THE 



PHILOSOPHICAL MAGAZINE 



AND 



ANNALS OF PHILOSOPHY. 



[NEW SERIES.] 



JUNE 1827. 



LXXVIII. Observations on the Crystalline Form ofSillimanite. 

 By W. Phillips, F.L.8. G.S. ^c* 



THIS mineral was first described by Bowen in the Ameri- 

 can Journal of Science for May 1 824, and is therein said 

 to occur in rhomboidal prisms of about 106° 30' and 73° 30', 

 the inclination of the base on the axis of the prism being 113°; 

 and as having one cleavage parallel to the longest diagonal 

 of the prism. In the translation by Haidinger, of Mohs's 

 System, &c. the measurements quoted are the same as those 

 given by Bowen. 



This mineral being always found, I believe, imbedded, and 

 chiefly in quartz, its prisms are frequently somewhat bent, 

 sometimes even twisted ; and their planes being generally far 

 from bright, and occasionally somewhat convex, are mostly 

 unfit for accurate measurement. Having attempted to mea- 

 sure several by means of the reflective goniometer, I was 

 unable to find any one affording the measurements above men- 

 tioned ; but almost uniformly found the prism bounded by 

 several longitudinal planes, of which however it was difficult 

 to obtain die measurements, for the reasons already stated. At 

 length I succeeded in detaching from the matrix some very 

 thin prisms presenting but four lateral planes, which afforded 

 constantly angles of about 88° and 92° ; but they were fur 

 from bright : these slender prisms are nearly transparent and 

 colourless, or have only a slight tinge of yellow ; and at first 

 some doubt arose whether they were in i-eality prisms.of silli- 

 manite. They agreed with it in being very hard ; but it oc- 

 curred to me that if I could procure the easy and brilliant 



# Communicated by the Author. 

 ■New Series. Vol. 1. No. 6. June 1827. 3 F plane 



