TUNGSTEN MINERALS. 213 



that it is distributed in minute quantities over a wider area of the 

 Tavoy field than is generally supposed. 1 It readily alters into 

 tungstite, which either forms on the surface or develops along minute 

 cracks. As the above descriptions show it may either separate simul- 

 taneously with the quartz in the parent veins or later than the 

 quartz matrix. It may also form as a secondary product due to 

 the decomposition of wolfram in the presence of solutions carrying 

 calcium salts. 



When pure scheelite contains 806 per cent, of tungsten trioxide 

 and 19*4 per cent, of lime. It has a specific gravity of about 6 

 and a hardness of 4'5-5, that is to say it is easily scratched by a 

 knife. It has a vitreous lustre and is either opaque or translucent. 

 It cleaves fairly well in four directions, possesses an uneven fracture 

 and is brittle. 



It is not an easy mineral to identify in the field without practice. 

 Some of its similarities have already been mentioned ; the mineral 

 with which it is more likely to be confused than others is barite, 

 the sulphate of barium, which has been found in Tavoy. Hess 

 has summarised tests as follows : — 



" Barite has cleavages in three directions, so perfect that if it is 

 coarsely crystallized it may be cleft into blocks that show rhombic 

 faces and that may almost immediately be distinguished from 

 scheelite. Barite is not so lustrous as scheelite, and it has a lower 

 specific gravitv (4*3 to 4*6). It is much softer than scheelite (its 

 hardness is 3), so that one means of identification is to scrape a smooth 

 surface of it with the back of a knife blade. A powder is in this 

 way much more easily made from barite than from scheelite. Barite 

 commonly crystallizes in thin plates, a habit totally unlike that of 

 scheelite. Some barite, however, is fine-grained and shows no 

 cleavage and its surface is so rough that the scraping test is incon- 

 clusive. Under such circumstances the test for tungsten should be 

 made and is conclusive " (Hess, pp. 30 and 31). 



Scheelite is of no economic importance as an ore mineral in Tavoy 

 owing to its occurrence in such insignificant quantities. It is in 

 fact rather more troublesome than otherwise as in small amounts 

 it gets into the separated tin ores and forms an undesirable impurity. 



1 A leaflet drawn up by the sun ey party and distributed through the field drawing 

 attention to those facts resulted in the discovery of soheelite at several new localities 



