METEORITE CLASSIFICATION

By MARK BOSTICK

 The moon is the only body in the universe, other then the earth, where mankind has visited and collected samples from. To learn more about our universe we have sent space probes into the far reaches of our galaxy.  But the information a space probe can tell us is limited.  To really understand the geological processes of planetary science, we need to have physical samples in our hands.  Thankfully, meteorites provide this to us.  By studying meteorites, we learn more about the universe and therefore our own planet. No matter what subject you are studying, whether it is animals, planets, or meteorites, the first step is to identify the different groups that exist among them. There are three main meteorite groups; Iron, Stony, and Stony-Iron.

 IRON METEORITES 

Iron meteorites represent about 6% of all meteorites known.   They are mostly nickel-iron with traces of other elements such as iridium, chromium and more. At one time they were one of the more common meteorites in our collections as they survive the weathering process of earth longer and were easier to find.  Now thanks to early meteorite pioneers, searches in African deserts and in Antarctica, this is no longer the case.  There are 13 recognized groups of irons, however almost 25% of the iron meteorites do not fit into any of these groups.  They are known as Iron Anomalous (IRANOM).

 ·           Ataxites (ATAX): Nickel-rich iron meteorites, with more then 12% nickel.   Because of the high nickel content, these meteorites do not show a widmanstatten pattern when etched.

 ·           Hexahedrties (HEX): Iron meteorites that when etched show a hexahedron or six-sided crystal structure. When one of these meteorites is cut, polished and acid etched it displays a series of parallel lines, known as Neumann lines.  Contains 4.5-6.5 percent nickel. 

 ·           Octahedrites (O): Iron meteorites that when etched show an octahedron or eight-sided crystal structure. Cut, polished and acid etched specimens show a series of lines, known as the Widmanstatten pattern. Contains 6-12 per cent nickel. 

STONY METEORITES

Stony Meteorites are first divided into two groups, chondrite and achondrite.  Chondrite meteorites are named for their small round chondrules found within their matrix. A feature that is unique to meteorites.  Achondrite means simply “without chondrule”.  Achondrites formed by a melting and recrystalization of a chondritic parent body.

 ACHONDRITE METEORITE FAMILY

Achondrites are separated into three basic groups, Calcium-rich, Calcium-poor and Primitive Achondrites.

Calcium-Poor Achondrites:

·           Aubrites (AUB): Calcium and iron poor meteorites consisting mostly of enstatite. A direct relationship to enstatite chondrites is possible. Named from a fall near Aubres, France.

·           Diogenites (DIO): An achondrite meteorite with an igneous origin.  A cumulate of almost pure orthopyrozene. It is made up of large crystals of iron-rich bronzite and hypersthene.  These large crystals formed out of cooling magma and accumulated at the bottom of a magma chamber.

·           Urelites (URE): Urelites show evidence of having been cooled rapidly and signs of a sudden pressure drop.  The common presence of this in all urelites, although some of them have diverse difference, shows a common parent body.  Many urelites have microscopic diamonds found in them that is associated with graphite inclusions and was likely formed with the graphite was put under tremendous pressure from an impact in space.

Calcium-Rich Achondrites:

 ·           Angrites (ANG): Named from a meteorite that fell into the Angrados Reis Bay in Brazil.  Angrites are composed primarily of pyroxene rich in calcium, aluminum and titanium.  Angrites show great variation in mineral composition.

 ·           Eucrites (EUC): The most common of all achondrites.  They are closely related to terrestrial basalt lava rocks and look a lot like them. Composed of small interlocking crystals, showing evidence of flow on its parent body.

 ·           Howardites (HOW): A brecciated achondrite meteorite made of diogenite and eucrite fragments. Very similar to Eucrites but with a higher percentage of diogenite (at least 20%).  They are polymict breccias and show damage from exposure to solar wind.

 Primitive Achondrites

 ·           Acapucolites (ACA): Acapucolites are igneous rocks composed primarily of olivine, bronzite, and plagioclase and with nickel-iron as principal minerals.  They also have accessory minerals of clinopyroxene, troilite, and schreibersite.  Lodranites show a coarse granular-granoblastic texture that is easily broken but vary in their respective grain sizes.

 ·           Brachintes: (BRA):  Named from a meteorite found near Brachina, South Australia in 1974. Brachinites are composed almost entirely of small olivine grains.  They have trace elements similar to those found in winonaites and silicate inclusions in iron meteorites.

