Geology

1. Hrematite, or Specular Iron, is ferric oxide, Fe₂0₃. Sp. gr. = 4.5-5.3; H = 6.5. Crystallizes in rhombohedrons, or more commonly, in nodular masses, which are composed internally of very flat crystals. The color is black or steel-grey, which becomes red when the mineral is finely powdered. Hsernatite frequently contains earthy and other impurities and is one of the most important ores of iron.
2. Limonite, or Brown Hsernatite, is hydrated ferric oxide (2 Fe₂0₃, 3 H₂0) containing more than 14% of water. It is softer than hsematite and of a yellow or brown color. Sp. gr. = 3. 6-4; H= 5-5.5.
3. Magnetite is the black oxide of iron, Fe₃0₄ (or FeO, Fe₂0₃).
   Sp. gr. = 4.9-5.2; H = 5.5-6.5. Crystallizes in the isometric system, usually in octahedrons, sometimes in dodecahedrons. This mineral is strongly magnetic and is black in color, with a bluish-black metallic luster, when viewed in reflected light. Magnetite is widely diffused in certain classes of rocks, and also occurs in veins and beds, which form an important source of supply of the metal.
4. Ilmenite is a titaniferous ferric oxide, (Ti, Fe)₂0₃ Sp. gr. = 4.5-5.2; H = 5-6. When crystallized, this mineral is rhombohedral, but is generally massive.
5. Siderite is ferrous carbonate, FeC0₃ Sp. gr. = 3.7-3.9; H = 3.5-4.5. Crystallizes in rhombohedrons, the faces of the crystals frequently much curved, and often the crystals are very much flattened. When fresh, the mineral is grey or brown. It is but slightly acted on by cold acids; hot acids dissolve it with effervescence. Mixed with clay, siderite forms clay iron-stone, a valuable ore.
6. Iron Pyrites, bisulphide of iron, FeS₂ Sp. gr. = 4.9-5.2; H = 6-6.5. Crystallizes in the isometric system, usually in cubes, sometimes in dodecahedrons, and has a very characteristic brassy luster and color, to which it owes the popular name of “fools’ gold.” It is very hard, cannot be scratched with a knife, and strikes fire, like flint, when struck with steel. The mineral is soluble in nitric acid: it is widely disseminated in the rocks.
7. Marcasite, or White Iron Pyrites, has the same composition as pyrites, but crystallizes in the orthorhombic system, in modified prisms, but more commonly occurs in nodular masses, with a radial structure. It has the same hardness as pyrites, but is not quite so heavy. Sp. gr. = 4.68-4.85. In color it is paler than pyrites, with a tendency to grey, green, or even black. It decomposes very readily and after a few months’ exposure, even to dry air, often crumbles to a whitish powder.

The iron minerals are seldom largely represented in any given rock, with the exception of the ore beds, but iron is one of the most widely diffused of substances, few rocks being altogether free from it, and its various compounds playa very important role as coloring-matter in the rocks. Ferrous carbonate gives no color to the rock in which it occurs, and such rocks are apt to have a blue or grey tint, due to other substances, both organic and inorganic. When such rocks are exposed to the action of air and moisture, ferric oxide and ferric hydrates are formed, the former giving a red color and the latter various shades of yellow and brown.

A blue clay containing ferrous carbonate will, when fired in a kiln, give rise to red bricks or pottery, by the conversion of FeC0₃ into Fe₂0₃. Exposure to moist air produces a similar effect in nature, and the contrast in color between the superficial and deep-seated layers of the same rock is often as great as between blue clay and red brick.

Weathered blocks stained red on the outside are often blue, grey, or nearly black on the inside, the change not having penetrated through the whole mass. Such changes are most conspicuous in the sandstones, because their porous character allows a comparatively free circulation of air and water through them, but similar effects are frequently to be observed in other rocks also.