Meteorite classifications and compositions

Most meteorites fall into one of four categories. The first three categories apparently have their origins in parent bodies that were gravitationally differentiated, as opposed to the fourth category.

• "Iron meteorites", also called "irons", are usually just one big blob of iron-nickel (Fe-Ni) metal, as if it came from a industrial refinery without shaping. The alloy ranges from 5% to 62% nickel from meteorite to meteorite, with an average of 10% nickel. Cobalt averages about 0.5%, and other metals such as the platinum group metals, gallium, and germanium are dissolved in the Fe-Ni metal. (Fe is the chemical symbol for iron.) While most "irons" are pure or nearly pure metal, the technical definition of an "iron" includes metal meteorites with up to 30% mineral inclusions such as sulfides, metal oxides and silicates. The irons represent the cores of former planetoids.
• "Stony irons" consist of mixtures of Fe-Ni metal of between 30% and 70% along with mixtures of various silicates and other minerals. The Fe-Ni metal can be present as chunks, pebbles and granules. Stony irons resemble the outer cores or mantles of planetoids or else a mix of materials due to a collision.
• "Achondrites" are silicate rich meteorites apparently formed by crustal igneous (i.e., molten or volcanic) activity in their parent bodies, and consist of a broad range of minerals. Achondrites are the result of gravitational differentiation in relatively large bodies by melting and gravitational separation of mineral phases, and most resemble the Earth's crust. Different types of achondrites average between 0 and 4% free Fe-Ni granules.
• "Chondrites" probably came from parent bodies that were too small to undergo a large degree of gravitational differentiation, or are collision ejecta from less than catastrophic collisions of slightly differentiated bodies. Chondrites are named after the tiny pellets of rock called "chondrules" embedded in them, a result of a kind of chemical fractionation unique to small bodies. If you were walking around in a field and saw a chondrite, it would be much more recognizable as being of nonterrestrial origin than the above achondrites.

Chondrites generally show much cooler histories than other meteorites. Some chondrites appear to be from noncollisional origins, e.g., a small archaic accretion.

Chondrites are crumbly, composed primarily of various silicates, with an Fe-Ni free metal content between 0.3 and 35%. Chondrites are often classified according to their free metal content:

• "Enstatite" (E) chondrites are around 35% free Fe-Ni granules.
• "High iron" (H) chondrites average about 19% Fe-Ni.
• "Low-iron" (L) chondrites average 9% Fe-Ni.
• "Low iron, low metal" (LL) and "high iron, low metal" (HL) chondrites are a technical scale that reflects different abundances of free metal versus metal oxides, in the neighborhood of 5% Fe-Ni granules plus about 15% to 30% iron oxide in minerals (e.g., magnetite, silicates), due to the level of oxygen depletion in the silicate mix.

The nonmetal ingredients of meteorites consist predominantly of silicates, oxides and sulfur minerals, which can be typically broken down as follows: silica (SiO₂) typically between 35% and 40%, magnesia (MgO) between a whopping 20% to 25% (in contrast to Earth's surface), aluminium (Al₂O₃) between only 2% and 3% (in contrast to Earth and the Moon's crusts), and calcia (CaO) around 2%. Iron sulfide (FeS), also called "troilite" (and "fool's gold"), usually occurs as around 6% of these meteorites.

As regards the precious (and "strategic") metals such as the platinum group, cobalt, gold, gallium, germanium, and others, the lower the Fe-Ni metal content, the more enriched the Fe-Ni metal is in these rare and precious metals and elements. These elements readily dissolve into the metal that exists, and the less metal that exists, the less diluted they are. Many asteroids are richer in most of these precious metals than the richest Earth ores which we mine. Further, these metals all occur in one ore when it comes to asteroids, not in separate ores. As discussed later in this chapter, the exact same process used to extract and separate these precious metals from the world's largest nickel ore mine at the Sudbury Astrobleme in Canada is easily used in space, and is a simple process using only carbon, sulfur and oxygen, all of which can be derived from asteroids, too.

Some chondrites are poor in volatiles, while others are rich in volatiles, such as water and carbon.

There are subcategories of chondrites, called "carbonaceous chondrites", which are further split up into five categories:

• "C1 carbonaceous chondrites" average about 10% water in a clay mineral matrix and as water of hydration (often in magnesium salts, 5% to 15%), 2% to 5% carbon in the form of graphite, hydrocarbons and organic compounds, several percent sulfur in elemental, iron sulfide and water soluble sulfate forms, some nitrogen and other volatiles, and 5% to 15% magnetite.
• "C2 carbonaceous chondrites" have very little magnetite, a little less water, carbon, and sulfur, and about 10% soluble sodium and magnesium salts, all in a mineral assemblage.
• "C3, C4 and C5 carbonaceous chondrites" are not really "carbonaceous" as their name implies but instead are very poor in water, carbon and other volatiles, but have other semblances to C1 and C2 carbonaceous chondrites.

It should be noted that there are meteorites that defy categorization in that they are significantly different from meteorites in any of the above classes. Some potentially new classes of meteorite are represented by only one specimen.

Two classes of unusual meteorites which have more than one unrelated specimen are worth noting:

• "Carbonados" have tiny black diamonds produced by the shock of astronomical impact on a carbon rich body.
• "Ureilite" achondrites typically contain about one percent industrial grade diamonds.

Likewise, there are sometimes major compositional differences within established classes of meteorites, and frequent overlap between classes in terms of the mineral distributions. For example, some achondrites have a few percent carbon. Some chondrites have veins of water soluble salts as if deposited by movement of water as in hydrothermal processes that occured on Earth. Enstatite meteorites (above, listed as a class) often contain large amounts of exotic and strange substances, such as titanium nitride, silicon oxynitride, and unusual metal sulfides.

Some readers from the mining industry are probably asking what mineral distributions exist in meteorites. Below are the approximate properties of four different typical asteroids which probably exist, based on four meteorites. (Chemical analysis in weight percent. Extracted from NASA SP-428, except for the iron meteorite.)