W. T. Schaller —Dumortierite. 211 



Art. XVI. — Dumortierite; by Waldemar T. Schaller, 

 IT. S. Geol. Survey. 



The following paper* is an excerpt of a paper on dumor- 

 tierite to appear in a forthcoming bulletin of the U.S. Geol. 

 Survey. 



The description is mainly of the lavender California dumor- 

 tierite and also of the Washington mineral. Several imperfect 

 crystals — two from California, one from Arizona and three from 

 New York — were found and some crystallographic data obtained. 



The California dumortierite occurs in a dike, a few miles 

 east of Dehesa, San Diego Co. The dike is in a disintegrated 

 biotite granite and has a length of about 1000 feet with a 

 thickness of from 30 to 40 feet and strikes S. 70° E., with a dip 

 of 70° N. 20° E. The upper part of the dike is a fine-grained 

 rock consisting of quartz and sillimanite, while the lower part 

 is a coarse rock composed of quartz and dumortierite, the lat- 

 ter in radiating masses several centimeters thick. They show 

 a transverse parting fairly well developed : the color of the 

 mineral is lavender instead of blue ; the pleochroism is from 

 colorless to red-purple. 



A number of thin sections of the dumortierite rock were 

 prepared and studied with the following results. 



Microscopically the minerals present in the lower part of the 

 dike are seen to be dumortierite and quartz with muscovite and 

 sillimanite in small quantities, together with accessory magne- 

 tite, titanite, rutile (?), apatite and zircon, with a number of 

 small undetermined inclusions. The dumortierite occurs in irre- 

 gular masses with ragged outline, and also in fan-shaped radia- 

 ting masses sometimes of large size. Irregular broken fibrous 

 masses also occur scattered through the slide. The quartz is 

 allotriomorphic and is bat slightly cracked. It is rather full 

 of inclusions in places. With the exception of a little musco- 

 vite, there are no secondary minerals present. 



The common form for the dumortierite is the radiated fan- 

 like masses that vary considerably in size. When the entire 

 piece is larger than a quadrant, parts of the black cross (seen 

 in spherulites) are seen when the nicols are crossed. These 

 fan-shaped pieces are probably the results of an incomplete 

 spherulitic growth. The most perfect one consisted of but 

 half a circle. On certain parts no radiated fibers are detect- 

 able, the (prismatic) cleavage lines being perfectly parallel. 

 Some of the pieces become decidedly fibrous towards the ends 

 and the various individual fibers depart somewhat from true 

 parallelism. In between these fibers, fine-grained aggregates 

 of muscovite can often be seen. 



* Published by permission of the Director of the U. S. Geol. Survey. 



