Prof. J. P. Cooke on the Vermiculites. 



247 



tion, I have called Sterlingite ; but it does not differ from the 

 original Damourite of Delesse except in the value of the optical 

 angle. We give in parallel columns the characters of the two 

 minerals : — 



Sterlingite. 



Mica-like in structure. An ag- 

 gregate of flexible and unelastic 

 laminae, frequently an inch in dia- 

 meter. 



Lustre, pearly. 



Colour, yellow or yellowish white. 



H = 2-3. 



Sp. gr. at 26° :— 

 1st ex. gave 2'832. 

 2nd „ 2-828. 



Cleavage basal, highly perfect, as 

 in mica. Jointed parallel to the 

 sides of a rhomb, having an angle 

 of 120°, and also parallel to the 

 shorter diagonal of the same. The 

 markings of these joints or clea- 

 vages visible on the surfaces of the 

 lamina. 



Double refraction strongly nega- 

 tive. 



Biaxial, with plane of axes parallel 

 to shorter diagonal. Divergence 

 about 70°. Dispersion of axes very 

 small. 



Before the blowpipe, fuses on the 

 edges with potash-flame. 



Damourite. 



Mica-like in structure, 

 gregate of fine scales. 



An ag- 



Lustre, pearly. 



Colour, yellow or yellowish white. 



H=2-3. 



Sp. gr. =2792. 



Double refraction negative, 

 axial divergence 1 0° to 12°. 



Bi 



SiO 2 



A1 2 3 

 Fe 2 3 



MgO 

 K 2 

 H 2 



Sterlingite. 

 . 43-87 



Oxygen. 

 23-40 



36-45 

 3-36 



17-00 

 1-00 



10-86 

 5-19 



1-84 

 4-61 



99-73 



Damourite. 

 1. 2. Oxygen. 



45-22 43-41 23-15 



37-85 



35-17 

 4-62 



1-40 



10-90 



4-50 



16-39 

 1-39 



11-20 

 5-25 



0-56 

 1-85 

 4-00 



99-52 100-00 



Of the two analyses of Damourite, No. 1 is by Delesse, of the 

 mineral found in the gangue of cyanite, at Pontivy in Brittany, 

 and No. 2 is by Igelstrom, of the similar miueral found at 

 Horrsjoberg, Wermland. The analysis of Sterlingite was made 

 by Mr. C. E. Munroe, Assistant in the Chemical Laboratory of 