 ·           Londranites (LO): Named from the Londran meteorite, which was the first of its type ever found.  Lodranites are basically the same as Acapucolites but are most recrystalization and therefore more metamorphosed.

 ·           Winonaites (WIN): Winonaites are thought to have formed by a meteorite impact with a parent body that had partially differentiated.  The impact mixed silicates into already molten metal to form silicated iron meteorites and the yet to be melted olivine-rich residues mixed into the unmelted silicates to form the winonaites.

 Chondrite Meteorite Family:

 Enstatite Chondrites:

  Also referred to by many as E-chondrites. Enstatite chondrites were formed in a highly reducing environment as shown by the extremely low nickel-iron content, and by the presence of the sulfide minerals, oldhamite, daubréelite, and alabandite (EL) or niningerite (EH).  Enstatites have almost no metal in the oxide form, an amount less than those found other chondrites, and even less then those found on the terrestrial planets. Metal occurs as low-nickel kamacite in both the high-iron, high-siderophile (EH) and low-iron, low-siderophile (EL) groups. The EH and EL groups are different from each other based on compositional, textural, and mineralogical differences, as well as by formation intervals and O-isotopic data, which tells that they were derived from separate, but closely related, parent bodies.

 Lunar Basalt and Breccias (LUN):

 Early meteorite curators wondered why there were no lunar meteorites, or lunites, in our collections.  Some thought that some that eucrites or other meteorites in our collections might be moon rocks. 

     When NASA moon walking astronauts brought back lunar samples, we were surprised not to find any meteorites in our collections that matched up to them.  This would soon change when a lunar meteorite was found in Antarctica by an American team during 1982.  After this meteorite was recognized, Japan was quick to recognize a meteorite they recovered during 1979 of lunar origin.  Now more then 40 moon meteorites have been found on earth, many of them are however paired.   These meteorites fall into two basic groups.   Anorthositic Regothic Breccias and Mare Basalts.

 ·           Anorthositic Regothic Breccia: Most of the lunites fall into this category.   These rocks are from the lunar highland and are composed of about 78-80% anorthic.  The anorthositic clasps are white and easy to recognize in the usually very dark matrix.

 ·           Mare Basalts:  Maria covers about 17% of the moon, for this reason, meteorites from this sample of the moon are very rare.  Mare Basalts contain feldspar in a matrix of pyroxene and chromite.  About 12% of these meteorites are yellow-white olivine.

 SNC's:

 Three types of Achondrite meteorites are grouped together to make the SNC subgroup.  S stands for Shergottite, N stands for Nakhites and C stands for Chassignites.  The origin of these meteorites was a debate for many years although many agreed a common origin.  It is now known that these meteorites came from the planet Mars

 ·           Shergottite:  The most common of all SNC’s.  Shergottites are basalt meteorites with compositions of pigeonite, augite and maskelynite.  The maskelynite is about 23% of the meteorite by volume and a result of plagioclase transformed during the impact that brought these meteorites to earth. 

 ·           Nakhite:  Nakhite’s are a cumulate meteorite composed of about 80% Augite, 5-10% olivine and other minerals.   The high augite percent gives these stones a greenish appearance.  Iddingsite can be found in veins running through the olivine.  Iddingsite usually occurs from iron-rich olivine altered by water.

 ·           Chassignite:  Chassignites are composed of almost 90% olivine.  These meteorite and very similar to the terrestrial rocks dunites and must have formed deep in a magma chamber.  Kaersuitite has been found in the olivine grains.  Kaersuitite is a water-bearing amphibole mineral and shows that the meteorite must have formed in the presence of water.

 Ordinary Chondrites:

 Around 85% of all meteorites seen to fall are in this class.  Hence they are given the nickname, ordinary chondrite.  They are not ordinary is the strictness sense of the word.  Ordinary Chondrites are further divided into different groups depending on their total iron and total metal content.

 ·           H Class: Stony meteorites containing olivine and bronzite pyroxene. 35% of observed chondrite falls are H-class.  H referring to high iron.  These meteorites contain 25-31% total iron by weight and around 15-19% iron in an uncombined metal state. Due to this high metal percentage, H-chondrites easily attract magnets.  The iron rich olivine, fayalite in H-Chondrites is between 16-20%. A stone with an olivine fayalite % at 16%, would be written, Fa16.

