78 MINERALOGY AND LITHOLOGY. 



These analyses agree closely with the formula given. The iolite is 

 usually found in large, flat pieces, with no crystalline planes except the 

 base. Crystals with prismatic faces are not rare, but they are commonly 

 much decomposed. 



53. Chloropiiyllite. 



Iolite, as already stated, is a mineral that is very easily altered, and 

 many of its decomposition products have been analyzed, and given dis- 

 tinctive names. When Dr. Jackson made his geological survey of this 

 state, he discovered at Unity (where iolite, also, is found) a hydrated 

 silicate, to which, on analysis, he gave the name of chlorophyllite. This 

 substance was also analyzed by Rammelsberg, who places it along with 

 several other like substances in a supplement to iolite. Dana classes 

 all these substances together under the name of fahlunite, a species 

 belonging with the hydrous silicates. The following are the analyses of 

 chlorophyllite from Unity : 



Jackson.* Rammelsberg. f 



Silica, 45.20 46.31 



Alumina, 27.60 25.17 



Iron sesquioxide, 9.17 10.99 



Manganese protoxide, ...... 4.08 tr. 



Magnesia, 9.60 10.91 



Lime, .58 



Water, 3.60 6.70 



99.25 100.66 



Rammelsberg remarks that his analysis gives, on calculation, a quantiva- 

 lent ratio, which is that of a definite hydrate of the species iolite, or a 

 species with the same formula as iolite, plus three or four molecules 

 of water. As Jackson's analysis, when compared with Rammelsberg' s, 

 shows wide variations, chlorophyllite must be regarded as a product 

 resulting from the decomposition of ioHte. 



Mica. 



Mica in our state is an important mineral from an economic stand- 

 point, and a very interesting mineral from a scientific. We have some 



* ReJ>. Geol. N. //., 1844. f Rammelsberg' s Min. Chem., 1875, p. 653. 



