430 Ford — Chemical Composition of Dicmortierite. 



and IsTo. Ill, 7*19 : 60, the ratios of all three being very close 

 to 7 : 60. Analysis ]^o. Ill shows the greatest variation from, 

 this ratio but the method of analysis which had to be employed 

 in this case, as explained above, would account for a high ratio 

 of the SiOj. The ratio as given leads to an acid having the 

 empirical formula, Hg^Si^O^^, and, on the assumption that the 

 hydrogens are wholly replaced by aluminium, the formula 

 becomes Al2oSi^044 or develo^Ded as a basic orthosilicate, 

 (A10)^6^l4(SiOJ,. In the foregoing formula it must be under- 

 stood that boron and a little hydrogen replace a portion of the 

 aluminium. In Analysis I, in which the ratio of B.Og : Si02 = 

 1:7, the formula approximates very closely to [AlOJigAl^B^ 

 (SiOJ,. The theoretical percentages corresponding to this 

 formula are as follows : 



SiO^ 29-83 



Al^Og 65-20 



BO,...- 4-97 



100-00 



It is of interest to note here a new occurrence of dumcr- 

 tierite. During the preparation of this article a blue mineral 

 was sent to Prof. E. S. Dana for determination by Mr. R. M. 

 Brereton of Woodstock, Oregon, and which was examined by 

 the present writer. It proved to be dumortierite of a some- 

 what new habit. The mineral occurs in small spherules, about 

 -j^mm j^ diameter, imbedded in a light-colored, iine-grained 

 siliceous gangue. Each spherule when broken shows a radi- 

 ated fibrous structure, and the mineral has a beautiful blue 

 color. When the gangue is pure white a polished specimen 

 of this material gives a very pleasing effect. This dumor- 

 tierite shows the pleochroism characteristic of the blue variety. 

 It was scarcely possible to obtain material sufficiently pure for 

 analysis, but a decisive qualitative test for boron and approxi- 

 mate determinations of the other constituents left no doubt as 

 to the identity of the mineral. The locah'ty of this occurrence 

 of dumortierite as given by Mr. Brereton is on the headwaters 

 of the ISTorth Fork of the Washougal River in Skamania 

 County, Washington. 



The writer wishes to express his obligations to Messrs. 

 English and Schermikow for the material which they gave for 

 analysis and to Prof. S. L. Penfield for his constant advice and 

 assistance. 



Sheffield Laboratory of Mineralogy, Yale Universitj^ New Haven. 