 ·           L Class: Stony meteorites containing olivine, hyperstene pyroxene. 45% of observed chondrite falls are L-class.  L referring to low metal.  While these meteorites do contain about 20-25% total metal, the amount of free metal is very low, around 4-10%.  The iron rich olivine, fayalite in L-Chondrites is between 23-26%. A stone with an olivine fayalite % at 24%, would be written, Fa24.

 ·           LL Class: Stony meteorites containing about 60% olivine. 8% of observed chondrite falls are LL-class. Most of the iron in the meteorites is in an oxidized form and very little free metal, usually less then 7%. The iron rich olivine, fayalite in L-Chondrites is between 27-32%. A stone with an olivine fayalite % at 31, would be written, Fa31.

 Carbonaceous Chondrites:

 A primitive class of stony meteorites that contain oxidized minerals and organic compounds.  They are similar to the pre-solar nebula by the abundance of elements. One of the better-known meteorites from this class is the Murchison meteorite.  More than 92 Amino acids have been found inside this meteorite.  Many believe these meteorites are the leftover of comets.

 ·           CI Chondrites: CI's get their name from the Ivuna meteorite fall.  These carbonaceous meteorites contain 3-5% carbon and around 20% water.

 ·           CM Chondrites: CM's et their name from the Mighei meteorite fall.  These carbonaceous meteorites contain 0.6-2.9% carbon elements and 10-13% water.  This is the most common of all carbonaceous groups.  These meteorites are all petrologic type 2 and contain high-temperature silicates, which can be seen in cut surfaces as light-colored inclusions.

 ·           CO Chondrites: CO's get their name from the Ornans meteorite fall.  These carbonaceous meteorites contain 0.2-1% carbon elements and around 1% water. These meteorites are all petrologic type 3.

 ·           CR Chondrites: CR's get their name from the Renazzo meteorite fall.  For a long time these meteorites were grouped into the CM class, but have a larger percentage of metal (about 10%) that is clearly visible in slices. This group was recognized because of discoveries in Antarctica.  CR Chondrites have hydrated minerals formed by hydrothermal alteration.   About 50% of the meteorite has large chondrules and metal can be found in and around many of them.  This is the best looking of the carbonaceous meteorites.

 ·           CK Chondrites: CK’s get their name from the Karoonda meteorite fall.  They are closely related to CV and CO chondrites.  CK chondrites have elemental abundances between those found in the CV and CO groups, as well as very similar O-isotope plots.  Also, when compared with CV’s and CO’s, CK's have a lower chondrule percent in its matrix (10-15%) with sizes between those found in CO and CV Chondrites.  This group shows high oxidations, which have resulted in a very low content of nickel-iron and a corresponding high content of magnetite and sulfides.

 Rumurutite Chondrites:

 When compared to other chondrite groups, Rumuruitites have a higher volume of olivine, a lower volume of pyroxene, and almost no nickel-iron.  They share similarities with ordinary chondrites including refractory element depletions and siderophile element abundances; they show different volatile element abundances and petrologic trends.  The difference in O-isotopic abundances between ordinary chondrites and rumurutite chondrites is greater than it is among the L, LL or H chondrites, further separating the R chondrites from other groups.

 Stony Iron Meteorites:

 Stony-Iron meteorites are a transitional type of meteorite between stony and iron meteorites.  The portions of metallic and non-metallic constituents vary but are close to 50/50.  There are two types of Stony-Irons; Mesosiderites and Pallasites.

 ·           Mesosiderites (MES): Mesosiderite meteorites are a combination of nickel-alloy and silicate minerals.  The metal is completely mixed into the meteorite.  Often different achondrite meteorite parts are found throughout the stones.  The achondrite fragments, usually diogenite and eucrite have no apparent association with the metal.  These meteorites were troublesome to those studying meteorites for many years. The origin of these stones now accepted is that an iron-nickel meteorite impacted an asteroid and mixed itself together with the surface material.

 ·           Pallasites (PAL): Named after Simon Pallas (1741-1811) a Russian explorer who found the first pallasite in 1772. Pallasites are the most common stony-iron, and arguable the most beautiful.  They consist of olivine crystals surrounded with a nickel-iron matrix that etches into the octahedrites. They most likely formed at the parent body's core-mantle boundary.