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An organic mineral is an
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. T ...
in
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
form. An organic compound is any compound containing carbon, aside from some simple ones discovered before 1828. There are three classes of organic mineral:
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
s (containing just
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
and
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
),
salts
In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively c ...
of
organic acids, and miscellaneous. Organic minerals are rare, and tend to have specialized settings such as fossilized
cacti and bat
guano. Mineralogists have used statistical models to predict that there are more undiscovered organic mineral species than known ones.
Definition
In general, an
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. T ...
is defined as any compound containing carbon, but some compounds are excepted for historical reasons. Before 1828, chemists thought that organic and inorganic compounds were fundamentally different, with the former requiring a
vital force
Vitalism is a belief that starts from the premise that "living organisms are fundamentally different from non-living entities because they contain some non-physical element or are governed by different principles than are inanimate things." Wher ...
that could only come from living organisms. Then
Friedrich Wöhler synthesized
urea
Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid.
Urea serves an important ...
by heating an inorganic substance called
ammonium cyanate
Ammonium cyanate is an inorganic compound with the formula . It is a colorless, solid salt.
Structure and reactions
The structure of this salt was verified by X-ray crystallography. The respective C–O and C–N distances are 1.174(8) and 1.1 ...
, proving that organic compounds could also be created through an inorganic process. Nevertheless, carbon-containing compounds that were already classified as inorganic were not reclassified. These include
carbide
In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece.
Interstitial / Metallic carbides
The carbides of th ...
s, simple oxides of carbon such as
carbon monoxide
Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
and
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
,
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...
s,
cyanide
Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms.
In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...
s and elemental carbon minerals such as
graphite
Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on lar ...
and
diamond
Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...
.
[
Organic minerals are rare and difficult to find, often forming crusts on fractures.] Early descriptions of organic minerals include mellite
Mellite, also called honeystone, is an unusual mineral being also an organic chemical. It is chemically identified as an aluminium salt of mellitic acid, and specifically as aluminium benzene hexacarboxylate hydrate, with the chemical formula Al2 ...
in 1793, humboldtine
Humboldtine is a rarely occurring mineral from the mineral class of "organic compounds" with the chemical composition FeC2O4•2H2O and is therefore a water-containing iron(II) oxalate or the iron salt of oxalic acid.
Humboldtine crystallizes i ...
in 1821 and idrialite
Idrialite is a rare hydrocarbon mineral with approximate chemical formula C22H14.
Idrialite usually occurs as soft orthorhombic crystals, is usually greenish yellow to light brown in color with bluish fluorescence. It is named after Idrija, town i ...
in 1832.[
]
Types
In the proposed 10th edition of the Nickel-Strunz classification, organic minerals are one of the ten primary classes of minerals. The class is divided into three subclasses: salts of organic acids, hydrocarbons, and miscellaneous organic minerals.
Hydrocarbons
As the name implies, hydrocarbon minerals are composed entirely of carbon and hydrogen. Some are inorganic forms of polycyclic aromatic hydrocarbon
A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...
(PAH) compounds. For example, a rare mineral known as either carpathite
Carpathite is a very rare hydrocarbon mineral, consisting of exceptionally pure coronene (C24H12), a polycyclic aromatic hydrocarbon. The name has been spelled karpatite and the mineral was improperly renamed pendletonite.
Discovery
The mineral wa ...
, karpatite or pendletonite is nearly pure coronene
Coronene (also known as superbenzene and cyclobenzene) is a polycyclic aromatic hydrocarbon (PAH) comprising seven peri-fused benzene rings. Its chemical formula is . It is a yellow material that dissolves in common solvents including benzene, tol ...
. Carpathite is deposited as pale yellow flakes in cracks between diorite
Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-sili ...
(an igneous rock
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...
) and argillite
:''"Argillite" may also refer to Argillite, Kentucky.''
Argillite () is a fine-grained sedimentary rock composed predominantly of indurated clay particles. Argillaceous rocks are basically lithified muds and oozes. They contain variable amounts ...
(a sedimentary rock); it is prized for a beautiful blue fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
under ultraviolet light
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
. Other PAH compounds appearing as minerals include fluorene as kratochvilite; and anthracene
Anthracene is a solid polycyclic aromatic hydrocarbon (PAH) of formula C14H10, consisting of three fused benzene rings. It is a component of coal tar. Anthracene is used in the production of the red dye alizarin and other dyes. Anthracene is col ...
as ravatite. Others are mixtures: curtisite contains several PAH compounds, including dibenzofluorine, picene
Picene is a hydrocarbon found in the Pitch (resin), pitchy residue obtained in the distillation of peat tar and of petroleum. This is distilled to dryness and the distillate repeatedly Recrystallization (chemistry), recrystallized from cymene. It ...
