MINERALOGY. 11/ 



which bears no resemblance to mica, which, however, is certainly related 

 to it in composition, which looks in the microscope like talc, and gives 

 its character to the rocks, and is what has been called sericite. Such 

 rocks occur at Northumberland, and at various points on the Connecticut. 

 Lasaulx shows this mineral to be a product of the decomposition of feld- 

 spar in some cases. 



In regard to margarodite, it may be noted that a visit to our mica 

 mines shows how quickly muscovite is turned to margarodite, after a lit- 

 tle exposure. The mica that is rejected, and thrown into piles on one 

 side, very quickly becomes hydrous, loses its transparency, and becomes 

 silvery. A piece exposed less than a year gave me 4.2 per cent, of water ; 

 and hence we might expect that the mica, wherever exposed, would be 

 hydrous. 



A hydrous mica is found at Enfield, associated with quartz, which 

 forms rounded mammillary forms, composed of excessively minute scales. 

 The whole appearance of the mica resembles prehnite, but it is nearly 

 infusible, and close examination reveals its micaceous structure. I 

 have examined this mica, and have found it to be a soda-potash mica, 

 intermediate between margarodite and paragonite. It resembles one 

 analyzed by Smith and Brush, from Litchfield, Conn.* The specimens 

 examined were furnished by Mr. Downs. These intermediate species 

 between the hydrous, potash, and soda micas, indicate that the dividing 

 lines between them are indefinite. 



Chlorite. 



Under this head it is proposed to describe those minerals which, 

 though having essential chemical differences, yet have those well known 

 properties in common that cause them to be all usually called chlorite. 

 The chlorites are hydrous silicates of magnesia, iron protoxide, and 

 alumina; their hardness varies between that of talc and gypsum; they 

 are foliated like mica, but their foliae are not elastic as are those of mica ; 

 and they are of various shades of green, according to the amount of iron 

 which they contain. The three most common species of chlorite are the 

 monochnic ripidolite, the rhombohedral penninite, and the hexagonal 

 prochlorite. The first is biaxial, the second and third uniaxial, though 



* See "Margarodite," Dana's Mineralogy. 



