- Ablation
- The wearing away of material by mechanical means. For a meteoroid, the heat of friction as it passes through the Earth's atmosphere melts and removes those materials at the surface of the object.
- Achondrite
- A stony meteorite that does not have chondrules. Compositionally, they contain hypersthene, plagioclase, diopside, olivine, and nickel-iron.
- Aerolite
- One of the three main categories of meteorites more commonly referred to as Stoney meteorites. This category is made up of two subgroups: Achondrites and Chondrites.
- Albedo
- The measure of how much light is reflected off an object. The albedo of snow is between 0.8 and 0.9.
- ALH84001
- A meteorite believed to be a piece of Mars, it was found in the Allan Hills region of Antarctica in 1984. It has been subject to intense scrutiny since 1996 when researchers proposed that it contained evidence of ancient microbial life from Mars.
- Allende
- A meteorite that fell in Mexico Feb. 8, 1969. More than two tons of material from this fall were found in an area or strewn field of approximately 500 square kilometers. Allende is a Carbonaceous Chondrite, Type CV3.
- Alloy
- A substance that has the properties of a metal and is made up of two or more elements. These elements are almost always metals. For example, bronze is an alloy of copper and tin.
- Amphoterite
- A type of Ordinary Chondrites, it is poor in metal and iron, with most of the iron coming from the olivine that is present.
- Astrobleme
- Used to refer to a large, crater-like feature on Earth thought to have been formed as a result of an impact event. The word is a combination of two Greek words: astron meaning star and bleme meaning blow.
- Ataxite
- One of the structure-based groups of Iron meteorites, designated Group-D, it is the most nickel-rich of the Iron structural-groups with more than 16% nickel.
- Aubrites
- One of the subgroups of the Achondrites group of meteorites, they are calcium-poor, iron-rich and enstatite-rich.
- Augite
- The most common mineral of the pyroxene group, it is compositionally (Ca,Na)(Mg,Fe,Al)(Si,Al)2)O6. Dark-green in color, it is most frequently found in igneous rocks.
- Basalt
- The most common of volcanic (igneous) rocks, it is both dark and fine-grained. Augite and olivine are common dark minerals in basalt.
- Bronzite
- A mineral of the pyroxene group, it it is compositionally (Mg,Fe)SiO3 and can be found in igneous rocks.
- Caliche
- A hard covering of calcium carbonate frequently found on meteorites that have been weathered in desert areas. It occurs when water containing calcium carbonate comes in contact with the meteorite and later evaporates, leaving behind the hard residue.
- Carbonaceous Chondrite
- One of the groups, Group-C, of ordinary Stoney Chondrites, meteorites in this group contain organic compounds and very little or no free metal.
- Chondrite
- One of the two subgroups of Stoney(Aerolite) meteorites, they are distinguished by the presence of chondrules and are they are rich in plagioclase, pyroxene and olivine.
- Chondrules
- Small (generally 0.1 to 2 mm), spherically shaped inclusions found embedded in some meteorites (like the chocolate chips in a cookie). Meteorites having chondrules are called chondritic. The word chondrule is a derivative of the Greek word for grain. They are rich in olivine and pyroxenes, like enstatite for example.
- Differentiated
- Those processes that result in a segregation (separation) of chemicals. For example, the Earth is a differentiated body.
- Diogenites
- One of the subgroups of the Achondrites group of meteorites, they are a calcium-poor, differentiated igneous meteorite.
- Enstatite
- A rock-forming silicate mineral, it is compositionally Mg2(Si2O6).
- Eucrites
- One of the subgroups of the Achondrites group of meteorites, they are a basaltic meteorite that is differentiated, igneous, silicate- and calcium-rich with pigeonite as the primary mafic mineral. These are the most frequently occurring achondrites.
- Fall
- Indicates that the meteorite in question was found as a result of observing it as a meteor.
- Feldspar
- An important rock-forming silicate mineral, it is compositionally (K,Ca,Na)(Al,Si)1-3O8.
- Find
- Indicates that the meteorite in question was found without having observed its entry as a meteor.
- Fireball
- A meteor that is much brighter than average and is produced by a meteoroid whose size is approximately at least the same as a pea. This is in contrast to the average meteor which is produced by an object the size of a grain of sand.
