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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Generally, we group some objects or specimens that share some common properties like, they originated by the same process or from the same location or made up by similar kind of materials or they can be used for a particular purpose and so on. The ultimate goal of meteorite classification is to group them based on some properties like they share a common origin; from a single parent body - that could be an asteroid, planet or moon that help to differentiate a particular meteorite from others. Based on the textures and bulk compositions meteorites can be classified into two major groups, chondrites, and nonchondritic meteorites; nonchondritic meteorites include the primitive achondrites and some secondary meteorites which have already undergone by igneous processes. They are further classified into different groups using their chemistry, mineralogy and the relative textures of different minerals and isotopic properties (mainly oxygen) (Figure 1). The goal of the classification schemes is to provide descriptive labels for different classes of meteorites that could be derived from the same asteroid or planetary body and to reveal possible genetic links between various classes. Here we will try to know more about the existing meteorite classification, some classification parameters like their textures, basic mineralogy and geochemical properties of different meteorite groups; interested readers can find more information in Rietmeijer (1998), Burbine et al., (2002) and Krot et al., (2005).
The primary group members of chondritic meteorites are – Carbonaceous Chondrites, Ordinary Chondrites (OCs) and Enstatite Chondrites (E-type).
R (Rumuruti-like) and K (Kakangari-like) chondrite groups are different from other chondrites and have been suggested to represent additional chondrite classes. Reference
Photo Sources [1] Chondritic meteorite - meteoritesaustralia [2] Microscopy of Chondrule : Wisc-SIMS Laboratory - Department of Geoscience - University of Wisconsin-Madison Beyond the Earth, the outer world phenomenon like formation of planetary bodies, meteorites etc. are studied in a particular branch of Geoscience, called of planetary science. Very few institutions in India work in this area. In this article, main aim is to discuss the basics of Meteorites. I believe we all have already seen at least one shooting star (Fig. a) on the dark night sky. Aren’t they beautiful? Bright trails of light are running on the sky. But most of us have no such clear idea about them, what are they actually? How they forms or from where they come. Image captions a) Popular picture of the night sky, where those shiny elongated trails are the ‘Shooting Stars’; b) When extraterrestrial rocks enter into the earth’s atmosphere due to friction heat they started burning; c) The Holsinger meteorite is the largest discovered fragment of the meteorite (Canyon Diablo Meteorite) that created Meteor Crater, Arizona, USA; d) Antarctica meteorite is being recovered by scientists. *Photo Source: a) Unknown source, collected from WWW; b) http://www.ufobevy.net/2012/03/nasa-confirm-200ft-asteroid-may-hit.html; f) Wikipedia – Owner: Mario Roberto Duran Ortiz; g) http://discovermagazine.com/2015/june/7-ask-discover; Photo: Katherine Joy/University of Manchester Shooting stars are the rapidly moving very small fragments of space-rocks (meteor), due to Earth’s gravitational pull when they enter into the Earth’s atmosphere due to frictional heat they started burning, that’s why we see the moving light trails on the sky. There is a size difference between shooting stars and meteorites. Theoretically meteorites are the piece of extraterrestrial rock fragments coming from outside of Earth. But when any object penetrates the Earth’s atmosphere, due to frictional heating they started igniting and become a fireball. A meteor is called Bolide if it explodes while crossing the Earth’s atmosphere. However, if it successfully reaches the Earth surface, it is called Meteorite (Fig. b-c). These extraterrestrial rock fragments are the broken/small parts of the planetary material which were formed during the formation of the solar system. Somehow they failed to form any planet but altogether they started rotating around the sun staying between Mars and Jupiter as space-rocks (extraterrestrial rocks). This zone is called asteroid belt because it looks like a conveyor belt where asteroids are rotating around the sun. Collectively these rock fragments/space debris are called meteoroids. Very big meteoroids are called as asteroids. If the length and the size of any such rock-mass are big enough then during crossing the Earth’s atmosphere they do not entirely burn down. Along with size, composition also plays a role. The remaining parts of this asteroid is called meteorite – which are successful to touch the ground. If any meteorite is observed to fall then after recovering it is classified as “falls”. If it is not directly observed to fall then it is called as “finds”. They are named based on which area they were recovered from, like the meteorite Dhurmsala [1] was found at Dhramsala area of Himachal Pradesh, India in 1860. One question may arise that if two meteorites are recovered from the same place, what will be the name of those meteorites? From the deserts of Africa and from Antarctica many meteorites have recovered. These meteorites are given name and numbers, sometimes with the year they were recovered. Allan Hills 84001 (ALH84001) is a meteorite that was found in Allan Hills, Antarctica in 1984. It is to be mentioned that, Antarctica is called the Store House for Meteorite [2] because the maximum number of meteorites recovered from Antarctica. Due to cold and arid climatic condition here the rate of weathering is very less that helps these extraterrestrial rock fragments to stay fresh for a long time. One more interesting aspect is the colour contrast between these meteorites and the ice sheet of the Antarctica. Due to frictional heating outer parts of the meteorites melts and transforms to dark black colored material that creates the colour contrast with the white ice sheet (Fig. d). It also helps the scientists to find them out quite easily. Most meteorites are the fragments of asteroids but some meteorites are fragments of the planets like Mars and Earth’s moon [4]. Among many others ‘Shergotty Mars Meteorite’ [3]is one of important Martian meteorite groups, where Shergotty (Sherghati, Located in Gaya district of Bihar state) meteorite is the first meteorite of this group of meteorites. Sherghati. In 1865 a meteorite of around 5 kg was recovered from this place and the name for Shergotty Martian Meteorite [5]. Unfortunately we know very little about this Shergotty meteorite due to the lack of basic knowledge about planetary science in our society. The Geological Survey of India (GSI) collected an amazing number of 5 kg Martian meteorite sample and reserved in their Kolkata branch repository. However, any significant work has yet to be done from this. This ignorance lies with our profit-oriented science/works, which can be overturned by allowing national and International scientists to access those samples and encourage people to take initiative to properly utilize our resources for the betterment of science and nation. Reference
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