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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: 190


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Subclass: Vanadates | Subclass: Molybdates
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