NWA 6704

AchondriteAn achondrite is a type of stony meteorite whose precursor was of chondritic origin and experienced metamorphic and igneous processes. They have a planetary or differentiated asteroidal origin where the chondritic parent body reached a sufficient size that through heating due to radioactive decay of 26Al (aluminum isotope) and gravitational Click on Term to Read More, ungroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). Click on Term to Read More
carbonaceous chondrite-related

Found 2010 in Algeria
no coordinates recorded At least forty-two conjoint stone 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 fragments, mostly devoid of fusion crustMelted exterior of a meteorite that forms when it passes through Earth’s atmosphere. Friction with the air will raise a meteorite’s surface temperature upwards of  4800 K (8180 °F) and will melt (ablate) the surface minerals and flow backwards over the surface as shown in the Lafayette meteorite photograph below. Click on Term to Read More, were found in Algeria in 2010. They were subsequently purchased by G. Hupé in February 2011 during the Tucson Gem and MineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More Show. Associated fragments were tracked down and acquired from various Moroccan dealers by G. Hupé over the succeeding four months. The total combined weight of these paired fragments was 8,387 g. An additional 5,100 g of fragments, designated NWA 6693, were acquired in March 2011 by E. Thompson and are considered likely paired with NWA 6704. One other group of paired stones previously obtained by G. Fujihara in January 2011 received a separate designation as NWA 6926. A portion of both NWA 6704 and NWA 6926 were submitted for analysis and classification to the University of Washington in Seattle (A. Irving and S. Kuehner), while a portion of NWA 6693 was submitted to the University of California in Los Angeles (P. Warren).

The meteorite is an igneous cumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber. Click on Term to Read More that originated on a large partially differentiated 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 distinct from all others known to date. As seen in the high-res photo above (courtesy of G. Hupé), NWA 6704 is composed primarily 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 (~70 vol%), yellowish-green in color, occurring as both mm-sized grains and larger cm-sized megacrysts that enclose other smaller grains such as 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 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 (Irving et al., 2011; Warren et al., 2013; Hibiya et al., 2018). The Fe-rich olivine grains (~14 vol%) are likely xenocrysts similar to those present in some quenched angrites (Hibiya et al., 2014). These olivine grains contain relatively high NiO (ave. 0.9 wt%). Other components include highly sodic plagioclase (~10 vol%, as albite), 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 (~7 vol%, as rims on orthopyroxene megacrysts), FeNi-metal (~0.3 vol%, as awaruiteNi-rich Fe metal, Ni3Fe, similar to taenite found in minor amounts in some meteorites. Awaruite is also known as josephinite, a mineral found as placer deposits in Josephine County, Oregon, and sometimes mistaken for a meteorite. Note: web.mineral.com incorrectly defines Awaruite as “Ni2Fe to Ni3Fe”, however the IMA Database of Click on Term to Read More), and chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More (~0.15 vol%), along with trace amounts of merrillite and sulfides (heazlewoodite and pentlanditeFe-Ni sulfide, (Fe,Ni)9S8, that is often associated with troilite, and found in the matrix and chondrules of CO, CV, CK and CR chondrites. The color is yellow-bronze with light bronze-brown streak and metallic luster. It typically forms during cooling of magmatic sulfide melts during the evolution of parent silicate melt. The Click on Term to Read More). Plagioclase forms a continuous 3-D framework within the orthopyroxene megacrysts, and is inferred to have crystallized in situ from an undercooled melt. The plagioclase contains tiny grains of the Ni-rich (~80 wt%) 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 awaruite. Curvilinear trains of micro-inclusions (mostly oxides) along with empty, smooth-walled bubbles are present in the orthopyroxene megacrysts, reflecting shock mobilization/injection (Irving et al., 2011; Warren et al., 2013; high-res image from Hibiya et al., 2018). Local compositional and textural variation, including an olivine-rich enclave, was observed in the NWA 6693 meteorite.

