26 THE MINERALS CONSTITUTINa ROCKS 



This, like magnetite, occurs in scattering granules as an 

 original constituent of many eruptives, and also in micaceous 

 lamellar and vein-like masses in other rocks. This form of iron 

 ore is extremely refractory to atmospheric agencies and is to be 

 found scarcely, if at all, changed in the residuary materials 

 resulting from the breaking down of the rocks in which it origi- 

 nally formed a part. 



Hematite (Specular Iron Ore). — Composition: Anhydrous 

 sesquioxide of iron, Fe^O^ == iron, 70.9% ; oxygen, 30.20%. H = 

 5.5-6.5. 



This mineral occurs in varying proportions and under vary- 

 ing conditions in rocks of all ages. In the form of minute 

 scales of a blood-red color, it is found in granitic and other 

 eruptive rocks. It occurs, also, in large beds, forming a valu- 

 able ore of iron. In the amorphous condition, it may form the 

 cementing constituent of sand-stones, and is the cause of the 

 red color of many rocks, both clastic and metamorphic, and of 

 soils as well. The usual coloring constituent is, however, limon- 

 ite or turgite, as noted below. The specular and massive forms 

 are best recognized by opacity, brilliant, black, metallic lustre, 

 and red streak. 



Limonite (Brown Hematite). — Composition: Hydrous ses- 

 quioxide of iron, HeFesO^ -f- FcaOg = iron sesquioxide, 85.6% ; 

 water, 14.4%. H = 5-5.5. 



This is a common constituent of rocks of all ages, but is 

 wholly secondary, resulting from the decomposition of ferrugi- 

 nous silicates, sulphides, and anhydrous oxides. As a coloring 

 constituent it is more abundant than hematite, and like it forms 

 a valuable ore of iron. (See p. 101.) Turgite (Fe4H207) in the 

 form of a brilliant red ochreous material is also a common con- 

 stituent of soils and clays resulting from the decomposition of 

 siliceous rocks, and is presumably, like limonite, a product of 

 the spontaneous hydration of the iron salts thus set free. (See 

 further under Color of Soils, p. 374.) 



Pyrite (Iron Pyrites). — Composition: Iron disulphide, FeSs 

 = iron, 46.7%; sulphur, 53.3%. H = 6-6.5. 



Two principal forms of iron disulphide occur in nature, alike 

 in chemical composition, but differing in forms of crystalliza- 

 tion and in density. The one is common pyrites which crys- 

 tallizes in the isometric system, and is easily recognized by its 

 strong brassy yellow color and hardness. Its usual form of 



