Since meteorites formed through a variety of processes on many different planetary bodies, they can have substantially different physical and chemical properties. Some meteorites, particularly primitive chondrites, are quite unlike any other type of rock found on Earth and can be readily identified. However, other meteorites, particularly achondrites, were produced by the same types of igneous processes that occur on Earth and may be very difficult to recognize. To illustrate the variations in their properties, brief descriptions of the different types of meteorites are given in this section.
These types of meteorites generally have a dark gray or black fusion crust and a lighter gray interior. Three basic structural components may be visible on broken surfaces. Perhaps the most prominent of these are chondrules. On a broken surface, parts of these millimeter-sized globular bodies may stick out and look like tiny half-buried eggs. Chondrules are immersed in the second component of primitive chondrites, which is a fine-grained, often soft, porous, and gray material, like spongy graphite, known as matrix. Unequilibrated ordinary chondrites are dominated by chondrules (up to 80 volume percent), while carbonaceous and enstatite chondrites contain far fewer chondrules (up to 30 volume percent) and in some cases consist entirely of matrix material. Both chondrules and matrix material are dominated by the minerals olivine and pyroxene (or their alteration products). Because these minerals have densities similar to those of most minerals in the Earth's crust, primitive chondrites will not feel unusually heavy for their size. However, they may contain small dispersed grains of metal which will appear as shiny specks on a ground or broken surface. These metal grains are particularly obvious in some unequilibrated ordinary chondrites.
The third component of primitive chondrites are refractory inclusions. Some of these objects are spherical, like chondrules, but typically they lack any well-defined shape and are thus called amoeboid. Refractory inclusions contain lighter colored minerals, like feldspar (which is also a major mineral in light-colored terrestrial granites), so they often look like white patches embedded in the gray matrix. The abundance of refractory inclusions in primitive chondrites varies; they are almost completely absent in unequilibrated ordinary and enstatite chondrites, but they may comprise as much as 15 volume percent of carbonaceous chondrites.
Most equilibrated chondrites are related to primitive ordinary chondrites; only a few are related to primitive carbonaceous or enstatite chondrites. Although primitive ordinary chondrites are usually gray, once they have been metamorphosed to an equilibrated state they may appear off-white, and are sometimes lightly tinted orange or yellow. Alternatively, if they have been shocked by impact processes on the surface of an asteroid, then they may be quite dark. The fusion crust, if not fresh, is often rusty orange. The amount of metal in these samples varies, and in some very weathered samples, may be completely gone. Such samples may resemble terrestrial sandstones. When fresh, however, shiny metal can be seen scattered throughout the rock, and in some cases, concentrated in veins.
Although these types of meteorites contain metal, they are dominated by olivine, pyroxene, and feldspar. Thus, their density is still comparable to that of many terrestrial rocks. The most important features distinguishing these meteorites from terrestrial rocks is their fusion crust and the presence of iron metal alloys.
In contrast to primitive chondrites and equilibrated chondrites, iron meteorites are very dense and nonporous specimens, and are thus much heavier than most comparably-sized rocks found in the Earth's crust. Iron meteorites also have metallic silver-colored interiors. These meteorites are well-known because the iron metal often crystallized in crisscrossing plates, known as a Widmanstätten pattern after the name of an Austrian count who was one of the first to describe them. However, this pattern is not commonly noticed in samples unless they have been chemically etched in a laboratory.
The fusion crust on these objects is usually a very thin brown coating. Often people mistake terrestrial magnetite for iron meteorites, because it is also heavy compared to most other terrestrial rocks and has a black to purplish-brown surface. However, samples of terrestrial magnetite have black to purplish-brown interiors, in contrast to the silver-colored interiors of iron meteorites. Iron meteorites also often have scalloped or fluted surfaces (like thumbprints pressed into clay) which are produced by ablation (severe frictional heating of the surface, but not interior, of the meteoroids) when they penetrated the Earth's atmosphere.
These types of meteorites are mixtures of metal and silicate material, which weather at different rates, and thus the surface of these meteorites may change with age. When fresh, they often have a smooth brown to black fusion crust similar to that around iron meteorites or achondrites. However, the surfaces of older falls may be quite knobby because of differential weathering and mottled with rusty orange and yellow colors. Because of the large amount of metal in pallasites, they are heavier than most similarly-sized terrestrial rocks.
Sawn surfaces of pallasites are readily identifiable, because of their complex network of green, yellow, or brown crystalline pods of olivine surrounded by a bright silver-colored iron-metal matrix.
Achondrites, including SNC's, are the most difficult specimens to differentiate from terrestrial rocks because they formed on bodies where the same processes that operate on Earth occurred. Consequently, their mineral assemblages, densities, and textures are similar to those of terrestrial rocks. An intact fusion crust is the best criteria for identifying candidates for future studies.
Some achondrites are breccias and thus their interiors may consist of a mixture of light and dark angular clasts. A related group of achondrites, called mesosiderites, are brecciated and metamorphosed. This latter group of meteorites are often lumped with pallasites as stony-iron meteorites, because the metal in them coagulated into large silvery blebs in an otherwise gray to brown silicate interior.
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