OF ARTS AND SCIENCES. 



39 



pose to bring into comparison with the vermiculites a class of hydrous 

 micas from which tlie water obtained is clearly water of constitution, 

 and this class of minerals we shall first describe. 



Damourites. — Delesse originally gave this name to a hydrous mica 

 which occurs in fine scales in Pontivy in Brittany. Since then, micas 

 of similar composition have been observed in several countries, and 

 shown to be not unusual constituents of granitic rocks.* Among these 

 we may distinguish several varieties, (or species ?) marked by slight 

 differences of composition and optical characters. But we would pro- 

 pose to give the name Damourite to the whole class, distinguisliing the 

 varieties by separate names only so far as may be thought necessary, 

 Under the family of Damourites, then, we class all unisilicate micas, 

 which are chiefly silicates of aluminum and potassium, but in which 

 a portion of the alkaline radical is replaced by hydrogen. 



Sterlingite. — A remarkable mineral of the Damourite type is found at 

 Sterling, Mass., associated with spodumene, in a vein of a large bowlder 

 rock. Tliis mineral, for the sake of distinction, 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 char 

 acters of the two minerals : — 



Sterlingitk. 



Mica-like in structure. An aggre- 

 gate of flexible and unelastic laminae, 

 frequently an inch in diameter. 



Lustre, pearly. 



Color, yellow or yellowish-white. 



H = 2 — 3. 



Sp. gr. at 26". 



1st ex. gave 2.832. 



2d „ „ 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 cleavages, visible on the sur- 

 faces of the laminae. 



Double refraction strongly negative. 



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, 

 gate of fine scales. 



An aggre- 



Lustre, pearly. 



Color, yellow or yellowish-white. 

 H = 2 — 3. 

 Sp.gr. = 2.792. 



Double refraction negative. Biax- 

 ial divergence, 10 to 12 degrees. 



* See the papers of Professor Haughton, cited below. 



