Olivine–pigeoniteLow-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, found as a major mineral in eucrites and shergottites. In order to be considered pigeonite, the clinopyroxene must contain 5 to 20 mol % of calcium (Wo5 - 20). Chondrites of petrologic types 4 and below contain significant low-Ca clinopyroxene. During metamorphism to higher temperatures, all existing
Found Spring 2000
~28° N., ~16° E. A single 40.03 g ureilite meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and Click on Term to Read More was found in the Libyan Sahara Desert. As with most ureilites, DaG 868 is composed of grains of olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion Click on Term to Read More (82 vol%) and pigeonite (11 vol%) along with carbonaceous material forming rims and veins. The small amount of metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More present has been extremely weathered. The olivine in DaG 868 has a high CaO content and a high fayalitePure* iron end-member (Fe2SiO4) of the olivine solid solution series and an important mineral in meteorites. When iron (Fe) is completely substituted by magnesium, it yields the the pure Mg-olivine end-member, forsterite (Mg2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo) Click on Term to Read More value of 20.6, which places it in Berkley’s subgroup I and Goodrich’s subgroup 1.
In contrast to most other ureilites, DaG 868 contains unshocked olivine without undulose extinctionIn astronomy, the dimming of starlight as it passes through the interstellar medium. Dust scatters some of the light, causing the total intensity of the light to diminish. It is important to take this effect into account when measuring the apparent brightness of stars. The dark bands running across portions Click on Term to Read More
, but still contains sub-millimeter-sized diamonds in the graphiteOpaque form of carbon (C) found in some iron and ordinary chondrites and in ureilite meteorites. Each C atom is bonded to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons. The two known forms of graphite, α (hexagonal) and β (rhombohedral), have Click on Term to Read More
that occur within pigeonite crystals. These diamonds have a solar signature inferred by their C and N isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: 1H, 2H (deuterium), and 3H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. Click on Term to Read More
compositions. It has been generally considered that diamonds found in ureilites, as well as those found in iron meteorites, were formed by impact-shock pressures and/or through chemical vapor depositionMethod for growing solids in which a gaseous precursor (containing fragments of the desired solid) is decomposed and deposited onto a desired surface. Chemical Vapor Deposition (CVD) is one of the most powerful synthetic methods in material science due to its remarkable flexibility. A variety of surfaces can be coated, Click on Term to Read More
(CVD) processes. While DaG 868 has forced a reconsideration of diamondOne of the naturally occurring forms of carbon found in meteorites. Each C atom is bonded through covalent sp3 hydrid orbitals to four others. The strength of the C-C bonds makes diamond the hardest naturally occurring substance (according to the Mohs scale) in terms of resistance to scratching. There are Click on Term to Read More
origins, a new mechanism, catalytic transformation of graphite to diamond, is under consideration to account for the production of diamonds in this ureilite. Under conditions of relatively low pressure and high temperature, certain molten metals can serve as solvent catalysts leading to diamond formation. An alternative scenario ascribing diamond formation to CVD processes has been propounded by Langendam and Tomkins (2012). They envision a smelting mechanism involving methane to explain observed smelting within fractures, as well as the finding of discontinuous smelting at grain boundaries. Still, the predominance of crystalline graphite accompanying diamond is more consistent with shock being the major driver of diamond synthesis in ureilites (Ross et al.
Conversely, the least-shocked, diamond-free ureilite, ALH 78019, lacks primordial noble gasesElement occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. Click on Term to Read More
in the graphite component, and contains a heavy N-isotopic signature in the graphite, observations which are inconsistent with graphite as a precursor to the nanodiamond which was subsequently formed through in situ
shock conversion processes (Rai, et al.
, 2002). Utilizing the ureilite NWA 4742, Guillou et al
(2009) studied this paradox in which graphite precursor material is depleted in noble gases, while the nanodiamonds into which it was transformed are noble gas-rich. Their investigation led to a proposal that a mixture of two diamond populations is present; i.e.
, an early population of unknown origin that contains noble gases, and a later population that was formed by shocked graphite depleted in noble gases. They further suggest that the presence of a noble gas-containing graphitic phase surrounding some nanodiamonds could be the result of back-transformation of the early population of diamonds under conditions of slow cooling following a late shock event.
To differentiate between the two competing scenarios for diamond formation on the ureilite parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. Click on Term to Read More
, impact shock vs.
chemical vapor deposition (CVD), Nagashima et al.
(2012) utilized micro-Raman spectroscopyTechnique of splitting electromagnetic radiation (light) into its constituent wavelengths (a spectrum), in much the same way as a prism splits light into a rainbow of colors. Spectra are not smooth but punctuated by 'lines' of absorption or emission caused by interaction with matter. The energy levels of electrons in
to study of carbonaceous material in a number of ureilite samples. The resulting spectral data obtained for the major parameters for diamond (peak position, band intensities, and full width at half maximum [FWHM]) were a better match to diamond produced under CVD rather than shock pressure. Moreover, they demonstrated that there was no correlation of the diamond:graphite ratio to the shock levelA petrographic assessment, using features observed in minerals grains, of the degree to which a meteorite has undergone shock metamorphism. The highest stage observed in 25% of the indicator grains is used to determine the stage. Also called "shock level". Click on Term to Read More
, and found the noble gasElement occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. Click on Term to Read More
and N-isotopic compositions of graphite, amorphousMaterial without the regular, ordered structure of crystalline solids. Amorphous substances, like glass, lack a definite repeating pattern in their atomic structures (crystallinity). There may be small regions of order, but, overall there is disorder. Click on Term to Read More carbonElement commonly found in meteorites, it occurs in several structural forms (polymorphs). All polymorphs are shown to the left with * indicating that it been found in meteorites and impact structures: a. diamond*; b. graphite*; c. lonsdalite*; d. buckminsterfullerene* (C60); e. C540; f. C70; g. amorphous carbon; h. carbon nanotube*. Click on Term to Read More
, and diamond to be in accordance with the CVD model, but not with the shock model. Their results suggest a scenario of chemical deposition of graphite, amorphous carbon, and diamond directly onto high-temperature condensates in the primitive solar nebulaThe primitive gas and dust cloud around the Sun from which planetary materials formed.
, with the formation of each phase being associated with specific variations in CH4:H2 ratios commensurate with temperature and pressure changes. The migration of carbonaceous material to silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the
grain contacts, as well as the occurrence of compressed graphite in conjunction with diamond, was the result of later shock events on the ureilite parent body.
Large sub-millimeter-sized diamonds of solar origin are also found in some unshocked meteorites, such as the enstatite chondriteType of meteorite high in the mineral enstatite and also referred to as E-chondrites. Although they contain substantial amounts of Fe, it is in the form of Ni-Fe metal or sulfide rather than as oxides in silicates. Their highly reduced nature indicates that they formed in an area of the Click on Term to Read More
Abee. By contrast, diamonds present in primitive chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More
are nanometer-sized, and contain anomalous C and N, reflecting a circumstellar origin. Remarkably, two types of diamonds are found in the meteorite Acfer 214—nanometer-sized diamonds similar to those found in primitive chondrites, and larger micrometer-sized diamonds with unique isotopic characteristics, combustion temperatures, and C/N ratios.
A synopsis of current models for ureilite formation is presented on the Kenna
page. The specimen of DaG 868 pictured above is a 0.39 g partial slice with crustOutermost layer of a differentiated planet, asteroid or moon, usually consisting of silicate rock and extending no more than 10s of km from the surface. The term is also applied to icy bodies, in which case it is composed of ices, frozen gases, and accumulated meteoritic material. On Earth, the Click on Term to Read More
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