Rocks with Quartz Veins in Brazil. 355 



Notes to the Foregoing Analyses. 



" The bracketed figures are either assumed or follow from direct 

 determinations in one of the other columns. 



An hour's treatment with concentrated sulphuric acid is suffi- 

 cient for extraction of all the soluble constituents. Probably very 

 prolonged action of hydrochloric acid would have had the same 

 effect. The silica from the decomposed silicates separated on 

 rotation in the insoluble form. The lithia belongs to a soluble 

 mineral, for but a faint trace was found in the residue. The P 2 6 

 is wholly extracted by dilute nitric acid; it can therefore hardly 

 exist as monazite. Moreover, careful tests failed to show a trace 

 of rare earths. Still the lime is quite insufficient to form apatite 

 with all the P 2 6 . 



No formula for the soluble portion can be calculated from the 

 analyses, but according to Professor Clarke it would seem to 

 approach in composition an impure earthy chlorite. 



The insoluble residue, after treatment by hydrofluoric and sul- 

 phuric acids, contained besides a large amount of unattacked 

 rutile, practically only silica and alumina in the proportions re- 

 quired lor cyanite. Assuming that the alkali of the residue be- 

 longs to sericite, as indicated by Professor Derby's letter accom- 

 panying the specimen, and deducting accordingly for the formula 

 of typical muscovite (KH 2 Al 3 Si 3 12 ), the following percentages 

 remain after excluding all Ti0 2 as rutile: Si0 2 19-62, A1 2 3 10-24, 

 H 2 -27. Again assuming that the alumina exists herein as 

 cyanite and deducting A1 2 3 and Si0 2 to correspond, there 

 remains 13*66 per cent of free silica. This can hardly be opaline 

 silica, for an hour's digestion of the insoluble residue with strong 

 potassium hydroxide solution extracted but a fraction of a per 

 cent of silica." 



In view of the apparent disaccord in some points of this 

 analysis with the results above recorded of the physical exam- 

 ination with batea, heavy liquids and microscope, the latter has 

 been repeated with special care and on a sufficient quantity (20 

 grams or more) to secure a reliable result. The presence in 

 considerable abundance of free quartz and of rutile was fully 

 confirmed, but no reliable trace of apatite or of zircon could 

 be found. The former if present at all, which is doubted, 

 must be in a proportion too small to account for more than an 

 extremely minute portion of the phosphoric acid and lime, and 

 the latter is certainly absent. The zirconia found in the analy- 

 sis must be from some other mineral and presumably from the 

 soluble constituents of the rock. A mineral closely resembling 

 zircon, and like it heavier than cyanite, is completely dissolved 

 out of a mixed residue by sulphuric acid, and in the solution 

 very satisfactory tests for phosphoric acid and cerium were 

 obtained, the latter being precipitated as oxalate, and verified 

 by the Florence method in both a borax and salt of phosphorus 



Am. Jour. Sol— Fourth Series, Vol. VII, No. 41. — May, 1899. 

 24 



