DIAMOND: OCCURRENCE IN BRAZIL 161 



The connection thus existing between the deposits of the plateau and those of the 



valley leads to the view that the mineral associations of diamond, whether they occur 



on the hills or in the valleys, are essentially the same over the whole of Minas Geraes. The 



material of these deposits consists mainly of grains and fragments of the surrounding rocks, 



from the weathering of which they have been derived, and includes besides the diamonds 



various minerals which may be in a fresh unaltered condition, or more or less weathered and 



decomposed. The mineral most frequently and abundantly present everywhere is quartz, of 



which the transparent and colourless varieties occur, as well as the compact varieties such as 



hornstoue, jasper, &c. All three modifications of titanium dioxide, namely, rutile, anatase, 



and brookite are met with, the last being represented by the variety known as arkansite ; 



crystals of anatase are sometimes completely altered to rutile, while preserving at the same 



time their own external form ; these pseudomorphs are known in Brazil as " captivos." 



Other minerals found in the deposits are oxides and hydroxides of iron, especially magnetite, 



ilmenite (titaniferous iron-ore), haematite, haematite having the external crystalline form of 



magnetite (the so-called martite), and limonite ; also iron-pyrites, either unchanged or 



altered into brown hydroxide of iron (gothite), tourmaline, various kinds of garnet, fibrolite, 



lazulite, psilomelane, talc, mica, yttrotantalite, xenotime and monazite, kyanite, various 



complex hydrated phosphates (goyazite, &c.), diaspore, staurolite, sphene, and topaz, both 



white and blue but not yellow. In addition to diamonds, gold is frequently washed for, 



and is associated with platinum, the latter, however, not in sufficient quantity to be of any 



commercial importance. Some of the minerals mentioned above are distinguished in the 



district by local na^mes ; thus the black rounded pebbles of tourmaline are known as " feijas ■" 



(that is, black beans), and the brown pebbles, consisting of a hydrated phosphate, or of 



titanium or zirconium oxides, are called " favas " (that is, broad beans). 



The minerals mentioned above are not of equally frequent occurrence ; the most 

 constant associates of diamond, after the diiferent varieties of quartz, are the oxides of 

 titanium (rutile, anatase, and brookite), haematite and martite, pebbles of black tourmaline, 

 and specially xenotime and monazite. Even these, however, occur in varying abundance 

 and frequency at different places ; thus the same minerals do not occur associated together 

 in the same way in every river, nor indeed in every part of the same river, this depending 

 on the fact that the lighter minerals are transported at a greater rate than the heavier, and 

 that some are more liable to be altered and reduced to powder than ai-e others. 



It should be mentioned here that while in the deposits of Salobro, in Bahia, corundum 

 is found associated with diamond, it is entirely absent from the State of Minas Geraes. 



Diamond-diggers are guided to a certain extent in their search for the precious stone 

 by the presence or absence of the minerals usually associated with it, which they refer 

 to as the formation. While by reason of their more sparing occurrence and small size 

 diamonds may easily be overlooked, the associated minerals occur usually in larger and more 

 conspicuous crystals and fragments, and are therefore more readily seen. Where the 

 " formation " is absent a search for diamonds is useless, and never undertaken, since they are 

 never found apart from their associates. It by no means follows, however, that diamonds are 

 to be found wherever the " formation " exists ; they may be altogether absent, or present in 

 numbers insufficient to pay for the labour of working. 



The different constituents of the "• formation " are not regarded alike by the diamond- 

 diggers. Those to which a special importance is attached, as being certain indicators of the 

 presence of diamonds, are tourmaline pebbles ("feijas"), the oxides of titanium (especially 

 anatase, less so rutile and brookite), iron oxides (magnetite, ilmenite, haematite, and 

 limonite), the phosphates (" favas "), &c. ; other minerals, such, for instance, as lazulite, are 



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