Achondrites chemical compositions

The Apollo 11 rocks contain large amounts of ilmenite, as can be seen from Tables 2 and 3 (high titanium content). We have plotted the chemical composition of rock sample 12018 in Fig. 3a vs. that of the carbonaceous chondrites (the most primitive of all meteorites), in Fig. 3b vs. the basaltic achondrite (eucrite) Juvinas (a class of meteorites which have undergone magmatic differentiation) and in Fig. 4 vs. the average composition of the Earth s... 

Winonaites have roughly chondritic mineralogy and chemical composition (Table 6), but achondritic, recrystallized textures (Eigures 16(c) and (d)). They are fine- to medium grained, mostly equigranular rocks but some (Pontlyfni and Mount Morris (Wisconsin)) contain what appear to be relic chondrules. Their mineral compositions are intermediate between those of enstatite and H chondrites, and EeNi-EeS veins that constitute the first partial melts of a chondritic precursor material are common (Benedix et al., 1998). 

In contrast to the small number of differentiated parent bodies represented by evolved achondritic meteorites, the number of parent bodies inferred from the chemical compositions of iron meteorites may be as large as 50 (Wasson, 1990). Of the 13 major iron meteorite groups, 10 appear to be from cores of differentiated meteorites. Many additional cores are inferred from the ungrouped irons, which make up —15% of iron meteorites. It is a puzzle why we appear to sample many more cores than mantles of these asteroids (see Chapter 1.12 for further discussion). 

Weigel A., Eugster O., Koeberl C., and Krahenbuhl U. (1997) Differentiated achondrites Asuka 881371, an angrite, and Divnoe noble gases, ages, chemical composition, and relation to other meteorites. Geochim. Cosmochim. Acta 61, 239-248. 

Notice that meteorites are classified first of all into three large categories stony, iron, and stony-iron. As their name suggests, stony meteorites are similar in appearance and chemical composition to rocks found on the Earth s surface. Stony meteorites are divided into two major categories chondrites and achondrites. Chondrites are stony meteorites that contain small spheres, called chondrules, with diameters of about 1 mm, consisting of minerals that were once melted and that have now aggregated to form the meteorite. Stony meteorites lacking chondrules are called achondrites. 

AEROLITES or STONY METEORITES possess a chemical composition reflecting solar abundances of nonvolatile elements and are subdivided as achondrites and chondrites. ... 

Crystallization of melts provides another way to fractionate chemical elements. Most crystals are denser than the melt from which they form, so crystal settling can separate the phases. Convection currents in hot magma can also entrain crystals and carry them to the bottom of a magma chamber. Accumulations of crystals are a common occurrence on the floors of terrestrial plutons, and many achondrites are cumulates formed in an analogous way in asteroids. Cumulates and the complementary melt have different elemental compositions. 

Anhydrous planetesimals, and especially the meteorites derived from them, provide crucial cosmochemical data. Spectroscopic studies of asteroids do not provide chemical analyses, but the spectral similarities of several asteroid classes to known meteorite types provide indirect evidence of their compositions. The few chemical analyses of asteroids by spacecraft are consistent with ordinary chondrite or primitive achondrite compositions. Laboratory analyses of anhydrous meteorites - chondrites, achondrites, irons, and stony irons - allow us to study important chemical fractionations in early solar system bodies. Fractionations among chondrites occur mostly in elements with higher volatility, reflecting the accretion of various components whose compositions were determined by high- and low-temperature processes such as condensation and evaporation. Fractionations among achondrites and irons are more complex and involve partitioning of elements between melts and crystals during differentiation. 

The petrology and composition of achondrites is a broad topic to cover, and only a fraction of the relevant literature can be cited. Mittlefehldt et al. (1998) presented a more complete treatment of the topic and referencing. Sources of petrologic and chemical data will generally be cited at the start of each achondrite section, while sources of ideas or very specific information will be cited where they are discussed. Here mg is molar 100 X MgO/ (MgO + FeO) and cr is molar 100 X (Cr203 + AI2O3). 

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