Minerals: Vanadates.

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1. Introduction.

Vanadates are a source of vanadium (V) which is a common element in the earths crust but it is only found in small quantities. Vanadium is used in industry to strengthen steel, it is used in making an iron alloy called ferrovanadium. Vanadium does not readily react with any other chemicals but it can absorb neutrons; it can be used in nuclear reactors. South Africa has one of the richest sources of minerals containing vanadium.

Vanadates are mined for other important elements which include zinc, copper, lead and uranium. Uranium is used in nuclear power plants as a heat source as it is radioactive.

2. Mineral identification.

Mineral class and sub-class:

Vanadates are a sub-class of the Phosphates class of minerals. Minerals in the class of phosphates have the same general chemical formula which is: (AO4)ˉ". The A stands for the either Vanadium (V), Antimony (Sb), Arsenic (As) or Phosphorous (P), all these elements are interchangeable. Many phosphates form solid solution series between the V, Sb, As and P. A solid solution series occurs when two or more structurally identical minerals can interchange elements within their chemistries without dramatically altering the crystal structure. they can occur in a series right next to each other in nature. Phosphates have a generally high density and can range in hardness from 4 to 7 on the Mohs' scale of hardness. They form many different minerals and join with other ions like hydroxide groups, uranyl groups, water and halogens. The (AO4) group forms a tetrahedron with 4 oxygen anions surrounding the (A) cation making a very dense crystal structure.

Vanadates are any minerals that contain the (VO4)ˉ" group and there are about 50 vanadate minerals that belong to many different groups called the apatite, adelite vanadium oxysalt and descloizite groups. Minerals in this sub-class form similar minerals to phosphates and arsenates but are less common. Some of the more common vanadate minerals are:

- Calciovolborthite (Calcium Copper Vanadate Hydroxide)

- Carnotite (Hydrated Potassium Uranyl Vanadate)

- Chervetite (Lead Vanadate)

- Clinobisvanite (Bismuth Vanadate)

- Curienite (Hydrated Lead Uranyl Vanadate)

- Descloizite (Lead Zinc Vanadate Hydroxide)

- Dreyerite (Bismuth Vanadate)

- Hewettite (Hydrated Calcium Vanadate)

- Hummerite (Hydrated Potassium Magnesium Vanadate)

- Mottramite (Lead Copper Vanadate Hydroxide)

- Pascoite (Hydrated Calcium Vanadate)

- Pucherite (Bismuth Vanadate)

- Schubnelite (Hydrated Iron Vanadate)

- Sengierite (Hydrated Copper Uranyl Vanadate Hydroxide)

- Sherwoodite (Hydrated Calcium Aluminium Vanadate)

- Steigerite (Hydrated Aluminium Vanadate)

- Tyuyamunite (Hydrated Calcium Uranyl Vanadate)

- Vanadinite (Lead Vanadate Chloride)

- Vanalite (Hydrated Sodium Aluminium Vanadate)

- Vanuralite (Hydrated Aluminium Uranyl Vanadate)

- Volborthite (Hydrated Copper Vanadate Hydroxide)

- Wakefieldite (Yttrium Cerium Lead Vanadate)

3. Mineral data

3.1 Vanadinite [Pb5(VO4)3Cl]

Vanadinite is a mineral in the apatite group and therefore has a hexagonal crystal structure (6/m) and crystal habit is usually hexagonal prisms and can also be found in the form of hollow prismatic structures that can be sharp or in rounded form. It is usually red to orange in colour and has a vitreous lustre, the streak is yellowish white. The crystals are generally translucent. The crystals have no cleavage and fracture is conchoidal. Hardness is 3 and specific gravity is between 6.6 and 7.1. Vanadinite forms a chemical series with Pyromorphite Pb5 (VO4)3Cl and Mimetite (Pb5 (AsO4)3Cl). Important sources of vanadinite include Arizona, USA; Mexico; Tsumeb, Namibia; Zambia; and South Africa.

3.2 Calciovolborthite: CaCuVO4(OH)

This mineral can be translucent to opaque and it can occur in various shades of green, lustre can be pearly or dull. It has an orthorhombic crystal system (2/m...
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