32 HILLEBRAND, MERWIN AND WRIGHT. fAprii 2$. 



Hewettite, 



Hewettite was rather abundant at the time of Mr. Hewett's visit 

 to Minasragra. It is wholly of superficial occurrence, derived by 

 oxidation from the vanadium sulphide, patronite. The principal 

 specimen examined by us was a lump about the size of a small apple. 



In the lumpy aggregates of pure mineral hewettite is deep red 

 (mahogany red)^ with a somewhat silky luster. Under the micro- 

 scope the needles measure usually less than o.oi mm. in width and 

 0.2 mm. in length. The extinction is parallel. The refractive in- 

 dices measured by immersion methods could be only approximately 

 found because of the extreme thinness of single blades and the lack 

 of entire parallelism of the blades in a group. Also, /? and y were 

 so high that slight heating was necessary to embed the mineral in the 

 standard refractive media, thus causing expulsion of an unknown 

 amount of water. For Li-light a^i.77, j3 = 2.i'&, y = about 2.35 

 to 2.4. Elongation is parallel to y. Pleochroism is strong; y dark 

 red, a and /? very light orange-yellow. The mineral is probably 

 orthorhombic. A determination of density, made by Mr. E. S. Lar- 

 sen, on air-dried material gave a value 2.618. A subsequent deter- 

 mination by one of the authors (M.), using material containing 9 

 molecules of water, gave 2.554. The apparent discrepancy between 

 these two determinations is explained by the fact that the material 

 used by Mr. Larsen was partially dehydrated. 



The mineral when heated passes through various color changes 

 (see p. 46) and melts readily, forming a dark red liquid ; it is slightly 

 soluble in water. 



The composition of an almost pure specimen, on which the fore- 

 going optical examination was made, is given under analysis i 

 (p. 40). Analysis la, in the footnote on pp. 40-41 (quoted by Mr. 

 Hewett, loc. cit., p. 311), represents a lump of ore showing little evi- 

 dences of crystallization but otherwise resembling closely the better 

 specimen, although the microscope shows it to be far from homogene- 

 ous. In spite of the similarity in appearance of the two specimens, the 



2 The specific color terms used in these descriptions are based on com- 

 parisons with Ridgway's standards. See " Color Standards and Color No- 

 menclature," Robert Ridgway, Washington, D. C, 1912. 



