212 



W. T. Schaller — Dumortierite. 



On a number of pieces a (secondary?) growth of fibers was 

 seen which are probably in all cases dumortierite, as with high 

 powers a faint pleochroism can be detected. They were at 

 first thought to be sillimanite, but they agree in all their 

 properties as far as can be determined with dumortierite. 

 These fibers can be classed in two parts. 



In the first part belong those which seem to have formed, as 

 a secondary growth, on the main masses of dumortierite. The 

 fibers branch out considerably and often form a radiating fringe 

 around an entire section of the mineral. They penetrate the 

 quartz grains and interstitial muscovite is absent. The line 

 where they join on to the main mass of the mineral is usually 



fairly well defined. In general, the fibers are normal to the 

 edge of the main mass, but locally they vary considerably at 

 times, especially where they form fan-like groups. 



The second class is clearly a stage in the alteration of the 

 dumortierite to muscovite. The solid mass of dumortierite 

 becomes fibrous and, at the edge breaks up into small fibers 

 which gradually become loose from the parent mass. The 

 space where they formerly joined is now occupied by a granular 

 mass of muscovite which also fills the spaces between the fibers. 

 Gradually this process goes on till finally we have a large mass 

 of granular mnscovite, in which are imbedded a few fibrous 

 prisms of the original mineral. Fig. 1 shows diagrammatically 

 a stage in the process. 



Basal sections present an entirely different appearance. The 

 macropinacoidal cleavage, so well developed on the New York 

 mineral, does not show on the sections of the California mineral. 



