Author: Anupam (Email) In our last article about meteorite classification we have discussed about chondrites – meteorites which contain chondrules. Meteorites without having chondrules are called nonchondrite. Like chondrites, nonchondrites are also classified into different groups based on their mineralogy, petrology, chemistry and isotopic properties (mainly oxygen) (Figure 1). Let's try to understand them! Compositionally (bulk composition) nonchondrites and chondrites are not much different but their textures are highly different. Nonchodritic meteorites don't have chondritic textures though most of the cases they are derived from chondritic materials by planetary melting and differentiation (differentiation is a process of separating out different constituents of a mixture based on their physical (like density) and chemical properties, if we talk about the planetary differentiation that indicates the process by which different planetary constituents get separated out from each other based on their physical and chemical properties: like heavier materials (Fe, Ni) settled in earth’s core and lighter materials (silicates) formed crust by earth’s differentiation after earth’s formation). They are (nonchondrites) generally thought to be residues of very low degrees of partial melting or ultra-metamorphosed chondrites (Prinz et al., 1983b). Due to melting, either partial or complete, they lose their initial textures. However, studies of non-chondrites provide unique opportunities to understand different planetary processes like, melting, differentiation and other relevant phenomenon of extra-terrestrial bodies. Based on the intensity of melting non-chondrites are categorised into two major subdivisions: primitive and differentiated. Degrees of melting events highly varies, some of these silicate-rich meteorites experienced a very low degree of melting. So, they largely retain their initial chondritic bulk compositions. To point out that character and intermediate status (relatively unchanged character after the low degree of melting of nonchondrites) to chondrites and highly differentiated meteorites one new sub-class have been introduced, called as primitive achondrites. Though these are categorically different from differentiated one but from the point of melting events (degrees of melting) they have no distinct boundary, it also depends on the specimens recovered. Primitive achondrite group include acapulcoites, lodranites, andwinonaites. Silicate bearing iron meteorite, IAB and IIICD, also thought to be originated from the same parent body as winonaites. Even some of the silicate inclusions within IIE irons also resemble to primitive achondrites. We will come to that point subsequently. Before that, we will try to understand the petrologic character of this group of meteorites. The acapulcoites and lodranites are fine to medium grained equigranular (grains sizes are similar) rocks composed of olivine (Fe-Mg silicates), pyroxene (rock-forming chain-silicate mineral, commonly found in igneous and metamorphic rock; general formula for pyroxene group is like XY(Si,Al)2O6 where X, Y are different +2 and +3 or +2 ions like Ca, Na, Mg, Cr, Al etc) mainly orthopyroxene and Cr-diopside (Cr bearing diopside: another type of pyroxene ), Na-plagioclase and other Fe-Ni rich metals like troilite (FeS), schreibersite etc. Although the mineral assemblage of this meteorite group is very much similar to that of ordinary chondrites but their overall composition, mineral abundance, grain size and texture differ highly from ordinary chondrites. In several acapulcoites rare relicts of chondrules have also been reported (Schultz et al., 1982; McCoy et al., 1996). Like primitive and differentiated nonchondrites there is no clear-cut distinction between acapulcoites and lodranites. Acapulcoites are typically fine grained with very similar chondritic abundances of silicate phases: olivine, pyroxene, plagioclase and metallic phases like troilite, whereas lodranites has relatively coarse grain size. Most importantly they are depleted in troilite and plagioclase (Nagahara, 1992, McCoy et al., 1996). Cosmic-ray exposure age calculation indicates all acapulcoites and most of the lodranites are formed between ~5.5 Ma and 7 Ma, possibly they were formed from a single impact event or from a common parent body (McCoy et al., 1996, 1997a,b). Winonaites also have similar chondritic mineralogy and chemical composition but achondritic texture where mineral grains are recrystallized and grain size varies from fine to medium in size. Winonaite is an equi-granular rock but sometimes it contains what appear to be relic chondrules. Their composition is intermediate between those of ordinary chondrites (H type chondrites) and enstatite (one type of chondrite). The presence of Fe-Ni-S metal veins also holds the signature of partial melting of a common chondritic precursor material (Benedix et al., 1998). Oxygen isotopes and other mineral composition also indicate the link between winonaites and the silicate inclusions in IAB and IIICD irons (Kracher, 1982; Kallemeyn and Wasson, 1985). Altogether they are also known as the WIN-IIAB-IIICD group. Well, we have already talked a lot about the different meteorite groups, hope now we all have a broad idea about some of those extra-terrestrial rocks but still we need some more time to cover them all. Hopefully, in next few articles of this section, we will be able to wrap-up them. Reference
Photo Sources [1] Astronomical Research Network (http://www.meteorites4sale.net/1_A_Guidline/Primative_AC.htm#Table_8) [2] To see patrographic images of different nonchondrites please feel free to visit this link - Natural History Pages – John Kashuba (http://johnkashuba.com/Ach%20Acapulcoites_and_Winonaites.html)
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