Carbon
Carbon is an element widely distributed in nature, occasionally appearing in its elementary form, as graphite or the diamond, but much more important in its compounds. Small quantities are present in the air as carbon dioxide, CO₂, immense quantities occurring in the carbonate minerals, which have been considered under their respective metallic salts, as calcite, malachite, siderite, cerrusite, smithsonite, witherite, etc., and still other large quantities being represented in organic compounds, which occur as rocks under the heads of petroleum, coal, etc. The occurrence of limestones, graphite, coal or petroleum is always indicative of the activity of living organisms, and in some cases is the only indication of life in the earlier rocks.
[Graphite]
C
Plumbago
Occurs in hexagonal scales or flakes, in layered masses, or earthy lumps; hardness, 1; specific gravity, 2.1; color black or steel-gray; streak gray; luster metallic; opaque on thin edges.
Like the diamond graphite is pure carbon, but in this case it is in non-crystalline form. It occurs in both igneous and metamorphic rocks. In the former case it is either in flakes in the rock, or in veins, and has been derived directly from the molten magmas, having either precipitated in the hardening granite or lava, or having been carried into the fissures and there precipitated to make the veins of graphite. In either case the graphite probably represents organic deposits which have been melted into the igneous magma at the time of its formation. Graphite may also occur in metamorphic rocks, beds of coal or other organic deposits being altered by the heat. Such beds are often of considerable extent and economic importance.
The extreme softness, greasy feel, and the dark-gray streak readily distinguish graphite.
It is widely used in making crucibles and furnace linings for foundries, lead pencils, paint, lubricating powders, etc.
Graphite is found at Brandon, Vt., Sturbridge, Mass., Ashford, Conn., in Essex, Warren and Washington Cos., N. Y., Clay, Chilton and Coosa Cos., Ala., Raton, N. M., Dillon, Mont., etc.
[Diamond]
C
Occurs in octahedral crystals; hardness, 10; specific gravity, 3.5; colorless to yellow, brown, blue, etc., luster adamantine; transparent on thin edges.
Like graphite the diamond is pure carbon, but in this case in crystal form. It is the hardest of all minerals, and as brilliant as any; so that in spite of being by no means the rarest, it may easily be considered the most popular of all gems. Tiny diamonds have been made artificially under great heat and pressure; so that this mineral is thought of as forming in Nature in dark igneous lavas at great depths. The diamond has good cleavage parallel to the octahedron faces, and, in spite of some traditions to the contrary, is brittle.
There are not many diamond localities, the most famous being the Kimberley district of South Africa, which produces many times as many diamonds as all the others put together, though all the time some are being found in Borneo and Brazil. A very few have been found in the United States, only one locality however yielding them in the original matrix. That is at Murfreesboro, Ark., where they are mined in a disintegrating peridotite (a dark lava, mostly peridot), which has been extruded through the sedimentary rocks of that region. This matrix is similar to the “blue earth,” the matrix of the diamonds of South Africa, which occurs in “pipes,” representing the necks of ancient volcanoes. The American diamonds are of small size, averaging considerably less than a third of a carat in weight, which does not allow great value to the individual diamonds.
From time to time, especially large diamonds have been found in different parts of the world, the largest being the Cullinan diamond, found at the Premier Diamond Mine of South Africa. It weighed 3025 carats or about a pound and a quarter, and was valued at over $3,000,000. It was presented to King Edward VII, who had it cut into 11 brilliants, four of which are larger than any other diamond yet found. Other famous diamonds, like the Kohinoor, 106 carats, found in India in 1304; the Regent, 136 carats, also found in India; the Orloff, 193 carats, set in the eye of an Indian idol; the South Star, 125 carats, the largest ever found in Brazil; the blue Hope, etc., have in many cases romantic and interesting stories woven about them.
Though for ages diamonds have been highly prized gems, it is only in comparatively recent times that cutting and polishing have been resorted to, for the purpose of enhancing their brilliancy. This is done by grinding reflecting faces on the original stone, by the aid of discs of iron or tin in which diamond dust has been embedded. Diamond chips and cloudy or imperfect diamonds are used for making tools for cutting glass, rock drills, etc.