Raydemarkite, the Natural Analogue of Synthetic α-MoO3·H2O, from Cookes Peak, Luna County, New Mexico, USA

Abstract

AbstractA new mineral species, raydemarkite, ideally MoO3·H2O, was discovered in an unnamed short adit on the Summit group of claims near Cookes Peak, Luna County, New Mexico, USA. It occurs as sprays of acicular or prismatic crystals on a matrix consisting mainly of quartz and pyrite. Individual crystals of raydemarkite are up to 1.00 × 0.10 × 0.06 mm. Associated minerals include sidwillite, ilsemannite, jordisite, powellite, anhydrite, gypsum, bouškaite, pyrite, and quartz. Raydemarkite is colorless in transmitted light and transparent with white streak and vitreous luster. It is flexible and has a Mohs hardness of ∼1½; cleavage is perfect on {100} and {001}. Twinning is common on (010). The measured and calculated densities are 3.44(5) and 3.41 g/cm3, respectively. Raydemarkite is insoluble in water or hydrochloric acid. An electron microprobe analysis yielded an empirical formula, based on 4 O apfu, of MoO3·H2O.Raydemarkite is the natural analogue of the α-form of MoO3·H2O, which was first synthesized over a century ago (Rosenheim & Davidsohn 1903). Its crystal structure was solved using single-crystal X-ray diffraction data. It is triclinic, crystallizing in space group and the unit-cell parameters a = 7.3750(2), b = 3.70920(10), c = 6.6833(2) Å, α = 108.1080(10), β = 112.779(2), γ = 91.7420(10)°, V = 157.828(8) Å3, and Z = 2. The crystal structure of raydemarkite is built up from isolated double chains of highly distorted, edge-sharing MoO5(H2O) octahedra parallel to [010] that are linked together through hydrogen bonds, accounting for its acicular/prismatic morphology and the marked flexibility. Synthetic hemihydrate MoO3·1/2H2O (monoclinic, space group P21/m) can be regarded as a combination of molybdite and raydemarkite both structurally and chemically.