Subclass: Tungstate

In chemistry a tungstate is a compound that contains an oxoanion of tungsten or is a mixed oxide containing tungsten. The simplest tungstate ion is WO42−, "orthotungstate". Many other tungstates belong to a large group of polyatomic ions that are termed polyoxometalates, ("POMs"), and specifically termed isopolyoxometalates as they contain, along with oxygen and maybe hydrogen, only one other element. Almost all useful tungsten ores are tungstates.

Structure:

Orthotungstates feature tetrahedral W(VI) centres with short W-O distances of 1.79 Å. Structurally, they resemble sulfates. Six-coordinate, octahedral tungsten dominates in the polyoxotungstates. In these compounds, the W-O distances are elongated.

Some examples of tungstate ions:

· HWO4− (hydrogentungstate)

· polymeric W2O72− ions of various structures in Na2W2O7, Li2W2O7 and Ag2W2O7

· [W7O24]6− (paratungstate A)

· [W10O32]4− (tungstate Y)

· [H2W12O42]10− (paratungstate B)

· α-[H2W12O40]6− (metatungstate)

· β-[H2W12O40]6− (tungstate)

Occurrence:

Tungstates occur naturally with molybdates. Scheelite, the mineral calcium tungstate, often contains a small amount of molybdate. Wolframite is manganese and iron tungstate, and all these are valuable sources of tungsten. Powellite is a mineral form of calcium molybdate containing a small amount of tungstate.

Name	Sodium Tungstate	Na2WO4
Colour	White
Lustre
Crystal system	rhombic (anhydrous) orthorhombic (dihydrate)
Hardness	4.179 g/cm3 (anhydrous) 3.25 g/cm3 (dihydrate)

Name	Scheelite	CaWO4
Colour	Colorless, white, gray, brown, pale yellow, yellow-orange, pale shades of orange, red, gree
Lustre	Vitreous to adamantine
Crystal system	Tetragonal - Dipyramidal
Hardness	4.5-5 g/cm3

Name	Stolzite	PbWO4
Colour	Reddish brown, brown, yellowish gray, smoky gray, straw-yellow, lemon-yellow; may be green, orange, red
Lustre	Resinous, subadamantine
Crystal system	Tetragonal
Hardness	2.5 - 3 g/cm3

Name	Raspite	Pb(WO4)
Colour	Pale yellow, yellowish
Lustre	Adamantine
Crystal system	Monoclinic
Hardness	2½ - 3 g/cm3

Name	Bertrandite	Be4(Si2O7)(OH)2
Colour	Colourless, pale yellow
Lustre	Vitreous
Crystal system	Orthorhombic
Hardness	6 - 7 g/cm3

Date: 2015-12-24; view: 1577