Carbonado
Encyclopedia
Carbonado, commonly known as the "Black Diamond", is a natural polycrystalline
diamond
found in alluvial
deposits in the Central African Republic
and Brazil
. Its natural colour is black or dark grey, and it is more porous
than other diamonds.
Carbonado diamonds are typically pea
-sized or larger porous aggregates of many tiny black crystals. The most characteristic carbonados have been found only in the Central African Republic and in Brazil, in neither place associated with kimberlite
, the source of typical gem diamonds. Lead
isotope analyses
have been interpreted as documenting crystallization of carbonados about 3 billion years ago. The carbonados are found in younger sedimentary rock
s.
Mineral grains included within diamonds have been studied extensively for clues to diamond origin. Some typical diamonds contain inclusions of common mantle
minerals such as pyrope
and forsterite
, but such mantle minerals have not been observed in carbonado. In contrast, some carbonados do contain inclusions of minerals characteristic of the Earth’s crust
: these inclusions do not necessarily establish formation of the diamonds in the crust, however, because these obvious crustal inclusions occur in the pores that are common in carbonados. These inclusions within pores may have been introduced after carbonado formation. Inclusions of other minerals, rare or nearly absent in the Earth’s crust, are found at least partly incorporated in diamond, not just in pores: among such other minerals are those with compositions of Si
, SiC
, and Fe
‑Ni
. No distinctive high-pressure minerals, including the hexagonal carbon
polymorph
, lonsdaleite
, have been found as inclusions in carbonados, although such inclusions might be expected if carbonados formed by meteorite
impact.
Isotope
studies have yielded further clues to carbonado genesis. The carbon isotope value is very low (little carbon‑13 compared to carbon‑12, relative to typical diamonds).
Carbonado exhibits strong luminescence (photoluminescence
and cathodoluminescence
) induced by nitrogen
and by vacancies existing in the crystal lattice. Luminescence halos are present around radioactive inclusions, and it is suggested that the radiation damage occurred after formation of the carbonados, an observation perhaps pertinent to the radiation hypothesis listed below.
None of these hypotheses for carbonado formation had come into wide acceptance in the scientific literature by 2008.
from Florida International University
, propose that their material source was a supernova
which occurred at least 3.8 billion years ago. After coalescing and drifting through outer space
for about one and a half billion years, a large mass fell to earth as a meteor
approximately 2.3 billion years ago, possibly fragmenting during entry into the Earth's atmosphere, and impacting in a region which would much later split
into Brazil
and the Central African Republic
, the only two known locations of carbonado deposits.
Polycrystalline
Polycrystalline materials are solids that are composed of many crystallites of varying size and orientation. The variation in direction can be random or directed, possibly due to growth and processing conditions. Fiber texture is an example of the latter.Almost all common metals, and many ceramics...
diamond
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...
found in alluvial
Alluvium
Alluvium is loose, unconsolidated soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel...
deposits in the Central African Republic
Central African Republic
The Central African Republic , is a landlocked country in Central Africa. It borders Chad in the north, Sudan in the north east, South Sudan in the east, the Democratic Republic of the Congo and the Republic of the Congo in the south, and Cameroon in the west. The CAR covers a land area of about ,...
and Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
. Its natural colour is black or dark grey, and it is more porous
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
than other diamonds.
Unusual properties
The characteristics of carbonado noted in this section are based mainly on the summary of Heaney et al. (2005), unless otherwise noted.Carbonado diamonds are typically pea
Pea
A pea is most commonly the small spherical seed or the seed-pod of the pod fruit Pisum sativum. Each pod contains several peas. Peapods are botanically a fruit, since they contain seeds developed from the ovary of a flower. However, peas are considered to be a vegetable in cooking...
-sized or larger porous aggregates of many tiny black crystals. The most characteristic carbonados have been found only in the Central African Republic and in Brazil, in neither place associated with kimberlite
Kimberlite
Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond in 1871 spawned a diamond rush, eventually creating the Big Hole....
, the source of typical gem diamonds. Lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
isotope analyses
Isotope analysis
Isotope analysis is the identification of isotopic signature, the distribution of certain stable isotopes and chemical elements within chemical compounds. This can be applied to a food web to make it possible to draw direct inferences regarding diet, trophic level, and subsistence...
have been interpreted as documenting crystallization of carbonados about 3 billion years ago. The carbonados are found in younger sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
s.
Mineral grains included within diamonds have been studied extensively for clues to diamond origin. Some typical diamonds contain inclusions of common mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
minerals such as pyrope
Pyrope
The mineral pyrope is a member of the garnet group. Pyrope is the only member of the garnet family to always display red colouration in natural samples, and it is from this characteristic that it gets its name: from the Greek for fire and eye. Despite being less common than most garnets, it is a...
and forsterite
Forsterite
Forsterite is the magnesium rich end-member of the olivine solid solution series. Forsterite crystallizes in the orthorhombic system with cell parameters a 4.75 Å , b 10.20 Å and c 5.98 Å .Forsterite is associated with igneous and metamorphic rocks and has also been found in meteorites...
