Penfield and Ford — Stibiotantalite. 77 



given on page 71, was determined. The majority of the 

 crystals examined range in specific gravity from 6* 6 to 6*7. 



Pyrognostics. — Stibiotantalite, when heated intensely at the 

 tip of the blue flame, is fusible at about 4 and imparts a pale 

 bluish-green color to the flame due to volatilization of anti- 

 mony. After driving off a part of the antimony there is left 

 an infusible mass of niobium and tantalum oxides generally 

 darkened by antimony. If the flame from the mineral is 

 directed against a piece of charcoal a considerable coating of 

 oxide of antimony collects on the coal.* When fused with 3 

 or 1 times its volume of sodium carbonate on charcoal, a coat- 

 ing of antimony oxides and small globules of metallic anti- 

 mony are obtained. The powdered mineral is not appreciably 

 attacked by ordinary acids, not even by boiling, concen- 

 trated sulphuric, but is readily soluble in hydrofluoric acid. 

 Unchanged when heated in closed and open tubes and gives 

 no characteristic reactions with the fluxes. 



Summary. — Stibiotantalite is a mineral first found in 

 rounded, water-worn pebbles in Australia and recently in well- 

 crystallized specimens in California, In chemical composition 

 it is an isomorphous mixture of (SbO) a Nb 2 6 and (SbO) 2 Ta 2 6 , 

 exhibiting a wide range in specific gravity, from 5*98 to 7*37, 

 depending upon the proportions of JN"b 2 5 and Ta 2 5 present. 

 The crystals belong to the hemimorphic group of the otho- 

 rhombic system, although, owing to twinning, they imitate the 

 symmetry of the normal group. In axial ratio, development 

 and occurrence of several forms, the mineral is related to 

 columbite. Stibiotantalite is characterized by an unusually 

 high index of refraction, above diamond, a high birefringence 

 and a wonderful luster. 



In closing, the writers take special pleasure in expressing 

 their thanks to Mr. Ernest Schernikow, of New York, who 

 has most generously placed at their disposal for study all of 

 the crystals which the locality, so far as known, has afforded. 

 He may well be proud that his eagerness to promote the science 

 of mineralogy has enabled him to bring to light a mineral of 

 such unusual beauty and scientific interest. 



Mineralogical Laboratory of the 



Sheffield Scientific School of Yale University, 

 New Haven, Conn., February, 1906. 



* Tried in the same way as when testing for zinc ; Brush-Penfield Deter- 

 minative Mineralogy and Blowpipe Analysis, p. 131. 



