Posts Tagged ‘iron’

Achondrite Meteorites

An achondrite is the name given to a type of stony meteorite containing very little iron or nickel and no chondrules. A chondrule is a rounded grain that originally formed as a molten droplet in Space.Achondrites have a basic mineralogiy i.e. similar to basalts. Because they are very similar to terrestrial basic igneous rocks, it is very difficult to locate them unless the landing area has been seen. According to reflection spectra studies, the majority of them are associated with the asteroid Vesta but there are a dozen or so subdivisions of various origins. One thing is certain though, because of the magmatic differentiation seen in these meteorites, they have come from a ‘differentiated’ object. Differentiation of magma (molten rock) occurs when a magma chamber is large enough and molten for long enough for crytals to have started to form. The heavier crystals sink slowly through the magma and a gradual chjange in composition can be seen after the magma has turned to rock. Because of their mode of formation, chondrules are not seen, hence their designation.

The public access area of the Hubble Space Telescope web site contains low resolution images of the surface of asteroid 4 Vesta if you are interested to see where most achondrites come from.

However, the achondrites are not exclusively from Vesta, the famous ‘Martian fossil’ meteorite found at Allan Hills in Antarctica is of this type. Other achondrites appear to be Lunar in origin. In terms of numbers, they are fairly rare and form about 4% of the known meteorites.

Achondrite Meteorite Falls

Station workers saw a fireball which fell on the Millbillillie estate in Western Australia. An object “with sparks coming off it” fell onto a plain to their north. No one went to have a look at the time but a decade later, in 1970, some remnants of an achondrite meteorite were found. Over the years, Aborigines walking the area have found other fragments, the largest fragment weighed in at 20kg and is on display in the Western Australia Museum. Others have been found in France, the USA, Mexico, Egypt, India and several other locations.

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