The following was retrieved from Wikipedia, the free encyclopedia :(en.wikipedia.org/wiki/Calcite) on February 26, 2020.
"Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as "calcite".
Other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite over timescales of days or less at temperatures exceeding 300 °C, and vaterite is even less stable." Etymology
Calcite is derived from the German Calcit, a term coined in the 19th century from the Latin word for lime, calx (genitive calcis) with the suffix -ite used to name minerals. It is thus etymologically related to chalk.
When applied by archaeologists and stone trade professionals, the term alabaster is used not just as in geology and mineralogy, where it is reserved for a variety of gypsum; but also for a similar-looking, translucent variety of fine-grained banded deposit of calcite. Properties Form
Over 800 forms of calcite crystals have been identified. Most common are scalenohedra, with faces in the hexagonal {2 1 1} directions (morphological unit cell) or {2 1 4} directions (structural unit cell); and rhombohedral, with faces in the {1 0 1} or {1 0 4} directions (the most common cleavage plane). Habits include acute to obtuse rhombohedra, tabular forms, prisms, or various scalenohedra. Calcite exhibits several twinning types adding to the variety of observed forms. It may occur as fibrous, granular, lamellar, or compact. A fibrous, efflorescent form is known as lublinite. Cleavage is usually in three directions parallel to the rhombohedron form. Its fracture is conchoidal, but difficult to obtain.
Scalenohedral faces are chiral and come in pairs with mirror-image symmetry; their growth can be influenced by interaction with chiral biomolecules such as L- and D-amino acids. Rhombohedral faces are achiral. Hardness
It has a defining Mohs hardness of 3, a specific gravity of 2.71, and its luster is vitreous in crystallized varieties. Color is white or none, though shades of gray, red, orange, yellow, green, blue, violet, brown, or even black can occur when the mineral is charged with impurities. Optical
Calcite is transparent to opaque and may occasionally show phosphorescence or fluorescence. A transparent variety called Iceland spar is used for optical purposes. Acute scalenohedral crystals are sometimes referred to as "dogtooth spar" while the rhombohedral form is sometimes referred to as "nailhead spar".
Single calcite crystals display an optical property called birefringence (double refraction). This strong birefringence causes objects viewed through a clear piece of calcite to appear doubled. The birefringent effect (using calcite) was first described by the Danish scientist Rasmus Bartholin in 1669. At a wavelength of ≈590 nm calcite has ordinary and extraordinary refractive indices of 1.658 and 1.486, respectively. Between 190 and 1700 nm, the ordinary refractive index varies roughly between 1.9 and 1.5, while the extraordinary refractive index varies between 1.6 and 1.4. Chemical
Calcite, like most carbonates, will dissolve with most forms of acid. Calcite can be either dissolved by groundwater or precipitated by groundwater, depending on several factors including the water temperature, pH, and dissolved ion concentrations. Although calcite is fairly insoluble in cold water, acidity can cause dissolution of calcite and release of carbon dioxide gas. Ambient carbon dioxide, due to its acidity, has a slight solubilizing effect on calcite. Calcite exhibits an unusual characteristic called retrograde solubility in which it becomes less soluble in water as the temperature increases. When conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together or it can fill fractures. When conditions are right for dissolution, the removal of calcite can dramatically increase the porosity and permeability of the rock, and if it continues for a long period of time may result in the formation of caves. On a landscape scale, continued dissolution of calcium carbonate-rich rocks can lead to the expansion and eventual collapse of cave systems, resulting in various forms of karst topography.
There are currently 118 known chemical elements exhibiting a large number of different physical and chemical properties. Amongst this diversity, scientists have found it useful to use names for various sets of elements, that illustrate similar properties, or their trends of properties. Many of these sets are formally recognized by the standards body IUPAC.
The following collective names are recommended by IUPAC: Alkali metals – The metals of group 1:
Li,
Na,
K,
Rb,
Cs,
Fr. Alkaline earth metals – The metals of group 2:
Be,
Mg,
Ca,
Sr,
Ba,
Ra. Pnictogens – The elements of group 15:
N,
P,
As,
Sb,
Bi.
(Mc
had not yet been named when the 2005 IUPAC Red Book was published, and its chemical properties are not yet experimentally known.) Chalcogens – The elements of group 16:
O,
S,
Se,
Te,
Po.
(Lv
had not yet been named when the 2005 IUPAC Red Book was published, and its chemical properties are not yet experimentally known.) Halogens – The elements of group 17:
F,
Cl,
Br,
I,
At.
(Ts had not yet been named when the 2005 IUPAC Red Book was published, and its chemical properties are not yet experimentally known.) Noble gases – The elements of group 18:
He,
Ne,
Ar,
Kr,
Xe,
Rn.
(Og had not yet been named when the 2005 IUPAC Red Book was published, and its chemical properties are not yet experimentally known.) Lanthanoids – Elements 57–71:
La,
Ce,
Pr,
Nd,
Pm,
Sm,
Eu,
Gd,
Tb,
Dy,
Ho,
Er,
Tm,
Yb,
Lu, Actinoids – Elements 89–103:
Ac,
Th,
Pa,
U,
Np,
Pu,
Am,
Cm,
Bk,
Cf,
Es,
Fm,
Md,
No,
Lr. Rare-earth metals –
ScY, plus the
lanthanoids Transition elements – Elements in groups 3 to 11 or 3 to 12.
