[Marcasite]
FeS₂
[Pl. 15]
white pyrite
Occurs in orthorhombic crystals, usually grouped to make fibrous or radiating masses, or non-crystalline in masses; hardness 6; specific gravity 4.8; color pale brassy-yellow; streak greenish-gray; luster metallic; opaque on thin edges.
Marcasite has the same chemical composition, as pyrite, and looks like it, but is lighter colored and usually occurs in fibrous masses. It is the commoner form in limestones and shales, while pyrite is more likely to occur in igneous and metamorphic rocks. It seems probable that marcasite is due to a more hasty precipitation from cold solutions, while pyrite is deposited more slowly from hot solutions.
Isolated crystals of marcasite are rare; but, if formed, they belong to the orthorhombic system. Usually some form of twinning is present, and because of the multiple character of the twinning, marcasite crystals usually show a ragged outline, with reentrant angles. It is most abundant in radiated masses, which appear fibrous on the broken surfaces. It decomposes easily, taking oxygen from the air and forming, even in museum cases, a white efflorescence or “flower,” which is iron sulphate or melanterite. In moist air it takes water and decomposes to sulphuric acid which may change the surrounding limestone to gypsum. Marcasite is found wherever limestones and shales are the country rock.
[Pyrrhotite]
Fe₁₁S₁₂
Magnetic pyrites
Occurs in masses; hardness 4; specific gravity 4.6; color bronze; streak grayish-black; luster metallic; opaque on thin edges.
Tabular crystals are known, but are very rare. They belong to the hexagonal system. This form is easily distinguished from the other yellow minerals by being magnetic. It is by no means as abundant as the two preceding sulphides of iron, but does occur fairly frequently in veins in igneous rocks, and less frequently in limestones, large quantities of sulphuric acid being made from a deposit in limestone at Ducktown, Tenn. It will be found in most states. When associated with nickel it is an important source for the latter mineral, as at Sudbury, Canada. Pyrrhotite is very like a substance found in meteorites, known as troilite.
The Lead Group
After learning how to get iron from the rocks by rude smelting methods, the early peoples tried heating various rocks, and some time around 500 B.C. stumbled upon lead, which is rather easily separated from its ores. This metal was used through Roman times to make pipes, gutters, etc.
Lead is a soft metal, fairly malleable, but with little ductility, and still less tensile strength. Though one of the commoner metals, it does not occur as pure metal in Nature. It is diffused in minute quantities through the igneous rocks, and also is found in the sedimentary rocks and in the sea water. Its minerals are few, galena, the sulphide of lead, being the commonest, and at the same time the form in which lead is primarily deposited. Galena may also represent a secondary deposition. The other minerals, cerrusite, anglesite, and pyromorphite are results of modification of the galena when it lies near enough to the surface to be acted on by weathering agents, like water and air. Lead minerals are usually associated with zinc minerals, there being but few places where the minerals of the one group occur without the other. Most lead when first smelted from its ore, contains a greater or less amount of silver in it, sometimes enough so that the lead ore is better worth working for the silver than for the lead.