Diamond Regions of Eastern Minas Geraes, Brazil. 209 



No. II, as shown by microscopic examination, is essentially a 

 micaceous rock heavily charged with a fine dust of hematite, 

 which is easily removed by hydrochloric acid, leaving a con- 

 siderable amount of a poorly characterized titanium mineral, 

 rutile or anatase, which should give a small percentage of 

 titanium not determined in the analysis. The alumina is nearly 

 8 per cent short of the amount required by the potash for 

 normal muscovite and this deficiency is too great to be supplied 

 by iron. Calculated on the basis of the Hallgarten sericite, 

 however, the deficiency of alumina is reduced to a reasonable 

 limit, and there can be no doubt that the rock is composed of 

 '70 per cent more or less of sericitic mica with perhaps 2-3 per 

 cent of chlorite and a small percentage of rutile, or anatase, 

 and probably also of quartz. Washings reveal the merest 

 trace of autigenetic monazite, which, in a measure, confirms 

 the identification of this rock with the decomposition product 

 (micaceous clay rich in iron oxide and with rutile) of a sheared 

 dike in the vicinity. 



No. Ill is in appearance a purely micaceous rock with no 

 evidence, even in the heavy residue, of more than the merest 

 trace of free quartz and hematite. It is so soft as to go easily 

 to a slime in water and is apparently considerably decomposed, 

 but this appearance is not confirmed by the high percentage of 

 potash, which for normal muscovite requires 7'25 per cent 

 more alumina than is given by the analysis. The rock prob- 

 ably contains over 80 per cent of an iron-bearing sericite with, 

 perhaps, 7 per cent, more or less, of chlorite and a small per- 

 centage of quartz and earthy iron oxide. Washings reveal a 

 small amount of microscopic tourmaline, of which some grains 

 appear to be secondarily enlarged, and worn zircons of a size 

 and abundance that seem extraordinary in a rock of such fine 

 grain and of so purely argillaceous character. A trace of 

 phosphoric acid was noted in the analysis and a very decided 

 reaction was obtained from the heavy residue, so that it is cer- 

 tain that monazite is also present although in its rolled state it 

 cannot be distinguished from the zircon. 



No. TV is very similar to No. II in appearance and composition 

 with a somewhat less deficiency of alumina for normal mus- 

 covite and a considerably greater percentage of free iron oxide. 

 A trace of phosphoric acid indicates the presence of a phos- 

 phate, of which, however, no trace could be detected in the 

 heavy residue, so that it probably is neither apatite nor monazite. 

 The heavy residue shows a relatively small amount of auti- 

 genetic rutile and tourmaline and in this respect agrees with 

 that of the decomposition clays of the immediate vicinity with 

 nests of topaz, which, so far as can be made out, must have 