This meteorite is unique because of its great abundance of highly-ferroan maficOne of the two broad categories of silicate minerals, the other being felsic, based on its magnesium (Mg) and/or iron (Fe) content. Mafic indicates silicate minerals that are predominantly comprised of Mg and/or Fe.The term is derived from those major constituents: Magnesium + Ferrum (Latin for iron) + ic (having Click on Term to Read More silicates (low-Ca 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: Mg# = 57–60; olivine: Fa50–53), its highly sodic plagioclase (Ab91–93), and its Ni-rich phases (olivine: ave. 0.9 wt% NiO; metal: ave. 81 wt% NiO; and sulfide: ~0.002 wt% NiO). These Ni-rich phases are indicative of formation in a highly oxidizingOxidation 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 environment (FMQ –2.6). Consistent with 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 conditions is the fact that the bulk rock and 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 compositions are nearly chondritic, although the meteorite does show depletions of S and highly 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. Elemental ratios (Mg/Si = 0.48 by wt.) indicate that very little 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 has occurred in the parent melt, and the compositional trends exclude this rock from being a refractory residue (restite) of a partial melt (Warren et al., 2011; 2013). The FeO/MnO ratios for low-Ca pyroxene in this meteorite (81–106; Irving et al., 2011) are distinctly higher than those of both HED and martian meteorites, and it was concluded that the parental source materal for the NWA 6704 pairing group was extrememly oxidizedOxidation 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 (FeO-rich).

Iizuka et al. (2013) report high abundances of highly siderophile elements (HSE) for this meteorite similar to those of some brachinite-like achondrites such as GRA 06128/9 and Zag (b). In their study of HSE abundances and Os-isotopic systematics in brachinites and other FeO-rich brachinite-like achondrites, Day and Warren (2015) found that NWA 6693[6704] has a high Pt/Os ratio indicative of a cumulate derived from a low-degree partial melt infused with a residual metal melt component. In addition, they reasoned that the variability observed in the Pd/Os ratios of NWA 6693[6704] and other brachinite-like achondrites reflects variable degrees of fractionation of such a metal melt prior to its incorporation. The high abundance of HSEs was attributed by Warren et al. (2013) to the highly oxidizing formation conditions which inhibited sequestration and removal of HSEs by FeNi-metal during differentiationA process by which a generally homogeneous chondritic body containing mostly metal, silicates and sulfides will melt and form distinct (differentiated) layers of different densities.   When the melting process continues for a long enough period of time, the once chondritic body will re-partition into layers of different composition including Click on Term to Read More. In a study of NWA 6704 conducted by Archer et al. (2016), it was determined that the bulk HSE abundances are nearly chondritic and exhibit only a low degree of fractionation, consistent with a scenario in which only minor, localized 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 partial meltingAn igneous process whereby rocks melt and the resulting magma is comprised of the remaining partially melted rock (sometimes called restite) and a liquid whose composition differs from the original rock. Partial melting occurs because nearly all rocks are made up of different minerals, each of which has a different melting Click on Term to Read More occurred with some loss of sulfides. Their results indicate that 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 had not proceeded to any great extent on the parent body by the time the meteorite was formed. It was further argued that the NWA 6704 chondritic precursor lithology was instantaneously melted during an impact event, and then rapidly cooled at ~1–100°C/hr under super-saturation conditions which promoted coarse dendritic growth of orthopyroxene (Hibiya et al., 2018). As temperatures fell below ~1100°C cooling proceeded much more slowly, consistent with burial ~100 m deep under an ejecta blanketGenerally symmetrical apron of ejecta surrounding a crater; it is thick at the crater's rim and thin to discontinuous at the blanket's outer edge. Click on Term to Read More (Hibiya et al., 2017, 2018).