- Fusion crust
- The outer covering that a meteorite acquires as a result of its passage as a meteor through the atmosphere. The friction meteoroid and the Earth's atmosphere heats the surface of the meteoroid to its melting point. Most of the melted material is ablated (removed): what remains resolidifies as the fusion crust. The thickness of these crusts can range from less than one millimeter up to several millimeters. Typically, the leading side of the meteoroid will have a thinner crust than the trailing side.
- Group
- As a part of the system by which meteorites are classified, a group identifies a number of meteorites that have similar physical and compositional characteristics. There must be at least five meteorites that share these common characteristics before they can be declared to form a Group. Groups can be further subdivided into categories called Types.
- Hematite
- A mineral that is often found in meteorites. It is an oxide of iron (Fe2O3) that is similar to magnetite. It does not attract a magnet. When it is rubbed against an object harder than itself, it leaves a reddish-brown stain. Hematite is also sometimes called bloodstone.
- Hexahedrite
- One of the structure-based groups of Iron meteorites, designated Group-H, they are 5% to 6% nickel and have crystals of kamacite.
- Howardites
- One of the subgroups of the Achondrites group of meteorites, they are a basaltic meteorite that is differentiated, igneous, silicate- and calcium-rich with pigeonite as the primary mafic mineral.
- Hypersthene
- A mineral of the pyroxene group, it it is compositionally (Mg,Fe)2Si2O6 and can be found in igneous rocks.
- Igneous
- A type of rock that is formed when magma cools. Igneous rocks can be either of volcanic or plutonic origin.
- Inclusion
- Some object, like a particle or crystal, that is embedded within another object. An analogy would be that of a chocolate chip in a chocolate chip cookie.
- Isotope
- A "flavor" of an element, each isotope of an element has the same number of protons but a different number of neutrons. This difference results in each "flavor" having somewhat different properties.
- Kamacite
- A mineral that is an alloy of nickel-iron.
- Lodranites
- One of the groups of Stoney-iron meteorite, it is composed of equal amounts of nickel-iron, olivine and pyroxene.
- Matrix
- In a geological context, it is the occurrence in a rock of small-grained materials that form a network in the spaces between larger-grained materials.
- Mesosiderites
- One of the groups of Stoney-iron meteorite, it is composed equally of nickel-iron and silicates, with the silicates being primarily hypersthene and plagioclase.
- Meteor
- The bright streak of light that appears in the sky as a result of friction between a meteoroid and the air molecules in our atmosphere. Entering the atmosphere at speeds between 10 and 70 kilometers per second, the friction-generated heat is hot enough to melt the surface layer of the object and ionize the air. The term originated with the Greeks circa 350 B.C. In its original Greek form, the word was meteora/meteoros and meant something raised up high into the air. Most of the meteors that are observed are produced by a meteoroid that is no larger than a grain of sand. The altitude at which the visible streak of light first appears is approximately 110 kilometers, +/- 20 kilometers.
- Meteorite
- An object of extraterrestrial origin that has survived passage as a meteor through the atmosphere.
- Meteoritics
- The study of meteorites.
- Meteoroid
- The term used to describe a meteorite producing object while it is still outside the Earth's atmosphere. Note that this term is reserved for those objects that can not be categorized as asteroids or comets.
- Mineral
- A naturally occurring inorganic solid. The internal crystalline structure of a mineral is controlled by its elemental composition. As an example of the way in which the elemental composition is expressed, the definition in this dictionary for "Augite" includes the term "(Ca,Na)(Mg,Fe,Al)(Si,Al)2)O6." What this means is that the first element can be either Calcium or Sodium, the second element can be either Magnesium, Iron, or Aluminum, the third element can be two atoms of either Silicon or Aluminum, and the final element is six atoms of Oxygen.
- Octahedrite
- One of the structure-based groups of Iron meteorites, designated Group-O, they are between 7% and 12% nickel. This group is subdivided based on the band-width of their kamacite lamellae.
- Olivine
- A glassy, rock-forming mineral that is olive-green in color. It is compositionally (Mg,Fe)2SiO4.
- Olivine-bronzite
- The group of Stoney Chondrite meteorites, also referred to as the H-group. Meteorites in this group contain roughly equal amounts of the minerals bronzite and olivine. Their nickel content ranges between 7 and 12 percent. Their free metal content is in the 16 to 21 percent range.
- Olivine-hypersthene
- One of the subgroups of ordinary chondrites, it is designated as Group-L. These meteorites contain olivine and hypersthene and a free metal content of 7-12%. They are richer in oxygen than their "sister" group, Group-H.
