SILICATES 



107 



greasy feeling and lustre, and of a color varying from dull 

 greenish and almost black, through all shades of yellow, brown- 

 ish, and red. It also occurs in fibrous and silky forms, filling 

 narrow veins in the massive rocks, and is known as amianthus, 

 or chrysotile. It is very doubtful if serpentine is ever an original 

 rock; it is rather an alteration product of other and less stable 

 magnesian minerals. Here will be considered only those which 

 have originated by a series of chemical changes known as meta- 

 somatosis, a process of indefinite substitution and replacement, 

 in simple mineral aggregates occurring associated with the 

 older metamorphie rocks. Such are the serpentines derived 

 from non-aluminous pyroxenes, like those of Montville, New 

 Jersey, and Moriah, New York, and those from Easton, Penn- 

 sylvania, derived from a massive tremolite rock. The analyses 

 given below will serve to illustrate the chemical changes which 

 occur in this process of metasomatosis, I being that of a nearly 

 white pyroxene, and II that of the serpentine derived therefrom. 



Analyses Showing Change of Pyroxene to Serpentine 



Constituents 



Silica (Si02) . . 

 Magnesia (MgO) . 

 Lime (CaO) . . 

 Alumina (AlgOs) . 

 Ferric oxide (FegOs) 

 Ferrous oride (FeO) 

 Ignition (H2O) 



99.946% 



11 



64.215% 



42.38 % 



19.82 



42.14 



24.71 



0.00 



0.59 



0.07 



0.20 



0.97 



0.27 



0.17 



0.14 



14.20 



99.83 % 



The pyroxene, it should be observed, occurs in nodular 

 masses in a crystalline granular dolomite. Various stages of 

 the process are shown in Fig. 8, in which the white and gray 

 central portions are nucleal masses of unchanged pyroxenes, 

 surrounded by the darker crusts of secondary serpentine.^ Ser- 

 pentine as an alteration product of the mineral chondrodite is 

 also known to occur, though this form is less common. At Brew- 

 ster, New York, are extensive deposits of this nature. (See fur- 

 ther on p. 137.) 



^See On the Serpentine of Montville, New Jersey, Proe. U. S. JSfational 

 Museum, Vol. XI, 1888, p, 105. 



