Stony Meteorites

Of all the rocks that fall from the sky, stony meteorites are by far the most common, making up 85-90% of all meteorites. They are less commonly found than these numbers would indicate, however, as they more closely resemble terrestrial rocks than do the iron meteorites. They are not as heavy, as they contain much less nickel-iron alloy (between 3 and 30%, as opposed to 98% for the irons). What nickel-iron they contain is usually in unoxidized grains of the alloy. They do usually have shiny, dark-brown to black fusion crusts, if exposure to the elements hasn't worn this away, but a fusion crust can easily be confused with the weathering rind which forms on some terrestrial rocks such as basalt.

A good test for almost any meteorite is to see if a magnet hanging on a string will swing toward it. Even the low-iron stony meteorites should show a response.

The primary minerals found in stony meteorites are olivines and pyroxenes, with the nickel-iron alloy coming in third. Common accessory minerals are troilite (an iron sulfide), magnetite, and plagioclase feldspar.



Chondrites

The vast majority of stony meteorites are known as chondrites from the small spherical occlusions (chondrules) in them. They were formed at approximately the same time as our sun. Chondrules are not found in terrestrial rocks because of the intense heat which occurred in the creation of the planet. The composition of the chondrules is similar to the surrounding matrix, except that it is in crystalline form. Chondrites are classified as ordinary (85% of the category), carbonaneous, E-chondrites, and R-chondrites.

Carbonaceous chondrites generally contain water-bearing minerals, and are so fragile that specimens in good condition are rare – particularly the CI subgroup. These meteorites originally formed in environments rich in oxygen. The Tagish Lake meteorite which was recovered in early 2000 appears to be a form of CI carbonaceous chondrite.

The E-chondrites contain a large percentage of a form of pyroxene that has no iron, called enstatite. They are less than 2 percent of known chondrites.

R-chondrites, or Rumurutites, are the newest grouping, named from a specimen collected from Rumuruti, Kenya, which is used as the benchmark. These chondrites contain almost no nickel-iron alloy, though iron is present in the olivine and sulfides. They are extremely rare.


Achondrites

The other kind of stony meteorites are achondrites, called so because they (usually) contain no chondrules. They form perhaps 7% of all stony meteorites. Like stony-iron and iron meteorites – and unlike the chondrites -- they were molten at some point in their development. As O. Richard Norton puts it in Rocks from Space, "In essence, achondrites are the igneous rocks of other worlds." (p.203)

There are a variety of achondrites, none of them with very many members. The eucrites (calcium-rich basaltic composition), the diogenites (calcium-poor basaltic composition), and the howardites (a brecciated mixture of eucrites and diogenites) appear to be fragments of Vesta, the third largest and most easily seen of the asteroids.

Aubrites are both calcium- and iron-poor, and appear related to the E-chondrites. These are unusual in having a grey-white to light tan crust when fresh. They may come from the Hungaria family of asteroids.

Ureilites contain carbon as the carbonaceous chondrites do. However, a significant portion of their carbon is in the form of microscopic diamonds, indicating that the parent body suffered an intense collision.

The SNC subgroup of achondrites (shergottites, nakhlites, and chassignites) are believed to be from Mars. They are the only achondrites that contain water. Evidence of microbial life has been found in a specimen of this subgroup, though whether this is the result of contamination has yet to be settled.

Lunaites are small meteorites which recent research has demonstrated to have originated on the moon.


Learn More about Stony Meteorites

New Mexico boasts a fine research facility, the Institute of Meteoritics, at UNM's main campus in Albuquerque. The Institute maintains a small museum in Northrop Hall, which is open 9-4, Monday through Friday; admission is free. You can also visit the Institute's website. Of special interest is their page on identifying meteorites.

Another good resource on meteorites is NASA Spacelink.

Mama's Minerals currently has a good collection of small, reasonably priced high-iron chondrites from NW Africa. We also carry books about meteorites.

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