Occurs in compact, mammillary, botryoidal, or stalactitic masses of dark red to black color, or in earthy masses of bright to dark red; hardness 6; specific gravity 5.2; color ochre red to black; streak cherry red to dark red; luster metallic, vitreous, or dull; opaque on thin edges.
Hematite is readily distinguished from other red minerals by its hardness and streak. It may occur in crystals, which belong to the hexagonal system, and are usually hemihedral forms of the double pyramid, or rhombohedrons. These rhombohedrons usually have the edges beveled, as in [Pl. 13], A; or are tabular in form as a result of the beveling of two of the opposite edges to such an extent that a form like [Pl. 13] B results. However the usual occurrence is in non-crystalline masses, which represent transformations from limonite by the loss of water of crystallization on the part of the limonite. In such cases we have fibrous, oolitic or compact masses, according to the form in which the limonite occurred. The transformation from limonite into hematite involves some heat to drive out the water of crystallization, but nothing like what is involved in metamorphism.
Hematite is the source of 90% of the iron mined in this country. Part of it comes from the famous Clinton iron ore, a layer a foot or more in thickness; starting in New York State, and extending all down the Appalachian Mountains to Alabama, where it is ten or more feet thick and the basis of the Birmingham iron industries. Then there are tremendous deposits of earthy to compact hematite, probably derived from limonite, around the west end of Lake Superior. This latter region yields today around 75% of the iron for this country.
Loose earthy masses of hematite are often known as “ochre red,” and were used by the Indians for war paint. Today the same sort of material is obtained by powdering hematite and using it for red paint. The red color in great stretches of rock is due to the presence of small amounts of hematite, acting as cementing material. The red of the ruby, garnet, spinel, and the pink of feldspars and calcite are due to traces of hematite.
This mineral is very common and found in every state.
[Magnetite]
Fe₃O₄
[Pl. 14]
Magnetic iron ore
Occurs in masses or in isometric octahedrons or dodecahedrons; hardness 6; specific gravity 5.8; color black; streak black; luster metallic; opaque on thin edges.
Magnetite is another important ore of iron, and is peculiar in being strongly magnetic; its name being derived, according to Pliny, from that of the shepherd Magnes, who found his iron pointed staff attracted by the mineral when he was wandering on Mount Ida. This magnetic property has been repeatedly used to locate beds of magnetite, and is very helpful in separating magnetite from the “black sands,” of which it so often forms a part. These sands however generally have magnetite with so much titanium in it that they are unfit for smelting.
Magnetite is found in association with igneous or metamorphic rocks, and often represents limonite or hematite which has been altered as the result of high temperatures. Some of it, in the igneous rocks especially, was undoubtedly in the molten magma and has crystallized out from the magma while it was still hot. It is the form of iron always indicative of former high temperatures. It is an ore mineral for about 3% of the iron in this country, but in Scandinavia and some other countries, it plays a leading role as the source of iron.
It is found in the Adirondack Mountains, in New Jersey, Pennsylvania, Arkansas, North Carolina, New Mexico, and California.