Carbonaceous chondrites mineral composition

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. 

Other meteorite classes like C2, CO and ordinary chondrites contain much smaller inclusions less than 1 mm (MacPherson et al. 1988) and require ion microprobe techniques to evaluate the isotopic compositions. On the least metamorphosed side. Cl have very few inclusions or oxide grains, but are the carrier of the greatest amounts of stellar nanodiamond and other carbides (Anders and Zirmer 1993). As will be shown for Cr anomalies in carbonaceous chondrites, the survival of the mineral carriers of the anomalies also depends on the metamorphic grade of the meteorites. Nevertheless, isotopic anomalies have also been formd in higher metamorphic grade from other classes, especially in the reduced enstatite chondrites. 

At the end of this section dealing with the question What are carbonaceous chondrites and before trying to answer the question Where do they come from and what is their age , it seems necessary to emphasise the fact that the carbon present in the Cl and CM and to a lesser extent in the CO and CV chondrites exists as elemental carbon (Cl, CM, CO, CV), organic molecules (Cl and CM) and an ill-defined macromolecular compound (Cl, CM, CO, CV). The exact composition of this complex mixture will be discussed below. The only point that we would like to stress in this section is that organic matter is always found intimately mixed with mineral matrices. 

Figure 2 Oxygen isotopic compositions of chondmles from all classes of chondritic meteorites ordinary (O), enstatite (E), carbonaceous (C), and Rumumti-type (R). The TF line and carbonaceous chondrite anhydrous mineral (CCAM) line are shown for reference in this and many subsequent figures. Equations for these lines are TF—5 = 0.525 and CCAM—5 = 0.9415 -4.00 (sources Clayton et al, 1983, 1984, 1991 Weisberg et al., 1991).

Engrand C., McKeegan K. D., and Leshin L. A. (1999) Oxygen isotopic compositions of individual minerals in Antarctic micrometeorites further links to carbonaceous chondrites. Geochim. Cosmochim. Acta 63, 2623-2636. 

Figure 13 A 5 jjim interplanetary dust particle. This is a carbon-rich particle with chondritic elemental composition. It is porous and entirely composed of anhydrous phases. This 10 ° g particle is an aggregate of >10 umelated and unequilibrated grains. The smooth grains are t5fpically single mineral grains such as Fo, En, or pyrrhotite or carbonaceous material, and the <0.5 p,m lumpy grains are usually GEMSs. This is a relatively t5fpical example of the particles that have entry speeds consistent with cometary origin.

The major-element compositions of 200 chondritic IDPs were measured by EDS (Table 1 and Figure 12). All of the particles were identified as extraterrestrial because they have approximately chondritic compositions or consist predominantly of a single mineral grain like forsterite or pyrrhotite (commonly found within chondritic IDPs) 37% of the particles are CSIDPs, 45% are CP IDPs, and 18% IDPs composed predominantly of a single mineral. Table 1 summarizes the compositions of the IDPs. Within a factor of 2 the abundances of oxygen, magnesium, aluminum, sulfur, calcium, chromium, manganese, iron, and nickel are approximately chondritic. CP IDPs are a closer match to Cl carbonaceous chondrites than CS IDPs, and they are closer to Cl bulk than to Cl... 

Figure 1. Oxygen three-isotope diagram showing isotopic compositions of minerals from Allende CAIs (CCAM = carbonaceous chondrite anhydrous minerals). The other line, labelled TF (terrestrial fractionation) is the locus of data points for terrestrial rocks and waters (data not shown). The bulk oxygen isotopic composition of the Earth is indicated by point E the expected oxygen isotopic composition of the Sun is indicated by point S. The S-notation on the...

Carbonaceous Chondrites Type of stony meteorite, characterized by the presence of hydrated, clay-type silicate minerals and abnndant organic matter. They represent among the most primitive material known in the solar system, and are thought to approximate the bulk composition of the accreting solar nebula. 

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