ARN Guidelines to Meteorite Classification
                                                              © 2006 ARN/ Kenneth Regelman
Since the beginning of the field of meteoritics many books and papers have been written about meteorites.
Now for the very first time in print a Guideline to meteorite Classification .
I do recommend one book Meteorites A Petrologic, Chemical and Isotopic Synthesis by Robert Hutchison


What I hope to accomplish here is to give everyone a set of definable parameters that can be used
in a lab anywhere and come up with a accurate classification of a meteorite.
There of course will be disagreement of what I have written but this is the start of something usable !
Lets start with the most common meteorite and work forward .  Ordinary Chondrites

Ordinary Chondrites are Extraterrestrial rocks containing chondrules and a matrix of minerals including
Iron and nickel combinations and any number of mineral combinations .
Chondrules are the basic building blocks of chondrites.  First formed by a flash of heat intense enough to
melt rock and metal matrix to liquid thus enabling the mass to form globules or spheres in this almost
weightless part of space .  Enough time and matter and the mass is changed as the heat leaves the spheres
and they solidify as solid spheres . Just as here on earth with the right cooling rates much of this material will
crystallizes to produce a wide variety of spheres or chondrules .  Many things may have happened at this
point to produce the varied textures of the chondrules and the mineral structure called the matrix or minerals
in between the chondrules .  This matrix or filler is also used to classify the type of Chondrite meteorites
being examined .  Where did the material come from to make chondrules ? Possibly an exploded  unstable
Planet . Just one of many possibilities !

Ordinary Chondrite (H, L, LL) Petrologic Types

Chondrites  H  ,  L  , LL
       The fayalite number  Fa .Fa = 100 x (Fe/(Fe + Mg)) in atomic units,  iron content of olivine .
        Fayalite  (Fe₂)SiO₄    /  Fosterite  %(Mg₂)SiO₄  
        Ferrosilite  ( Fs)  FeSiO3 Fs = 100 x (Fe/(Fe + Mg + Ca)) in atomic units , iron content of pyroxene .
        The iron-rich half of the Ca-poor proxenes                                                   

N.C. = No Chondrules visible completely recrystallized into
polygonal and subhedral grain matrix. May contain relic chondrules .

Table #2 Chondrites 3.0 to 3.9  Defining parameters

The terms below help describe the condition of the chondrite
but are not necessarily needed for classification .

Chondrule Types
Type IA  FeO-poor PO or BO chondrules
Type IB  FeO-poor PP chondrules
Type IAB  FeO-poor POP usually with small rounded olivines
poikilitically enclosed within pyroxene
Type IIA  FeO-rich Po or Bo chondrules
Type IIB  FeO-rich PP chondrules
Type IIAB  FeO-rich POP chondrules

Porphyritic olivine  ( PO )
Chondrules have olivine/pyroxene ratios > 10

Porphyritic Pyroxene  ( PP )
Chondrules have olivine/pyroxene ratios <0.1

Porphyritic olivine-pyroxene  ( POP )
Chondrules have intermediate ratios

Barred olivine  ( BO )
BO chondrules consist of skeletal olivines whose crystalites form sets of
subparallel bars across the interior .

Poikilitic chondrules
This refers to an igneous texture in which rounded crystals of one kind (e.g. olivine)
are completely surrounded by another kind (e.g. pyroxene).
These chondrules have CA-poor pyroxene phenocrysts that may be intergrown ,
enclosing rounded olivine grains .

Relic Grains
This is a grain in a chondrule that did not crystallize in situ. 
They may be grains that were not fully melted during the last episode of chondrule formation. 
Most relict grains in chondrules are composed of olivine.

Radial pyoxene   ( RP )
Chondrules composed of fibrous crystallites of Ca-poor proxene that radiate from one
or more points near the inside chondrule margins .

Granular olivine  ( GO ) Dark zoned or agglomeratic  chondrules
Rare olivine-rich, subspherical to irregular frains of fragmented silicate minerals ,
plus triolite and metal , with no significant matrix in the chondrule .

Granular olivine -pyroxene  ( GPO )
These chondrules have significant proxene grains within a matrix of granular olivine .

Cryptocrystalline  ( C )
C chondrules are a third non-porphyritic type, which is to say have no optically
resolvable crystals ,  and consist of submicroscopic proxene-rich material .