Separate O-isotopic analyses were conducted for NWA 6704 and NWA 6693 initially at Okayama University in Japan, and at Seoul National University in the Republic of Korea, respectively. It was ascertained by both institutions that this ungrouped achondrite plots within the field of the 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 (see 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 three-isotope plot). However, mafic silicates in these paired meteorites are significantly more ferroan and the feldsparAn alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of feldspars and due to their abundance, feldspars are used in the classification of igneous rocks. A more complete explanation can be found on the feldspar group page. Click on Term to Read More significantly more sodic than members of the acapulcoite–lodranite clan. While this diagnostic method suggests a connection might exist with the acapulcoite–lodranite clan, its O-isotopic composition also plots within the CR/CB/CH field, presenting yet another possibility for a connection (see oxygen three-isotope plot). The O-isotopic composition of NWA 6704 also plots close to the ungrouped LEW 88763, and although their mineralogy is completely different it is notable that they have complementary HSE patterns (Day and Warren, 2015). Notably, new analyses of LEW 88763 by Day et al. (2015) led them to propose its reclassification as an anomalous achondrite, possibly related to NWA 6704 and pairings. A diagram depicting the O-isotopic plot for NWA 6704 and NWA 6693, the acapulcoite–lodranite clan, and brachinites is shown below courtesy of Achim Raphael.

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Northwest Africa 6704 — Oxygen isotopes (R. Tanaka, OkaU): replicate analyses by laser-fluorination produced δ17O = 1.015‰, 0.880‰; δ18O = 3.922‰, 3.613‰; Δ17O = -1.048‰, -1.020‰
Northwest Africa 6693 — Oxygen isotopes (B-G. Choi and I. Ahn, Seoul-NU): replicate analyses by laser-fluorination produced δ17O = 1.19‰; δ18O = 4.32‰; Δ17O = -1.08‰

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Additional constraints on the origin of this meteorite were established through studies of the Cr-isotopic systematics (Sanborn et al., 2013). The resulting ε54Cr value of +1.69 (±0.07) resolves NWA 6704 from the acapulcoite–lodranite clan (ε54Cr = –0.75; Göpel and Birck, 2010); discrimination between these groups had not been attained through the use of O-isotopic values alone. Continued efforts to better resolve the relationship that exists among the numerous anomalous meteorites has been ongoing (e.g., Bunch et al., 2005, [#2308]; Floss et al., 2005, [MAPS vol. 40, #3]; Irving et al., 2014 [#2465]; Sanborn et al., 2014 [#2032]). As provided in the Sanborn et al. (2014) abstract, a coupled Δ17O vs. ε54Cr diagram is one of the best diagnostic tools for determining genetic relationships among meteorites. The diagrams below include the paired stones NWA 6704 and 6693, and it is apparent that they plot within the CR chondriteClass named for the Renazzo meteorite that fell in Italy in 1824, are similar to CMs in that they contain hydrous silicates, traces of water, and magnetite. The main difference is that CRs contain Ni-Fe metal and Fe sulfide that occurs in the black matrix and in the large chondrules Click on Term to Read More field.
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014)

17O vs. ε54Cr and ε50Ti for CR Carbonaceous Achondrites

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Diagrams credit: Sanborn et al., GCA, vol. 245, pp. 577–596 (2019)
‘Carbonaceous Achondrites Northwest Africa 6704/6693: Milestones for Early Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. Chronology and Genealogy’
(https://doi.org/10.1016/j.gca.2018.10.004) However, a plot of Δ17O vs. olivine Mg# (Mg/[Mg+Fe]) resolves the NWA 6704 pairing group from other meteorite groups as well as ungrouped meteorites with similar O-isotopic compositions, as it displays a significantly more ferroan composition (Warren et al., 2013). Further resolution of the parental source group for NWA 6704 was obtained by Hibiya et al. (2017, 2018) through a titanium 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 analysis. It is demonstrated on both Δ17O vs. ε50Ti and ε54Cr vs. ε50Ti coupled diagrams that the meteorite plots within the field for carbonaceous 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 (see diagrams below). Based on the results of their petrologic, geochemical, and isotopic analyses of NWA 6704, Hibiya et al. (2017, 2018) concluded that an undifferentiated asteroid experienced an impact-generated melting event followed by rapid cooling. Presumably, subsequent burial beneath an ejectaFractured and/or molten rocky debris thrown out of a crater during a meteorite impact event, or, alternatively, material, including ash, lapilli, and bombs, erupted from a volcano. Click on Term to Read More blanket ushered an extended period of slow cooling. ε50Ti vs. 17O and ε54Cr for NWA 6704

