246 Prof. J. P. Cooke on the Vermiculites. 



this ratio requires must represent water of crystallization, while 

 all required to complete the ratio must represent water of con- 

 stitution; and we thus have a means of distinguishing between 

 these two states of combination where the class of compounds 

 to which the mineral belongs is known. 



Now it is true of each of the minerals we have distinguished 

 among the vermiculites, — 



First. That the water is driven off at a temperature below a 

 red heat. 



Secondly. That the loss of the water is attended with a com- 

 plete disintegration of the mineral. 



Thirdly. That the amount of the basic radicals, exclusive of 

 the hydrogen, is sufficient to saturate the silicon, and that the 

 amount of hydrogen is wholly in excess of the amount which the 

 atomic ratio 1 : 1 requires. 



Could it be proved that the vermiculites are orthosilicates, the 

 last of the three facts just stated would be alone sufficient to 

 establish the correctness of our conclusion. Unfortunately such 

 absolute proof cannot be obtained; and we only claim that the 

 crystallographic and chemical relations of the vermiculites to 

 each other, and to the magnesian micas, give a very high degree 

 of probability to our theory that they are orthosilicates. 



From these facts we have concluded that the water which 

 enters into the composition of the vermiculites is water of crys- 

 tallization. For the evidence of the facts, we refer to the descrip- 

 tions of the several species given below. But, further, in order 

 to justify our conclusion, we propose to bring into comparison 

 with the vermiculites a class of hydrous micas from which the 

 water obtained is clearly water of constitution ; and this class of 

 minerals we shall describe first. 



Damourite. — 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 op- 

 tical characters. But we would propose to give the name 

 Damourite to the whole class, distinguishing 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 aluminium and potassium, but in 

 which a portion of the alkaline radical is replaced by hydrogen. 



Sterlinyite. — A remarkable mineral of the Damourite type is 

 found at Sterling, Mass., associated with spodumene, in a vein 

 of a large boulder rock. This mineral, for the sake of distinc- 

 * See the papers of Professor Haughton, cited below. 



