Rocks with Quartz Veins in Brazil. 



353 



Both the monazite and the xenotime of the clays and rocks 

 above described and of the concentrated washings of the mines 

 to which they have contributed, present peculiarities of form 

 that distinguish them from the types that have hitherto been 

 found in place in Brazilian, or other, rocks. The monazite 

 crystals are invariably in elongated prisms with only the pina- 

 coid plane developed giving forms that exactly resemble simple 

 prisms of zircon with broken terminations. The extremities 

 are most frequently broken, but occasionally one or two ter- 

 minal planes are observed as in the accompanying figure 

 kindly drawn by Dr. Hussak. The smallness of the extinction 





\ 





x/ 





\ 



y/\ 









\ $) 









#/ 



a 







_V 





b 





b 







d 





V 



y 







d 



a (100), b (010), x (101), w (101). 



angle (l°-4°) increases the resemblance to zircon, which is so 

 close that only by chemical tests and the measurement of the 

 angles (for which material of sufficient size was fortunately 

 found in the diamond concentrates) could their true character 

 be determined. The xenotimes are invariably elongated prisms 

 with pyramidal terminations in strong contrast with the octa- 

 hedral type, which is the only one hitherto found in the numer- 

 ous rocks containing the mineral that have been examined. 

 The crystals when perfect are as transparent and lustrous as 

 zircons, but are quite subject to a milky alteration and are 

 extremely brittle. They rarely sink to the extremely minute 

 size of the generality of the monazite crystals and show a ten- 

 dency to assume macroscopic proportions. Brown opaque 

 crystals of this prismatic type up to 10 millimeters or more in 

 length, are quite abundant in the concentrates of the mines 

 about the village of Dattas, a few miles to the southward of 

 the localities above described, and there can be no donbt that 

 they come from quartz and kaolin veins like those of Sao Joao 

 da Chapada and Sopa. The octahedral type which commonly 

 occurs in the monazite-bearing granites and porphyries, and 

 which was actually found in one of the clays from Sopa, is 

 comparatively rare in the concentrates examined from the Dia- 

 mantina region, but is abundant and of macroscopic propor- 

 tions in those from the Lencoes region in Bahia. Perhaps its 

 tendency to alteration noted in the Sopa clay may explain its 

 rarity. 



