A chondrule (from

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

χόνδρος ''chondros'', grain) is a round grain found in a

chondrite A chondrite is a stony (non-metallic) meteorite that has not been modified by either melting or planetary differentiation, differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar Syste ...

. Chondrules form as

molten Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which incre ...

or partially molten droplets in space before being accreted to their parent

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

s. Because chondrites represent one of the oldest solid materials within the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

and are believed to be the building blocks of the

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

ary system, it follows that an understanding of the formation of chondrules is important to understand the initial development of the planetary system.

Abundance and size

Different kinds of the stony, non-metallic

meteorite A meteorite is a rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...

s called

s contain different fractions of chondrules (see table below). In general,

carbonaceous chondrite Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 8 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites. The C chondrites represent only a small propo ...

s contain the smallest percentage (by volume) of chondrules, including the

CI chondrite CI chondrites, also called C1 chondrites or Ivuna-type carbonaceous chondrites, are a group of rare carbonaceous chondrites, a type of stony meteorite. They are named after the Ivuna meteorite, the type specimen. They represent the most chemica ...

s which, paradoxically, do not contain ''any'' chondrules despite their designation as chondrites, whereas ordinary and

enstatite Enstatite is a mineral; the magnesium endmember of the pyroxene silicate mineral series enstatite (MgSiO3) – ferrosilite (FeSiO3). The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and m ...

chondrites contain the most. Because ordinary chondrites represent 80% of the meteorites that fall to earth, and because ordinary chondrites contain 60–80% chondrules, it follows that (excluding dust) most of the meteoritic material that falls on earth is made up of chondrules. Chondrules can range in diameter from just a few micrometers to over . Again, different kinds of chondrites contain different ranges of chondrule sizes: they are smallest in CH, CM, and CO chondrites (see

meteorite classification In meteoritics, a meteorite classification system attempts to group similar meteorites and allows scientists to communicate with a standardized terminology when discussing them. Meteorites are classified according to a variety of characteristics ...

), moderately large in CR, CV, L, LL, and R chondrites, and largest in some CB chondrites (see table). Other chondrite groups are intermediate between these.

Mineralogy and petrology

Most chondrules are composed primarily of the

silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...

minerals

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

and

pyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...

, surrounded by

feldspathic Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocla ...

material that may either be

glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

y or crystalline. Small amounts of other minerals are often present, including Fe sulfide (

troilite Troilite () is a rare iron sulfide mineral with the simple formula of FeS. It is the iron-rich endmember of the pyrrhotite group. Pyrrhotite has the formula Fe(1−x)S (x = 0 to 0.2) which is iron deficient. As troilite lacks the iron deficiency ...

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

lic Fe-Ni, oxides such as

chromite Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the chemical formula of Iron, FeChromium, Cr2Oxygen, O4. It is an oxide mineral belonging to the spinel group. The ...

, and

phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...

s such as merrillite. Less common types of chondrules may be dominantly composed of feldspathic material (again either glassy or crystalline),

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

, or metallic Fe-Ni and sulfides. Chondrules display a wide variety of textures, which can be seen when the chondrule is sliced open and polished. Some show textural evidence for extremely rapid cooling from a molten or nearly completely molten state.

Pyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...

-rich chondrules that contain extremely fine-grained, swirling masses of fibrous crystals only a few micrometers in size or smaller are called ''cryptocrystalline'' chondrules. When the pyroxene fibers are coarser, they may appear to radiate from a single

nucleation In thermodynamics, nucleation is the first step in the formation of either a new Phase (matter), thermodynamic phase or Crystal structure, structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically def ...

site on the surface, forming a ''radial'' or ''excentroradial'' texture.

Olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

-rich chondrules may contain parallel plates of that mineral, surrounded by a continuous shell of olivine and containing feldspathic glass between the plates; these are known as ''barred'' textures. Other observed textural features that are clearly the result of very rapid cooling are

dendritic Dendrite derives from the Greek word "dendron" meaning ( "tree-like"), and may refer to: Biology *Dendrite, a branched projection of a neuron *Dendrite (non-neuronal), branching projections of certain skin cells and immune cells Physical *Dendri ...

and hopper-shaped olivine grains, and chondrules that are composed entirely of glass. More commonly, chondrules display what is known as a

porphyritic Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning ...

texture. In these, grains of olivine and/or pyroxene are equidimensional and sometimes

euhedral Euhedral and anhedral are terms used to describe opposite properties in the formation of crystals. Euhedral (also known as idiomorphic or automorphic) crystals are those that are well-formed, with sharp, easily recognised faces. The opposite i ...

. They are named on the basis of the dominant mineral, i.e. ''porphyritic olivine'' (PO), ''porphyritic pyroxene'' (PP), and ''porphyritic olivine-pyroxene'' (POP). It seems likely that these chondrules cooled more slowly than those with radial or barred textures, however they still may have solidified in a matter of hours. The composition of olivine and pyroxene in chondrules varies widely, although the range is usually narrow within any single chondrule. Some chondrules contain very little iron oxide (FeO), resulting in olivine and pyroxene that are close to

forsterite Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich Endmember, end-member of the olivine solid solution series. It is Isomorphism (crystallography), isomorphous with the iron-rich end-member, fayalit ...

() and

() in composition. These are commonly called ''Type I'' chondrules by scientists, and often contain large amounts of metallic Fe. Other chondrules formed under more

oxidizing Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

conditions and contain olivine and pyroxene with large amounts of FeO (e.g., olivine with the formula ). Such chondrules are called ''Type II''. Most chondrites contain both Type I and Type II chondrules mixed together, including those with both porphyritic and nonporphyritic textures, although there are exceptions to this.

Formation

Chondrules are believed to have formed by a rapid (flash) heating (within minutes or less) and melting of solid dust aggregates of approximately Solar composition under temperatures of about 1000 K. These temperatures are lower than those under which CAIs are thought to have formed. However, the environmental setting, the energy source for the heating, and the precursor material are not known. The

solar nebula There is evidence that the formation of the Solar System began about 4.6 bya, billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, whil ...

or a protoplanetary environment are possible places of formation. Proposed heating mechanisms are: *Impacts between molten

planetesimals Planetesimals () are solid objects thought to exist in protoplanetary disks and debris disks. Believed to have formed in the Solar System about 4.6 billion years ago, they aid study of its formation. Formation A widely accepted theory of pla ...

Meteor A meteor, known colloquially as a shooting star, is a glowing streak of a small body (usually meteoroid) going through Earth's atmosphere, after being heated to incandescence by collisions with air molecules in the upper atmosphere, creating a ...

ablation *Hot inner

nebula A nebula (; or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regions, such as in the Pillars of Creation in ...

* FU Orionis-type outburst of the early sun * Energetic bipolar-shaped outflows * Nebular lightning * Magnetic flares *

Shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...

s in the protoplanetary disk shocks *

Supernova A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...

radiation and shock wave

Isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

studies indicate a nearby supernova explosion added fresh material to what became the Solar System. The Ningqiang

contained

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

-36 derived from

chlorine Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...

-36. As chlorine-36 has a half-life of only 300,000 years, it could not have travelled far from its origin. The presence of

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

-60 also indicates a nearby supernova.G. Quitte et al. (2007). "Correlated iron 60, nickel 62 and zirconium 96 in refractory inclusions and the origin of the solar system", ''Astrophysical Journal'' (655): 678–84 Such proximity implies the radiation and shock wave would have been significant, although the degree of heating is not known. In contrast, the fine grained matrix, in which the chondrules are embedded after their accretion into the chondrites parent body, is assumed to have been condensed directly from the solar nebula.

Types

Chondrules may be organized into textural types according to such characteristics as their appearance, color, texture, and crystalline structure.

References

External links

A Pictorial of Chondrules
– Meteorites Australia (Meteorites.com.au)
Chondrules and their Origins
{{Authority control Chondrite meteorites Meteorite mineralogy and petrology