Washington — Kaersutite from Linosa and Greenland. 191 



While in general the probable error for this method should not 

 exceed 2°, the intense color of the amphibole has evidently 

 affected this limit appreciably, as it is not probable that differ- 

 ences exist in the chemical composition of the material suffi- 

 ciently great to cause the optic axial angle to vary 9°. The 

 optic axial angle apparently lies between the two extremes, 

 71° and 79° 54/, but it is thought that the latter more closely 

 approaches the true value. 



Chemical Composition. — For the chemical analysis several 

 grams of selected crystal fragments were coarsely crushed, 

 washed free from dust, and the material (dried at 110°) care- 

 fully picked out under a lens. The only adherent impurities 

 were feldspar, limonite, and particles of the scoria, and all frag- 

 ments showing traces of these were excluded. Thin sections 

 of fragments showed but very few small inclusions of magnet- 

 ite, but these were separated by treatment with an electro- 

 magnet, only a very small amount being thus extracted. It is 

 believed that the material as finally pulverized for analysis was 

 practically free from extraneous matter. Treatment with acid 

 for purification was not resorted to, as the mineral is partially 

 decomposed by acids. * 



The methods of analysis employed were those advocated by 

 Hillebrandf and the writer,;}; about one gram being taken in 

 each case for silica, alumina, etc., and for the alkalies ; about 

 one-half a gram for ferrous iron; and 0*8 gram for fluorine. 

 The alkalies were determined by Lawrence Smith's method, 

 titanium colorimetrically (a mean of three closely agreeing 

 determinations), and manganese by precipitation with bromine. 

 Ferrous iron was determined twice by the simple Pratt method, 

 freshly standardized permanganate solution being used. The 

 figure given (/>*96) is the mean of 3*99 and 3*94. These results 

 indicate that there was no appreciable oxidation of the FeO in 

 the finely ground powder during the interval of a year which 

 elapsed between the two determinations. 



*In a recent criticism (Geol. Mag., dec. v. vol. iv, p. 161, 1907) of a pre- 

 vious paper of mine, Mr. T. Crook says that evidence is needed of the absence 

 of inclusions of ilmenite, etc., in this hornblende, and he expresses doubts 

 as to " the view that titanium enters vitally and in any serious quantity into 

 the composition of ordinary ferromagnesian minerals." The amount of Ti0 2 

 found would imply, if existent only as ilmenite, the presence of 17 or more 

 per cent of this, and it is hoped that the details given here will suffice to 

 show that but minimal amounts, if any, of ilmenite or titaniferous magnet- 

 ite were present. As regards his latter doubt, Mr. Ci'ook seems to be unaware 

 of much recent and highly trustworthy work which has been done in chem- 

 ical mineralogy, and which proves conclusively that titanium does enter 

 vitally and often in considerable amount into the composition of the ferro- 

 magnesian minerals. 



f W. F. Hillebrand, Bull. No. 305, U. S. Geol. Surv., 1907. 



% H. S. Washington. Manual of the Chemical Analysis of Eocks, New 

 York, 1904. 



