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;
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.
REFERENCEPovarennykh, A. S. Kristallokhimicheskaia klassifikatsiia mineral’nykh vidov. Kiev, 1966.
L. F. BORISENKO 4-834-5