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
no coordinates recorded
A single 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
weighing 56.2 g was found in Western Sahara and subsequently purchased by S. Ralew in Midelt, Morocco; a search for further pairings have proven unsuccessful. This small stone was analyzed over an extended period at the Museum für Naturkunde, Berlin, Germany (A. Greshake) and the Planetary and Space Sciences Research Institute, The Open University, Milton Keynes, UK (I. Franchi and R. C. Greenwood). Northwest Africa 4042 is substantially composed of equigranular 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
crystals (~93.3 vol%), along with minor 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 Click on Term to Read More
and accessory FeNi-metal, phyrrotite, and Mg–Al–Ti-chromite.
As a result of five test runs, the O-isotopic composition of NWA 4042 was determined to plot near several resolved fields—the aubritesAubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8). They contain large white crystals of enstatite as Click on Term to Read More
, brachinites, and winonaites. Therefore, this meteorite was classified as an ungrouped achondrite. A subsequent O-isotopic analysis employing an acid leaching technique demonstrates that NWA 4042 plots within the brachinite field (Greenwood et al.
, 2007). A newly compiled O-isotope diagram
for brachinites and other planetary achondrites, based on published data from Rumble III et al.
(2008), demonstrates that NWA 4042 (Δ17
O value of –0.177) plots within a select grouping of brachinites including NWA 3151, NWA 595, and Zag (b); these investigators believe that this meteorite should probably be lumped with the brachinites (see also this MetBull 90 oxygen three-isotope plot
However, 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 4042 and similar brachinite-like achondrites were not likely genetically related (i.e.
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
) 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. 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.
Northwest Africa 4042 shows evidence of very weak shock (S2), and a low degree of weathering (W2). The specimen of NWA 4042 shown above is a 0.63 g partial slice. The top photo shown below is the remaining 16.19 g main massLargest fragment of a meteorite, typically at the time of recovery. Meteorites are commonly cut, sliced or sometimes broken thus reducing the size of the main mass and the resulting largest specimen is called the "largest known mass". Click on Term to Read More
, and below that is a 40× magnified view of a cut section depicting a coarse-grained, recrystallized texture.
Photos courtesy of Stefan Ralew—SR–Meteorite