NWA 2656

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
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 2004
no coordinates recorded A 386 g 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 Sahara Desert in 2003. This was later determined to be part of an ~7.5 kg mass, which, along with other recovered fragments, compose a meteorite with a total weight of ~10 kg. The 386 g fragment was purchased by N. Oakes from a Moroccan dealer, and a sample was submitted for analysis and classification (T. Bunch and J. Wittke, NAU; A. Irving, UWS; D. Rumble III, CIW). Northwest Africa 2656 was determined to be an acapulcoite, exhibiting a recrystallized, polygonal, granular texture. Portions of this meteorite have been analyzed by NAU under several different names, including at least NWA 2699 (1,294 g), NWA 2714 (100 g), NWA 2866 (213 g), and NWA 2871 (3,467 g).

As a member of the lodranite/acapulcoite group, NWA 2656 has been distinguished from the members of the winonaitea partially differentiated asteroid that was disrupted just as it began to form an Fe core and a silicate-rich crust. This disrupting impact mixed silicates into molten Ni-Fe metal forming the silicated IAB irons, and mixed olivine-rich residues of partial melts into unmelted silicates, forming the winonaites. A few winonaites Click on Term to Read More group, which exhibits similar recrystallized textures, through a plot of the Fa content 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 vs. the Δ17O-isotopic value. From this diagram, it is apparent that these two groups plot in separate regions, and NWA 2656 is clearly resolved within the lodranite/acapulcoite group (D. Rumble, III et al., 2005). Northwest Africa 2656 consists of orthopyroxeneOrthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg2+ substitutes for Fe2+ up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca2+ contents occur Click on Term to Read More, olivine, and plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite Click on Term to Read More, along with minor FeNi-metal, troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. Click on Term to Read More, 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, Cr-diopside, and chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More, and exhibits a grain size of <1 mm. This meteorite has been shocked to stage S2 and has been weathered to grade W3.

A division of the acapulcoite–lodranite meteorite clan based on metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. Click on Term to Read More stage was proposed by Floss (2000) and Patzer et al. (2003).

1. primitive 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: near-chondritic (Se >12–13 ppmParts per million (10⁶). Click on Term to Read More [degree of sulfide extraction])
2. typical acapulcoites: Fe–Ni–FeS melting and some loss of sulfide (Se ~5–12 ppm)
3. transitional acapulcoites: sulfide depletion and some loss of plagioclase (Se <5 ppm)
4. lodranites: sulfide, 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, and plagioclase depletion (K <200 ppm [degree of plagioclase extraction])
5. enriched acapulcoites (addition of feldspar-rich melt component)

Because both acapulcoites and lodranites are derived from the same 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 and have the same O-isotopic ratios, the plagioclase content in this meteorite is an important factor in making the distinction between acapulcoite and lodranite. Lodranites contain no plagioclase (or only trace amounts) since it was depleted from the restite during the partial melt phase. The two groups also have similar mineralogies, thermal histories, and cosmic ray exposure ages. Additionally, lodranites and acapulcoites have identical cosmogenic nuclideA nuclear species characterized by Z protons and N neutrons. Click on Term to Read More abundances and similar shielding conditions. Another factor which distinguishes acapulcoites from lodranites is their grain size. The grain size of NWA 2656 is more consistent with the finer-grained acapulcoites than with the coarser-grained lodranites—the division has been established by some at 500 µm, and the average grain size of NWA 2656 is 400 µm; however, further studies indicate an average grain size for portions of this meteorite of 0.6–0.7 mm, consistent with a lodranite classification. With many more samples to 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.

For more complete amd current formation scenarios of the acapulcoite–lodranite parent body, visit the Monument Draw and Lodran pages. The specimen shown above is a partial slice of NWA 2656 weighing 1.21 g.