Gujba

CBa, bencubbinite

Fell April 3,1984
11° 29′ 30′ N., 11° 39′ 30′ E. At 6:30 in the evening, a bright fireballA fireball is another term for a very bright meteor, generally brighter than magnitude -4, which is about the same magnitude of the planet Venus as seen in the morning or evening sky. A bolide is a special type of fireball which explodes in a bright terminal flash at its end, often with visible fragmentation. Click on Term to Read More approaching from the west was seen and heard by local residents. The cone-shaped mass that landed in a corn field near the village of Bogga Dingare in Yobe, Nigeria, was estimated to have weighed ~100 kg, but most of the mass was broken up into small pieces and dispersed. Gujba is a primitive, polymict, chondritic brecciaWork in Progress ... A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts). A breccia may also be distinguished by the origin of its clasts: (monomict breccia: monogenetic or monolithologic, and polymict breccia: polygenetic or polylithologic). The proportions of these fragments within the unbrecciated material Click on Term to Read More, the first fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If Click on Term to Read More of the bencubbinite group. It contains 0.4–8 mm-diameter, rounded, metallic globules (~41 vol%) and 0.8–15 mm-diameter 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 globules (~20 vol%), cemented together by a dark-colored, silicate-rich, impact-melt matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. Click on Term to Read More composed of mm-sized fragments of both silicate and 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 globules (~39 vol%).

Metal globules can contain up to ~1 vol% troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. Click on Term to Read More, which is positively correlated with the abundance of volatileSubstances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.). siderophile elements. FractionationConcentration or separation of one mineral, element, or isotope from an initially homogeneous system. Fractionation can occur as a mass-dependent or mass-independent process. Click on Term to Read More of siderophile elements in Gujba was controlled by volatility rather than by oxidationOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More/sulfidation processes or magmatic crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. Click on Term to Read More (Krot et al., 2002). Siderophile elementLiterally, "iron-loving" element that tends to be concentrated in Fe-Ni metal rather than in silicate; these are Fe, Co, Ni, Mo, Re, Au, and PGE. These elements are relatively common in undifferentiated meteorites, and, in differentiated asteroids and planets, are found in the metal-rich cores and, consequently, extremely rare on correlations are inconsistent with a nebular condensation model. It is generally assumed that a protoplanetary impact gave rise to a vapor cloud with high enough partial pressures to generate a metal-enriched gas. The metal globules then condensed as liquids from this gas and were sorted by size and densityMass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much Click on Term to Read More, thereby establishing the high metal/silicate ratio of the group. The CB group reflects a sequence of increasingly lighter Fe isotopes from Gujba through HaH 237 to Isheyevo (Zipfel and Weyer, 2006). This wide Fe isotopic range provides further evidence of a formation within an impact vapor plume rather than in a nebulaAn immense interstellar, diffuse cloud of gas and dust from which a central star and surrounding planets and planetesimals condense and accrete. The properties of nebulae vary enormously and depend on their composition as well as the environment in which they are situated. Emission nebula are powered by young, massive Click on Term to Read More setting. See the HaH 237 page for a more detailed scenario of the CB group formation process ascertained by Fedkin et al. (2015) through kinetic condensation modeling.

In a nm-scale study of Gujba, a two-phase (kamaciteMore common than taenite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Kamacite, α-(Fe,Ni), contains 4-7.5 wt% Ni, and forms large body-centered cubic crystals that appear like broad bands or beam-like structures on the etched surface of a meteorite; its name is derived from the Greek word Click on Term to Read More and taeniteLess common than kamacite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Taenite, γ-(Fe,Ni), has 27-65 wt% Ni, and forms small crystals that appear as highly reflecting thin ribbons on the etched surface of a meteorite; the name derives from the Greek word for "ribbon." Click on Term to Read More) metal particle was observed comprising ~30 individual grains that demonstrate a reheating episode occurred at temperatures of ~675°C (Goldstein et al, 2011). It is estimated that a subsequent cooling to ~550°C occurred within a time period of a month. A similar metal particle having a similar thermal history was found in the CBb bencubbinite HaH 237. Through 3-D mapping of Gujba at a µm scale, at least five types of metal particles of differing Ni content (~5 to ~8.2 wt%) and sulfide content were identified (Berlin et al., 2013). These metal particles are consistent with an origin in an impact plume, followed by accretionAccumulation of smaller objects into progressively larger bodies in the solar nebula leading to the eventual formation of asteroids, planetesimals and planets. The earliest accretion of the smallest particles was due to Van der Waals and electromagnetic forces. Further accretion continued by relatively low-velocity collisions of smaller bodies in the Click on Term to Read More to a secondary 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, where they experienced impact-associated secondary heating. Based on their examination of sulfide phases embedded in metal grains within the Gujba and Weatherford CBa meteorites, and through comparisons of Fe–S–Cr phase diagrams, Srinivasan et al. (2013) concluded that both the reheating of these sulfide phases and the injection of silicate melt into metal and silicate host components were likely concurrent with this impact.

