AcapulcoitePrimitive achondrite that belongs to a small group named after the Acapulco meteorite that was observed to fall in Mexico in 1976. Acapulcoites are made mostly of fine-grained olivine (Fo3-14), orthopyroxene(En86-97), Ca-rich pyroxene (En51Wo44), plagioclase (An12-31), Ni-Fe metal, and troilite. They are transitional between primordial chondritic matter and more differentiated Click on Term to Read More, FeO-rich Acapulcoite–LodraniteRare type of primitive achondrite named after the Lodran meteorite that fell in Pakistan in 1868. Initially, lodranites were grouped with the stony-iron meteorites because they contain silicates (olivine, orthopyroxene, and minor plagioclase) and Fe-Ni metal in nearly equal proportions. However, since discovery of the closely related acapulcoite group, lodranites Click on Term to Read More Clan
Purchased November 2005 no coordinates recorded A 222 g [231 g] partially fusion-crusted stone, which was found in Algeria, was eventually marketed in Erfoud, Morocco. The stone was purchased by American collectors and a portion was submitted for analysis to both the Northern Arizona University (J. Wittke and T. Bunch) and the University of Washington in Seattle (A. Irving). Northwest Africa 2775 was classified as an acapulcoite (see MetBull 91).
While NWA 2775 exhibits the typical recrystallized texture of acapulcoitesPrimitive achondrite that belongs to a small group named after the Acapulco meteorite that was observed to fall in Mexico in 1976. Acapulcoites are made mostly of fine-grained olivine (Fo3-14), orthopyroxene(En86-97), Ca-rich pyroxene (En51Wo44), plagioclase (An12-31), Ni-Fe metal, and troilite. They are transitional between primordial chondritic matter and more differentiated Click on Term to Read More, its 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 and 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 have FeO contents among the highest measured thus far in other acapulcoites. Notably, grain sizes have a range of 0.35–0.8 mm (ave. 0.55 mm), which is consistent with lodranites (>0.5 mm) according to one of the parameters that help distinguish lodranites from acapulcoites. However, with the many new samples available for study, it is now evident that a continuum exists for the grainsizes of these two groups, and it has been proposed by Bunch et al. (2011) that an arbitrary group division is no longer justified; the term ‘acapulcoite–lodranite clan’ should therefore be applied to all members of the combined group.
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 isotopes were analyzed at the Carnegie Institute, Washington D.C. (D. Rumble III), and the Δ17O was found to be the lowest among all acapulcoites measured to date, but was determined to be consistent with an acapulcoite classification. A method to distinguish acapulcoites from winonaites was recently devised by Rumble III et al. (2005). Utilizing their diagram comparing the Fa mol% of olivine vs. the Δ17O‰, it is demonstrated that NWA 2775 (Fa14.5; ave. Δ17O –0.75‰) plots in a unique location at the edge of the acapulcoite field. The reason for the observed correlation between the Fa content in olivine and the Δ17O value for acapulcoites was considered by Irving et al. (2007). They suggested the possibility that a metal-poor impactor with a Δ17O value plotting close to the terrestrial 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 line, similar to a brachinite, was mixed into the 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 of a body having olivine and an O-isotopic composition similar to a CH chondriteRare carbonaceous chondrite class that is chemically very close to the CRs and CBs as evidenced by the CH/CBb meteorite Isheyevo. The "H" stands for "high metal" since the CH chondrites contain up to 15 vol. % Fe-Ni metal. The first CH chondrite was found in the Antarctic Allan Hills Click on Term to Read More; subsequent to the collision, the mixture was thermally equilibrated.
The closure of the Hf–W chronometer on the ACA–LOD 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 occurred 4.5621 (±0.0014) b.y. ago, or 6.4 (±1.3) 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 (Touboul et al., 2007). A slightly older Hf–W age of 3.84 (+3.6/–3.1) m.y. after CAI formation was calculated by Schulz et al. (2010). With other factors considered, they concluded that the 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 Moremelting pointTemperature at which a solid changes to a liquid. A completely pure crystalline substance has an exact temperature at which it melts. Impure substances and amorphous substances will begin melting at one temperature and finish at another. For these substances the term "melting range" is more appropriate. Under some conditions Click on Term to Read More, or the cooling point at which redistribution of Hf and W between metal and 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 ended, occurred 4.1 (+1.2/–1.1) 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. Although the ACA–LOD parent body reached higher temperatures than did the H chondriteOrdinary chondrites with a high content of free Ni-Fe metal (15-19 vol. %) and attracted easily to a magnet. Their main minerals are olivine (Fa16-20) and the orthopyroxene bronzite (Fs14.5-18.5), earning them their older name of bronzite chondrites. Chondrules average ~0.3 mm in diameter. Comparison of the reflectance spectra of Click on Term to Read More parent body by assimilating a higher abundance of radiogenic nuclides during its earlier accretionary period, it also cooled more rapidly at high temperatures, possibly reflecting a smaller-sized planetesimal and/or a near-surface residence for the acapulcoites (Kleine et al., 2007). Another possibility for its early rapid cooling could be the fact that it experienced a collisional disruption early in its history forming sub-km- to multi-km-sized fragments, which eventually succumbed to gravitational reassembly.
For more complete amd current formation scenarios of the acapulcoite–lodranite parent body, visit the Monument Draw and Lodran pages. The specimen of NWA 2775 shown above measures 35 mm × 19 mm and weighs 2.68 g. The photo below shows a magnified image of this acapulcoite. Photos courtesy of S. Turecki