426 Ford — Chemical Composition of Dumortierite, 



Art. XL. — On the Chemical Composition of Dwinortierite ^ 

 by W. E. Ford. 



Introduction. — Dumortierite was first discovered near Beau- 

 nan, France, by Gonnard, who recognized it as a new mineral 

 and named it after the palaeontologist, Eugene Dumortier.* 

 The mineral occurs at the original locality rather sparingly as 

 tine grains or needles enclosed in pegmatite, associated with a 

 gneiss. Sufficient material, however, was obtained for an 

 analysis which was made by Damourf while the optical prop- 

 erties of the mineral were studied by Bertrand.:}: Dumorti- 

 erite was next found near Harlem, Manhattan Island, I^ew 

 York, where it occurs in a pegmatoid portion of a biotite 

 gneiss. The mineral from this locality was first thought to be 

 indicolite, the blue variety of tourmaline. An analysis, how- 

 ever by Riggs, and a study of the optical properties by Diller 

 proved that it was not tournaaline, but presumably a new min- 

 eral. § Later it was shown by E. S. Dana to be identical with 

 the dumortierite of the French locality. ! A little later the 

 mineral was discovered at Clip, Yuma County, Arizona, and 

 two analyses of material from this last locality, together with 

 one of the Harlem material, were made by W bitfield. •[ 



All of the five analyses thus far published show considerable 

 variation in the composition of the mineral and none of them 

 yield a satisfactory formula. The original analysis by Damour 

 showed the mineral to be essentially an aluminium silicate con- 

 taining a small amount of water. The presence of boron evi- 

 dently was not suspected. The analysis, it should be stated, 

 was made on a limited quantity of material, only 0-41 gr. being 

 used. The analysis by Riggs of the Harlem dumortierite 

 showed the presence of B^Og and also a considerable amount of 

 alkalies. The presence of alkalies has given rise to some ques- 

 tion as to the purity of the material analyzed, while Whitfield 

 ascribes the B^Og to the possible presence of tourmaline. The 

 amount of B^O, found by Riggs was about 4 per cent ; hence, 

 since tourmaline contains only about 10 per cent of B^Og, it 

 would be necessary, according to Whitfield's hypothesis, to 

 assume the presence of over 40 per cent of tourmaline in the 

 material analyzed, a supposition quite' untenable, as indicated 

 by the proportions of the other constituents found by Biggs. 



The analyses by Whitfield of dumortierite from Harlem and 

 Clip are discordant, since only a trace of B2O3 is reported in 

 the Harlem mineral, while varying amounts, 4*94: and 2*62, are 

 indicated in the two analyses of material from Clip. Of the 

 Harlem material only 0*217 gr. was available for analysis and a 



*Bull. Soc. Min., iv, 2, 1881. f Ibid., iv, 6, 1881. 



X Ibid., iv, 9, 1881. § This Journal (8), xxxiv, 406, 1887. 



II Ibid. (3), xxxvii, 216, 1889. TIbid., xxxvii, 216, 1889. 