Hydrocode modeling employing a homogeneous nucleation theory demonstrates that the very high densitiesMass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much Click on Term to Read More and temperatures that would lead to the formation of mm- to cm-sized metal globules are consistent with an impact vapor plume origin rather than a nebular origin (Anic et al., 2005). A high velocity collision is most likely to produce those conditions conducive to producing the particle size that exists in Gujba. An alternative model has been described whereby the metal was melted to form globules, while S and volatile siderophiles were subsequently evaporated out. The metal globules have varying Ni contents and exhibit quench textures (Rubin et al., 2003), and since no diffusionMovement of particles from higher chemical potential to lower chemical potential (chemical potential can in most cases of diffusion be represented by a change in concentration). Diffusion, the spontaneous spreading of matter (particles), heat, or momentum, is one type of transport phenomena. Because diffusion is thermally activated, coefficients for diffusion Click on Term to Read More has occurred among globules in contact with each other, it can be inferred that they were accreted at cold temperatures after being isolated from the hot condensation region.

The silicate globules in Gujba exhibit skeletal 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 textures, contain no FeNi-metal or troilite, and have low concentrations of volatile elementsChemical elements that condense (or volatilize) at relatively low temperatures. The opposite of volatile is refractory. Volatile elements can be divided into moderately volatile (Tc = 1230–640 K) and highly volatile (Tc < 640 K). The moderately volatile lithophile elements are: Mn, P, Na, B ,Rb, K, F, Zn. The moderately Click on Term to Read More, features indicative of quenching from a molten state; i.e., condensation from a hot, impact-generated vapor plume (Krot et al., 2004). CAIsSub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they Click on Term to Read More have been found in some bencubbinites including HaH 237, QUE 94411, and Gujba, as well as the transitional member Isheyevo; however, since their O-isotopic values plot along the CCAM line instead of the CR trend line, they represent solar nebulaThe primitive gas and dust cloud around the Sun from which planetary materials formed. material rather than condensates from the impact vapor plume (Fedkin et al., 2015).

Raman spectra have identified the first carbonaceous chondriteCarbonaceous chondrites represent the most primitive rock samples of our solar system. This rare (less than 5% of all meteorite falls) class of meteorites are a time capsule from the earliest days in the formation of our solar system. They are divided into the following compositional groups that, other than Click on Term to Read More occurrence of several high pressure phases within barred olivine fragments and the matrix components of Gujba; these include majorite garnetMineral generally found in terrestrial metamorphic rocks, although igneous examples are not uncommon. Garnet is a significant reservoir of Al in the Earth's upper mantle. The garnet structure consists of isolated SiO4 tetrahedra bound to two cation sites. The A site holds relatively large divalent cations (Ca2+, Mg2+, Fe2+, Mn2+); the Click on Term to Read More, majorite-pyrope solid solutionCompositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an isostructural material. This results in a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions. Solid solutions can be complete (with, and wadsleyiteHigh pressure polymorph of olivine, β-Mg2SiO4, found on Earth and in some meteorites. It is thought to make up 50% or more of Earth's mantle between depths of 400 and 525 km. Wadsleyite transforms into ringwoodite at high pressure, but the exact pressure depends strongly on composition. At lower pressures,, along with minor grossular-pyrope solid solution and coesiteHigh-pressure polymorph of silicon dioxide (SiO2). Has the same chemical composition as cristobalite, stishovite, seifertite and tridymite but possesses a different crystal structure. Coesite forms at intense pressures of above about 2.5 GPa (25 kbar) and temperature above about 700 °C, and was first found naturally on Earth in impact Click on Term to Read More (Weisberg and Kimura, 2010). These high pressure phases formed either from solid-state transformation of pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification category. Click on Term to Read More or crystallization from an impact melt during a heterogeneous, planetesimal wide impact shock event reaching minimum pressures of ~19 GPa and temperatures of ~2000°C. The investigators argue that these high pressure phases are inconsistent with the subsequent formation of chondrulesRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at  ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine Click on Term to Read More within an impact plume since at such high temperatures these phases would be rapidly back-transformed to their low temperature polymorphs. Moreover, the measured cooling rates of chondrules (ave. 100K/hr) are much too slow than that at which shock veins with high pressure polymorphs would survive (~1000K/hr). Therefore, they determined that the barred chondrules and metal in CB 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 were formed prior to the impact event that produced the high pressure polymorphs.

The bencubbinites constitute a small group having similar oxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only Click on Term to Read More and nitrogenPrincipal constituent of the Earth’s atmosphere (78.08 vol. % at ground level). Nitrogen is the fifth most abundant element in the universe by atom abundance. Nitrogen comprises only 3.5 vol. % of the atmosphere of Venus and 2.7 vol. % of Mars’s atmosphere. Nitrogen has two isotopes: 14N (99.632 %) and 15N Click on Term to Read More isotopic compositions as well as similar petrologic characteristics. They have highly reducedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More silicates, bulk metal abundances of 60–70 vol%, Cr-bearing troilite, metal with near solar Ni/Co ratios, and similar elemental abundances. Among chondriteChondrites 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 groups, the bencubbinites show a significant enrichment of 15N, with Gujba having an intermediate content within the group. The bencubbinites have been divided into two petrologic subgroups, CBa and CBb, representing those with cm-sized metal and silicate globules, and those with mm-sized globules, respectively. Further information on the formation of bencubbinites can be found on the Bencubbin, Isheyevo, and NWA 1814 pages.

