Thin Section of an Achondrite
September 29, 2013
Photographer: Mila Zinkova; Mila's Web site
Summary Author: Mila Zinkova
In a prior Earth Science Picture of the Day, thin sections of a chondrite meteorite were discussed. Chondrites are the most common type of stony meteorites, making up as much as 80 percent of all meteorites found on the Earth. Achondrites are another type of stony meteorite. They're much rarer than chondrites -- comprising between 4 and 8 percent of all meteorites. Achondrites were ejected from their parent bodies (the Moon, large asteroids like Vesta, and planets such as Mars or Mercury) as a result of the impact of an asteroid or a comet.
Their story is different from that of chondrites. To begin with, they have no chondrules; nor do they have the primitive solar composition of chondrites. Many achondrites resemble terrestrial volcanic rocks. It's been suggested that such chemical differentiation took place in the same way it does in terrestrial rocks --- by the separation of newly-formed crystals from a cooling mass of molten magma.
Shown above is a microscopic image of a thin section of an achondrite (Ureilite). Ureilites are a small group of unusual and poorly understood meteorites. They seem to combine two processes quite often observed in other meteorite types -- chemical processing and alteration by shock waves produced during meteorite impacts. However, the matrix of ureilites is atypical of that of most all other meteorites.