24° 50′ N., 31° 40′ E. Five stones with black, bubbly crustOutermost layer of a differentiated planet, asteroid or moon, usually consisting of silicate rock and extending no more than 10s of km from the surface. The term is also applied to icy bodies, in which case it is composed of ices, frozen gases, and accumulated meteoritic material. On Earth, the Click on Term to Read More were found by a member of the Geological Survey of Egypt in a farm field on the Nile River near Luxor. Today only a single 23 kg specimen remains preserved.
Recent studies have determined that systematic changes occur in amoeboid olivine aggregatesMillimeter sized, fine-grained inclusions present to a few volume-percent in most carbonaceous chondrites. They can be round but can also be irregularly shaped like an amoeba (thus the name amoeboid). They are forsterite (Mg-rich olivine) and Ca-Al-Ti mineral aggregates. The most characteristic texture of AOAs is an anorthite core (sometimes Click on Term to Read More
(AOAs) with increasing subtype, which is directly linked to increasing aqueous and thermal metamorphism (Chizmadia et al.
, 2002; Grossman and Rubin, 2006). For example, textures and morphologies of AOAs show changes, olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion Click on Term to Read More
in AOIs becomes progressively FeO-rich, troiliteBrass colored non-magnetic Fe sulfide, FeS, found in a variety of meteorites.
becomes more prevalent, and trace elements become more equilibrated. Because of their smaller grain size, olivines in AOAs are better indicators of alteration processes (such as the substitutionReplacement of one ion or ionic group for another in the same structural site in a mineral yielding a solid solution. Most substitution in minerals is of cations which are smaller and essentially sit in a lattice of oxygen anions. Anionic substitution does occur in halides. Substitutions are classified based
of Fe for Mg) than the chondrulesRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine Click on Term to Read More
previously utilized to determine subtype. As a result of this study, a refinement in the subtypes of the CO3 chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More
was proposed; the CO carbonaceous chondriteCarbonaceous chondrites represent the most primitive rock samples of our solar system. This rare (less than 5% of all meteorite falls) class of meteorites are a time capsule from the earliest days in the formation of our solar system. They are divided into the following compositional groups that, other than Click on Term to Read More
group would span a petrographic sequence from type 3.0, represented by Colony, to type 3.8, represented by Isna.
Chizmadia and Bravo-Ruiz (2013) employed a similar method to that of Grossman and Rubin (2006) to classify CO3 chondrites by the degree of aqueous alteration; they utilized the Fe-Mg composition and distribution in olivines in AOAs. However, Chizmadia and Bravo-Ruiz extended their study to comprise the entire metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. Click on Term to Read More
range. Based on their study, they proposed that Isna should be assigned to petrologic typeMeasure of the degree of aqueous alteration (Types 1 and 2) and thermal metamorphism (Types 3-6) experienced by a chondritic meteorite. Type 3 chondrites are further subdivided into 3.0 through 3.9 subtypes.
3.75. They also better resolved Colony as type 3.05, and assigned the MET 00694 pairing group to the highest CO3 petrologic type of 3.8.
In a different analysis of CO3 petrologic types conducted by Bonal et al.
(2005), they found that an accurate comparison could be made between the metamorphic grades of the CO and the ordinary chondrites using Raman spectrometry combined with petrographic analysis. Their method is based on the structural order of the chondritic organicPertaining to C-containing compounds. Organic compounds can be formed by both biological and non-biological (abiotic) processes. Click on Term to Read More
matter, which was initially accreted in the same proportions in both CO and ordinary chondrites. Based on their data, the CO group would span a petrographic sequence from 3.03 as represented by ALHA77307, to 3.7 as represented by both Warrenton and Isna.
D.W.G. Sears (2016) conducted an in-depth petrographic study of CO chondrites in an effort to bring a measure of consistency to the wide diversity of classification schemes that now exist for this meteorite group. He studied a significant number of the ‘MIL’ and ‘DOM’ CO chondrites that were found in Antarctica (representing low petrographic types), and updated the petrologic classification of five of the six CO meteorites that were recovered as fresh falls around the world (representing the higher petrographic types). Computer software was used to ascertain which of the many metamorphic properties can best serve as accurate indicators of petrologic type. The results of this ‘Principle Component Analysis’ revealed that 83% of the correlation between metamorphic alteration and petrologic type can be explained by three component types, none of which are decisive when considered alone: 30% is explained by bulk properties
(bulk composition, bulk C content, trapped inert gas content, reflectance spectra at 0.8 µm), 28% is explained by metamorphism-induced phase changes
(TL sensitivity, matrix composition, graphitization), and 25% is explained by Fe diffusionMovement of particles from higher chemical potential to lower chemical potential (chemical potential can in most cases of diffusion be represented by a change in concentration). Diffusion, the spontaneous spreading of matter (particles), heat, or momentum, is one type of transport phenomena. Because diffusion is thermally activated, coefficients for diffusion Click on Term to Read More
processes (olivine composition and heterogeneity, Ni, Co, and Cr content in kamaciteMore common than taenite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Kamacite, α-(Fe,Ni), contains 4-7.5 wt% Ni, and forms large body-centered cubic crystals that appear like broad bands or beam-like structures on the etched surface of a meteorite; its name is derived from the Greek word Click on Term to Read More
