eqn [1], and this explains the fact that tungstates withW in its highest oxidation state þ6 dominate the mineralogy of the element in both the primary and the secondary environments.. Ta
Trang 1eqn [1], and this explains the fact that tungstates with
W in its highest oxidation state (þ6) dominate the
mineralogy of the element in both the primary and the
secondary environments
WO24 þ 2H2O þ 2e ! WO2þ 4OH ½1
Tungstenite, WS2, is an extremely rare species,
iso-morphous with its common congener molybdenite,
MoS2 Table 1 provides a list of known naturally
occurring tungstates The so-called tungstic acids of
stoichiometry WO3 nH2O are not listed because
they are simple lattice compounds However, due to
possible confusion as to the attribution of certain
species to the tungstate class, Table 1 does include
certain species that are in fact related For example,
alumotungstite and ferritungstite are derivatives of
WO3 with the pyrochlore structure Substitution
of W by Al or Fe gives a positive charge discrepancy
that is compensated by incorporation of other cations
in a vacant lattice site In related fashion, raspite,
PbWO4, the dimorph of stolzite, contains chains
of edge-linked WO6 octahedra Pinalite, russellite,
and tungstibite are complex layer structure oxides;
cerotungstite-(Ce) and yttrotungstite-(Y) are complex
secondary oxyhydroxide species containing chains of
WO6octahedra The same situation may obtain for
anthoinite and mpororoite, two poorly characterized minerals that require further investigation
Primary Tungstates
All of the primary tungstates, including ferberite, hu¨bnerite, paraniite-(Y), sanmartinite, and scheelite, contain the simple WO24 ion Paraniite-(Y) and san-martinite are both extremely rare minerals, but other members of the group have great economic import-ance, constituting the only minerals of tungsten to have value commercially Scheelite takes up molyb-denum in the lattice and a complete solid solu-tion extends to the isomorphous mineral powellite, CaMoO4 Primary scheelite, however, does not usually contain much molybdenum The term ‘wolf-ramite’ was formerly applied to members of the fer-berite–hu¨bnerite series and the nomenclature is still commonly in use Solid solution in the series is in fact limited to about 20 mol% in each end-member and the separate end-member terms are now applied
to individual specimens, depending on the exact composition
Economically valuable deposits of the Alpine Cleft
or sedimentary types of scheelite are well documented, but the main settings of scheelite, ferberite, and hu¨b-nerite mineralization are either with acid-intrusive rocks or associated skarns Frequent associates of the tungstates are cassiterite, SnO2, molybdenite, MoS2, base metals such as copper, and minor amounts of gold Important mines were once located in all contin-ents, but at present deposits in China, Indochina, and Brazil are the main sources of tungsten
Secondary Tungstates
Scheelite is also well known as a secondary mineral, especially associated with the oxidation of ferberite and hu¨bnerite In this setting, it also may incorporate molybdate in its lattice Other secondary tungstates are rare, with perhaps the most frequently encoun-tered species being stolzite Notable locations of stol-zite include the Clara mine and other mines in the Black Forest in Germany, and at Broken Hill and the Cordillera mine in New South Wales, Australia The Clara, Broken Hill, and Cordillera mines have produced raspite as a mineralogical curiosity and the latter is renowned for the association of the dimorphs with cuprotungstite
As with molybdate (see Rocks and Their Classifi-cation), polymerization of tungstate in acid solutions yields polymeric species These are represented in the mineral kingdom by phyllotungstite and rankachite Both are much rarer than are their molybdenum-bearing congeners and are of academic interest
Table 1 Tungstate(VI) minerals
Mineral Chemical composition
Simple tungstates
Ferberite MnWO4
Hubnerite FeWO4
Paraniite (Y) Ca2Y(AsO4)(WO4)2
Sanmartinite ZnWO4
Scheelite CaWO4
Basic double salts
Anthoinite WAl(O,OH) 3 (?)
Cuprotungstite Cu 3 (WO 4 ) 2 (OH) 2
Mpororoite WAlO 3 (OH) 3 2H 2 O(?)
Pinalite Pb 3 WO 5 Cl 2
Russellite Bi2WO6
Tungstibite Sb2WO6
Complex uranium salts
Uranotungstite (Fe,Ba,Pb)(UO2)2(WO4)(OH)4 12H2O
Polytungstates
Phyllotungstite (Ca,Pb)Fe3H(WO4)6 10H2O
Rankachite CaFeV4W8O36 12H2O
Other complex species
Cerotungstite (Ce) CeW2O6(OH)3
Ferritungstite (W,Fe)(O,OH)3
Alumotungstite (W,Al)(O,OH)3
Yttrotungstite (Y) YW2O6(OH)3
a Other cations, including Ca, Na, and Pb, may substitute in a site
vacancy to compensate for charge imbalance (see text).
MINERALS/Tungstates 587
... saltsAnthoinite WAl(O,OH) (? )< /small>
Cuprotungstite Cu (WO ) (OH) 2
Mpororoite WAlO (OH) 2H O( ?)< /small>
Pinalite Pb WO...
Ferritungstite (W,Fe)(O,OH)3
Alumotungstite (W,Al)(O,OH)3
Yttrotungstite (Y) YW2O6(OH)3... salts
Uranotungstite (Fe,Ba,Pb)(UO2)< /small>2(WO4 )( OH)4 12 H2O