38 COLORADO FERBERITE AND THE WOLFRAMITE SERIES. 



Wolframite: A monoclinic mineral containing the ferberite mole- 

 cule (FeWO 4 ) and the hiibnerite molecule (MnWOJ in all propor- 

 tions between 20 per cent FeWO 4 and 80 per cent MnWO 4 , and 80 

 per cent FeWO 4 and 20 per cent MnWO 4 . 



Under this scheme the analyses show ferberites from 13 widely 

 separated localities Boulder County, Colo., Cave Creek, Ariz., and 

 Hill City, S. Dak., in the United States; British Columbia; Greenland; 

 France; Saxony; Italy; Sierra Almagrera and Estremadura, Spain; 

 New South Wales; India; Siberia; and Japan. The ferberite of Japan 

 was described as reinite and replaces scheelite. From Limoges, 

 France (No. 68), is shown a ferberite that contains nearly the maxi- 

 mum amount of manganese. The purest ferberite, showing no 

 manganese and only a small excess of iron oxide (0.8 per cent), is 

 that from Japan. Minerals from Gordon Gulch (No. 58) and Ward 

 (No. 49), in the Boulder tungsten field, fall well within the wolfram- 

 ites, although from the most remarkable ferberite locality known. 

 They merely emphasize the characteristic variability of the hubnerite- 

 ferberite mixture. 



EXCESS OF FERROUS OXIDE AND MANGANOUS OXIDE. 



A point to which considerable importance has been attached by 

 different writers is the excess of FeO in the ferberites, but the table 

 of analyses shows that they are not unique in this, though some 

 ferberites do carry large excesses of FeO. 



The proneness of WO 3 to form a lower oxide on heating and its 

 solubility make it seem probable that many determinations of the 

 oxide are low, so that the appearance of an excess of 0.1 or 0.2 per 

 cent FeO may be neglected. It is also probable that in many of the 

 analyses the MnO is too high, a common error when the determina- 

 tion is made gravimetrically, and a very small excess of MnO would 

 cause a somewhat larger apparent excess in the FeO. Only a few 

 of the analyses show the presence of alumina, but many wolframites 

 occur in a gangue containing muscovite or feldspar or both, and 

 small quantities are likely to be mixed with the mineral analyzed. 

 If the mineral is fused, the alumina, unless specially separated, will 

 be precipitated with the iron oxide, and, if the iron is determined 

 gravimetrically, will by so much make it appear larger. 



Iron oxide is commonly present as such, for in the oxidized zone the 

 breaking down of minerals such as siderite, pyrite, chalcopyrite, or wolf- 

 ramite setsfree iron oxide, which mayformfilms over the residual wolf- 

 ramite or fill cavities of microscopic or larger size. A cut and polished 

 crystal of brown ferberite from the Winnebago claim near Rollins- 

 ville shows a very porous structure (see PL IV, A, p. 13) and the 

 cavities are coated with hydrous iron oxide. This material is similar 

 to that found on part of the Rogers tract and would surely give a 



