1. Haematite

Haematite, 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. The colour is black, steel-grey, or red, and always is red when the mineral is finely powdered. Haematite frequently contains earthy and other impurities and is one of the most important ores of iron.

2. Limonite

Limonite, or Brown Haematite, is hydrated ferric oxide (2 Fe203, 3 H20) containing more than 14% of water. It is softer than haematite and of a yellow or brown colour. Sp. gr. =3.6-4;

H = 5-5.5.

3. Magnetite

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 colour, with a bluish-black metallic lustre, 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

Ilmenite is an oxide of iron and titanium (Ti, Fe)203. Sp. gr. = 4.5-5.2; H = 5-6. When crystallized, this mineral is rhom-bohedral, but is generally massive.

5. Siderite

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. Pyrite

Pyrite, or 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 lustre and colour, 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

Marcasite, or White Iron Pyrites, has the same composition as pyrite, 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 pyrite, but is not quite so heavy. Sp. gr. =4.68-4.85. In colour it is paler than pyrite, 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 play a very important role as colouring-matter in the rocks.