Our sun has many rocks circling around it. The biggest rocks form some of the planets, while rocks that are too small to be called planets are called "asteroids." Smaller chunks of rock, broken off from asteroids, comets, planets or planetary moons, are called "meteoroids."
When a meteoroid's orbit brings it close to a planet it may be captured in the planet's gravitational pull. As it makes its brief, fiery descent toward the planet’s surface it is called a "meteor."
Most meteors break apart and burn up in the atmosphere. But occasionally, large pieces of a meteor make it all the way to the ground. The bits and pieces of meteor debris that reach a planet's surface are called "meteorites."
There are three main types of meteorites: iron meteorites, stony meteorites and stony-iron meteorites. Meteorites are made of the same material as the asteroids or planetary bodies from which the original meteoroid broke off.
The majority of meteorites that fall to Earth are stony. Stony meteorites are made primarily of silicate minerals.
Silicate minerals are composed of a salt containing a silicon and oxygen (Si-O) anion, combined with wide range of cations. According to the International Meteorite Collectors Association, the most common silicates found in meteorites are olivine and pyroxene. Olivines are magnesium iron silicates. Pyroxenes are silicates with calcium, sodium, iron or magnesium cations; or more rarely zinc, manganese or lithium cations.
As such, stone meteorites are composed primarily of the elements oxygen and silicon with smaller quantities of magnesium, iron, nickel and other metallic elements.
Iron meteorites are composed of two minerals: nickel-poor kamacite and nickel-rich taenite. These two metal alloys occur in varying concentrations from one meteorite to the next.
Iron meteorites can be separated into three categories based on the chemical composition and structure of the nickel-iron alloy. All iron meteorites are mostly iron. Hexahedrites, made entirely of kamacite, contain less than 6 percent nickel. Octahedrites, the most common type of iron meteorites, are 7 to 10 percent nickel and contain both kamacite and taenite. Ataxites, the rarest group of iron meteorites, are pure taenite and may be more than 16 percent nickel.
Stony-iron meteorites have roughly equal amounts of nickel-iron alloy and stone. Stony-iron meteorites are divided into two groups: pallasites and mesosiderites.
Pallasites are characterized by beautiful olivine crystals suspended in a nickel-iron matrix. Mesosiderites contain equal amounts of nickel-iron alloys and silicate stone. The stony portion of mesosiderites consists mainly of an igneous rock called basalt.
The most common elements in stony-iron meteorites are iron, silicon and oxygen, with varying amounts of magnesium, nickel and other metallic elements.
