220 W. T. Schaller — Dumortierite. 



W. Schimpff showing a strong test for boric acid. In 1902 

 Ford gave us three analyses of American dumortierite and also 

 mentioned two new localities. During the winter of 1903-4 

 the writer made an analysis of the California dumortierite col- 

 lected by himself during the previous summer. An analysis was 

 also made at the same time of the Washington dumortierite 

 kindly furnished by Mr. Brereton. 



Believing that one good analysis is better than several poorer 

 ones for the exact determination of the formula of the mineral, 

 there will be presented an analysis of the California dumor- 

 tierite made by the writer. From this a formula has been cal- 

 culated for the mineral, and it will be shown how the other 

 analyses agree with this proposed formula. Before giving the 

 results a few preliminary words will not be out of place. 



An analysis of dumortierite is a difficult operation. The 

 small amount of silica makes it difficult to get a good fusion. 

 On preliminary trials on an analysis of the California dumortie- 

 rite, it was found that what was weighed as silica, in the regular 

 course of the analysis, contained more or less of the undecom- 

 posed mineral. Results of about 30 to 32 per cent were 

 obtained. This is, of course, on the uncorrected silica. It was 

 found that a second fusion of the supposed silica was not only 

 very beneficial but also necessary. What was then weighed as 

 silica contained but little residue. The handling of so large an 

 amount of alumina is very cumbersome and an accurate deter- 

 mination of the alumina (plus iron and other oxides here pre- 

 cipitated) is difficult. The boric acid determination is a tedious 

 and difficult one. The accurate determination of the water, 

 which is only given off at a high temperature, is also not easy. 

 One can thus see that an analysis of such a mineral is a difficult 

 operation and that a greater allowance must be made than for 

 most silicate mineral analyses. 



The specimen analyzed was selected in the field, an exceed- 

 ingly pure piece weighing about ten grams being selected. The 

 specific gravity of this piece was taken by suspension in water, 

 giving 3*306. This was broken into small pieces and carefully 

 examined for muscovite, quartz, or other minerals. Sec- 

 tions of the mineral showed that the dumortierite was free 

 from any inclusions. No grains of any titanium mineral abun- 

 dant in the quartz could be detected. When the mineral was 

 powdered it was treated with heavy solution of sp. gr. 3'10, 

 and a minute amount of mineral (muscovite ? ) stayed on top 

 and was removed. The separation was repeated several times, 

 the powder dried at 100° and carefully examined under the 

 microscope for impurities. None were found. The mineral 

 was unquestionably pure. 



The general analytical methods used were those pursued in 

 the Survey laboratory for silicate analysis with a few modifica- 

 tions. The mineral was fused with sodium carbonate, the 



