A chondrite (, ) is a stony (non-metallic) meteorite that has not been modified by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form primitive asteroids. Some such bodies that are captured in the planet's gravity well become the most common type of meteorite by arriving on a trajectory toward the planet's surface. Estimates for their contribution to the total meteorite population vary between 85.7% and 86.2%.
Their study provides important clues for understanding the origin and age of the Solar System, the synthesis of organic compounds, and the origin of life and the presence of water on Earth. One of their characteristics is the presence of chondrules (from the Ancient Greek χόνδρος chondros, grain), which are round grains formed in space as molten or partially molten droplets of distinct minerals. Chondrules typically constitute between 20% and 80% of a chondrite by volume.
Chondrites can be distinguished from iron meteorites by their low iron and nickel content. Non-metallic meteorites that lack chondrules are achondrites, which are believed to have formed more recently than chondrites. There are currently over 27,000 chondrites in the world's collections. The largest individual stone ever recovered, weighing 1770 kg, was part of the Jilin meteorite shower of 1976. Chondrite falls range from single stones to extraordinary showers consisting of thousands of individual stones. An instance of the latter occurred in the Holbrook fall of 1912, in which an estimated 14,000 stones were grounded in northern Arizona.
Origin and history
Chondrites were formed by the accretion of particles of dust and grit present in the primitive Solar System which gave rise to asteroids over 4.54 billion years ago. These asteroid parent bodies of chondrites are (or were) small to medium-sized asteroids that were never part of any body large enough to undergo melting and planetary differentiation. Dating using 206Pb/204Pb gives an estimated age of 4,566.6 ± 1.0 Ma, matching ages for other chronometers. Another indication of their age is the fact that the abundance of non-volatile elements in chondrites is similar to that found in the atmosphere of the Sun and other stars in the Milky Way galaxy.
Although chondritic asteroids never became hot enough to melt based upon internal temperatures, many of them reached high enough temperatures that they experienced significant thermal metamorphism in their interiors. The source of the heat was most likely energy coming from the decay of short-lived radioisotopes (half-lives less than a few million years) that were present in the newly formed Solar System, especially 26Al and 60Fe, although heating may have been caused by impacts onto the asteroids as well. Many chondritic asteroids also contained significant amounts of water, possibly due to the accretion of ice along with rocky material.
As a result, many chondrites contain hydrous minerals, such as clays, that formed when the water interacted with the rock on the asteroid in a process known as aqueous alteration. In addition, all chondritic asteroids were affected by impact and shock processes due to collisions with other asteroids. These events caused a variety of effects, ranging from simple compaction to brecciation, veining, localized melting, and formation of high-pressure minerals. The net result of these secondary thermal, aqueous, and shock processes is that only a few known chondrites preserve in pristine form the original dust, chondrules, and inclusions from which they formed.
Characteristics
Prominent among the components present in chondrites are the enigmatic chondrules, millimetre-sized spherical objects that originated as freely floating, molten or partially molten droplets in space; most chondrules are rich in the silicate minerals olivine and pyroxene.
Chondrites also contain refractory inclusions (including Ca–Al inclusions), which are among the oldest objects to form in the Solar System, particles rich in metallic Fe-Ni and sulfides, and isolated grains of silicate minerals. The remainder of chondrites consists of fine-grained (micrometre-sized or smaller) dust, which may either be present as the matrix of the rock or may form rims or mantles around individual chondrules and refractory inclusions. Embedded in this dust are presolar grains, which predate the formation of the Solar System and originated elsewhere in the galaxy. The chondrules have distinct texture, composition and mineralogy, and their origin continues to be the object of some debate. The scientific community generally accepts that these spheres were formed by the action of a shock wave that passed through the Solar System, although there is little agreement as to the cause of this shock wave.
An article published in 2005 proposed that the gravitational instability of the gaseous disk that formed Jupiter generated a shock wave with a velocity of more than 10 km/s, which resulted in the formation of the chondrules.
Chondrite classification
Chondrites are divided into about 15 distinct groups (see Meteorites classification) on the basis of their mineralogy, bulk chemical composition, and oxygen isotope compositions (see below). The various chondrite groups likely originated on separate asteroids or groups of related asteroids. Each chondrite group has a distinctive mixture of chondrules, refractory inclusions, matrix (dust), and other components and a characteristic grain size. Other ways of classifying chondrites include weathering and shock.
Chondrites can also be categorized according to their petrologic type, which is the degree to which they were thermally metamorphosed or aqueously altered (they are assigned a number between 1 and 7). The chondrules in a chondrite that is assigned a "3" have not been altered. Larger numbers indicate an increase in thermal metamorphosis up to a maximum of 7, where the chondrules have been destroyed. Numbers lower than 3 are given to chondrites whose chondrules have been changed by the presence of water, down to 1, where the chondrules have been obliterated by this alteration.
A synthesis of the various classification schemes is provided in the table below.
