Black Forest.

Arsenuranospathite proper is a new mineral found at Menzenschwand and also at Wittichen in the Central Black Forest. It forms lathlike or wedge-shaped crystals of orthorhombic symmetry. Forms: {100}, {010}, {110}, {001}. Cleavage {001} perfect, {100} and {010} good. Hardness probably about 2, ρcak2·54 g cm −3. White to pale yellow. Streak white. Fluoresces greenish in ultra-violet light with variable intensity.

Optical properties: β ≈ 1·538, γ 1·542±0·003, 2Vα about 52·, r > v, straight extinction, length positive. Unit-cell dimensions: a 7·16, c 30·37 Å, Z = 2, probable space group P42/n. Strongest lines of the powder pattern: 14·62 (10) 002; 7·62 (10) 004; 5·03 (8) 110, 006, 111; 3·49 (9) 202.

Analytical work showed arsenuranospathite to be an aluminium-bearing uranyl arsenate. According to analyses by the electron microprobe the content of Al2O3 amounts to ≈ 2·8 % for the partially dehydrated mineral, which is compatible with the formula (HAl)0·5(UO2)2 (AsO4)2.20H2O derived for the fully hydrated mineral.

Arsenuranospathite, like uranospathite, is unstable under normal conditions and converts to a lower hydrate containing about 10 H2O per formula unit. Optical properties of the lower hydrate: α 1·564, colourless, β 1·594, γ 1·596, yellowish, all ±0·002, 2Vα about 28°, r > v. Unit-cell dimensions: a 7·15, c 20·52 Å, Z = 2, ρcak 3·20 g cm−3.

Strongest lines of the powder pattern: 10·54 (10) 002; 5·13 (6) 004, 110; 3·54 (10) 105, 201. The experiments aimed at synthesizing fully hydrated arsenuranospathite were not successful and only yielded a hydrate with the composition (HAl)0·5(UO2)2(AsO4)2.16H2O. This compound changes to a phase with 10 1120 identical with the dehydration phase of natural arsenuranospathite.

Mineralogical Magazine; March 1978 v. 42; no. 321; p. 117-128; DOI: 10.1180/minmag.1978.042.321.18
© 1978, The Mineralogical Society
Mineralogical Society (www.minersoc.org)