550 Palache cud Warren — Chemical Com/position and 



The total of the analysis is a little high, a fault that is almost 

 or quite unavoidable in a long- silicate analysis made in a labo- 

 ratory located in an excessively dusty part of a large city, par- 

 ticularly in the summer when the windows have to be open. 

 If the TiO„ is deducted with a proportionate amount of FeO 

 to form iltnenite, the ratios derived from the. analysis are: 

 Si0 2 : R 2 3 : (RO+KO + H 5 0)=8 : 0-90 : 4-76. Tliese may 

 be apportioned between appropriate molecules as follows : .Na 2 

 1-Y, Si 4 O ia = 0-582 : R^SiA^ 0-834 : Si0 2 left = 0-058. The 

 excess of silica is considerable. The cause of this is not entirely 

 clear. A little feldspar was present in the material analyzed, 

 perhaps also unnoticed bits of quartz, and if the alumina in 

 the analysis be taken out and combined with a proportion- 

 ate amount of potash and silica to form microcline, the excess 

 of silica is considerably reduced and the metasilicate ratios 

 become more satisfactory. The potash seems rather high and 

 may be in error. 



A comparison of the Quincy riebeckite with other varieties 

 of the mineral has been made elsewhere* and need not be re- 

 peated here further than to call attention to the fact that, owing 

 to its relatively low ferric and high ferrous oxide, it contains 

 only 42$ of the Na 2 Fe 2 Si 4 12 molecule, showing in this respect 

 a rather close agreement with the riebeckite from Colorado, 

 described by Koenig,f that from Red Hill, New Hampshire, 

 described by Pirsson and Washington,^: which contains 43 and 

 44 per cent respectively, but departs widely from the riebeck- 

 ite from Socatra, analyzed by Sauer,§ which contains from 

 68 to 69 per cent of this molecule. The Quincy mineral also 

 corresponds quite closely in the same way to several croci- 

 dolites whose analyses are quoted by JDana.| This last 

 resemblance is especially interesting in the present connec- 

 tion because, in the central pocket of one of the pegmatites 

 in which the riebeckite is found, there occurs an abundant 

 crystallization of a black needle-like amphibole and crocidolite 

 which seem to be, if not identical with the riebeckite in coin- 

 position, at least very nearly so. 



Aegirite. — The aegirite of the central pocket is also pris- 

 matic in development, sometimes, and especially in the smaller 

 crystals, showing distinct and measurable terminations. There 

 is, however, even in the best crystals much facetting and curva- 

 ture of part of the terminal planes, especially in those highly 

 inclined to the vertical axis. The faces of the prism zone are 



*The Pegmatites of the Quincy Granite, etc., Wan-en and Palache. Proc. 

 Amer; Acad. Arts and Sciences, 1911. 



f Zs. Kr., i, 430, 1877. }This Journal, xxxiii, 439, 1907. 



£Zs. D. Geol. Ges., xl, 138, 1888. See also Dana's System of Minera- 

 logy, p. 400. 



|| Dana's System of Mineralogy, p. 400. 



