Lonsdaleite

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Lonsdaleite
Lonsdaleite.png
Crystal structure of lonsdaleite
General
Category Mineral
Formula
(repeating unit)
C
Strunz classification 1.CB.10b
Crystal system Hexagonal
Crystal class Dihexagonal dipyramidal (6/mmm)
H-M symbol: (6/m 2/m 2/m)
Space group P63/mmc
Unit cell a = 2.51 Å, c = 4.12 Å; Z = 4
Structure
Jmol (3D) Interactive image
Identification
ColorGray in crystals, pale yellowish to brown in broken fragments
Crystal habit Cubes in fine-grained aggregates
Mohs scale hardness7–8 (for impure specimens)
Luster Adamantine
Diaphaneity Transparent
Specific gravity 3.2
Optical propertiesUniaxial (+/-)
Refractive index n = 2.404
References [1] [2] [3]

Lonsdaleite (named in honour of Kathleen Lonsdale), 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 graphite's hexagonal crystal lattice. Lonsdaleite was first identified in 1967 from the Canyon Diablo meteorite, where it occurs as microscopic crystals associated with diamond. [4] [5]

Kathleen Lonsdale Irish crystallographer

Dame Kathleen Lonsdale, DBE, FRS was an Irish pacifist, prison reformer and crystallographer. She proved, in 1929, that the benzene ring is flat by using X-ray diffraction methods to elucidate the structure of hexamethylbenzene. She was the first to use Fourier spectral methods while solving the structure of hexachlorobenzene in 1931. During her career she attained several firsts for female scientists, including being one of the first two women elected a Fellow of the Royal Society (FRS) in 1945, first woman tenured professor at University College London, first woman president of the International Union of Crystallography, and first woman president of the British Association for the Advancement of Science.

Crystal structure Ordered arrangement of atoms, ions, or molecules in a crystalline material

In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns that repeat along the principal directions of three-dimensional space in matter.

Graphite Allotrope of carbon, mineral, substance

Graphite, archaically referred to as plumbago, is a crystalline form of the element carbon with its atoms arranged in a hexagonal structure. It occurs naturally in this form and is the most stable form of carbon under standard conditions. Under high pressures and temperatures it converts to diamond. Graphite is used in pencils and lubricants. Its high conductivity makes it useful in electronic products such as electrodes, batteries, and solar panels.

Contents

Hexagonal diamond has also been synthesized in the laboratory (1966 or earlier; published in 1967) [6] by compressing and heating graphite either in a static press or using explosives. [7] It has also been produced by chemical vapor deposition, [8] [9] [10] and also by the thermal decomposition of a polymer, poly(hydridocarbyne), at atmospheric pressure, under argon atmosphere, at 1,000 °C (1,832 °F). [11] [12]

Chemical synthesis is the artificial execution of useful chemical reactions to obtain one or several products. This occurs by physical and chemical manipulations usually involving one or more reactions. In modern laboratory uses, the process is reproducible, reliable, and established to work the same in multiple laboratories.

Chemical vapor deposition chemical process used in the semiconductor industry to produce thin films

Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.

Poly(hydridocarbyne)

Poly(hydridocarbyne) (PHC) is one of a class of carbon-based random network polymers primarily composed of tetrahedrally hybridized carbon atoms, each having one hydride substituent, exhibiting the generic formula [HC]n. PHC is made from bromoform, a liquid halocarbon that is commercially manufactured from methane. At room temperature, poly(hydridocarbyne) is a dark brown powder. It can be easily dissolved in a number of solvents, forming a colloidal suspension that is clear and non-viscous, which may then be deposited as a film or coating on various substrates. Upon thermolysis in argon at atmospheric pressure and temperatures of 110 °C to 1000 °C, decomposition of poly(hydridocarbyne) results in hexagonal diamond (Lonsdaleite).

It is translucent, brownish-yellow, and has an index of refraction of 2.40 to 2.41 and a specific gravity of 3.2 to 3.3. Its hardness is theoretically superior to that of cubic diamond (up to 58% more), according to computational simulations, but natural specimens exhibited somewhat lower hardness through a large range of values (from 7 to 8 on Mohs hardness scale). The cause is speculated as being due to the samples having been riddled with lattice defects and impurities. [13]

Refractive index dimensionless number that describes how fast light propagates through the material

In optics, the refractive index or index of refraction of a material is a dimensionless number that describes how fast light travels through the material. It is defined as

Specific gravity Relative density compared to water

Specific gravity is the ratio of the density of a substance to the density of a reference substance; equivalently, it is the ratio of the mass of a substance to the mass of a reference substance for the same given volume. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance for liquids is nearly always water at its densest ; for gases it is air at room temperature. Nonetheless, the temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 atm (101.325 kPa).

Hardness is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. Some materials are harder than others. Macroscopic hardness is generally characterized by strong intermolecular bonds, but the behavior of solid materials under force is complex; therefore, there are different measurements of hardness: scratch hardness, indentation hardness, and rebound hardness.

