CaCO 3 . A type of anhydrous carbonate mineral. Hexagonal or trigonal crystal system. Polymorphic with aragonite (orthorhombic crystal system). Specific gravity 2.71, hardness 2.7-3. Vitreous or pearlescent. Colorless and transparent, sometimes white, pale pink, pale brown, etc. due to the inclusion of impurities. White or off-white streaks. Complete cleavage. Extremely high birefringence, used as Nicol prisms. Occurs as a polyhedron (trigonal scalenohedron) surrounded by rhombohedral or scalene triangular faces, as well as fibrous, leaf-like, earthy, or nodular crystals. While aragonite is stable under high pressure conditions, calcite has a stable region on the low pressure side, and is stable at 1 atm. It is part of the calcite group of minerals along with magnesite, siderite, rhodochrosite, and smithsonite. At temperatures above 600°C, it forms a continuous solid solution with rhodochrosite. At temperatures above 1100°C, it also forms a continuous solid solution with dolomite. Natural calcite may contain impurities such as magnesium, manganese, and iron. It occurs on a large scale as the main component of limestone, and is commonly found as a gangue mineral in various hydrothermal deposits or as an accessory mineral in igneous rocks. It is also one of the main components of fossil shells. Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information |
CaCO3 。無水炭酸塩鉱物の一種。六方晶系あるいは三方晶系。霰石 (斜方晶系) と同質異像。比重 2.71,硬度 2.7~3。ガラス光沢ないし真珠光沢。無色透明,ときに不純物の混入により白色,淡桃色,淡褐色などを呈する。条痕は白色ないし灰白色。劈開 に完全。複屈折がきわめて高く,ニコルプリズムなどとして利用される。菱面体または不等辺三角形状の面で囲まれた多面体 (三方スカレノヘドロン) の結晶として産出するほか,繊維状,葉片状,土状,団塊状を呈して産出することもある。霰石が高圧条件下で安定であるのに対し,低圧側に安定領域を有し,1気圧では方解石が安定である。マグネサイト,菱鉄鉱,菱マンガン鉱,菱亜鉛鉱などとともに方解石族鉱物群を構成する。 600℃以上の高温では,菱マンガン鉱との間に連続固溶体を形成する。また 1100℃以上の高温では,ドロマイトとの間にも連続固溶体を形成する。天然産方解石は不純物としてマグネシウム,マンガン,鉄などを含んでいることがある。石灰岩の主成分鉱物として大規模に産出するほか,各種熱水鉱床の脈石鉱物あるいは火成岩の副成分鉱物として普遍的に産出する。貝殻化石の主成分鉱物の一つでもある。
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