Occurs in crystals or in lamellar, nodular or granular masses; hardness 3; specific gravity, 4.5; colorless, white or almost any color; luster vitreous; transparent on thin edges.
Barite occurs in orthorhombic crystals, which are tabular in form, and usually have the edges beveled, as in figure A, [Plate 48]. There is cleavage in three directions, a rather perfect basal cleavage, and two less perfect cleavages, which are at right angles to the basal cleavage plane, and intersect each other at 78°.
The tabular form, the cleavage, the heavy weight, and the fact that a piece of barite put into the flame colors it green, all serve to distinguish this mineral.
Barite is a secondary mineral of aqueous origin, which has been deposited in veins and cavities in igneous, metamorphic, or sometimes sedimentary rocks. It is most likely to occur in veins in igneous or metamorphic rocks, the barium having been dissolved from certain feldspars and micas by percolating water, and then redeposited in the fissures, as the water came into them. If in sedimentary rocks, the barite veins are usually in limestones. Barite is quite likely to be a gangue mineral for the ores of lead.
It is found at Hatfield and Leverett, Mass., Cheshire, Conn., Pillar Point, N. Y., Cartersville, Ga., in Virginia, North Carolina, South Carolina, Missouri, Kentucky, Tennessee, Alabama, Illinois, Wisconsin, Nevada, California, Alaska, etc.
[Witherite]
BaCO₃
[Pl. 48]
Occurs in crystals, or in granular or columnar masses; hardness, 3.5; specific gravity, 4.3; color white to gray; luster vitreous; translucent on thin edges.
Witherite is not an abundant mineral. Its crystals are really orthorhombic, but they are usually twinned, three crystals growing through each other in such a manner that the resulting crystal appears like a six-sided double pyramid, similar to the one figured on [Plate 48]. The commonest mode of occurrence is in compact masses. Witherite effervesces when cold acid is dropped upon it, which, with its heavy weight, and the green color it gives to the flame, serves to distinguish the mineral. It is used for medicines, in chemical industries, and a considerable amount is made into rat poisons. The chief locality for witherite is in northern England, but in this country it is found along with barite, especially at Lexington, Ky., and in Michigan.
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.