Meteorites! Pieces of the Cosmic Rosetta Stone

Title: Meteorites! Pieces of the Cosmic Rosetta Stone

Author: Matt Morgan, Colorado Geological Survey

Publication: The Outcrop, March 2000, p. 1, 8

Throughout earth’s history, debris from space has shaped our planet. Impact craters and possibly the demise of the dinosaurs are testaments to the forces unleashed during a major impact. Meteorites also may have carried the building blocks of life to earth; amino acids, exotic minerals, liquid water, and possibly primitive life have been discovered in meteorites.

Meteorites are of three types: iron, stony-iron, and stone, based on their content of nickel-iron alloy (about 98%, about 50%, and less than 23%, respectively). Nickel-iron is so rare in terrestrial rocks that the presence of any amount usually indicates that a rock is a meteorite.

About 85% of known meteorites are stone meteorites. Many of these contain chondrules, and are thus termed chondrites. Chondrules are spheres of pyroxene or olivine smaller in diameter than about 1 mm. Chondrules probably formed by melting of solid particles that were part of the original solar nebula. Chondrules record varying degrees of shock or hydrothermal alteration of the parent body. Classification of a chondritic meteorite involves analyzing mineralogy and chondrule deformation, which also indicate the body from which a chondrite originated.

Achondrites are stony meteorites that lack chondrules. Achondrites are rarer than chondrites, and their histories often are complex. Recent evidence suggests that some achondrites may derive from the surfaces of asteroids. “Millbillillie”, an achondrite from Australia, is composed primarily of Ca-pyroxene and plagioclase, like earth basalt. The spectral patterns of this meteorite match closely the spectral reflectance of the surf ace of the asteroid Vesta. Some achondrites may have originated on Mars. SNC’s (pronounced “snicks”, after the meteorites found in Shergotty, India, Nakhla, Egypt, and Chassigny, France) contain trapped gases with compositions very similar to the Martian atmosphere. SNC’s most likely derive from volcanic dikes, sills, and flows.

Iron meteorites are classified based on their percentages of germanium, gallium, iridium, nickel, and iron. Also considered is the width of kamacite and taenite crystals in their matrix. The crystals cause the familiar crosshatched texture, termed a Widmanstatten figure, seen on polished faces treated with dilute nitric acid. The exteriors of iron meteorites often are jagged and have a dark orange patina. Iron meteorites probably originated near the center of a moon or asteroid as the molten core cooled and solidified. Over time, the parent body was smashed by impacts with other bodies.

Iron meteorites have varied cooling histories. This results in a wide range of Widmanstatten patterns, from very fine to very coarse. A coarse pattern presumably indicates a slow cooling rate.

The stony-iron meteorites combine characteristics of stone and iron meteorites. Pallasites and mesosiderites comprise a major portion of the stony-iron group. Pallasites are roughly half olivine and half nickel-iron. These are favorites among collectors, especially when the meteorite is sawn very thin, rendering the gem-quality, bright lime-green olivine crystals transparent.

Scientists have used pallasites to interpret the internal structure of the earth. Due to the immiscibility between olivine and the nickel-iron matrix, it is believed that pallasites formed in their parent body near a cor􀀰mantle boundary.

While not quite as photogenic as the pallasites, the mesosiderites contain nickel-iron fragments set in an achondritic matrix. They may have formed during the collision of asteroids of differing compositions.

Meteorites are exotic because they represent pieces of other worlds. They also help us to understand how the solar system began and evolved. They have taught us much about the structure of our own planet. So the next time you see a fireball light up the sky and burst into millions of colored fragments, think about how far it traveled, and where it might have been … and wonder.

A stony-iron meteorite sawn and polished, showing transparent olivine crystals (lighter colored) in nickel-iron matrix.