CONSTITUENT MINERALS OF ROCKS. 57 



36, sesquioxyd of iron and manganese, 5.0, potash, 10, fluorine, 1.0. A variety 

 called lepidolite has often a violet color and contains lithia. 



b. Phlogopite. — A magnesia-mica. Crystals right prisms, rhombic or hexa- 

 gonal. Color, yellowish-brown to white, often a little like copper in its reflections. 

 By polarized light, biaxial, with the angle between the axes 5° to 20°. Contains, 

 besides silica, alumina, and magnesia, some potash and oxyd of iron. Found 

 mostly in crystalline limestones. 



c. Biotite. — A magnesia-mica, usually containing much oxyd of iron. Crys- 

 tals right or oblique prisms. Color, often black, and greenish black ; rarely 

 white. By polarized light, nearly or quite uniaxial, the angle between the axes, 

 when distinctly biaxial, but 1° or 2°. Contains, besides silica, alumina, and mag- 

 nesia, much oxyd of iron and some potash. 



Fig. 36. Fig. 37. 



57. (4.) Garnet. — Crystals usually dodecahedrons (fig. 36, imbedded in the 

 rock), trapezohedrons (fig. 37), and combinations of these forms, and commonly 

 imbedded in gneiss, mica slate, and other crystalline rocks. Color, clear wine and 

 cinnamon red to reddish-brown and black; rarely green. Hardness equal 

 to that of quartz, and therefore not scratched by a knife. G. = 3.1-4.3, the 

 heavier kinds containing much iron. Before the blowpipe, fuses rather easily. 

 Composition : Silica and alumina, with either lime, magnesia, or oxyds of iron, 

 and sometimes protoxyd of manganese or chromic oxyd. The lime variety 

 (cinnamon-stone) contains silica, 40.1, alumina, 22.7, lime, 37.2. But some oxyd 

 of iron is generally present; and common garnet contains silica, 36.3, alumina, 

 20.5, protoxyd of iron, 43.2. The ratio of protoxyds, peroxyds, and silica is 

 1:1:2. 



Idocrase resembles garnet closely in color, composition, and fusibility, but 

 is square-prismatic in its crystals, and brownish in color like some brown 

 garnet and tourmaline. 



(5.) Lettcite. — In trapezohedral crystals like garnet, but white or gray and 

 without cleavage. Hardness nearly that of feldspar. G. = 2.45-2.5. Occurs 

 in Vesuvian lavas in crystals and grains. Composition : Silica, 55.1, alumina, 



23.4, potash, 21.5. 



58. (6.) Epidote. — Crystals oblique prisms (monoclinic). Also occurring 

 massive, with a granular or columnar structure. Prevailing color, a peculiar 

 yellowish green, but varying to brown, ash-gray, and white. Hardness = 6-7, or 

 nearly that of quartz. G. = 3.2-3.5. Composition : Constituents, same essen- 

 tially as in garnet, but ratio 3:2:3. There is a lime variety, containing silica, 



42.5, alumina, 31.5, lime, 26.0 ; and a lime-and-iron variety, containing silica,, 

 39.5, alumina, 19.5, peroxyd of iron, 17.0, lime, 24.0. 



