Vanadates, Natural

a group of minerals of various composition and complexity; salts of orthovanadic acid, H₃(V0,₄). A total of about 40 mineral forms are known, most of them very rare. The most widespread natural vanadates are:

pucherite, Bi[VO₄], a simple anhydrous vanadate;

volborthite, Cu₃[VO₄]₂ · 3H₂O;

carnotite, K₂(UO₂)₂[VO₄]₂·3H₂O;

tyuyamunite, Ca(UO₂)₂[V₂O₈]₂·8H₂0;

metahewettite, Ca[V₆O₁₆]·3H₂O, simple and complex natural vanadates with water of crystallization;

vanadinite, Pb₅[VO₄]₃Cl; and

descloizite, Pb(Zn,Cu)[VO₄]OH, vanadates with additional anions.

The crystal chemistry of natural vanadates has been inadequately studied (1969). Island structures with [VO₄]³-tetrahedrons linked only by cations (Na⁺, K⁺, Ca²⁺, Ba²⁺, Cu²⁺, Pb²⁺, Zn²⁺, Mn²⁺, Bi³⁺, and Al²⁺) have been definitively established. Natural vandates crystallize in hexagonal (vanadite), rhombic, monoclinic, and triclinic systems; usually they occur as powders, films, and encrustations and rarely as well-formed crystals. Owing to the [VO₄]^3- anion, the color is mainly yellow, red, or brownish-red. The color of natural vanadates is also affected by cations—for example, the Cu cation imparts a green tint. The hardness of natural vanadates is 1-4 on the mineralogical scale; their density is on the order of 2,500-7,000 kg per cu m. Most natural vanadates are secondary minerals; depending on their origin, they fall into two classes: vanadates formed in an oxidation zone of sulfide deposits (descloizite, cuprodes-cloizite, vanadinite, and pucherite) and vanadates of sedimentary epigenetic deposits (carnotite, tyuyamunite, volborthite, and so on), which formed after the rock containing them. In a number of cases, natural vanadates are good ores for extracting vanadium and sometimes uranium.