Calcite

[Chemical composition] often contains Mn, Fe, Zn, Mg, Pb, Sr, Ba, Co, TR and other similar materials; when they reach a certain amount, can form manganese calcite , iron calcite, zinc calcite, Magnesium calcite and other varieties. In addition, brucite, dolomite, iron hydroxide and mechanical mixtures such as oxides, sulfides and quartz are also commonly found in crystals.

[Crystal Structure] Trigonal System; Rhombohedral unit cell: a _rh = 0.637 nm, α = 46 ° 07 '; Z = 2; if converted into a hexagonal (dual body) lattice, then: a _h = 0.499 nm, c _h = 1.706 nm; Z = 6 . The calcite structure is described in the overview section of Mineralogy Chapter 22.
[Form] Commonly intact crystals. There are various forms, and there are more than 600 different shapes. It is mainly in the form of parallel and [0001] developed columnar and parallel {0001} developed plate-like and various states of rhombohedron or complex trigonal scalenohedron (Fig. H-1). Calcite often forms a contact twin crystal according to (0001), more often (01 2) Forming polycrystalline twin crystals, the orientation of this polycrystalline double crystal grain on the cleavage plane is different from that of dolomite (Fig. H-2). In nature, the appearance of this polycrystalline twin crystal can be used to illustrate the formation of calcite. After that, it was subjected to geological stress.

Figure H-1 Crystals and twins of calcite

(quoted from Pan Zhaoyu, 1993)

Parallel double-sided: c{0001}; hexagonal column: m{10 0}; rhombohedron: r{10 1}, e{01 2}; complex three-party triangular facet: v{2141}

Figure H-2 Comparison of the performance of polycrystalline twin crystals of calcite and dolomite on the rhombohedral cleavage plane

(quoted from Pan Zhaoyu, 1993)

(a) Polycrystalline twin crystals of calcite parallel (0112), (b) Polycrystalline twin crystals of dolomite parallel (0221)

Figure H-3 Calcite clusters

The aggregate form of calcite is also diverse. From flaky (plate-like) or fibrous calcite, parallel or nearly parallel continuum, called calcite and fiber calcite. There are also dense blocks (limestone), granular ( maolite ), earthy ( white ), porous (travertine), stalactite (stone stalactite) and braided, bean-like, tuberculous, grape-like, membranous And clusters, etc. (Figure H-3). The crystal morphology of calcite is related to the formation conditions. With the decrease of temperature during formation, the crystal form has a tendency to evolve from a plate-like, obtuse rhombohedral crystal form to a complex three-sided triangular facet, a hexagonal column and an acute-angled rhombohedral crystal form (Fig. H-4).

Figure H-4 Variation of calcite morphology with temperature

(quoted from Pan Zhaoyu, 1993)

[Physical properties] It is colorless or white, and is sometimes dyed as light yellow, light red, purple, and brown by elements such as Fe, Mn, and Cu. Colorless and transparent calcite known as Iceland spar (icespar). Cleavage {10 1}complete; under the influence of stress, along {01 2} The polycrystalline twin is slipped into a split. Hardness 3. The relative density is 2.6 to 2.9. Some calcites have an optical rotation.

[genesis and occurrence] Calcite is one of the most widely distributed minerals, with various genetic types. Mainly: 1 sedimentary type, when CaCO _{3 in} seawater reaches supersaturation, it can deposit a large amount of limestone, shale limestone, etc.; 2 hydrothermal type, common in medium and low temperature hydrothermal deposits, veined or seen in empty, the crystalline form with good; â‘¢ magma, the main rock-forming minerals calcite magmatic carbonate and carbonate lava, often coexist with dolomite, phlogopite and the like; â‘£ weathering, limestone, marble in During the weathering process, the groundwater dissolves easily to form calcium carbonate Ca(HCO ₃ ) into the solution. When the pressure is reduced or evaporated, a large amount of CO ₂ is released, and the calcium carbonate can be reprecipitated to form stalactites, stalagmites, stone pillars and the like. Its reaction formula: Ca(HCO ₃ ) ₂ = CaCO ₃ + H ₂ O + CO ₂ .

[identification feature] crystal form, {01 2} polycrystalline twin crystal, {10 1} Three groups are completely cleavage, the hardness is small, and the relative density is small. Add HCl to foam rapidly. The calcined pieces after burning are placed on the litmus paper for alkaline reaction. There is a flame reaction of calcium (orange).

[Main use] Limestone, marble, chalk and other rocks composed of calcite are widely used in chemical, metallurgical, construction and other industrial sectors, such as for burning lime and cement. Beautiful marble can be used as building decoration material. High purity limestone is an important raw material for plastics and nylon. Because the ice stone has strong birefringence and polarization, it is widely used in the field of optics, such as polarizing microscope prisms, polarizers, photometers and so on.

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