- Oxidation
- This is a form of chemical weathering that destroys meteorites, especially irons. It is a chemical reaction in which oxygen in the environment combines chemically with a substance and electrons are lost. It is this process that causes iron to rust - the oxygen combines with the metallic iron to form iron oxides.
- Pallasites
- One of the subgroups of Stoney-iron meteorites, it is a group of differentiated silicate-rich meteorites. These meteorites have a matrix of nickel-iron with embedded grains of olivine.
- Petrology
- The study of rocks and their origin.
- Piezoglypts
- See Regmaglypts.
- Plagioclase Feldspar
- A silicate mineral, it it is compositionally (Na,Ca)(Al)1-2Si2-3O8.
- Plessite
- A mineral that consists of a mixture of kamacite and taenite with a bulk nickel content in excess of 120 mg/g. In iron meteorites, it is concentrated in the gaps between the Widmänstatten lines.
- Plutonic
- An igneous rock, it forms beneath the surface, as opposed to volcanic rocks which form on the surface, as magma cools.
- Pyroxene
- One of the major groups of silicate (silicon and oxygen bearing) minerals.
- Radiant
- The point in the sky from which a meteor or meteor shower appears to originate.
- Refractory
- Elements that are refractory have a high vaporization temperature. Conversely, these will be the first elements to solidify in a cooling environment.
- Regmaglypts
- The small crater-like indentations found on the surface of a meteorite. A similar effect can be obtained by pushing ones finger into a ball of soft clay or playdough. These hollows are the result of the erosion of areas on the meteoroid as material is ablated on its passage through the atmosphere. Regmaglypts are most noticeable on iron meteorites.
- Shatter Cone
- A rock that exhibits a pattern of fractures caused by a shock wave generated by an impact event. The apex of the fracture lines tend to point towards the source direction of the shock wave.
- Siderites
- The category of meteorites commonly referred to as Irons. Classified based on their chemistry, there are today 13 different chemical groups of Irons. These groups are defined based on the relative abundance of nickel(Ni), gallium(Ga), germanium(Ge), and iridium(Ir). Irons are also categorized by their structural type. There are three structural categories: ataxite, hexahedrite and octahedrite. The bulk of the material in Iron meteorites is kamacite and taenite.
- Siderolites
- The category of meteorites commonly referred to as Stoney-Irons. The three classes of Siderolites are Lodranites, Mesosiderites and Pallasites.
- Siderophile
- Any element that prefers to combine with iron rather than some other element. Frequently referred to as "iron loving."
- Silicate
- SiO4 (Silicon and Oxygen) in the shape of a tetrahedron is the heart of this most important of the rock-forming minerals. It has been estimated that up 95 percent of the Earth's crust is made up of silicate minerals.
- Strewn Field
- This is the area, generally elliptical in shape, over which the meteorites from a particular fall are distributed.
- Taenite
- A mineral that is found in meteorites. It consists of a nickel-iron alloy with the amount of nickel present varying between 27 percent and 65 percent.
- Tektite
- Black in color, these small, mostly glass objects are of unknown origin. Found mostly in Australia, Java, Philippines and Indochina, they are thought to be formed from material ejected at the time of a crater-creating impact. The term is derived from the Greek word tektos which means melted.
- Troilite
- A mineral found in meteorites whose composition is FeS (iron sulfide).
- Tunguska
- On June 30, 1908, a large object entered the Earth's atmosphere over Siberia. The object was most likely either a stoney meteoroid or a comet. The object exploded at an altitude of approximately 8 kilometers with the force of a 20 megaton atomic bomb. The sound of the explosion was heard as far as 1000 kilometers away. The explosion, which occurred north of the Podkamennaya Tunguska River, leveled roughly 2,000 square kilometers of forest.
- Urelites
- A differentiated, igneous, calcium-poor achondrite meteorite containing mostly olivine and pigeonite. These relatively rare meteorites contain microscopic-sized diamonds.
- Widmänstatten
- Named after Count Widmänstatten of Vienna who first wrote about the cross-hatched pattern of lines he saw in the iron of meteorites after polishing and etching with acid. The visual pattern is a result of the interlocking of bands of kamacite and taenite. The nature of these bands is a result of the rate of cooling for the meteorite in question.
The Astrodigital Online Dictionary of Meteoritics document was conceived and written by Jim Plaxco.
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