Meteorites are pieces of rock from space which have survived the perilous trip through the Earth’s atmosphere. They are fragments of various space bodies, such as asteroids, which have been knocked out of orbit, or which have acquired an orbit which crosses that of the Earth. Multitudes of meteorites land on Earth daily. Most are too small to be seen or noticed.
The most recognizable meteorites are those composed primarily of nickel-iron. They are extraordinarily heavy and often have fantastically sculpted shapes. These pieces of extraterrestrial metal often lay on the ground rusting for years before they are recognized as meteorites. These are the most common ‘finds’, but scientists have found that ‘irons’ are not the most common type of meteorite to fall.
Chondrites, a class of “stony” meteorite, fall to earth much more commonly than “irons”, but unfortunately, these stony masses look much like other rocks found on Earth, and are much more difficult to recognize than irons. Chondrites are distinguished from most earthbound rocks by a component of their internal structure called chondrules. Chondrules are spherical grains composed of silicate minerals, usually suspended in a ground mass of other silicates with a minor free ‘iron’ content. The rare carbonaceous chondrites sometimes contain amino acids, the building blocks of life, and have estimated formation ages which predate the Earth.
Some stony meteorites don’t even give us identification clues like the chondrites. The achondrite group includes many varieties that are very similar to the igneous rocks which comprise much of the Earth’s crust. These meteorites must be found soon after their fall, while the shiny fusion crust on the outside has not yet been weathered. After they lay too long on the ground, they look very much like ordinary rocks.
The most spectacular meteorites are the pallasites. These are coarsely crystalline aggregates of nickel-iron with contained olivine crystals. The yellow-green olivine set against a ground mass of metal makes these rarities easily recognized, even decades after their fall to Earth.