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Lecture8 - Some meteorites have clearly been altered by...

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Classification Some meteorites have clearly been altered by heat pressure, mechanical shock, or other processes ¾ Sometimes the heating is gentle, so they are only slightly altered ¾ Others have completely melted, allowing the separation of heavy and light materials: differentiation Stones are the least differentiated and are the most common Irons are almost pure metal (mostly Ni and Fe) and the most differentiated. Stony-Irons are a mixture of the two. How does differentiation occur? ¾ Imagine heating a solid mixture of primordial material so that it melts ¾ High density metals such as iron and nickel, along with elements with a chemical affinity for iron (e.g., Co, Ni, Ru, Rh, Pd, Os Ir, and Pt) drain to the centre to form a metal core ¾ Lighter, silica-associated elements float to the surface to form stony mantle. ¾ Collisions then produce fragments with different compositions. ¾ Later collisions can weld fragments together, forming breccias .
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The stony meteorites are further subclassified into chondrites - those that have chondrules (glassy spherules) - and achondrites. ¾ All irons and stony-irons are achondrites ¾ Achondrites have non-solar chemical compositions, and have clearly been differentiated ¾ Produced when parent material melted, destroying any chondrites Carbonaceous chondrites are the least differentiated ¾ Formed at low temperature and never reheated ¾ Unusually high concentration of volatiles, organic compounds (which would boil off at high T) ¾ Low densities ¾ Contain heavier elements in nearly original proportions ¾ No evidence for heating above 500 K ¾ Originate from distant solar system (>2.5 AU) Ordinary chondrites are the most numerous meteorite and are similar to carbonaceous chondrites, but slightly more processed ¾ Probably originate from inner asteroid belt
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