Calcium carbonate, CaCO3
Name origin: From the Latin calx, meaning lime.
One of the most common minerals on Earth, calcite is the mineral that comes in the most crystal forms. It crystallizes in the trigonal system, but habits are many: rhombohedral, prismatic, thin to thick tabular crystals, scalenohedral, dipyramidal, and twinned crystals of a multitude of types. As the most common carbonate mineral and the centerpiece of the Calcite Group, it is closely related to its cousins magnesite, rhodochrosite, siderite, and smithsonite. Colors range from colorless to white, gray, yellow, pink, light green, brown, red, violet, blue, and black.
Several variety names exist for calcite. Iceland Spar is an ice-clear variety that demonstrates the effect of double refraction or birefringence — look through a piece at a sharply defined object like type on paper and you will see the effect strongly, which creates a twinning of the light rays that reach your eye. Dog-tooth Spar is a variety with sharply terminating scalenohedral crystals that approximate canine teeth. Nail-head Spar has crystals with shallow terminations that appear to stack up like a nailhead. As for color varieties, cobaltoan calcite is strongly colored dazzling hot pink by cobalt. Manganoan calcite is softly colored pink by manganese.
Calcite is so common it occurs in a wide range of conditions. It’s a rock-forming mineral in sedimentary and metamorphic zones. It comprises most the material in rocks such as limestone and marble. It exists in caves and cavities and makes up much of the rock called travertine that forms from springs. It exists in biological settings such as shells, fossils, corals, and the skeletons of sea creatures.
Calcite is widespread and collectable specimens come from thousands of locations. Especially notable places in the United States include Houghton Co., Michigan; Franklin and Sterling Hill, Sussex Co., New Jersey; Paterson, Passaic Co., New Jersey; Wood Co., Ohio; York Co., Pennsylvania; and Shullsburg, Lafayette Co., Wisconsin. Superb calcite specimens come from the Cave-in-Rock Dist., Hardin Co., Illinois. An enormous range of magnificent calcites originates from the lead-zinc deposit of the Tri-State Dist., Missouri, Kansas, and Oklahoma, chiefly from the area centered on Joplin, Missouri. Great calcite specimens also come from the area of Carthage, Smith Co., Tennessee.
Young mountain ranges in Mexico and South America also host fine localities for calcite. They include Chihuahua, Chihuahua; the Santa Eulalia Dist., Chihuahua; Mapimí, Durango; Guanajuato, Guanajuato; and Charcas, San Luis Potosí; all Mexico; and Minas Gerais, Brazil. Important Canadian localities include Hastings Co., Ontario and several important areas in Québec.
European localities of interest include old-time mines in the Czech Republic, Denmark, France, the Harz Mountains of Germany, Italy, and Norway. Important mines in England include several in Cumbria such as at Alston Moor and Egremont, and from Weardale, Durham. Leadhills, Strathclyde, Scotland, and Bleiberg, Carinthia, Austria, are important sources.
Spectacular calcites come from the Dal’negorsk Region, Primorskiy Kray, Far Eastern Russia, and other Russian locations. In recent years a flood of superb Chinese calcites has hit the market, and those from Hunan Prov. and from the Guangxi Zhuangzu Autonomous Area are really fine. Excellent calcite specimens also come from the extensive Deccan traps in India.
|Crystal System: Trigonal||Luster: Vitreous
|Hardness: 3||Tenacity: Brittle
|Density System: 2.7 g/cm3||Streak: White
Nickel-Strunz, 10th edition, classification: 5.AB.05
Calcite Group, Calcite-Rhodochrosite Series
All specimens from the David J. Eicher Mineral Collection; images © David J. Eicher