ORDER Y. RHOMBOHEDRAL PEARL-MICA. 205 



Cleavage, R oo highly perfect Traces of 

 P 4- oo. Fracture not observable. Surface, 

 R 00 triangularly, P and P + oo horizontally 

 streaked, though faintly. 



Lustre, common pearly upon R oo, both as fa- 

 ces of crystallisation and of cleavage, vitreous 

 upon the other faces. Colour pale pearl-grey, 

 passing into reddish- and yellowish-white. Streak 

 white. Translucent. 



Rather brittle. Hardness = 8-5 . . . 4-5. Sp. Gr. = 

 3-032. 



Compound Varieties. Massive : composition 

 granular, individuals of various sizes, faces of com- 

 position seldom observable, rough, sometimes 

 smooth. 



OBSERVATIONS. 



1. The determination of the crystalline system of the 

 forms belonging to the present substance is not altogether 

 to be depended on, from a want of sufficient observation. 

 It may prove in future to be prismatic. 



2. A variety of rhombohedral Pearl-mica has yielded to 

 Mr Du MENIL. 



Silica 37-00. 



Alumina 40*50. 



Oxide of Iron 4-50. 



Lime 8-96. 



Soda 1-24. 



Water 1-00. 



On account of the considerable loss of 6-80, Mr Du 

 MEVIL wishes to have the analysis repeated. 



3. Rhombohedral Pearl-mica has been found in a bed in 

 primitive rocks, mixed with and engaged in the variety of 

 prismatic Talc-mica, called foliated Chlorite, at Sterzing in 



