W. T. Sohaller — Dumortierite. 219 



For staurolite the values are taken from Goldschmidt's Winkel- 

 tabellen with the a and e axes interchanged. 



There was some question in the writer's mind as to whether 

 the orientation chosen was the best or whether the present 

 prism {210 \ had not better be made the unit prism. The 

 cleavage is parallel to this prism and the twinning also bears 

 some relation to this form. Moreover, the ratio above given 

 for staurolite, though adopted by Goldschmidt, is not, in the 

 writer's opinion, the best one for that mineral. The one given 

 by Dana, who makes the a axis just half as long, would be the 

 better one. The common form of staurolite is prismatic with 

 a prismatic angle of 50° 40 7 (Dana). 



For andalusite, on the other hand, the prismatic angle is 89° 

 12' and the a axis should be -9861 as given. Should dumor- 

 tierite be classed crystallographically with staurolite or with 

 andalusite? Unfortunately, the evidence is almost too meager 

 to decide this question. On the California crystals the unit 

 prism is poorly developed, and entirely absent from the E"ew 

 York crystals, the strongest form next to the brachypinacoid 

 being the prism { 320 \ . The prismatic cleavage is parallel to 

 {210 1, and making the cleavage form the unit prism the a 

 axis should be given half its present value. On the other hand, 

 the Arizona crystal is of the typical andalusite habit and mak- 

 ing the prism the unit one, we obtain the axes as here given. 

 Giving staurolite the (approximate) axes as andalusite and 

 dumortierite, the prismatic cleavage is parallel to the same form 

 as in dumortierite, namely {210 J. 



Chemical. 

 In 1881, after the first announcement of the discovery of 

 the mineral had been made by Gonnard, Damour gave the 

 following analysis of the mineral : 



Si0 2 ._ 29-85 



AI 2 O a 66-02 



Fe 2 3 1-01 



MgO -45 



Ign. 2-25 



99-58 

 Sp. gr. 3*36 



From this analysis the formula 4Al 2 3 .3Si0 2 was calcu- 

 lated. Damour did not suspect the presence of boric acid in 

 the mineral and what was weighed as alumina probably con- 

 tained several per cent B 2 3 . In 1887 Riggs gave an analysis 

 of the New York mineral in which he found 4 per cent B 2 0,. 

 Two years later Whitfield gave several analyses of American 

 dumortierite, all showing the presence of B 2 3 . In 1899 Linck 

 gave some observations on the mineral, including an analysis by 