Type Subtype Distinguishing features/Chondrule character Letter designation Enstatite chondrites Abundant E3, EH3, EL3 Distinct E4, EH4, EL4 Less distinct E5, EH5, EL5 Indistinct E6, EH6, EL6 Melted E7, EH7, EL7 Ordinary chondrites H Abundant H3–H3,9 Distinct H4 Less distinct H5 Indistinct H6 Melted H7 L Abundant L3–L3,9 Distinct L4 Less distinct L5 Indistinct L6 Melted L7 LL Abundant LL3–LL3,9 Distinct LL4 Less distinct LL5 Indistinct LL6 Melted LL7 Carbonaceous chondritesIvuna Phyllosilicates, Magnetite CI Mighei Phyllosilicates, Olivine CM1–CM2 Vigarano Olivines rich in Fe, Ca minerals and Al CV2–CV3.3 Renazzo Phyllosilicates, Olivine, Pyroxene, metals CR Ornans Olivine, Pyroxene, metals, Ca minerals and Al CO3–CO3.7 Karoonda Olivine, Ca minerals and Al CK Bencubbin Pyroxene, metals CB Loongana Chondrules and CAIs, metals CL High Iron Pyroxene, metals, Olivine CH Tagish Lake Phyllosilicates, Magnetite, Ca-Mg-Fe carbonates TAG Kakangari-type K Rumurutiites Olivine, Pyroxenes, Plagioclase, Sulfides R
Enstatite chondrites
Enstatite chondrites (also known as E-type chondrites) are a rare form of meteorite thought to comprise only about 2% of the chondrites that fall to Earth.Norton, O.R. and Chitwood, L.A. Field Guide to Meteors and Meteorites, Springer-Verlag, London 2008 Only about 200 E-Type chondrites are currently known. The majority of enstatite chondrites have either been recovered in Antarctica or have been collected by the American National Weather Association. They tend to be high in the mineral enstatite (MgSiO3), from which they derive their name.
E-type chondrites are among the most chemically reduced rocks known, with most of their iron taking the form of metal or sulfide rather than as an oxide. This suggests that they were formed in an area that lacked oxygen, probably within the orbit of Mercury.
Ordinary chondrites
Ordinary chondrites are by far the most common type of meteorite to fall to Earth: about 80% of all meteorites and over 90% of chondrites are ordinary chondrites. They contain abundant chondrules, sparse matrix (10–15% of the rock), few refractory inclusions, and variable amounts of Fe–Ni metal and troilite (FeS). Their chondrules are generally in the range of 0.5 to 1 mm in diameter. Ordinary chondrites are distinguished chemically by their depletions in refractory lithophile elements, such as Ca, Al, Ti, and rare earths, relative to Si, and isotopically by their unusually high 17O/16O ratios relative to 18O/16O compared to Earth rocks.
Most, but not all, ordinary chondrites have experienced significant degrees of metamorphism, having reached temperatures well above 500 °C on the parent asteroids. They are divided into three groups, which have different amounts of metal and different amounts of total iron: H chondrite have high total iron and high metallic Fe (15–20% Fe–Ni metal by mass), and smaller chondrules than L and LL chondrites. They are formed of bronzite, olivine, pyroxene, plagioclase, metals and sulfides and ~42% of ordinary chondrite falls belong to this group (see Meteorite fall statistics). L chondrites have low total iron contents (including 7–11% Fe–Ni metal by mass). ~46% of ordinary chondrite falls belong to this group, which makes them the most common type of meteorite to fall on Earth. LL chondrites have low total iron and low metal contents (3–5% Fe–Ni metal by mass of which 2% is metallic Fe and they also contain bronzite, oligoclase and olivine). Only 1 in 10 ordinary chondrite falls belong to this group.
An example of this group is the NWA 869 meteorite.
Carbonaceous chondrites
Carbonaceous chondrites (also known as C-type chondrites) make up less than 5% of the chondrites that fall on Earth. They are characterized by the presence of carbon compounds, including amino acids. They are thought to have been formed the farthest from the sun of any of the chondrites as they have the highest proportion of volatile compounds. Another of their main characteristics is the presence of water or of minerals that have been altered by the presence of water.
There are many groups of carbonaceous chondrites, but most of them are distinguished chemically by enrichments in refractory lithophile elements relative to Si and isotopically by unusually low ratios of 17O/16O relative to 18O/16O, when compared to Earth rocks. All groups of carbonaceous chondrites except the CH group are named for a characteristic type specimen: CI (Ivuna type) chondrites entirely lack chondrules and refractory inclusions; they are composed almost exclusively of fine-grained material that has experienced a high degree of aqueous alteration on the parent asteroid. CI chondrites are highly oxidized, brecciated rocks, containing abundant magnetite and sulfate minerals, and lacking metallic Fe. It is a matter of some controversy whether they once had chondrules and refractory inclusions that were later destroyed during formation of hydrous minerals, or they never had chondrules in the first place. CI chondrites are notable because their chemical compositions closely resemble that of the solar photosphere, neglecting the hydrogen and helium. Thus, they have the most "primitive" compositions of any meteorites and are often used as a standard for assessing the degree of chemical fractionation experienced by materials formed throughout the Solar System. CO (Ornans type) and CM (Mighei type) chondrites are two related groups that contain very small chondrules, mostly 0.1 to 0.3 mm in diameter; refractory inclusions are quite abundant and have similar sizes to chondrules.