[Siderite]
FeCO₃
Pl. [13] & [14]
Spathic iron
Occurs in fibrous botryoidal masses or rhombohedral crystals, sometimes with curved faces; hardness 3.5; specific gravity 3.8; color gray-brown; streak white; luster vitreous; translucent on thin edges.
Like hematite this mineral belongs to the hexagonal system, and crystallizes in hemihedral form, making the rhombohedron. Its faces are often curved, which is rare in minerals, only a few forms like this and dolomite having other than plane faces. When siderite crystals grow in clusters, the crowding often results in growth on one face only, making a mass of fibrous character, and in such cases the surface of the mass is botryoidal in contour. The mineral is likely to oxidize, losing its gray-brown color, and becoming limonite. In the United States it is scarcely ever used as an ore for iron, but in Germany and England a great deal of iron is smelted from this mineral.
It occurs in Massachusetts, Connecticut, New York, throughout the Appalachian Mountains, and also in Ohio.
[Pyrite]
FeS₂
Pl. [15] & [16]
iron pyrites
Occurs as cubes, octahedrons and pyritohedrons, or in compact masses, scales or grains; hardness 6; specific gravity 5.1; color brassy yellow; streak greenish-black; luster metallic; opaque on thin edges.
This is one of the commonest of all minerals. It is found in all kinds of rocks, with all kinds of associations, in all parts of the world. Its crystals are isometric, and cubes and octahedrons are abundant. The pyritohedron is also a common form, and characteristic of this mineral. It is a hemihedral form derived from a 24-sided form, i.e. the cube with four faces on each side. On this 24-sided form each alternate face has developed and the others have disappeared, resulting in a 12-sided form, known as the pyritohedron, which differs from the dodecahedron in that each of its faces is five-sided instead of rhomboidal. When in crystals pyrite can not be easily confused with any other mineral; but when in masses it is often mistaken for gold, chalcopyrite, pyrrhotite or marcasite. From the first two, the color should be sufficient to distinguish it, for they are golden yellow. Pyrrhotite is bronze yellow, and marcasite is paler yellow. Then too in hardness pyrite is much harder than any of these minerals except marcasite. This last is the one which is most likely to cause real difficulty. Its lighter color, and the fact that it usually comes in fibrous masses are the best distinctions.
In spite of being so abundant pyrite is scarcely ever used as an ore for iron, because the sulphur makes the metal “short,” or brittle, and the sulphur is not easily gotten entirely out of the iron; but pyrite is used largely in the manufacture of sulphuric acid, so important to many of our industries.
Other sulphides are commonly mixed with pyrite, such as chalcopyrite, arsenopyrite, argentite, etc.; but the most important impurity is gold, which is often scattered through the pyrite in invisible particles, and sometimes in quantities enough to make it worth while to smelt it for the gold.
Pyrite is particularly the form in which the sulphur compounds of iron appear in rocks which have been highly heated, and is to be expected in metamorphic rocks and also igneous rocks, especially in fissures and veins leading from the igneous rocks. It may occur in sedimentary rocks, but in these last it is usually marcasite.