Ferrodimolybdenite, FeMo³⁺₂S₄ from Daba-Siwaqa, Jordan - The first natural compound of trivalent molybdenum

Ferrodimolybdenite with ideal formula FeMo³⁺₂S₄ (C2/c, a = 11.8249(8) Å, b = 6.5534(3) Å, c = 13.0052(10) Å, β = 114.474(9)°, V = 917.27(12) ų and Z = 8) was discovered in a differentiated sulfide nodule composed of troilite and pentlandite parts. The nodule was detected in the central zone of a diopside-anorthite-tridymite oval paralava body, ∼30 metres in diameter, within the pyrometamorphic Hatrurim Complex in Daba-Siwaqa, Jordan. Ferrodimolybdenite is the first trivalent molybdenum compound discovered in Nature. Its synthetic analogue crystallises in the C1c1 space group. Ferrodimolybdenite with the empirical formula (Fe²⁺0.99Cu²⁺0.07Ni²⁺0.04)Σ1.10Mo³⁺1.94(S^2-3.98P^3-0.02)Σ4.00 was identified in the troilite part of the differentiated sulfide nodule. The nodule contains inclusions of tetrataenite, nickelphosphide, molybdenite, galena and rudashevskyite. Ferrodimolybdenite forms platy crystals with dimensions ranging from 3×100 m to 20×40 m. The mineral exhibits a grey colour and a dark grey streak. It is opaque with a metallic lustre, and its Mohs hardness is ∼3. The cleavage observed in the mineral is perfect on {001}, good on {100} and poor on {010}. Its tenacity is sectile, and its fracture is smooth. The calculated density of 5.445 g·cm^-3 was derived from the empirical formula and unit cell volume refined from single-crystal X-ray diffraction data. In reflected light, ferrodimolybdenite appears grey to light grey with a blueish tinge. It is anisotropic, with a reflectance in the range of 34-40%. The crystallisation of ferrodimolybdenite occurred in reduced conditions in monosulfide Fe(+Ni) melt at a temperature of 1000-1100°C and at low pressure.