Various Forms Of Calcite

2021-08-25

Summary:Calcite is what the majority of friends are; "Listen more, see less". People often use single crystals as examples, so these indispensable minerals in life beco

Calcite is what the majority of friends are; "Listen more, see less". People often use single crystals as examples, so these indispensable minerals in life become invisible. For us, calcite is a state of existence. The "otoliths" in the ears and ear bubbles of many animals, including humans, are the calcareous crystals stacked with calcite. The main function of these crystals is to feel. The center of gravity changes in order to adjust the balance of the body to maintain the activities of the creatures. Such an important mineralization of the human body will make us hate it if the location is different, just like the "breast calcification" formed by the calcite piled crystals in the breast after the human breast is mineralized... In natural life, we often see some calcite microstructures. Crystal aggregates, such as corals in the ocean, shells, mussels and snails in rivers, exoskeletons of creatures, and travertine attached to kettles.

Calcite is an inorganic compound, that is, calcium carbonate and lime we often say. Its chemical composition content is CaO: 56.03%, CO2: 43.97%. It often contains MgO, FeO, MnO, etc. to form isomorphic variants (such as aragonite) , Chalk, etc.), sometimes containing Zn, Pb, Sr, Ba, Co, Tr and other isomorphic substitutes.

Calcite is one of the important minerals in the crustal rock formation, and it is also an important member of the earth's element cycle. It has been in a cycle for generations and generations, and it plays an important control role in the five major circles of the earth.

Calcite belongs to the trigonal crystal system; D63d-R3c. Rhombohedral unit cell: arh=6.37, α=46°5′, Z=2. Hexagonal unit cell; ah=bh=4.9896, ch=17.061, Z=6. The CO2-3 in the calcite unit cell structure is a plane triangle perpendicular to the third axis and arranged in layers. The CO2-3 triangles in the same layer have the same direction, and the CO2-3 triangles in the adjacent layers have the opposite direction. Ca is also arranged in layers perpendicular to the direction of the cubic axis and alternately distributed with CO2-3. The coordination number of calcium is 6, forming [CaO6] octahedron.

Calcite often develops intact crystals of various shapes, with more than 600 kinds of shapes. It often forms contact twin crystals according to {0001}, and more often forms poly-sheet twin crystals according to {0112}. The aggregates of calcite are also very large in number, including plate-shaped calcite, fibrous fibrous calcite, dense massive limestone, granular marble, earthy chalk, porous limestone, stalactite, and oolitic. Shape, bean shape, nodule shape, grape shape, envelope shape, crystal cluster...there are various forms. It is common in the sedimentary crystallization of crevices, fissures, cavities and caves.

The purer calcite is colorless to white, and the colorless and transparent one is often called "Ice Island Stone" in China. However, most of the calcites show light yellow, light red, purple, brown, black... various colors due to the impurity elements such as Fe, Mn, and Cu. Cleavage {1011} is complete. Under the influence of stress, it can slip along the direction of {0112} poly-sheet twin crystals to form cracks.

The Mohs hardness of calcite is 2.50~3.75, and the hardness is higher on the (0001) surface. The specific gravity is 2.6~2.9g/cm, and the density change is mainly affected by the impurity content. Calcite can fluoresce under ultraviolet fluorescence, but the fluorescence color is related to the impurity elements contained in calcite and its appearance. Heating calcite can produce elastic deformation and thermoluminescence. The excitation factors of thermoluminescence are the influence of radioactive impurities, the presence of trace impurities and the degree of crystal deformation. Under a polarized light microscope, calcite shows uniaxial crystal (-) optical properties. Sometimes it is a biaxial crystal with a small optical axis angle, Ne=1.4864, N0=1.6584 (λ=589nm), and high direfringence.

The colorless and transparent birefringence effect of calcite is its important optical property, and it has the highest birefringence and polarization performance among white and transparent crystalline minerals. This high birefringence rate is determined by the structure of calcite. When the light propagates along the c-axis (the cubic axis of symmetry), in the CO2-3 coordination triangle plane, the polarization of oxygen ions is increased due to the interaction of adjacent oxygen ions. When the light propagates perpendicular to the c-axis, the electric field acts In the perpendicularly coordinated triangular plane, the polarization of oxygen ions is weakened by the interaction of neighboring cations. Therefore, the refractive index N0 of the light wave vibrating in the triangular plane coordinated by CO2-3 is much greater than the refractive index Ne of the light wave vibrating perpendicularly to the triangular plane coordinated by CO2-3. That is, N0≥Ne.

Limestone is a sedimentary carbonate composed mainly of calcite, sometimes containing dolomite, clay minerals and detrital minerals. Limestone is not easily soluble in water but easily soluble in acid. When calcined at 1000~1300℃, the limestone is decomposed and converted into high-calcium quicklime CaO. The quicklime is deliquescent in water, and the hydrate is high-calcium slaked lime [Ca(OH)2]. The slaked lime + water is adjusted into a mortar that is easy to harden in the air. The calcium carbonate precipitate produced by the introduction of CO2 gas into the slaked lime can be filtered, dried, and finely ground to produce light calcium carbonate powder.

There are many ways to cause calcite. The common ones are the sedimentation and mineralization of biological remains, algae, sponges, and the shells of porous zooplankton, etc., as well as the weathering of rocks, the precipitation of the ground carrying carbon dioxide into the ground...the fluids carry and account for the remains of other organisms, etc. Etc. The common ones are cave stalactites, stalagmites, stone spears, calcite calcite of plants, calcareous root canals, and calcite of biological remains.

The lime we are most familiar with is made from calcite minerals. These limes are used in construction, decoration materials, glass and ceramic ingredients (to reduce the firing temperature) and so on. In metallurgy, it can be used as an alkaline solvent, and in chemical industry, it is mostly used to prepare calcium carbide, soda, calcium carbonate, potassium carbonate... The caustic soda produced is also used in rayon, chemicals, dyes, pulp, rubber, and coatings. In industry, fertilizer can be produced in agriculture.

With the development of science and technology, calcite is now also used in the field of nano-calcium carbonate. Calcium carbonate with a particle size of <100nm is called nano-calcium carbonate. Due to the ultra-fineness of nano-scale calcium carbonate particles, the crystal structure and surface electronic structure have changed. As a result, the quantum size effect, small size effect, surface effect and macroscopic quantum effect that ordinary calcium carbonate does not have are produced. Compared with conventional materials, it shows superior performance in terms of magnetism, catalysis, photothermal resistance and melting point.

In addition to functional mineralization, the human body also has this pathological mineralization. This mineralization called calcification, calculus, or calcification is often manifested in meningioma, breast cancer, thyroid cancer, cardiovascular disease, and so on. These mineralizations are mostly in the form of μm-level flakes or irregular granular and mixed aggregates, and they are mostly characterized by calcite structure or hydroxyapatite.

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