Glossary Of Meteorite Terms

achondrite

Without chondrules; the chondrules originally present have been destroyed by melting.

ataxite

High nickel content iron meteorite. Composed nearly entirely of Taenite with microscopic Kamacite "spindles."

breccia

Consisting of angular (sharp, broken) rock fragments.

chondrite

Chondrites are stony meteorites that have never been melted. A diagnostic feature of chondrites is the presence of chondrules. (See below) About 85% of meteorites recovered are chondrites.

chondrule

Chondrules are millimeter sized silicate mineral spheres embedded in a fine grain matrix which includes cosmic dust. Chondrules are believed to be the oldest solid material in the solar system. Chondrules were formed by rapid heating and melting of precursor material then cooling in the zero gravity of space. The source of the heating is unknown.

Diogenite

Diogenites are achondritic stony meteorites that originate from deep within the crust of the asteroid 4 Vesta. Diogenites are composed of igneous rocks of plutonic origin, having solidified slowly enough deep within Vesta's crust to form crystals which are larger than in the eucrites. These crystals are primarily magnesium-rich orthopyroxene, with small amounts of plagioclase and olivine.

Enstatite/Hypersthene

A solid solution of magnesium and iron ranging from Enstatite with up to 15 % iron and Hypersthene with greater iron content. Formed from a melt at temperatures slightly under 1100° C, at higher temperatures pyroxene is produced.

Eucrite

Eucrites are achondritic stony meteorites that originate from the surface of the asteroid 4 Vesta. Eucrites consist of basaltic rock from the crust of 4 Vesta. They are mostly composed of Ca-poor pyroxene, pigeonite, and Ca-rich plagioclase (anorthite).

Fayalite ratio

Meteorite classification frequently will include the ratio of Fayalite to Forsterite in the olivine present. The ratio is expressed as Fa25 which means that the olivine contains 25% Fayalite and 75% Forsterite.

hexahedrite

Low nickel content iron meteorite. Composed nearly entirely of Kamacite usually as a single crystal.

lithology

The gross physical character of a rock or rock formation.

magma

Molten rock located beneath the surface of a rocky planet.

mesosiderite

A brecciated meteorite containing unequal silicate and metallic components; the silicates are mostly igneous rock fragments which include eucritic and diogenitic components.

metamorphic

Rocks which have undergone some chemical or physical change subsequent to their original formation.

octahedrite

Octahedrites are composed of inter-grown Taenite and Kamacite crystals. The structure depending on the amount of nickel available as the molten mass cooled below 1200° C. The growth is octahedral and this crystal attribute provides the name for this broad physical group of nickel-iron alloy meteorites. The crystals may be expressed as geometric structures when a polished surface is etched with any of several agents commonly used to differentially dissolve the Taenite and Kamacite crystals present. These etched patterns are called "Widmanstätten bands," named for their discoverer.

olivine (Chrysolite)

A solid solution of Forsterite (magnesium silicate) and Fayalite (iron silicate). Chrysolite has a vitreous luster with an olive to brown color. Formed by crystallization of magmas low in silica and rich in magnesia. Chrysolite commonly alters through the oxidation of the iron becoming reddish brown. The process ends with the crystal replaced with the oxides of iron, Limonite and Hematite.

plagioclase

A feldspar group which can range from pure sodium aluminum silicate (Albite), to pure calcium aluminum silicate (Anorthite.) Characterized by a general lack of color. Common in terrestrial rocks.

plessitic

A fine-grain mixture of Kamacite and Taenite.

pyroxene

Chiefly a calcium, magnesium meta-silicate which may contain more, or less, iron. This is a common mineral in terrestrial rocks, forming from a melt at about 1500° C.

regolith

As impacts large and small break up the surface of bodies with no atmosphere they excavate overlapping craters. The surface is pulverized and stirred as if ploughed repeatedly. The resulting fractured mixture is called "regolith." The soil found by the Apollo astronauts on the moon is an example of a regolith. Some meteorite breccias consist of regolith compacted on the surface of a small planetary body.

regmaglypt

Any small, well-defined indentation or pit on the surface of a meteorite. Regmaglypts are caused by the ablation of certain minerals as the meteorite passes through Earth's atmosphere.

xenolithic

Composed of mixed lithologies. That is two or more rock types mixed.