Researches on the Mineralogy of South America, 139 



ment, when the oxide of iron and the metal have the same lustre, 

 the indications of polarization disappear. Lastly, when the pla- 

 tinum appears more brilliant than the oxide, the rays which 

 come from the metal to the eye are polarized in the plane pass- 

 ing through the axis and the perpendicular, which shows that 

 then it is the reflected light which predominates in the pencil 

 coming from the platinum. 



XVIII. Researches on the Mineralogy of South America. 

 By David Forbes, F.R.S., fyc* 



III. 

 f^iAS SITE RITE. — As previously mentioned, this mineral 

 ^ occurs in considerable quantity in the sands of the Kiver 

 of Tipuani in Bolivia, where, from its greater specific gravity, it 

 accumulates in the washing-apparatus (Lavaderos) during the 

 process for extracting the gold : this was particularly found to 

 be the case at Playa Gritada, at the establishment of Don llde- 

 fonso Villamil, and at this place a considerable quantity was 

 collected for examination. 



The Cassiterite was generally in small semi-rounded nodules, 

 frequently, however, showing one or more crystalline faces, and 

 varying in size from a mustard- seed up to that of a hazel-nut. 

 It was generally pure, that is, without attached rock-substance; 

 but occasionally pieces were found with adherent quartz. The 

 associated minerals found along with the Cassiterite were, besides 

 gold and metallic tin, black tourmaline, garnet, haematite, chias- 

 tolite, quartz, and small fragments of minerals much resembling 

 ruby and topaz, but as yet not examined. 



The colour of the Cassiterite itself was most generally a 

 dirty brownish white or dark black, but fragments were occa- 

 sionally met with of a fine wine-yellow colour, or even colourless 

 and transparent. A chemical examination of these different 

 varieties yielded the following results : — 



Cassiterite, colourless. — Upon examination this was found to 

 be nearly pure binoxide of tin. The specific gravity, taken at 

 60° Fahr. upon 97*38 grs., in fragments, was found to be 

 6*8432. In order to see whether any change in this specific 

 gravity could be effected by heating, the above quantity was 

 ignited to a full red heat for half an hour; after cooling, it was 

 found to have lost 0*06 gr. in weight, and a second determi- 

 nation of its specific gravity gave the number 6 8439; so that 

 it may safely be considered as having remained constant. In 

 external appearance the only observable change was in colour, 

 which had now a slight reddish-yellow tint, and in lustre, which 

 was more waxy. 



* Communicated by the Author. 



