Iron, IIIB, octahedriteMost Common type of iron meteorite, composed mainly of taenite and kamacite and named for the octahedral (eight-sided) shape of the kamacite crystals. When sliced, polished and etched with an acid such as nitric acid, they display a characteristic Widmanstätten pattern. Spaces between larger kamacite and taenite plates are often Click on Term to Read More
Found before 1950 14° 40′ S., 74° 30′ W. A mass of 141 kg was found in 1949 in a remote part of the Andes Mountains near the village of Tambo Quemado in Peru. It was brought to the Geological Museum in Lima, Peru in 1950. On average, the mass has lost less than 1 mm from its surface during its terrestrial residence, but has been artificially heated to about 1000°C for ~1 hour at some point in its history (Buchwald, 1975). This heating has melted inclusions of the phosphide schreibersiteNi-Fe phosphide mineral, (Fe,Ni)3P, yellowish in color and predominantly found in iron and stony-iron meteorites. Schreibersite can also be found in a variety of other meteorites including some acapulcoites, aubrites, enstatite chondrites and achondrites, lunars, ureilites, winonaites and a smattering of other meteorite types like CM, CO and CB. Schreibersite Click on Term to Read More and the phosphate graftonite. As the graftonite recrystallized, phosphoran-wustite and silicaSilicon dioxide, SiO2. were added to the inclusionFragment of foreign (xeno-) material enclosed within the primary matrix of a rock or meteorite. Click on Term to Read More to form a new silicophosphate. Migration of lithophile trace elements was also facilitated during this period.
In another study involving Tambo Quemado (Sugiura and Hoshino, 2003), it was suggested that the absence of phosphates was the result of its formation under more reducingOxidation 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 conditions (the IIIA Thunda is also phosphate-free). Consistent with such highly reducing conditions is the fact that Tambo contains the highest Mn and Cr in its sulfides compared to other IIIAB irons. Highly reducing conditions have also produced low Mn in chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More and high Cr in 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. The phosphides in Tambo are associated with metal, in contrast to their association with sulfide in the phosphate-bearing meteorites. Also present in Tambo Quemado are many inclusions of the carbide coheniteFe-Ni-Co carbide, (Fe,Ni,Co)3C, that occurs as an accessory constituent in several iron meteorites, and coarse octahedrites with < 7 wt. % Ni. Click on Term to Read More, which make the cutting of this 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 very difficult and expensive. Upon etching, the 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 laths display a medium bandwidth with well-contrasted plessiteA fine-grained intergrowth of kamacite and taenite that fills in the wedges between wide kamacite and taenite bands in octahedrites. The name derives from the Greek word for "filling." Click on Term to Read More fields making for a remarkable appearance.
A formation history was constructed based on isotopic chronometry and metallographic cooling rates. The initial 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 of the 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 must have been completed ~1.7 m.y. after CAISub-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 formation to provide an appropriate quantity of heat-producing radiogenic 26Al. Results of age studies conducted by Kruijer et al. (2012, 2013), utilizing 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 Hf–W chronometry for those IIIAB and other iron group members that have the lowest CRE ages, indicate coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. Click on Term to Read More formation occurred ~1.0–1.5 m.y. after 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. Eventually, sufficient heat was produced resulting in a differentiated molten core on an asteroid measuring ~28 km in diameter, forming a core with a diameter of ~11 km. After a period of rapid cooling, continued accretion (or development of a significant regolithMixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles). Click on Term to Read More) 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 the cooling rate, and complete core solidification occurred ~3 m.y. later. The 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 of phosphides then created the various phosphate minerals present in many IIIAB irons (but not Tambo Quemado). In addition, they established segregation models for groups IIIAB and IVA, which began segregating ~1.5–2.0 m.y. after CAIs, and for groups IVB and IID, which iron–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 segregation occurred later still, ~2.0–3.0 m.y. after CAIs.
The main-group pallasites have metal compositions and O-isotopes that are nearly identical to those of an evolved, high-Au, high-Ni IIIAB melt following ~80% 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 of the core. Therefore, it has been considered that both groups originated on a common parent body; however, several recent technical studies have ruled out this scenario (Yang and Goldstein, 2006; Scott, 2007; Ziegler and Young 2007; Greenwood et al., 2008; Yang et al., 2010). The specimen of Tambo Quemado shown above is a 61.8 g etched partial slice.