Based on the U–Pb isotopic chronometer using the Shallowater standard (at that time corrected to 4.5613 [±0.0008] b.y. by Connelly et al., 2012), the chondrules in Gujba (CBa) and HaH 237 (CBb) were calculated to have formed simultaneously 4.56168 (±0.00051) b.y. ago; this age reflects a more recent formation event in comparison to other chondrite groups. Employing the corrected I–Xe data from Gilmour et al. (2009) for Gujba and that from Pravdivtseva et al. (2014) for HaH 237, respective closure ages of 4.5632 (±0.0013) b.y. and 4.56101 (±0.00087) b.y. were obtained. The age difference between these CBa and CBb chondrules was attributed to possible heterogeneity of the I-isotopic compositions in the two meteorites’ respective formation regions within the impact vapor-melt plume (Bollard et al., 2015). Furthermore, other chronometers have provided ages consistent with those cited above, with a Hf–W age anchored to CAIs of 4.5622 (±0.0024) b.y., and a Mn–Cr age anchored to D’Orbigny of 4.5633 (±0.001) b.y. (Bollard et al., 2015). In their high-precision study of four Gujba chondrules, Bollard et al. (2015) derived a weighted average age of 4.56249 (±0.00021) b.y.; this equates to 4.8 (±0.3) m.y. after CAIs, or 1.2 (±0.6) m.y. after the formation of the youngest known nebular chondruleRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at  ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine Click on Term to Read More. Subsequent to this, high precision isotopic studies involving HaH 237 were conducted by Pravdivtseva et al. (2015, 2016), which led them to suggest a refinement in the absolute I–Xe age for the Shallowater standard of 4.5624 (±0.0002) b.y. Based on this new refinement, the age of HaH 237 relative to Shallowater was ascertained to be 4.5621 (±0.0003) m.y., which is consistent with the U-corrected Pb–Pb age determined for Gujba chondrules by Bollard et al. (2015) above, as well as that determined for HaH 237 silicates by Krot et al. (2005) of 4.5619 (±0.0009) b.y. All of these ages attest to a relatively late formation of the CB-group chondrites from a vapor-melt plume following a catastrophic impact between two planetary embryos.

The CRE age of Gujba (26 ±7 m.y.) is identical within uncertainties to that of Bencubbin (27.3 m.y.), and both have similar 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 concentrations (also similar in some respects to the enstatiteA mineral that is composed of Mg-rich pyroxene, MgSiO3. It is the magnesium endmember of the pyroxene silicate mineral series - enstatite (MgSiO3) to ferrosilite (FeSiO3). Click on Term to Read More chondrites; Nakashima and Nagao, 2009), which attests to a common ejection event on their parent body. However, while the metal and silicate globules in Gujba are mostly complete, undistorted spheres, those in Bencubbin and Weatherford are fragmented and distorted. Gujba and other CB chondrites exhibit multiple characteristics that are consistent with a severe shock subsequent to its formation, including melting, brecciationThe formation of a breccia through a process by which rock fragments of of various types are recemented or fused together. Click on Term to Read More, and deformation. The presence of certain high-pressure phases in Gujba—majorite and wadsleyite, produced by the conversion of low-Ca pyroxene and olivine, respectively—attests to the occurrence of a significant shock event of ~19 GPa at 2000°C (Weisberg and Kimura, 2004) consistent with a shock stageA 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 of S2. As in Bencubbin, shock-associated structures identified in Gujba include stishoviteDense, high-pressure phase of quartz; so far identified only in shock-metamorphosed, quartz-bearing rocks from meteorite impact craters. Stishovite was synthesized in 1961 before it was discovered at Meteor Crater, Arizona. Its structure consists of parallel chains of single SiO6 octahedra. The octahedra are on their sides, sharing opposing edges. Image Click on Term to Read More, 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 to poorly graphitized 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, ordered 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, rounded to euhedral diamonds, nanodiamond clumps, and rare bucky-diamonds, along with carbonaceous nanoglobules (Garvie et al., 2011).

The CB, CH, and CR chondrites constitute the CR clan, comprising groups which likely formed in the same isotopic reservoir under similar conditions in the solar nebula; current evidence argues for an origin of the metal-rich carbonaceous chondrites in a common collision between planetary embryos (Krot et al., 2009). The Gujba specimen pictured above is an 18.1 g polished slice sectioned from a 282 g fragment that was originally purchased in 2000 in Gidan Wire, Nigeria. See also a most spectacular 81.05 g full slice of this special bencubbinite, courtesy of the Stephan Kambach collection, which exhibits the finest details of both metallic and silicate chondrules. A beautiful high-resolution exterior view of Gujba, courtesy of Paul Swartz (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 Picture of the Day, 1 Oct 2014), can be seen here. The photo below shows an awesome 2,365 g end piece, part of the Jay Piatek Collection, that was sectioned from a 3,440 g complete Gujba mass.

Photo courtesy of Dr. J. Piatek