); the remainder (17%) of the correlation to metamorphic grade can be explained by several less accurate properties such as O- and C-isotopic values and AOAMillimeter sized, fine-grained inclusions present to a few volume-percent in most carbonaceous chondrites. They can be round but can also be irregularly shaped like an amoeba (thus the name amoeboid). They are forsterite (Mg-rich olivine) and Ca-Al-Ti mineral aggregates. The most characteristic texture of AOAs is an anorthite core (sometimes Click on Term to Read More
textures. After assessing each of these parameters for the CO chondrites in his study as well as for the known falls (except Moss), a petrologic grade was assigned to each sample (see the following). Based on this CO chondriteMeteorite class named after the Ornans meteorite that fell in France in 1868, are related in chemistry and composition to the CV chondrites and may, with them, represent a distinct clan of carbonaceous chondrites that formed in the same region of the early solar system. However, COs are usually blacker Click on Term to Read More
study, D.W.G. Sears argues that it is ill-advised to construct a petrologic classification scheme for a common application among different chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More
groups, and he contends that resolution of the metamorphic grade to an accuracy greater than a single decimal place is not warranted for this group given the current techniques. ALHA77307: 3.0
MIL CO3 chondrites: 3.2
DOM CO3 chondrites: 3.2
Felix, Lancé, and Ornans: 3.4
Isna: 3.7 Isna represents one of the most highly metamorphosed CO meteorites, both thermally and aqueously. The peak metamorphic temperature of Isna was ~530°C, reflected by its higher Ni and Co content, lower Cr in kamacite concentration, very coarse texture, decreased abundance of presolar diamonds, and increased TL sensitivity due to feldsparAn alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of feldspars and due to their abundance, feldspars are used in the classification of igneous rocks. A more complete explanation can be found on the feldspar group page. Click on Term to Read More crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. Click on Term to Read More
from chondruleRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine Click on Term to Read More
glass. Compared to other CO chondrites, Isna has an exceptionally low cosmic ray exposure ageTime interval that a meteoroid was an independent body in space. In other words, the time between when a meteoroid was broken off its parent body and its arrival on Earth as a meteorite - also known simply as the "exposure age." It can be estimated from the observed effects Click on Term to Read More
of ~0.15 m.y. A high solar noble gasElement occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. Click on Term to Read More
content attests to a regolithMixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles). Click on Term to Read More
A high hydrothermal alteration is reflected in the increased size and abundance of rimmed AOAs, an apparant increase in chondrule size, and melilite-rich refractory inclusionsInclusions found predominantly in carbonaceous chondrites and are rich in refractory elements particularly calcium, aluminum and titanium that in various combinations form minerals such as spinel, melilite, perovskite and hibonite. There are two types of refractory inclusion: • Ca Al-rich inclusions (CAIs) • Amoeboid olivine aggregates (AOAs) Refractory inclusions were Click on Term to Read More
altered to fine-grained, spinel–pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification Click on Term to Read More
inclusions. In addition, 16
O compositions are lowest in the highest petrographic subtypes, reflecting an increased oxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only Click on Term to Read More
exchange in an 16
O-poor water reservoir during hydrothermal metamorphism.
Some theories have linked the CO3 group to the K-type asteroids of the Eos family, an asteroid group that possesses S-type spectra in visible wavelengths and C-type spectra in the near-infrared. Based on reflectance spectra and albedoRatio of the amount of light reflected by an object and the amount of incident light. Albedo is used as a measure of the reflectivity or intrinsic brightness of an object. A white, perfectly reflecting surface has an albedo of 1.0 while a black perfectly absorbing surface would have an Click on Term to Read More
, the asteroids 221 Eos (104 km-diameter) and 653 Berenike (39 km-diameter) were found to be good analogs for the olivine-rich CO3 meteorites. These asteroids are located near 3 AUThe astronomical unit for length is described as the "mean" distance (average of aphelion and perihelion distances) between the Earth and the Sun. Though most references state the value for 1 AU to be approximately 150 million kilometers, the currently accepted precise value for the AU is 149,597,870.66 km. The Click on Term to Read More
at the 9:4 resonance, where meteorites are expected to become Earth-crossers on timescales greater than tens of millions of years (see diagram below). However, utilizing more advanced spectrographic techniques comparing a broader spectrum, it was determined that a better fit to the Eos family was the ungroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). Click on Term to Read More achondriteAn achondrite is a type of stony meteorite whose precursor was of chondritic origin and experienced metamorphic and igneous processes. They have a planetary or differentiated asteroidal origin where the chondritic parent body reached a sufficient size that through heating due to radioactive decay of 26Al (aluminum isotope) and gravitational Click on Term to Read More Divnoe
(Mothé-Diniz and Carvano, 2005).
Diagram credit: M. M. M. Meier et al.
, Earth and Planetary Science Letters
, vol. 490 (2018) ‘Cosmic history and a candidate parent asteroid for the quasicrystal-bearing meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and Click on Term to Read More Khatyrka’
Isna has an average porosityThe volume percentage of a rock that consists of void space. Vesicular porosity is a type of porosity resulting from the presence of vesicles, or gas bubbles, in igneous rock such as the pumice presented here. Vesicular porosity is very rare in meteorites and is often associated with slag, one Click on Term to Read More
of 14.5% (Macke et al.
, 2011). For additional information on the formation of the CO chondrites and their metamorphic heirarchy, visit the Colony
page. The specimen of Isna shown above is a 4.5 g partial slice.