Virgilluethite: A New Mineral and the Natural Analogue of Synthetic β-MoO3·H2O, from Cookes Peak, Luna County, New Mexico, USA

Abstract

Abstract The new mineral virgilluethite, ideally β-MoO3·H2O, was discovered in an unnamed short adit on the Summit group of claims near Cookes Peak, Luna County, New Mexico, USA. All virgilluethite crystals are pseudomorphs after sidwillite and occur as aggregates of sub-parallel platy crystals. Associated minerals include sidwillite, raydemarkite, tianhuixinite, ilsemannite, jordisite, powellite, fluorite, baryte, pyrite, and quartz. Virgilluethite is pale yellow-green in transmitted light, transparent with white streak and vitreous luster. It is flexible with a Mohs hardness of ∼2; cleavage is perfect on {010}. No twinning was observed visually. The measured and calculated densities are 3.71(5) and 3.69 g/cm3, respectively. Virgilluethite is insoluble in water or hydrochloric acid. An electron probe microanalysis yielded an empirical formula (Mo1.00)O3·H2O based on 4 O apfu. Virgilluethite is the natural analogue of the β-form of MoO3·H2O, which was first synthesized over a century ago (Rosenheim & Davidsohn 1903). It is monoclinic, space group P21/c, with unit-cell parameters a = 7.2834(3), b = 10.6949(6), c = 7.4861(3) Å, β = 112.779(2)°, V = 583.03(5) Å3, and Z = 4. The crystal structure of virgilluethite, which is topologically identical to that of tungstite (WO3·H2O), is characterized by highly distorted and elongated MoO5(H2O) octahedra that share four corners in the equatorial plane with one another to form sheets parallel to (010). These sheets, analogous to those in sidwillite, are held together by H-bonding between the H2O molecule and the O atom in the axial position in the adjacent sheets. Virgilluethite and raydemarkite are dimorphs of MoO3·H2O. Unlike virgilluethite, the MoO6 octahedra in raydemarkite share edges to form isolated double chains, resembling those found in zhenruite, (MoO3)2·H2O.