Glass-rich and glass chondrules
Glass-rich chondrules contain50-90 vol% glass or microcrystalline mesostasis.
Glass chondrules contain 90 vol% vol% glass or microcrystalline mesostasis
The former are porphyritic and latter contain rare skeletal olivines and
Ca-pyroxenes distinguishing them from Cryptocrystalline chondrules .

Al-rich, Ca-,Alrich,  Ca-Al,Na-rich and Al-Cr-rich  chondrules
These are by definition at least 10wt% Al2O3 and with igeous textures
may be droplet or clast type .  Texture and mineralogically intermediate
between ferromagnesian and CAIs . Usually have primary Ca- and AL-rich
 mineral such as plagioclase, aluminous Ca- proxene and spinel .
Rare chondrules are rich in chromite.

Metallic and metal-sulfide chondrules
Chondrules with subspherical metallic or metal-sulfide objects with dendritic
or polycrystalline textures .

Breccia definitions
In describing a meteorite it may be necessary to describe its
Brecciated structure .

Monomict Breccia
Consist of fragments of one chemical-petrologic type

Genomict Breccia
Consist of fragments of one chemical group but with more than one
petrologic type .

Polymict Breccia
Consist of fragments of more than one chemical group but and more
than one petrologic type . Polymict breccia are meteorites within
meteorites eg H3-

 

This chart below may be one of the most important tools in meteorite classification today .

Oxygen Isotopes Explained?

Oxygen isotopic compositions of primitive materials in chondrites plot along a line that suggests addition or subtraction of ¹⁶O . An old explanation for this trend was that pre-solar grains were enriched in ¹⁶O, but now cosmochemists and astronomers believe that it is more likely to be a chemical effect produced by irradiation of carbon monoxide (CO) by ultraviolet light. Observations of molecular clouds indicate that ultraviolet radiation can dissociate preferentially CO inside the cloud made with ¹⁷O or ¹⁸O. Ultraviolet light that can dissociate CO made with ¹⁶O cannot penetrate beyond the surface of the cloud. The oxygen released from dissociated CO can combine with hydrogen to produce water ice that is rich in ¹⁷O and ¹⁸O . This dynamic process, called "self-shielding," can produce large variations in the proportions of ¹⁶O relative to the other two isotopes. The theory predicts that the planets were made of material that contained excess water ice that was preferentially enriched in ¹⁷O and ¹⁸O, and that the Sun should have an oxygen isotopic composition like those meteoritic grains richest in ¹⁶O in CAIs. Ion microprobe measurements of solar wind implanted onto some metallic grains on the lunar surface (which of course contain no oxygen originally) suggest that the Sun's oxygen isotopic composition is like those of the CAIs.

Fa , Fs, Wo information
Solid solution - this describes the ability of certain minerals to have a range of compositions.  A mineral may have a crystalline structure that allows more than one element to fit into a specific site in that structure, giving rise to compositional variability.  An example of this is the mineral Olivine.  The structure of this mineral consists of units of SiO4 attached to two crystal sites that can contain an element such as iron (Fe) or magnesium (Mg).  In the case of Fe and Mg, olivine forms a solid solution from fayalite [Fe2SiO4] to forsterite [Mg2SiO4].  Fayalite and forsterite are end-members of the olivine solid solution.  In the real world, an olivine grain will typically have a mixture of Fe and Mg filling those two crystal sites, and the formula for olivine is written (Mg, Fe)2SiO4. One can express the composition of an olivine either by the percentage of forsterite (Fo) or fayalite (Fa). Another important solid solution occurs for common pyroxene (Ca,Mg,Fe)SiO3, in which the elements Ca, Mg, and Fe can substitute for one another.  The composition of a pyroxene can be expressed by the endmember proportions it contains of En or enstatite [MgSiO3], Fs or ferrosilite [FeSiO3], and Wo or wollastonite [CaSiO3].  So a pyroxene said to have the composition of Wo5En80Fs15 (or simply Wo5En80, or Wo5Fs15) would have 5% of the wollastonite component, 80% of the enstatite component, and 15% of the ferrosilite component. 

 Fayalite    %(Mg₂)SiO₄ / % Fosterite (Fe₂)SiO₄