click on image for a magnified view

Diagram credit: Hibiya et al., GCA, vol. 245, pp. 597–627 (2019)
‘The origin of the unique achondrite Northwest Africa 6704: Constraints from petrologyScience dealing with the origin, history, occurrence, chemical composition, structure and classification of rocks. Click on Term to Read More, chemistry and Re–Os, O and Ti isotope systematics’
(https://doi.org/10.1016/j.gca.2018.04.031) An Ar–Ar and 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 study was conducted by Fernandes et al. (2013) to constrain the petrologic history and to determine the CRE age of this meteorite. The oldest possible 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 age was determined to be 4.56 (±0.29) b.y., or 4–5 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. This is in agreement with the U–Pb age of 4.56280 (±0.00046) b.y. determined by Iizuka et al. (2013), and the most precise and accurate age of 4.56238 (±0.00049) b.y. determined by P. Koefoed (2017). A refined Pb–Pb isochron age of 4.56278 (±0.00018) b.y. was calculated by combining data obtained from both the Australian National University and UC Davis (Huyskens et al. (2017). In a subsequent investigation, Amelin et al. (2019) employed the 238U/235U value directly measured for NWA 6704 of 137.7784 to calculate the most precise and accurate Pb–Pb isochron age of 4.56276 (+0.00022/-0.00030) b.y. In a similar manner, the best age estimate for NWA 6693 was determined to be 4.56263 (+0.00029/-0.00021) b.y. Other isotopic chronometers (e.g., Al–Mg, Mn–Cr [Sanborn et al., 2019], Rb–Sr, Re–Os) give ages that are consistent with the Pb–Pb age and attest to rapid cooling, crystallization, and isotopic closure. Based on initial Sr chronometry data, Amelin et al. (2019) determined that 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 NWA 6693/6704 parent body occurred within 3.6 m.y. of the formation of CAIs.

The occurrence of an impact-resetting event prior to complete cooling of the planetesimal is indicated by the loss of fluid from micro-inclusion bubble trains present in orthopyroxene crystals. This event may be related to a resetting event that occurred 4.199 (±0.032) b.y. ago as revealed by Amelin et al. (2018) through Ar–Ar chronometry. In addition, they provided further evidence of another thermal event that occurred ≤2.12 b.y. ago. The CRE age was calculated based on Ne and Ar systematics to be 30 (±3) m.y. (Fernandes et al., 2013). Furthermore, the pre-atmospheric size of the meteoroidSmall rocky or metallic object in orbit around the Sun (or another star). was calculated to have been at least 100 cm in diameter.

Further studies have been conducted by Sanborn et al. (2018) of new anomalous ungrouped meteorites recovered in Northwest Africa. Utilizing a coupled Δ17O vs. ε54Cr diagram, they demonstrated that NWA 6704 and pairings, NWA 011 and pairings, and NWA 6962/7680 all plot within the CR/CH 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 field represented by CR2 Renazzo and CH3 NWA 2210, which suggests a possible genetic linkage among them (see diagram below). Chromium vs. Oxygen-isotope Plot

