Silicon chondrites

The largest class of meteorite finds is stony meteorites, made principally of stone. The general stony classification is divided into three subclasses called chondrites, carbonaceous chondrites and achondrites, and it is at this level of distinction at which we will stop. Before looking at their mineral and isotopic structure in more detail, it is useful to hold the composition of the Earth s crust in mind here for comparison. The Earth s crust is 49 per cent oxygen, 26 per cent silicon, 7.5 per cent aluminium, 4.7 per cent iron, 3.4 per cent calcium, 2.6 per cent sodium, 2.4 per cent potassium and 1.9 per cent magnesium, which must have formed from the common origin of the solar system. 

Hoppe P, Amari S, Zinner E, Ireland T, Lewis RS (1994) Carbon, nitrogen, magnesium, silicon and titanium isotopic compositions of single interstellar silicon carbide grains from the Murchison carbonaceous chondrite. Astrophys J 430 870-890... 

Huss GR, Meshik AP, Smith JB, Hohenberg CM (2003) Presolar diamond, silicon carbide, and graphite in carbonaceous chondrites implications for thermal processing in the solar nebula. Geochim Cosmochim Acta 67 4823-4848... 

The only in situ chemical data for asteroids are from the NEAR Shoemaker spacecraft, which orbited 433 Eros in 2000-1, and from the Japanese Hayabusa spacecraft, which visited 25143 Itokawa in 2003. NEAR obtained numerous measurements of the surface composition using X-ray fluorescence and gamma-ray spectrometers, and Hayabusa carried an XRF. The magnesium/silicon and aluminum/siUcon ratios for both asteroids are consistent with the compositions of chondrites. However, sulfur is depleted in Eros relative to chondritic compositions, possibly due to devolatilization by impacts or small degrees of melting. 

Bulk techniques still have a place in the search for presolar components. Although they cannot identify the presolar grain directly, they can measure anomalous isotopic compositions, which can then be used as a tracer for separation procedures to identify the carrier. There are several isotopically anomalous components whose carriers have not been identified. For example, an anomalous chromium component enriched in 54Cr appears in acid residues of the most primitive chondrites. The carrier is soluble in hydrochloric acid and goes with the colloidal fraction of the residue, which means it is likely to be submicron in size (Podosck el al., 1997). Measurements of molybdenum and ruthenium in bulk primitive meteorites and leachates from primitive chondrites show isotopic anomalies that can be attributed to the -process on the one hand and to the r- and /7-processes on the other. The s-process anomalies in molybdenum and ruthenium correlate with one another, while the r- and /7-process anomalies do not. The amounts of -process molybdenum and ruthenium are consistent with their being carried in presolar silicon carbide, but they are released from bulk samples with treatments that should not dissolve that mineral. Thus, additional carriers of s-, r-, and/ -process elements are suggested (Dauphas et al., 2002). 

Huss, G. R. and Smith, J. A. (2007) Titanium isotopes in isotopically characterized silicon carbide grains from the Orgueil Cl chondrite. Meteoritics and Planetary Science, 42, 1055-1075. 

Silicates - Si02, or compounds of various cations with various complex ions of silicon and oxygen. Cations can combine with individual Si044- tetrahedra, chains of Si032, and sheets of Si40104. Silicates are the most abundant minerals in chondrites, IDPs, and most types of differentiated materials. 

Volatile element abundances in CV chondrites (normalized to Cl chondrites and silicon) lie along a linear array on semi-log plots versus their 50% condensation temperatures. This depletion pattern persists, whether the elements are siderophile, lithophile, or chalcophile. 

Concentrations of metallic iron and oxidized iron, normalized to silicon, indicate that both oxidation/reduction and loss of metal are required to explain the compositions of various chondrite classes. 

Hua, X., Huss, G. R., Tachibana, S. and Sharp, T. G. (2005) Oxygen, silicon, and Mn-Cr isotopes of fayalite in the oxidized Kaba CV3 chondrite constraints for its formation history. Geochimica et Cosmochimica Acta, 69, 1333-1348. 

Noble gas abundances in planetary atmospheres and Cl carbonaceous chondrites, relative to silicon and solar abundances. After Porcelli and Pepin (2004). 

Chondrite-normalized element/silicon ratios measured in chondritic porous IDPs. Cl chondrite ratios are shown by the vertical dashed line. Modified from Schramm et al. (1989). 

The bulk chemical composition of the dust, obtained by averaging the compositions of particles in numerous tracks (Fig. 12.11a) and impact crater residues (Fig. 12.11b), is chondritic for iron, silicon, titanium, chromium, manganese, nickel, germanium, and selenium, within the 2o confidence level (Flynn el al., 2006). Copper, zinc, and gallium are... 

Huss, G. R. (1990) Ubiquitous interstellar diamond and silicon carbide in primitive chondrites abundances reflect metamorphism. Nature, 347, 159-162. 

Debris from earlier supemova(s) condensed into the Solar Nebula about 6 billion years ago (Faure, 1998, 22). By about 4.5 billion years ago, the planets had largely condensed from the nebula and the core of the Sun became dense enough to ignite through fusion Reaction 3.1. Based on the chemistry of chondrite meteorites (Wasson and Kallemeyn, 1988, 536), the original Solar Nebula had about 6.79 arsenic atoms for every one million atoms of silicon (Table 3.1) and 2.72 x 1010 atoms of hydrogen (Faure, 1998 Anders and Ebihara, 1982, 15). 

Some of the chondritic meteorites contain grains (including crystalline and amorphous silicates, diamonds, silicon carbide, graphite, metal oxides, and metal nitrides) that have been identified as presolar based on non-solar isotopic ratios (Zinner 1988 Anders Zinner 1993 Bematowicz et al. 2006), particularly for... 

Bulk isotopic compositions of chondrites. Isotopic compositions of bulk chondrites are essentially uniform within variations of 0.1-0.01% except for light elements such as H, C, N, and O and for presolar grains (see e.g. Lodders 2003 Palme Jones 2003). Presolar grains have isotopic compositions significantly different from those of Solar System materials, suggesting that they were dust particles formed in circumstellar environments and incorporated into the proto-solar molecular cloud (Chapter 2 and see e.g. Nittler 2003 Zinner 2005). Presolar grains are thus considered to be the first dust components that formed in the proto-solar disk. The rarity of presolar grains in chondrites (several ppb for silicon nitride to 200 ppm... 

Chondrite matrices are also the carriers of presolar grains. These phases cover a broad mineralogy and include diamonds and graphite, silicon carbide (SiC), titanium carbide (TiC), silicon nitride (SisN, ), corundum, spinel, silicates, and even rare metal grains. 

Fig. 3a. (Upper diagram). Abundances of chemical elements in lunar rock 12018 vs. the abundances in carbonaceous chondrites type 1 (C 1) normalized to silicon. Data for 12018 were taken from all authors of the Second Lunar Science Conference. Geochim. Cosmochim. Acta, Supplement 2,2 (1971), data for C 1 from the compilation of Mason17 ...

Besmehn A. and Hoppe P. (2001) Silicon- and calcium-iso-topic compositions of presolar sihcon nitride grains from the Indarch enstatite chondrite. In Lunar Planet. Sci. XXXII, 1188.TheLunarandPlanetary Instimte,Houston(CD-ROM). 

Huss G. R., Hutcheon I. D., and Wasserburg G. J. (1997) Isotopic systematics of presolar silicon carbide from the Orgueil (Cl) carbonaceous chondrite implications for solar system formation and stellar nucleosynthesis. Geochim. Cosmochim. Acta 61, 5117-5148. 

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