220 0. A. Derlnj — Notes on Monazite. 



origin. In the schists from Sopa and Sao Joao da Chapada, 

 which are almost purely sericitic, the monazite is free from 

 inclusions, but in the chlorite-cyanite schist from the Serra do 

 Gigante the monazite, like the accompanying chlorite and 

 cyanite, is full of minute grains of rutile and no essential dif- 

 ference in the character and mode of occurrence of the inclu- 

 sions in the three minerals can be detected. Whatever may 

 have been the original nature of this rock, the silicate elements 

 and the rutile are clearly secondary, and there is no escaping 

 the conclusion that the monazite must be so also. 



In a small gold-working called Ogo* a few hundred meters 

 away from the great diamond mine of Sao Joao da Chapada, 

 monazite of the ordinary type occurs with rutile in the peculiar 

 partings, composed almost exclusively of bright green musco- 

 vite, of a quartz vein in diabase. The crystals are here of the 

 type known as turnerite and both they and the associated 

 muscovite and rutile closely resemble those of the microscopic 

 residue obtained from a specimen of the Binnenthal gneiss 

 faced on one side with the well known macroscopic crystals of 

 that locality. In the case of the Brazilian crystals, however, 

 rutile and muscovite are included in them in such a way as to 

 show contemporaneous crystallization. The rock is a peculiar 

 one and in its present condition may be suspected to be a 

 secondary product, but this cannot be proven. A decomposed 

 massive quartz-muscovite rock from the same mine, which in 

 appearance is of the same nature as the partings except for 

 being finer-grained and more quartzose, did not give monazite. 

 The exposure is unsatisfactory and as in the field examination 

 the greenish rock was taken to be a chloritic alteration phase 

 of the diabase, no special attention was given to it beyond tak- 

 ing a specimen that quite unexpectedly proved of great interest. 

 The quartz vein with its peculiar micaceous accompaniment 

 suggests a comparison with the quartz-kaolin (pegmatitic ?) 

 veins of the same vicinity and may be suspected to be a special 

 type of granitic apophyses. 



If the possibility of a secondary formation of monazite be 

 admitted, a more plausible explanation than that given in the 

 paper above cited (that of an intimate interlamination of erup- 

 tive and clastic material, can be given for the micaceous part- 

 ings and selvages of the Sopa quartz veins that afford perfectly 

 fresh monazite mixed with well-worn zircons. 



* Ogo, which is apparently a term of African origin, is the name given to the 

 line heavy concentrate of microscopic minerals obtained in panning for gold and 

 diamonds, when the predominant color is yellow. It may consist of monazite, 

 xenotime or rutile, or a mixture of all three, the first generally predominating. In 

 panning the miner, seeing the formation of a heavy yellow concentrate, is often 

 deceived and on cleaning up exclaims in disgust " the gold turned to ogo ". 