The property of lonsdaleite as a discrete material has been questioned, since specimens under crystallographic inspection showed not a bulk hexagonal lattice, but instead cubic diamond dominated by structural defects that include hexagonal sequences. [14] A quantitative analysis of the X-ray diffraction data of lonsdaleite has shown that about equal amounts of hexagonal and cubic stacking sequences are present. Consequently, it has been suggested that "stacking disordered diamond" is the most accurate structural description of lonsdaleite. [15] On the other hand, recent shock experiments with in situ X-ray diffraction show strong evidence for creation of relatively pure lonsdaleite in dynamic high-pressure environments such as meteorite impacts. [16] [17]

Crystallography The scientific study of crystal structure

Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids. The word "crystallography" is derived from the Greek words crystallon "cold drop, frozen drop", with its meaning extending to all solids with some degree of transparency, and graphein "to write". In July 2012, the United Nations recognised the importance of the science of crystallography by proclaiming that 2014 would be the International Year of Crystallography. X-ray crystallography is used to determine the structure of large biomolecules such as proteins. Before the development of X-ray diffraction crystallography, the study of crystals was based on physical measurements of their geometry. This involved measuring the angles of crystal faces relative to each other and to theoretical reference axes, and establishing the symmetry of the crystal in question. This physical measurement is carried out using a goniometer. The position in 3D space of each crystal face is plotted on a stereographic net such as a Wulff net or Lambert net. The pole to each face is plotted on the net. Each point is labelled with its Miller index. The final plot allows the symmetry of the crystal to be established.

In situ is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in many different contexts. For example, in fields such as physics, geology, chemistry, or biology, in situ may describe the way a measurement is taken, that is, in the same place the phenomenon is occurring without isolating it from other systems or altering the original conditions of the test.

Properties

According to the traditional picture, Lonsdaleite has a hexagonal unit cell, related to the diamond unit cell in the same way that the hexagonal and cubic close packed crystal systems are related. The diamond structure can be considered to be made up of interlocking rings of six carbon atoms, in the chair conformation. In lonsdaleite, some rings are in the boat conformation instead. At the nanoscale dimensions cubic diamond is represented by diamondoids while hexagonal diamond is represented by wurtzoids. [18] In diamond, all the carbon-to-carbon bonds, both within a layer of rings and between them, are in the staggered conformation, thus causing all four cubic-diagonal directions to be equivalent; while in lonsdaleite the bonds between layers are in the eclipsed conformation, which defines the axis of hexagonal symmetry.

Diamond Allotrope of carbon often used as a gemstone and an abrasive

Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. At room temperature and pressure, another solid form of carbon known as graphite is the chemically stable form, but diamond almost never converts to it. Diamond has the highest hardness and thermal conductivity of any natural material, properties that are utilized in major industrial applications such as cutting and polishing tools. They are also the reason that diamond anvil cells can subject materials to pressures found deep in the Earth.

Crystal system Classification of crystalline materials by their three dimensional structural geometry

In crystallography, the terms crystal system, crystal family, and lattice system each refer to one of several classes of space groups, lattices, point groups, or crystals. Informally, two crystals are in the same crystal system if they have similar symmetries, although there are many exceptions to this.

In chemistry, diamondoids are variants of the carbon cage molecule known as adamantane (C10H16), the smallest unit cage structure of the diamond crystal lattice. Diamondoids also known as nanodiamonds or condensed adamantanes may include one or more cages (adamantane, diamantane, triamantane, and higher polymantanes) as well as numerous isomeric and structural variants of adamantanes and polymantanes. These diamondoids occur naturally in petroleum deposits and have been extracted and purified into large pure crystals of polymantane molecules having more than a dozen adamantane cages per molecule. These species are of interest as molecular approximations of the diamond cubic framework, terminated with C−H bonds. Cyclohexamantane may be thought of as a nanometer-sized diamond of approximately 5.6×10−22 grams.

Lonsdaleite is simulated to be 58% harder than diamond on the <100> face and to resist indentation pressures of 152 GPa, whereas diamond would break at 97 GPa. [19] This is yet exceeded by IIa diamond's <111> tip hardness of 162 GPa.

Miller index describing crystal lattice planes

Miller indices form a notation system in crystallography for planes in crystal (Bravais) lattices.

Diamond type is a method of scientifically classifying diamonds by the level and type of their chemical impurities. Diamonds are separated into four types: Type Ia, Type Ib, Type IIa, and Type IIb. The impurities measured are at the atomic level within the crystal lattice of carbon atoms and so, unlike inclusions, require an infrared spectrometer to detect.

Material properties of diamond Physical properties of the mineral

Diamond 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. Diamond is the hardest naturally occurring material known. Yet, due to important structural weaknesses, diamond's toughness is only fair to good. The precise tensile strength of bulk diamond is unknown;however, compressive strength up to 60 GPa has been observed, and it could be as high as 90–100 GPa in the form of nanometer-sized wires or needles ,with a corresponding local maximum tensile elastic strain in excess of 9%. The anisotropy of diamond hardness is carefully considered during diamond cutting. Diamond has a high refractive index (2.417) and moderate dispersion (0.044) properties that give cut diamonds their brilliance. Scientists classify diamonds into four main types according to the nature of crystallographic defects present. Trace impurities substitutionally replacing carbon atoms in a diamond's crystal structure, and in some cases structural defects, are responsible for the wide range of colors seen in diamond. Most diamonds are electrical insulators and extremely efficient thermal conductors. Unlike many other minerals, the specific gravity of diamond crystals (3.52) has rather small variation from diamond to diamond.

Occurrence

Lonsdaleite occurs as microscopic crystals associated with diamond in several meteorites: Canyon Diablo, Kenna, and Allan Hills 77283. It is also naturally occurring in non-bolide diamond placer deposits in the Sakha Republic. [20] Material with d-spacings consistent with Lonsdaleite has been found in sediments with highly uncertain dates at Lake Cuitzeo, [21] in the state of Guanajuato, Mexico, by proponents of the controversial Younger Dryas impact hypothesis. Its presence in local peat deposits is claimed as evidence for the Tunguska event being caused by a meteor rather than by a cometary fragment. [22] [23]

See also

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References

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Further reading