Derby — Occurrence of Topaz near Ouro Preto, Brazil. 25 



Art. II. — On the Mode of Occurrence of Topaz near Ouro 

 Preto, Brazil ; by Orville A. Derby. 



The current treatises on mineralogy, in the brief reference to 

 the association of the yellow Brazilian topaz of the Ouro Preto 

 district with talcose or chlorite schist, give an idea of a mode 

 of occurrence quite different from that of any of the other 

 known localities of the mineral. This statement is made on 

 the authority of Esch wege, who in the early part of the century 

 (1811-1822) spent a number of years at Ouro Preto and was 

 very familiar with the mines during a period of active work- 

 ing. A fuller statement of Eschwege's observations, given in 

 various writings, but most fully in his Pluto Brasiliensis pub- 

 lished in 1833, is as follows. The topaz occurs in a narrow belt 

 of country only a few hundred meters wide, extending for 

 several kilometers from Saramenha, a suburb of Ouro Preto 

 westward, in a nearly straight line for a distance of about 20 

 kilometers, by the mines of Boa Yista and Jose Correia to 

 Capao de Lana,* with indications of a second less important 

 belt a few kilometers to the northward. The topaz here 

 occurs in situ and exclusively of a yellowish or rose color in 

 contradistinction to the northern region near Minas Novas on 

 the Jequitinhenha, where only white and blue stones occur 

 exclusively in the state of rolled pebbles. The mineral, asso- 

 ciated with quartz, specular iron, rutile and euclase, occurs in 

 layers and nests of a tine scaly friable lithomarge, white or 

 colored by iron oxide, all of these minerals being well crys- 

 tallized but invariably broken at the base and irregularly 

 mingled, as if kneaded into the lithomarge. The topaz-bearing 

 nests and layers are enclosed in a decomposed unctuous schist 

 which provisionally was called talc or chlorite schist, and which 

 in turn are intercalated in the decomposed argillaceous schists 

 of the region that were referred to the primary formation. 



Mawe,f and Spix and Martius,^: who visited the region about 

 the same time gave descriptions of the mines in substantial 

 accord with that of Esehwege, except that they refer the topaz 

 to veins and the latter authors contest the classification of the 

 enclosing schist as talcose, calling its characteristic mineral a 

 modified mica, 



No additional information of value regarding the topaz was 

 given until 1882, when Grorceix§ described the principal mines 



* Not Ulana nor Lane, as frequently given. Capao is the Indian name for an 

 isolated group of trees or grove aod Lana is probably the name of a former pro- 

 prietor. 



f Travels in the Interior of Brazil. London, 1812. 



% Reise in Brasilieu, Munich, 1831. 



§ Annaes da Escola de Alinas de Ouro Preto, No. 1. 



