Chondrule - AbsoluteAstronomy.com

Chondrules are round grains found in chondrite

Chondrite

Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids...

s. Chondrules form as molten

Mölten

Mölten is a comune in South Tyrol in the Italian region Trentino-Alto Adige/Südtirol, located about 60 km north of Trento and about 12 km northwest of Bolzano .-Geography:...

or partially molten droplets in space before being accreted

Accretion (astrophysics)

In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...

to their parent asteroid

Asteroid

Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s. Because chondrites represent the oldest solid material within our solar system

Solar System

The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

and are believed to be the building blocks of the planet

Planet

A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

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 chondrite

Chondrite

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

Carbonaceous chondrite

Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...

s contain the smallest percentage (by volume) of chondrules, including the CI chondrites which, paradoxically, do not contain any chondrules despite their designation as chondrites, whereas ordinary

Ordinary chondrite

The Ordinary chondrites are a class of stony chondritic meteorites. They are by far the most numerous group and comprise about 87% of all finds...

and enstatite chondrite

Enstatite chondrite

Enstatite chondrites are a rare form of meteorite thought to comprise only about 2% of the chondrites that fall on Earth. Only about 200 E-Type chondrites are currently known....

s 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 most of the meteoritic material that falls on earth (excluding dust) is made up of chondrules.

Chondrules can range in diameter from just a few micrometers to over 1 cm. Again, different kinds of chondrite

Chondrite

s contain different ranges of chondrule sizes: they are smallest in CH, CM, and CO chondrites (see meteorite classification

Meteorites classification

The ultimate goal of meteorite classification is to group all meteorite specimens that share a common origin on a single, identifiable parent body. This could be a planet, asteroid, Moon, or other current Solar System object, or one that existed some time in the past...

), 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.

Table 1: Chondrule sizes and abundances
Chondrite group	abundance (vol%)	avg. diam. (mm)
CI	0	–
CM	20	0.3
CO	50	0.15
CV	45	1
CK	45	1
CR	50-60	0.7
CH	70	0.02
CB	20-40	10 (a subgroup), 0.2 (b subgroup)
H	60-80	0.3
L	60-80	0.7
LL	60-80	0.9
EH	60-80	0.2
EL	60-80	0.6
R	>40	0.4
K	30	0.6

Mineralogy and petrology

Most chondrules are composed primarily of the silicate

Silicate

A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...

minerals olivine

Olivine

The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....

and pyroxene

Pyroxene

The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...

, surrounded by feldspathic

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

material that may either be glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

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

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 FeS which is iron deficient...

), metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

lic Fe-Ni, oxides such as chromite

Chromite

Chromite is an iron chromium oxide: FeCr2O4. It is an oxide mineral belonging to the spinel group. Magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite ; substitution of aluminium occurs leading to hercynite .-Occurrence:Chromite is found in...

, and phosphate

Phosphate

A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...

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

Pyroxene

-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

Nucleation

Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...

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

Olivine

The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....

-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 (crystal)

A crystal dendrite is a crystal that develops with a typical multi-branching tree-like form. Dendritic crystal growth is very common and illustrated by snowflake formation and frost patterns on a window. Dendritic crystallization forms a natural fractal pattern...

and hopper-shaped olivine grains, and chondrules that are composed entirely of glass.

More commonly, chondrules display what is known as a porphyritic

Porphyritic

Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...

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

Euhedral

Euhedral crystals are those that are well-formed with sharp, easily recognised faces. Normally, crystals do not form smooth faces or sharp crystal outlines. Many crystals grow from cooling liquid magma...

. 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

Forsterite is the magnesium rich end-member of the olivine solid solution series. Forsterite crystallizes in the orthorhombic system with cell parameters a 4.75 Å , b 10.20 Å and c 5.98 Å .Forsterite is associated with igneous and metamorphic rocks and has also been found in meteorites...

(Mg₂SiO₄) and enstatite

Enstatite

Enstatite is the magnesium endmember of the pyroxene silicate mineral series enstatite - ferrosilite . The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rocks...

(MgSiO₃) 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

Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

conditions and contain olivine and pyroxene with large amounts of FeO (e.g., olivine with the formula (Mg,Fe)₂SiO₄). 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 formed by a rapid heating (within minutes or less) of solid precursor material to temperatures between 1500°C and 1900°C and subsequent melting. This is followed by a cooling within one to several hours. However, the environmental setting, the energy source for the heating, and the precursor material are not known. The solar nebula

Solar nebula

In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation...

or a protoplanetary environment are possible places of formation.

Proposed energy sources are:

Impact melting
Meteor
METEOR
METEOR is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision...

ablation
Hot inner nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...
FU Orionis-type outburst of the early sun
Energetic bipolar-shaped outflows
Nebular lightning
Magnetic flares
Accretion shocks
Nebular shocks
Supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

radiation and shock wave

Isotope

Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

studies indicate a nearby supernova explosion added fresh material to what became our solar system. The Ningqiang carbonaceous chondrite

Carbonaceous chondrite

contained sulfur

Sulfur

Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

-36 derived from chlorine

Chlorine

Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

-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

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

-60 also indicates a nearby supernova. 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.

External links

A Pictorial of Chondrules - Meteorites Australia (Meteorites.com.au)
Chondrules and their Origins

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Abundance and size

Mineralogy and petrology

Formation

See also

Further reading

External links