>ER.] 
DUMORTIERITE. 
Ill 
For staurolite the values are taken from Goldschmidt's Winkelta- 
frellen with the a and c 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. More- 
over, the ratio above given for staurolite, though adopted by Gold- 
schmidt, is not, in the writer's opinion, the best one for the 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 prism angle of 50° -10' (Dana). 
For andalusite, on the other hand, 
the prismatic angle is 89° 12' and 
the a axis should be .9861, as given. 
Should dumortierite be classed crys- 
lallographically with staurolite or 
with andalusite? Unfortunately, 
the evidence is almost too meager 
to decide this question. The unit 
prism is poorly developed on the 
California crystals and entirely ab- 
sent from the New York, the stron- 
gest form next to the brachypin 
acoid being the prism {320}. The 
prismatic cleavage is parallel to 
plO}, 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, making the prism the unit one, we obtain the axes as here given. 
Giving staurolite the (approximate) same axes as andalusite and 
dumortierite, the prismatic cleavage is parallel to the same form as in 
dumortierite {210}. 
It may be worth noting that as staurolite and dumortierite are 
related crystallographically more closely than either with andalusite, 
the chemical composition of the two former is much more complex 
than that of the last named species. 
Fig. 8. — Dumortierite. 
PHYSICAL, PROPERTIES. 
GENERAL PROPERTIES. 
Macroscopic crystals are exceedingly rare, the mineral usually 
occurring in prismatic fibrous forms showing no crystal faces. The 
Washington mineral occurs in spherulitic forms which are but a 
special arrangement of fibers. 
