CONSTITUENT MINERALS OF ROCKS. 



65 



72. (33.) Hematite (or Specular Iron). A common iron-ore, having often a 

 high metallic lustre, though often also red and earthy. Powder deep red or 

 brownish red. Not attracted by the magnet. As hard as feldspar. Composi- 

 tion: Oxygen, 30, iron, 70. Titanic iron (Ilmenite) resembles hematite closely, 

 but has a black powder, and contains titanium with the iron. 



(34) Magnetite (or Magnetic Iron-Ore). Crystals octahedrons, and some- 

 times cubes or dodecahedrons. Iron-black. Powder black. Strongly attracted 

 by a magnet. GK = 4.9-5.2. Composition: Protoxyd of iron, 31.03, peroxyd 

 of iron, 68.97; or, Oxygen, 27.6, iron, 72.4. 



(35.) Limonite (Hydrous Sesquioxyd of Iron). — In massive forms j often 

 also stalactitic and mamillary; black and imperfectly metallic in lustre, or 

 brown to yellowish-brown, and earthy. Powder yellowish-brown. Gr. = 3.6-4. 

 Composition: Sesquioxyd of iron, 85.6, water, 14.4; or the same as hematite, 

 excepting the water. The color of the powder distinguishes this species, as well 

 as the magnetite, from hematite. 



Hematite, Magnetite, and Limonite are the three most common ores of iron. 

 They are distinguished by the color of their powders. 



73. (36.) Fluor Spar (Fluorid of Calcium). In cubes, octahedrons, and 

 other forms, with a perfect and easy cleavage on the angles of the cube ; also 

 massive, granular. Often transparent and glassy ; also translucent. Colors, 

 clear and handsome, blue, purple, yellow, reddish, white; sometimes banded; 

 receiving a high lustre when polished. When powdered coarsely and thrown 

 on a shovel heated to just below redness, it phosphoresces finely. Heated 

 with sulphuric acid, it gives out vapors which corrode glass. Composition: 

 Fluorine, 48.7, calcium, 51.3. 



(37.) Barytes (or Heavy Spar, Sulphate of 

 Baryta). In tabular crystals (fig. 57), rect- 

 angular or rhombic. Color, white, sometimes 

 yellowish, reddish. Remarkable for its weight. 

 G. = 4.3-4.7. Composition: a sulphate of 

 baryta = Sulphuric acid, 34.33, baryta, 65.67. 



(38.) Apatite (Phosphate of Lime). — In 

 hexagonal prisms, often large ; no good cleav- 

 age; also massive. Color, sea-green, bluish- 

 to yellowish-white, often resembling beryl, but 

 much softer, being scratched with the point of 

 a knife. Transparent to opaque. Gr. = 3.25. 

 Composition: Phosphoric acid, 42.26, lime, 

 50.0, fluorine, 3.77, calcium, 3.97. 



(39.) Clay. — Clay is not a simple mineral. 

 It is the material of such rocks as contain feldspar, ground up to an impal- 

 pable powder. It is therefore often a mixture of finely-ground feldspar and 

 quartz, in which the former predominates, together usually with other ingre- 

 dients derived from the rocks, as lime, magnesia, and oxyd of iron, in small 

 or large proportions. Clay, when baked, makes brick or pottery ; and, if it 

 contains oxyd of iron, it burns red, the oxyd of iron losing in the process the 

 water which was in combination with it. 



6 



Fig. 57. 



