Meteorites

Meteorites are basically any object from any Solar System source that reach the surface of the Earth after surviving passage through the atmosphere. They can be any size from gravel-sized pieces of rock or even large objects several metres across. Some rare meteorites are large enough to affect the climate upon impact. (Hutchison and Graham, 1994.)

Mineralogical classification

The following table lists the different classes of meteorite, based upon their mineralogy. (Source: Hutchison and Graham, 1994.)

Type / Name		Mineralogy
STONY	Chondrites	Usually between 2 and 20% Fe* and Ni*; sulphide minerals such as "troilite".  Contain chondrules1.  Occasionally contain water and organic compounds, with little Fe* or Ni*.
	Carbonaceous Chondrites	Chondrules1 are common, in a watery matrix containing C*, H*, O*, and N* organic compounds.  The matrix often contains diamond and "silicon carbide" crystals, thought to form in the atmospheres of red giant stars. Also contain some extremely rare, heat-resistant minerals based on Ca* and Al* oxides, which could represent the first minerals to condense out of a solar nebula infused with minerals from dying red giants.
	Achondrites	These formed as molten (igneous) rock solidified in chondritic bodies such as asteroids or planets. Indeed, most originate from asteroids, and a few from the Moon and Mars. Most are chemically similar to basalts, containing olivine2, but do show some chemical variation as some are carbon-rich, perhaps related to carbonaceous chondrite bodies.  They contain no chondrules1.  Some achondrites are lumps of regolith from the surface of asteroids, called "howardites". The minerals in howardites are damaged by particles streaming out from the Sun in the solar wind, so we know they formed on a body with no atmosphere.  SNC (or Shergotty Nakhla Chassigny) meteorites originate from Mars, because their young ages of between 1300 and 200 Ma3 means they formed long after other solar system bodies ceased to be active.
IRON		These consist primarily of Fe* and Ni* along with a few other minerals including olivine2. Iron meteorites usually contain around 7 to 15% Ni*, by weight.  Most originally formed in asteroids' cores, while the rest formed in association with non-metallic material further out.  Kamacite is present if an iron meteorite contains 5 to 6% Ni*, whereas it will not be present with concentrations of over 15%.  "Widmanstätten" structure occurs when Fe*-Ni* alloys form two minerals which intergrow while cooling from temperatures exceeding 700°C. One mineral contains 5% Ni* and the other around 40% Ni*.
STONY - IRON		Two main types: "pallasites" (Fe*, Ni* and olivine2) and "mesosiderites" (Fe*-Ni*, achondritic, glassy and metallic material).  Likely to originate from a large, layered asteroid.  Pallasites formed from cooling and cracking of a body early in its history, with metal migration along fractures, while mesosiderite structure suggest formation was impact-based.
Key:- * : Al = aluminium; C = carbon; Ca = calcium; Fe = iron; H = hydrogen; Mg = magnesium; N = nitrogen; Ni = nickel; O = oxygen; Si = silicon. 1: Chondrules are roughly spherical solidified droplets of melt, on the millimetre scale, often containing water and olivine2. 2: Olivine is a common mineral in Earth's mantle, as well as in meteorites. Chemical formula: (Mg,Fe)2 [SiO4]. 3: In geology, long periods of time are measured in millions of years (Ma) or billions of years (Ga).

 

A list of this month's meteor showers is kept on the Night Sky page.