284 



REPOKT OF NATIONAL MUSEUM, 1899. 



perfect basal cleavage, whereby thej' split readily into thin, trans- 

 lucent to transparent, more or less elastic sheets. Chemically they 

 are in most cases orthosilicates of aluminum with potassium and 

 hydrogen, and in some varieties magnesium, ferrous, and ferric iron, 

 sodium, lithium, and more rarely barium, manganese, titanium, and 

 chromium. Seven species of mica are commonly recognized, of which 

 but three have any commercial value, though a fourth form, lepidolite, 

 may perhaps be utilized as a source of lithia salts. Of these three 

 forms, the white mica, muscovite, and the pearl gray, phlogopite, are 

 of greatest importance, the black variety, biotite, being but little used. 

 Muscovite, or potassium mica, is essentially a silicate of aluminum 

 and potassium, with small amounts of iron, soda, magnesia, and water. 

 Its color is white to colorless, often tinted with brown, green, and violet 

 shades. When crystallized it takes on hexagonal or diamond-shaped 

 forms, as do also phlogopite and biotite as shown in samples (Speci- 

 mens Nos. 62377 and 30763, U.S.N.M.). Its industrial value lies in its 

 great power of resistance to heat and acids, its transparency, and its 

 wonderful fissile property, in virtue of which it may be split into 

 extremely thin, flexible sheets. It has been stated, though I know 

 not how correctly, that sheets but one two hundred and fifty thou- 

 sandths (1/250000) of an inch in thickness have been obtained. Phlog- 

 opite, or magnesian mica, difl'ers from muscovite in being of a darker, 

 deep pearl gray, sometimes smoky, often yellowish, brownish red, or 

 greenish color. Biotite, or magnesia iron mica, difiers in being often 

 deep, almost coal black and opaque in thick masses, though trans- 

 lucent and of a dark brown, yellow, green, or red color in thin folia. 

 It further differs from the preceding in that its folia are less elastic, 

 and the sheets of smaller size. Lepidolite, a lithia mica, is much more 

 rare than either of the above, is of a pale rose or pink color, folia 

 usually of small size, commonly occurring in scaly granular forms 

 without crystal outlines. The following table will serve to show the 

 varying composition of the four varieties mentioned: 



