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About Minerals

Calcite

Calcite is one of the most common minerals, second only to the silicates (quartz, agates, jasper, opal, etc.) It is a mineral chemically found usually as calcium carbonate. It forms into hexagonal crystals, and is found in limestone, chalk and marble. It forms in large veins as a gangue mineral. A gangue mineral is the worthless rock or vein matter in which other valuable metals or minerals occur.

It precipitates from sea water to build up limestone, and secondarily is deposited from solutions of limestone and other rocks. Localities are far too numerous to list. Crystals may be flat, clear plates, steep golden crystals or transparent masses a foot thick. Some of the large transparent crystals are considered to be the original Iceland Spar. Marble, cave formations, travertine and onyx are all calcite varieties. Oolitic calcite sand forms on the shores of the Great Salt Lake in Utah. Calsite is part of the group of minerals or carbonites called the Calcite Group, that includes minerals such as rhodochrosite, magnesite, siderite and smithsonite. The carbonites form in various ways: as primary minerals, separating from hot solutions freshly springing from inside the earth, and from cold solutions on the surface of the earth, near the surface, or from the very ocean itself. The carbon dioxide of the air combines with water to form a mild acid, carbonic acid, which attacks the surface minerals. Some of the elements dissolve, often to reappear in other solid forms as carbonates. This corrosion by carbonic acid is one of the principle mechanisms in the weathering of rocks. The same attacker alters many of the metal ores, the sulfides, when they are exposed on the surface. When such ore deposits are in the rocks that are predominantly carbonates like limestone (calcium carbonate, better known as calcite) the metals are found concentrated and immobilized as carbonates in the upper, weathered ore zones.

There are two significant crystal forms in the carbonates. The first series, formed in rhombohedral crystals, is know as the Calcite Group. The second series, formed of orthorhombic crystals, is known as the Aragonite Group. Although their structures are different, they have some properties in common. All of the carbonates are soft, all of them are light colored and translucent to transparent, all are soluble in acid, some more easily than others, as bubbles of carbon dioxide escape. They are predominantly, but not exclusively, secondary in origin.

Calcite is usually found in its natural form in a crystal state. It is extrememly varied in appearance, from tabular to prismatic or needle like crystals. Scalenohedron and rhombohedron crystals are most common. The crystal formations are micro-crystalline to coarse. Calcite is frequently flourescent. A small amount of manganese is enough to make it glow bright red under some wave lengths of ultraviolet light. Flawless transparent calcite is used in optical instruments, especially in geological (polarizing) microscopes.