O. A. Derby — Notes on Monazite. 219 



tral portion surrounded by perfectly clear material having the 

 same optical orientation, but, like all the grains of the residue, 

 without distinct crystalline form. The appearance is that of 

 a rounded and worn grain with secondary enlargements prin- 

 cipally in the form of clusters of spindle-shaped prolongations 

 of the two poles of an original ovate grain. So far as can be 

 made out from the color and from rather unsatisfactory optical 

 tests, the clear outer shell is of monazite material, and this 

 conclusion is apparently confirmed by microchemical tests, 

 since grains in which the central nucleus appeared to be en- 

 tirely protected from the action of the acid gave very satis- 

 factorily the cerium and phosphoric acid reactions. As there 

 was a possibility that the enlargement might be the newly dis- 

 covered cerium-aluminium phosphate, norencite, which, when 

 the form is not distinct, is readily confounded with monazite, 

 a small quantity of the residue was tested by wet analysis for 

 alumina, but with a negative result. It thus seems tolerably 

 certain that in the clastic rock from which the residue comes 

 there had been a new formation of monazite such as has been 

 shown for tourmaline and several other minerals in similar 

 rocks. The main objection to this conclusion is that monazite, 

 as an autigenetic element, is thus far only positively known in 

 rocks of eruptive or presumably eruptive origin, but this 

 objection is weakened by evidence presented below indicative 

 of a possible secondary origin. 



Inclusions. — The monazite thus far extracted from granites, 

 gneisses and porphyries, and that found in secondary deposits 

 derived from these rocks, is free from inclusions, and in these 

 cases the mineral, together with the associated accessories 

 (zircon, magnetite or ilmenite, or both, and occasionally the 

 phosphates xenotime and apatite), has evidently been one of 

 the first to crystalize out of an eruptive magma. Quite recently, 

 however, evidence has come to hand, that it may also be 

 formed in other ways and probably by secondary processes.. 

 In addition to the secondary enlargements above noticed, 

 which were evidently formed in a metamorphosed clastic rock, 

 the following cases have been noted. 



From the residues of diamond washings at Sao Joao da 

 Chapada and Sopa near Diamantina, and from a gold washing 

 at Bandeirinha in the same neighborhood, the monazite crys- 

 tals of the peculiar prismatic habit described in a recent num- 

 ber of this Journal (vol. vii, p. 353) are frequently found 

 heavily charged with scales of hematite and more rarely with 

 minute needles and grains of rutile. This type of monazite 

 has been traced home to some peculiar schists of the region 

 that are presumed (in part on the evidence of the perfectly 

 fresh condition of the monazite crystals), to be of eruptive 



Am. Jour. Sci. — Fourth Series, Vol. X, No. 51.— September, 1900. 

 15 



