Stones represent the third main group of meteorites,
by far the most diverse group of the three. Nine out of ten meteorites
that fall from the sky are stone meteorites. Fewer are found, however,
primarily because they resemble Earth rocks, and are therefore harder to
recognize. Stone meteorites also decompose more rapidly under terrestrial
conditions than irons.
Stone meteorites are divided into two main groups; those with
the chondrites and those without, the achondrites. Chondrules, from the Greek word for "seed", are the inclusions for which
the two types are named. These round spheres of silicates range in size
from the microscopic, to the size of marbles. Most, however, are the size
of steel shot pellets. These chondrules are time-travelers, dating from
the very beginnings of the solar system, some 4.6 billion years ago.
Formed as clouds of pre-solar dust began to condense, these tiny
balls were packed into larger and larger masses, eventually forming small
moons and asteroids. Some of the "parent bodies" were destroyed before
they accumulated enough matter to cause re-melting of their core material.
Bits of this material usually contains chondrites in their earliest form.
Other parent bodies continued to accrete material until the mounting
high-pressures within destroyed all evidence of the original chondritic
structure. Bits of this material represent the achondrites, or those without