by Jeff Barton
While not exactly "common", meteorites are the easiest "pieces of outer space" we can get our hands on to study in a lab. What we've learned about terrestrial geology and mineralogy informs how we understand the ways in which meteorites form. This is a thin section (a 30-micron-thin slice) of a specimen of the famous Allende, Mexico, meteorites that fell in 1969. The two colorful, round objects are chondrules. These have recrystallized (once they were probably clear, like plain glass) and you can see different shapes inside them. The colors are due to optical birefringence and tell us the nature and index of refraction of the minerals.
The dark border around the larger one is a layer of oxidized "matrix": a very fine-grained collection of tiny bits of olivine and other silicates, plus a hefty dose of oxidized Iron (rust), and sulfides. What we've learned about carbonaceous chondrites like Allende is that they are the most ancient material remaining in their natural state in the Solar System -- the stuff from which our planets originally formed. The isotopes from these meteorites indicate an age of about 4.55 billion years.
Image made with a Pentax DSLR using a petrographic microscope and light polarized in two different planes, August 31, 2008. The horizontal dimension of this image is about 2mm.