Natural Molybdates

minerals of the molybdate class; salts of molybdic acid. In the simplest case, their chemical composition can be expressed as R[MoO₄] or R[MoO₄] · n/H₂O, where R= Ca, Pb, (UO₂), or, less frequently, Cu, Bi, or Fe³⁺ · More than 15 natural molybdates are known; a considerable portion of them are uranium molybdates.

The basic structure of natural molybdates is formed by distorted [MoO₄]²⁺ tetrahedrons, which are similar to [WO₄]²⁺ tetrahedrons in their crystal-chemical properties; this is the reason for the natural occurrence of isomorphic mixtures of molybdates and tungstates. Anhydrous natural molybdates exhibit a detached structure resembling that of tetragonal scheelite, Ca[WO₄]. The natural molybdates include powellite, Ca[MO₄]; wulfenite, Pb[MoO4]; chillagite, Pb[(Mo,W)O₄]; and koechlinite, (BiO)₂[MoO₄]. Basic and aqueous natural molybdates have a more complex, often lamellar structure; the most common such molybdates are ferrimolybdite, lindgrenite, Cu₃[MoO₄]2(OH)₂; and betpakdalite, CaFe₂³⁺-[As₂Mo₅O₂₄] · 14H₂O.

Natural molybdates are formed primarily under exogenic conditions (in the zone of oxidation of molybdenum, tungstenmolybdenum, and copper-molybdenum deposits) during the transformation of molybdenite into the corresponding pseudo-morphs (powellite or ferrimolybdite) or into earthy deposits that are pale yellow (powellite) or bright yellow (ferrimolybdite) in color. Minute tabular or tetragonal-dipyramidal powellite crystals sometimes occur along the cracks in such beds. Natural molybdates may also be produced under hydrothermal conditions (wolframite-containing powellite).

A separate group, the uranium molybdates (discovered in the 1960’s), includes umohoite, (UO₂)[MoO₄] × 4H₂O; iriginite, [UO₂[Mo₂O₇](H₂O2) × H₂O; calcurmolite, Ca(UO₂)_3- [MoO₄]3(OH)₂ · 8H₂O; and sedovite, U[MoO₄]₂. They are formed in the oxidation zone of molybdenum-uranium deposits and occur as powdery precipitates, crusts, incrusted aggregates, and concretions, generally ranging in color from black, dark green, and blue (umohoite) to reddish brown (sedovite) and canary yellow (iriginite).

Secondary natural molybdates (for example, powellite) may sometimes be extracted along with molybdenite in molybdenum deposits with a well-developed zone of oxidation.