670 



UNITED STATES MINERAL RESOURCES 



est, may contain from 5 to 50 ppm. The accessory 

 iron-oxide and titanium minerals may have tungsten 

 values gi-eater than 10 ppm, and the content of tung- 

 sten in niobium-tantalum minerals may reach 1 per- 

 cent or more, owing to the substitution of W+'' for 

 Nb+'' and Ta+\ The hypogene manganese-oxide min- 

 erals psilomelane and cryptomelane locally contain 

 large amounts of tungsten. 



Tungsten is one of the Group VI elements in the 

 periodic table and has atomic number 74 and an 

 atomic weight of 184. It is a lithophile element in ter- 

 restrial environments and occurs in nature as W+'' 

 (ionic radius 0.70 A) and W+^ (ionic radius 0.62 A). 

 The closest relative of tungsten, molybdenum, has 

 similar ionic radii and electro-negativities, the same 

 valences in natural materials, and about the same 

 abundance in the earth's crust. Other geochemically 

 similar elements are niobium and tantalum, close 

 neighbors of tungsten in the pei*iodic table. 



Separation of molybdenum and tungsten in natur- 

 al environments occurs because of the tendency of 

 molybdenum to form the sulfide, molybdenite, and 

 tungsten to form the tungstates, scheelite and 

 wolframite. 



The geochemical cycle of tungsten begins with the 

 formation of a tungsten deposit by processes related 

 to magmatism. In a crystallizing magma, the tung- 

 sten gradually becomes concentrated in the residual 

 fluids as either volatile compounds (not quantitative- 

 ly appreciable, according to thermodynamic calcula- 

 tions) or soluble components of the water-rich phase. 

 Experimental evidence indicates that tungsten is 

 carried in ore-forming solutions chiefly as tungstate 

 ion, tungstic acid, sodium tungstate, or a heteropoly- 

 acid, the relative amounts of these being controlled 

 by temperature, pH, and the silica content of the 

 solutions (Krauskopf, 1970). At a critical stage of 

 physical conditions and of solution and wallrock 

 chemistry, tungsten is precipitated as either wolf- 

 ramite or scheelite ; which tungsten mineral forms is 

 controlled by relative amounts and activity of iron, 

 manganese, calcium, carbon dioxide, and fluorine. 



During weathering of a tungsten deposit, most of 

 the wolframite and scheelite enter surface waters as 

 detritus because both are fairly insoluble in the pH 

 range of these waters. The high specific gravities and 

 insolubilities of wolframite and scheelite predispose 

 their accumulation in eluvial and alluvial placers 

 from which some production has been made, but 

 these are not a significant source of tungsten at pres- 

 ent. The brittleness of these minerals causes them 

 to break down during transport into very fine par- 

 ticles which are dispersed as fine sediment and thus 

 lost within relatively short distances. 



Narrow tungsten "halos" in soils around ore de- 

 posits suggest some mobility of the metal in near- 

 surface waters. Secondary enrichment of tungsten 

 is known, but is minor and very local. Alkaline solu- 

 tions probably have little effect on tungsten miner- 

 als; the influence of organic matter is uncertain. 

 Tungsten minerals are slowly attacked by acid sur- 

 face waters and the released tungsten is thought to 

 be partly dissolved as HW04~ and partly converted 

 to some form of tungstic oxide (tungstite, hydro- 

 tungstite, f erritungstite) . Some of the tungsten in 

 solution may be removed by adsorption on oxides of 

 iron and manganese. The part remaining in solution 

 is stable as polytungstate ions such as eHCWO^)- 

 and (HWsOsi)"'' and heteropolyacids such as HsSi 

 (W207)o having silicon as the central atom in the 

 complex. The concentration in natural waters is 

 generally very low. Much of the tungsten is even- 

 tually dispersed in sedimentary rocks in amounts 

 about the same as in ig-neous rocks. Enrichment may 

 occasionally occur in ferruginous sandstone and 

 shale. In soils tungsten is slightly enriched in the B 

 horizon or lower A horizon and maj'^ form halos 

 around ore deposits. 



Almost nothing is known about the biogeochemis- 

 try of tungsten. Some indication of enrichment in 

 plants in mineralized zones has been documented. 

 There is little information on black shales, but high 

 values for graphitic phyllites and schists in Uganda, 

 Ruanda, and India suggest a syngenetic sedimentary 

 origin of the tungsten and a correlation with carbon 

 content. An important biogeochemical role is unlike- 

 ly, since tungsten in more than traces is poisonous 

 to most organisms. 



The behavior of tungsten in metamorphic proc- 

 esses is unknown. It has been speculated that tung- 

 sten occurring in minute amounts in ordinary rocks 

 may be mobilized by fluids generated during meta- 

 morphism and transported long distances to be de- 

 posited where the solutions change character. Thus, 

 some investigators have suggested that most tung- 

 sten deposits, even those intimately associated with 

 granites, are derived by metamorphic processes from 

 the small accumulation of tungsten in sediments. 



MINERALOGY 



Although more than 20 species of tungsten-bear- 

 ing minerals are known, only members of the wolf- 

 ramite group and scheelite are important as ore min- 

 erals. Many are very rare in occurrence and several 

 have been described from only one locality. Only the 

 more common minerals are listed in table 145. 



In many deposits throughout the world, tungsten 

 occurs with minerals of other commodities. In the 