, and chrysene, while the most common components of idrialite
Idrialite is a rare hydrocarbon mineral with approximate chemical formula C22H14.
Idrialite usually occurs as soft orthorhombic crystals, is usually greenish yellow to light brown in color with bluish fluorescence. It is named after Idrija, town i ...
are tribenzofluorenes.[ One theory for their formation involves burial of PAH compounds until they reach a temperature where ]pyrolysis
The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...
can occur, followed by hydrothermal
Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
transport towards the surface, during which the composition of minerals that precipitate out depends on the temperature.[
]
Salts of organic acids
A salt of an organic acid is a compound in which an organic acid is combined with a base. The largest such group is the oxalate
Oxalate (IUPAC: ethanedioate) is an anion with the formula C2O42−. This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (Na2C2O4), and several esters such as dimethyl ...
s, which combine with cations. A large fraction have water molecules attached; examples include weddellite, whewellite
Whewellite is a mineral, hydrated calcium oxalate, formula Ca C2 O4· H2O. Because of its organic content it is thought to have an indirect biological origin; this hypothesis is supported by its presence in coal and sedimentary nodules. Howev ...
, and zhemchuzhnikovite. Oxalates are often associated with particular fossilized biological materials, for example weddellite with cacti; oxammite with guano and egg shells of birds; glushinskite with lichen; humboldtine, stepanovite and whewellite with leaf litter; and humboldtine, stepanovite and whewellite with coal
Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen.
Coal is formed when ...
. Where plant material such as tree roots interacts with ore bodies, one can find oxalates with transition metals ( moolooite, wheatleyite).[
Other salts include salts of formate () such as formicaite and dashkovaite; and salts of acetate () such as ]acetamide
Acetamide (systematic name: ethanamide) is an organic compound with the formula CH3CONH2. It is the simplest amide derived from acetic acid. It finds some use as a plasticizer and as an industrial solvent. The related compound ''N'',''N''-dimet ...
and calclacite.[ Joanneumite is the first isocyanurate mineral to be officially recognized.
]
Miscellaneous
Some organic minerals do not fall into the above categories. These include nickel porphyrin
Porphyrins ( ) are a group of heterocyclic macrocycle organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). The parent of porphyrin is porphine, a rare chemical com ...
(), closely related to biological molecules such as heme
Heme, or haem (pronounced / hi:m/ ), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver.
In biochemical terms, heme is a coordination complex "consis ...
(a porphyrin with iron as the cation) and chlorophyll (a magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
cation), but does not itself occur in biological systems. Instead, it is found on the surface of fractures in oil shales.[ ]Urea
Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid.
Urea serves an important ...
derived from bat guano and urine also occurs as a mineral in very arid
A region is arid when it severely lacks available water, to the extent of hindering or preventing the growth and development of plant and animal life. Regions with arid climates tend to lack vegetation and are called xeric or desertic. Most ...
conditions.[ In the Dana and Strunz classifications, ]amber
Amber is fossilized tree resin that has been appreciated for its color and natural beauty since Neolithic times. Much valued from antiquity to the present as a gemstone, amber is made into a variety of decorative objects."Amber" (2004). In ...
is considered an organic mineral, but this classification is not approved by the International Mineralogical Association (IMA). Other sources call it a mineraloid because it has no crystal structure.
Carbon Mineral Challenge
As of 2016, the IMA recognized ten hydrocarbon minerals, ten miscellaneous organic minerals, 21 oxalates and over 24 other salts of organic acids.[ However, ]Robert Hazen
Robert Miller Hazen (born November 1, 1948) is an American mineralogist and astrobiologist. He is a research scientist at the Carnegie Institution of Washington's Geophysical Laboratory and Clarence Robinson Professor of Earth Science at George ...
and colleagues analyzed the known species of carbon-bearing minerals using a statistical technique called the Large Number of Rare Events In statistics, large number of rare events (LNRE) modeling summarizes methods that allow improvements in frequency distribution estimation over the maximum likelihood estimation when "rare events are common".
It can be applied to problems in lingui ...
(LNRE) model, and predicted that at least 145 such minerals are yet to be discovered. Many undiscovered organic minerals may be related to known species by various substitutions of cations. Hazen et al. predict that at least three more PAH crystals ( pyrene, chrysene and tetracene) should occur as minerals. There are 72 known synthetic oxalates, some of which could occur in nature, particularly near fossil organisms.[ To encourage the discovery of more carbon minerals, the Deep Carbon Observatory launched an initiative known as the Carbon Mineral Challenge.]
See also
* Classification of organic minerals
* Kerogen
Kerogen is solid, insoluble organic matter in sedimentary rocks. Comprising an estimated 1016 tons of carbon, it is the most abundant source of organic compounds on earth, exceeding the total organic content of living matter 10,000-fold. It ...
References
Further reading
*
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