, but such mantle minerals have not been observed in carbonado. In contrast, some carbonados do contain inclusions of minerals characteristic of the Earth’s crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
: these inclusions do not necessarily establish formation of the diamonds in the crust, however, because these obvious crustal inclusions occur in the pores that are common in carbonados. These inclusions within pores may have been introduced after carbonado formation. Inclusions of other minerals, rare or nearly absent in the Earth’s crust, are found at least partly incorporated in diamond, not just in pores: among such other minerals are those with compositions of Si
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, SiC
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
, and Fe
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
‑Ni
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
. No distinctive high-pressure minerals, including the hexagonal carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
polymorph
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
, lonsdaleite
Lonsdaleite
Lonsdaleite , also called hexagonal diamond in reference to the crystal structure, is an allotrope of carbon with a hexagonal lattice. In nature, it forms when meteorites containing graphite strike the Earth. The great heat and stress of the impact transforms the graphite into diamond, but retains...
, have been found as inclusions in carbonados, although such inclusions might be expected if carbonados formed by meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
impact.
Isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
studies have yielded further clues to carbonado genesis. The carbon isotope value is very low (little carbon‑13 compared to carbon‑12, relative to typical diamonds).
Carbonado exhibits strong luminescence (photoluminescence
Photoluminescence
Photoluminescence is a process in which a substance absorbs photons and then re-radiates photons. Quantum mechanically, this can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon...
and cathodoluminescence
Cathodoluminescence
Cathodoluminescence is an optical and electrical phenomenon whereby a beam of electrons is generated by an electron gun and then impacts on a luminescent material such as a phosphor, causing the material to emit visible light. The most common example is the screen of a television...
) induced by nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and by vacancies existing in the crystal lattice. Luminescence halos are present around radioactive inclusions, and it is suggested that the radiation damage occurred after formation of the carbonados, an observation perhaps pertinent to the radiation hypothesis listed below.
Theories on origin
The origin of carbonado is controversial. Some proposed hypotheses are as follows:- Direct conversion of organic carbon under high-pressure conditions in the Earth's interior, the most common hypothesis for diamond formation
- Shock metamorphismShock metamorphismShock metamorphism or impact metamorphism describes the effects of shock-wave related deformation and heating during impact events. The formation of similar features during explosive volcanism is generally discounted due to the lack of metamorphic effects unequivocally associated with explosions...
induced by meteoritic impact at the Earth's surface - Radiation-induced diamond formation by spontaneous fission of uranium and thorium
- Formation inside an earlier-generation giant star in our area, that would have long ago exploded in a supernova.
- An origin in interstellar space, due to the impact of an asteroid, rather than being thrown from within an exploding star.
None of these hypotheses for carbonado formation had come into wide acceptance in the scientific literature by 2008.
Extraterrestrial origin hypothesis
Supporters of an extraterrestrial origin of carbonados, such as Dr. Stephen Haggerty, a geoscientistEarth science
Earth science is an all-embracing term for the sciences related to the planet Earth. It is arguably a special case in planetary science, the Earth being the only known life-bearing planet. There are both reductionist and holistic approaches to Earth sciences...
from Florida International University
Florida International University
Florida International University is an American public research university in metropolitan Miami, Florida, in the United States, with its main campus in University Park...
, propose that their material source was a supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
which occurred at least 3.8 billion years ago. After coalescing and drifting through outer space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....
for about one and a half billion years, a large mass fell to earth as a meteor
METEOR
METEOR is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision...
approximately 2.3 billion years ago, possibly fragmenting during entry into the Earth's atmosphere, and impacting in a region which would much later split
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
into Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
and the Central African Republic
Central African Republic
The Central African Republic , is a landlocked country in Central Africa. It borders Chad in the north, Sudan in the north east, South Sudan in the east, the Democratic Republic of the Congo and the Republic of the Congo in the south, and Cameroon in the west. The CAR covers a land area of about ,...
, the only two known locations of carbonado deposits.
See also
- Material properties of diamondMaterial properties of diamondDiamond is the allotrope of carbon in which the carbon atoms are arranged in the specific type of cubic lattice called diamond cubic. Diamond is an optically isotropic crystal that is transparent to opaque. Owing to its strong covalent bonding, diamond is the hardest naturally occurring material...
- Amsterdam DiamondAmsterdam DiamondThe Amsterdam Diamond is a black diamond weighing , and has 145 facets. It is in a pear shape, and cut from a rough. It was sold in 2001 for $US352,000, the highest price paid for a black diamond at auction.-External links:...
- Spirit of de Grisogono DiamondSpirit of de Grisogono DiamondThe Spirit of de Grisogono is the world's largest cut black diamond and the world's fifth largest diamond overall. Starting at an uncut weight of , it was taken from its origin in west central Africa and cut by Swiss jeweler De Grisogono. The resulting mogul-cut diamond weighs and is set in a...
External links
- Photo of porous carbonado at National Science Foundation
- Photo of glossy carbonado and article on possible extraterrestrial origins at PBS Nova
- Mystery Diamonds: Geoscientists Investigate Rare Carbon Formation ScienceDaily (June 1, 2007) Story
- Diamonds From Outer Space: Geologists Discover Origin Of Earth's Mysterious Black Diamonds ScienceDaily (January 9, 2007) Story.