Criteria for Classifying Chondrites Based on Petrologic Type
Posted on by skyfallmeteorites
Open Popup
Feedback
Criteria for Classifying 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 Based on 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.
Van Schmus and Wood (1967) with modifications by Sears and Dodd (1988), Brearley and Jones (1998), and this Weisberg, McCoy and Krot (2006). Modifications to be added soon ...
| Â | Â | Petrologic Type | ||||||
| Â | Criteria | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| 1 |
Compositional Homogeneity of |
– | Olivine   > 5% Mean Deviation Pyroxene > 5% Mean Deviation |
< 5% Mean Deviation | 0% Mean Deviation (Homogeneous) | |||
| 2 | Structural State of Low-Ca Pyroxene |
– | Predominantly Monoclinic Crystals | Crystals > 20% Monoclinic | Crystals < 20% Monoclinic | Orthorhombic Crystals | ||
| 3 | Secondary 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. Development | – | Absent | Grains < 2 µm | 2 µm < Grains < 50 µm | Grains > 50 µm Clear InterstitialTerm applied to ions or atoms occupying sites between lattice points. Glass | ||
| 4 | Igneous Glass in 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 | – | Mostly Altered, Some Preserved | Clear, IsotropicSame in all directions. | Devitrified or Turbid if Present | Absent | ||
| 5 | MetalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn: Maximum Ni (wt%) | – | Ni < 20%, TaeniteLess common than kamacite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Taenite, γ-(Fe,Ni), has 27-65 wt% Ni, and forms small crystals that appear as highly reflecting thin ribbons on the etched surface of a meteorite; the name derives from the Greek word for "ribbon." Minor or Absent | Ni > 20%, 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 and Taenite in ExsolutionSegregation, during cooling, of a homogeneous solid solution into two or more different solids. Relationship | ||||
| 6 | Sulfides: Average Ni (wt%) | – | Ni > 0.5% | Ni < 0.5% | ||||
| 7 | Chondrule Texture | No 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 | Chondrules Very Sharply Defined | Chondrules Well Defined | Chondrules Readily Distinguished | Chondrules Poorly Defined | No Chondrules | |
| 8 | MatrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. Texture | All Fine Grained, Opaque | Predominantly Opaque | Opaque | Transparent Microcrystalline Matrix | Recrystallized Matrix | ||
| 9 | Bulk CarbonElement commonly found in meteorites, it occurs in several structural forms (polymorphs). All polymorphs are shown to the left with * indicating that it been found in meteorites and impact structures: a. diamond*; b. graphite*; c. lonsdalite*; d. buckminsterfullerene* (C60); e. C540; f. C70; g. amorphous carbon; h. carbon nanotube*. (wt%) | 3% < C < 5% | 1.5% < C < 2.8% | 0.1%< C < 1.1% | < 0.2% | |||
| 10 | Bulk H2O Content (wt%) | 18% < H2O < 22% | 3% < H2O < 11% | H2O < 2% | ||||
