308 0. A. Derby — Occurrence of Xenotime as 



an 



Art. XXXVI. — On the Occurrence of Xenotime as an Acces- 

 sory Element in Hocks / by Orvtlle A. Derby. 



In the examination of the heavy residues obtained by con- 

 centration in the batea, or Brazilian miner's pan, of decom- 

 posed, or crushed, samples of a large number of Brazilian crys- 

 talline rocks, the rare mineral xenotime has been found to be a 

 tolerably constant accessory of one group, viz : the muscovite 

 granites. In comparison with the biotite, amphibole and py- 

 roxene rocks, the muscovite rocks are comparatively rare in 

 Brazil and in the present investigation no opportunity has 

 offered for testing any of a gneissoid type. The samples ex- 

 amined represent an area extending from Ceara on the north 

 to Bio Grande do Sul on the south, the greater portion being 

 from the central part of this area in the States of Bio de 

 Janeiro, Sao Baulo and Minas Geraes. As regards their geolog- 

 ical occurrence the greater number are from small dykes cut- 

 ting gneiss or metamorphosed (Cambrian?) schists, a few being 

 from large bosses which, in one case at least, are newer than 

 the schists. These masses are, for the most part, totally decom- 

 posed to kaolin, and in some cases have been referred to the 

 muscovite granites from the inspection of the decomposition 

 products alone, so that it is possible that some highly feld- 

 spathic rocks of other types, or in which the muscovite is sec- 

 ondary after biotite, have been included. It should be noted 

 that the ones subject to this suspicion are included in the small 

 number that failed to yield xenotime, while in others all the 

 usual accessories were uncommonly rare, and the absence of 

 the mineral may, perhaps, be due to a deficiency in the quan- 

 tity washed. In these tests about a cubic decimeter was treated 

 and in several instances some of the heavy accessories were so 

 rare that only two or three microscopic grains could be de- 

 tected in the residue. The usual associates are zircon and 

 monazite, the second rarely, the first never failing to appear in 

 a residue containing xenotime.' As the lighter and the iron- 

 bearing accessories, such as tourmaline, garnet, magnetite and 

 titaniferous iron were removed by the use of heavy liquids and 

 the electro-magnet in the preparation of the residues for micro- 

 scopic examination, no record was kept of their occurrence, 

 though, with the exception of garnet, they were almost con- 

 stantly found. 



The xenotime occurs in the well known octahedral form with 

 the prismatic faces usually barely susceptible. The elongated 

 crystals with highly developed prismatic faces that characterize 

 this mineral in the diamond gravels of various Brazilian local- 