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Diagram credit: Sanborn et al., 49th LPSC, #2296 (2018) Sanborn et al. (2018) also determined that the absolute Mn–Cr age (anchored to D’Orbigny) calculated for NWA 6962/7680 (4.56376 [±0.00176] b.y.) is concordant with the ages calculated for both 6704/6693 (Mn–Cr age from Sanborn et al., 2019: 4.56217 [±0.00076] b.y.) and NWA 011/2400/2976 (U–Pb age from Bouvier et al., 2011: 4.56289 [±0.00059] b.y.). It has been proposed by many investigators that a large (~400 km diameter) differentiated CR parent body formed in the early history of the Solar SystemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with and subsequently experienced a collisional disruption. For more information pertaining to this scenario, see the LPSC abstract ”Primitive’ and igneous achondrites related to the large and differentiated CR parent body’ by Bunch et al. (2005), the MetSoc abstract ‘Tafassasset and Primitive Achondrites: Records of Planetary Differentiation’ by Nehru et al. (2014), and the LPSC abstract ‘Collisional Disruption of a Layered, Differentiated CR Parent Body Containing Metamorphic and Igneous Lithologies Overlain by a Chondrite Veneer’ by Irving et al. (2014).

The reflectance spectra of NWA 6704 was acquired and compared to that of known asteroid groups (Le Corre et al., 2014). The analysis of a large-sized sample was comparable to spectra from the S(VI) asteroid group, which is thought to represent formation as a partial melt residue (e.g., winonaites). The spectra of a smaller, grain-sized sample plotted between the S(V) and S(VI) asteroid fields, the former thought to represent formation as a metamorphosed 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 or a primitive achondriteAchondrite with an almost chondritic composition with age similar to the primordial chondrites. These should be better classified as "metachondrites". (e.g., 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/lodranites). However, direct spectral comparisons of NWA 6704 to known asteroids only produced a relatively close match to V-type asteroids (e.g., VestaThird largest and fourth brightest asteroid; it was discovered in 1807 by Heinrich Olbers and named for the ancient Roman goddess of the hearth. 4 Vesta has a basaltic surface composition and an average density not much less than that of Mars. Evidently lava once flowed here indicating that the). Moreover, direct spectral comparisons of NWA 6704 to HED suite meteorites demonstrated a close similarity to the basaltic and cumulate eucritesMost common type of achondrite meteorite and a member of the HED group. Eucrites are basalts composed primarily of pigeonite and anorthite (An60-98). Eucrites have been placed into three subgroups based on mineralogical and chemical differences. • Non-cumulate eucrites represent the upper crust that solidified on a magma ocean after Click on Term to Read More as well.

A separate 940 g stone designated NWA 10132 was found at a location different from the NWA 6704 pairing group, but it has a similar mineralogy, an almost identical O-isotopic composition, and a matching U–Pb age (Irving et al., 2015; Koefoed et al., 2015). Based on these facts, NWA 10132 is thought to be a possible genetic relative, likely co-magmatic and/or launch paired. Consistent with this hypothesis, Y. Amelin (2017) determined that the Rb–Sr data for NWA 10132, NWA 6693, and NWA 6704 are similar and establish an isochron age of 4.543 (±0.035) b.y. In addition, the ε54Cr value for NWA 10132 determined by Sanborn et al. (2018) matches that of the NWA 6704 pairing group, and they calculated an age for NWA 10132 based on Mn–Cr systematics anchored to D’Orbigny of 4.56276 (±0.00041) b.y., as well as an age based on U–Pb systematics of 4.56271 (±0.00019) b.y.; both of these ages are concordant with the ages previously calculated for the NWA 6704 pairing group. Employing the 238U/235U ratio value of 137.7784 (Huyskens et al.), a similar Pb–Pb isochron age of 4.56212 (±0.00067) b.y. was calculated for NWA 10132 by P. Koefoed (2017). A fourth chronometric age for NWA 10132 was calculated by Huyskens et al. (2019) based on Al–Mg systematics. They derived an absolute age anchored to D’Orbigny of 4.56304 (±0.00059) b.y. Their compiled diagram incorporating multiple chronometric system ages for four different achondrite parent bodies that accreted in the CR reservoir is shown below.
Diagram credit: Huyskens et al., 50th LPSC, #2736 (2019) The meteorites composing this unique pairing group were found to have features consistent with a low weathering grade described as generally unshocked (S1) after significant annealing (Warren et al., 2013). The specimen of NWA 6704 framed above is a 1.56 g fragment.

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