The sulphide of iron, pyrite, occurs in many crystalline rocks; but, owing to the difficulty of separating the iron and sulphur, is not used as an ore of iron. The mineral much resembles in external appearance a yellow ore of copper, called chalcopyrite, from which it may be distinguished in that it will strike fire with steel. A specimen of pyrites containing large crystals is an interesting subject of study. These crystals are cubical in shape, but generally massed together so that no single crystal form may be observed as complete. Peculiar striations on the cube faces may often be noted. The striations of no two adjoining faces are continuous; but rather a striation of one face bears to that of another in direction the relation of the stem of a printed T to the top, or vice versa. Owing to the affinity of each component element for oxygen, pyrite often changes to vitriol, or else forms the oxide of iron, limonite, described below.
The black oxide of iron, magnetite, occurs widely distributed. As its name indicates, it sometimes displays the properties of a magnet. If a fragment of unequal dimensions be suspended freely by a string, the longer dimension will gradually swing into a north and south direction. The property possessed by magnetite of attracting other bits of iron appears to have been known to the ancients, and by them the name lodestone was applied to the mineral. Since the power to attract other particles of iron is not apparent in all specimens of magnetite we must consider other more distinguishing characteristics. The ore is very heavy; particles of it are attracted by an artificial magnet, in which regard it differs from the other minerals we have mentioned; if a piece of the ore be scratched across the surface of a harder substance, _e. g._, smoky quartz, a black "streak" will be left. Pure magnetic iron ore is intensely black, with no coloring.
In a series of ore beds formerly operated by a mining company of northern New York, four distinctions of the crude ore were made, two varieties of blue, one of black, and one of gray. The blue coloring is apparently due to the presence of impurities; the black ore is evidently magnetite; and the steel-gray mineral, failing in the characteristic properties of magnetite, finds its class place under hematite. Hematite differs from magnetite in representing a higher degree of oxidation. It is often found, as indicated above, in beds distributed in close conjunction with those of magnetite. This ore is a valuable source of iron. Hematite commonly occurs in earthy materials, as red ochre. Its streak is red. All rocks of a reddish or red color owe the color to this oxide of iron.