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LXIX. On the Crystalline Form of Leucophane. 

 Bxj R. P. Greg, Esq.* 



MINERALOGISTS have not hitherto satisfactorily deter- 

 mined the crystalline form of leucophane, from the want 

 probably of good crystals. In the last edition (fourth) of Dana's 

 ' Mineralogy ' it is described as prismatic, with cleavages in three 

 directions inclined to each other at angles of 53i° and 36i° ; 

 while in Brooke and Miller's late edition of ' Phillips/ leucophane 

 is stated as belonging to the anorthic system. 



As this mineral, from its chemical composition, locality, and 

 associated minerals, forms an interesting and important species, 

 I propose briefly to describe a large crystal in my possession, 

 and which I believe is the only tolerably perfect crystal of leuco- 

 phane yet obtained. The accompanying figure is drawn so as 

 to represent the most perfect portion of the crystal, the dotted 

 lines indicating those faces or edges which are imperfect or 

 broken in the crystal itself. 



The measurements given by Professor Miller were made with 

 a reflective goniometer constructed for measuring the angles of 

 large crystals ; those given by myself were taken with the common 

 goniometer, for applying which the crystal is well adapted. From 

 the nature of the crystal it is not easy to determine all its forms, 



* Communicated by the Author. 



