218 0. A. Derby — Notes on Monazite. 



sulphates are so similar that a confusion with xenotime is also 

 to be guarded against. It has recently been found also that 

 soluble silicates containing zirconium give very similar forms 

 in the sulphuric acid test, but these are readily distinguishable 

 by the absence of phosphoric acid. Confirmatory tests with 

 oxalic acid have not proved uniformly successful and can 

 usually be dispensed with. The Florence test by crystalliza- 

 tions in a blowpipe bead requires from a half a dozen to a 

 dozen grains of the usual size and is more successful with the 

 salt of phosphorus than with the borax bead in which the pre- 

 sence of phosphoric acid appears to exercise a disturbing influ- 

 ence, although with patience the crystals characteristic of cerium 

 can be obtained. 



When, as is usually the case, the grains are transparent, the 

 micro-spectroscope will also usually give a very satisfactory 

 test on a single grain of the usual size, by means of the absorb- 

 tion band of didymium. This can also be obtained by either 

 reflected or transmitted light with an ordinary hand, or rain- 

 band, spectroscope, when a number of grains can ,be brought 

 close together in balsam on a microscopic slide. 



Natural etching. — The grains of monazite from decomposed 

 pegmatites or muscovite-granites are frequently etched, though 

 not to the point of completely obliterating their original form 

 and faces, bat the phenomenon has rarely been observed in the 

 decompositiou-products of other types of rocks. In a miner's 

 residue from the Cavallo Morto (Dead Horse) diamond mine 

 near Diamantina the extraordinarily abundant monazite grains 

 (the mineral is lacking or rare in the residues of most of the 

 mines of the district) are extremely fresh in appearance, pre- 

 senting a strong contrast with the well-worn aspect of the 

 associated zircons, and under the microscope this difference is 

 seen to be due to the profound natural etching of the grains. 

 The deposit is reported by a competent observer to be a 

 decomposed metamorphosed conglomerate like others of the 

 vicinity, and in this case the monazite grains should be equally 

 worn with those of zircon. On the contrary, however, they 

 have been completely rejuvenated in appearance and amongst 

 thousands passed in review under the microscope, none show- 

 ing what could be positively identified as original or worn 

 faces were seen except when, as in the case of the latter, these 

 had been protected by the secondary enlargements described 

 below. In similar deposits in the neighborhood in which the 

 cement of the ancient conglomerate was evidently highly 

 argillaceous, a similar etching of the included quartz grains is 

 almost universal and is often very beautiful. 



Secondary enlargement. — In the above-mentioned residue 

 from the Cavello Morto mine, many grains show a darker cen- 



