Primitive AchondriteAchondrite with an almost chondritic composition with age similar to the primordial chondrites. These should be better classified as "metachondrites"., 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
(Brachinite-like; Brachinite in MetBull 86)
Purchased November 9, 2000
no coordinates recorded A small, highly weathered (W3/4), but complete stone, weighing 196 g was purchased in M’hamid, Morocco and given the name Northwest Africa 595. Northwest Africa 595 is an olivine-rich primitive 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 with subchondritic chemistry and mineralogy, and despite some anomalous features like more magnesian silicates, has been grouped by some investigators with the brachinites. Its 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 constituents include minor troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. 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.
The brachinite group consists of chemically and mineralogically diverse members. A study of NWA 595 by Irving et al.
(2005) revealed an anomalous 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
composition and an O-isotopic composition that plots away from the brachinite group. However, in a followup O-isotopic analysis and petrographic study incorporating a more thorough acid-washing procedure, Irving and Rumble III (2006) did show that NWA 595 is both isotopically and petrologically similar to the brachinite NWA 3151, as well as to the primitive achondrite NWA 4042. The Fe-Mn-Mg relations of NWA 595 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
show correlations to other brachinites, and as found in many brachinites, fine-grained assemblages 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
and opaques lining olivine grain boundaries are present throughout (Goodrich, 2010). At the same time, these three Saharan meteorites have more positive O-isotopic values than Brachina. In their concluding statement (Irving and Rumble III, 69th MetSoc, #5288
) they suggest that ‘if all these specimens (including Brachina) derive 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
, then it must be isotopically quite heterogeneous.’
This study and a study by Greenwood et al.
(2007) both revealed a disparity in O-isotopes between Brachina and some Saharan brachinites, which is consistent with an origin for these Saharan brachinites on a parent body separate from that of Brachina. Other evidence supports such a multiple parent body scenario: some brachinite members exhibit characteristics of primitive achondrites, i.e.
, have near-chondritic compositions, while others appear to have experienced igneous 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
with elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of Click on Term to Read More
depletions and contain melt inclusions in olivine. Evidence is also ambiguous among brachinites as to whether they represent cumulates or metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. Click on Term to Read More
processes. It could be inferred that this varied group represents a diversity of petrogenetic models representing more than a single parent body. However, it is also plausible that the isotopically and petrologically diverse suite of brachinites originated on a very heterogeneous common parent body. If the latter viewpoint is true, then the range of the brachinite group might be too narrowly defined, and perhaps some of the brachinite-like primitive achondrites are also genetically related. Goodrich et al.
(2017) determined that brachinites and brachinite-like achondrites have a distinct redoxOxidation 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
trend and a higher Fe/Mg ratio compared to all other primitive achondrites, consistent with formation in a similar 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
reservoir; therefore, they suggest that brachinites and brachinite-like achondrites be called the brachinite clan.
According to published studies, several factors indicate that NWA 595 and similar brachinite-like achondrites may not be members of the brachinite group. Despite its similarities to the brachinites in chemical composition and Xe-isotopic ratios, the O-isotopic ratios of NWA 595 plot outside of the brachinite field towards the TFL. In addition, FeO/MnO ratios of both olivine and clinopyroxene are lower than for typical brachinites and plot outside of the brachinite field. Moreover, NWA 595 contains more magnesian olivine, lacks 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 ALH 84025 and Eagles Nest), and contains an orthopyroxene abundance (10–15 vol%) significantly higher than in typical brachinites; notably, this is 10× the next highest abundance, measured for Hughes 026. The brachinite-like MIL 090206 has similar abundances of opx and magnesian olivine to NWA 595 (Goodrich et al.
, 2012). Moreover, through studies of highly 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
(HSE) abundances, and upon examining the metal-sulfide segregation processes, it was determined by Day et al.
(2012) that NWA 595 and similar brachinite-like achondrites were not likely genetically related (i.e.
from the same parent body) to brachinites, but rather, originated on similar volatile-rich, 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
, chondritic precursor asteroids while experiencing similar petrologic processes during their formation history.
Many of the known brachinites have disparate cosmic-ray exposure ages, indicating that they represent numerous separate ejection events. According to a study by Patzer et al.
(2003), the CRE ages of EET 99402/407, Hughes 026, and Eagles Nest form a cluster at ~48 m.y., and those of Reid 013 and ALH 84025 coincide at ~10 m.y. In a separate study by Ma et al.
(2003), the cosmogenic nuclideA nuclear species characterized by Z protons and N neutrons. Click on Term to Read More
calculations establish a range of CRE ages from 4 m.y. for Brachina to ~25.5 m.y. for Eagles Nest. From their 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
analyses of 15 brachinite and brachinite-like meteorites, together with the literature values for seven others, Beard et al.
(2018) identified three potential CRE age clusters. The intermediate cluster reflects a possible ejection event that occurred ~25.0 (±3.4) m.y. ago, comprising the five brachinites LEW 88763, NWA 3151, NWA 4874, NWA 7297, and RaS 309, and the three brachinite-like meteorites NWA 595, NWA 6077, and NWA 8777. It should be noted that although the FeO-rich LEW 88763 is currently classified as a brachinite, new analyses by Day et al.
(2015) led them to propose a reclassification as anomalous achondrite, with a possible relationship to the ungrouped achondrite NWA 6704 pairing group. Importantly, two of the resolved CRE age clusters include both brachinite and brachinite-like meteorites, which attests to a common parent body for all of these meteorites (see diagram below). click on image for a magnified view
Diagram credit: Beard et al.
, 81st MetSoc, #6170
(2018) A transmitted light view of a petrographic thin sectionThin slice or rock, usually 30 µm thick. Thin sections are used to study rocks with a petrographic microscope.
of NWA 595 can be seen on John Kashuba’s page
. The specimen of NWA 595 shown above is a 0.81 g partial slice.