|
Meteorite Classification Table
|
|
Category
|
Composition Type
|
Distinguishing Features/ Chondrule
Character
|
Letter Designation
|
|
Chondrites
Stony Meteorites are
characterized by chondrules--small
spheres (average diameter of 1 mm) of formerly melted minerals that have
come together with other mineral matter to form a solid rock. Chondrites are
believed to be among the oldest rocks in the solar system.
Number designations for chondrites:
Most classified chondrites are given a number designation. The
number refers to the alteration of the chondrules. A designation of "3"
indicates unaltered chondrules. Numbers greater than 3 indicate increasing
thermal metamorphism. A designation of 7 indicates complete obliteration of
chondrules. Numbers less than three indicate increasing aqueous alteration.
A designation of 1 indicates that chondrules have been obliterated by
aqueous alteration. A lack of numeric designation means that no alteration
designation has been assigned.
Further reading,
King, ed., Chondrules and
Their Origins. 1983
Note on type 7 Chondrites.
There is no firm agreement among meteorite researchers on the definition of
type 7 chondrites. Many meteorites previously described as type 7 are either
type 6 or are impact melts.
Impact melting is not considered a proper basis for type 7 classification.
Future research is expected to clear up this issue.
|
Enstatite
Chondrites
Letter Designations: The
E stands for Enstatite. H indicates high metallic iron and L indicates low
metallic iron. Very Rare--1.5% of falls. Minerals: Enstatite, metal,
sulfides, plagioclase, and occasional olivine (EH).
|
Abundant
|
E3, EH3, EL3
|
|
Distinct
|
E4, EH4. EL4
|
|
Less Distinct
|
E5, EH5, EL5
|
|
Indistinct
|
E6, EH6, EL6
|
|
Melted*
|
E7
|
|
Ordinary Chondrites
The most common meteorite from observed
falls--73.5%.
The initial letter designation for
ordinary Chondrites, "O", is usually deleted. Brecciated meteorites with
clasts of more than one type of alteration are designated with the range of
alteration present in the meteorite. The letter designation refers to the
iron content for the meteorite as a whole. E.g., Zag bears the designation
H3-6.
Some meteorites are classified as
transitional between the main groups, e.g., H/L, L/LL
When a researcher is not sure of a class,
it may be designated with a notation like L(LL) with the less certain class
being in parentheses.
|
H Chondrites
High Iron (12 to 21%
metallic iron)
(also called Bronzite Chondrites) 31.4% of falls. Minerals: Olivine, pyroxene,
metal, plagioclase, sulfide.
|
Abundant
|
H3-H3.9*
|
|
Distinct
|
H4
|
|
Less Distinct
|
H5
|
|
Indistinct
|
H6
|
|
Melted*
|
H7
|
|
L Chondrites
Low Iron (5 to 10%
metallic iron)
(also called Hypersthene Chondrites) 34.8% of falls. Minerals: Olivine, pyroxene,
plagioclase, metal, sulfide.
|
Abundant
|
L3-L3.9*
|
|
Distinct
|
L4
|
|
Less Distinct
|
L5
|
|
Indistinct
|
L6
|
|
Melted*
|
L7
|
|
LL Chondrites
Low Metal Content (about 2%
metallic iron)
(also called Amphoterites) Principle minerals are
bronzite,
olivine, and
minor oligoclase.
7.2% of falls.
|
Abundant
|
LL3-LL3.9*
|
|
Distinct
|
LL4
|
|
Less Distinct
|
LL5
|
|
Indistinct
|
LL6
|
|
Melted [* these rocks can be considered
primitive achondrites and not Ordinary Chondrites]
|
LL7
|
|
Carbonaceous Chondrites
These rare meteorites contain elemental
carbon, a basic
building block for life. 3.6% of falls.
Explanation of Letter Designations:
The "C" stands for Carbonaceous Chondrite.
The second letter refers to type localities (except the H designation). The
number refers to alteration.
|
Ivuna
|
Friable, more water. Minerals:
Phyllosilicates, magnetite
|
CI
|
|
Mighei
|
Friable, less water. Minerals:
Phyllosilicates, tochilinite, olivine
|
CM1-CM2
|
|
Vigarano.
|
Fe rich
olivine, CAIs
|
CV2-CV3.3
|
|
Renazzo
|
Minerals: Phyllosilicates, pyroxene,
olivine, metal
|
CR
|
|
Ornans
|
Minerals: Olivine, Pyroxene, CAIs,
metal.
|
CO3-CO3.7
|
|
Karoonda
|
Minerals: Olivine, CAIs
|
CK
|
|
Bencubbin
|
Minerals: Metal, Pyroxene. [ongoing
research suggests the product of asteroidal collisions.]
|
CB
|
|
High Iron
|
Minerals: Pyroxene, metal, olivine.
[May be related to Bencubbinites]
|
CH
|
|
Kakangari-type
|
|
K
|
|
Rumurutiites
|
Minerals: Olivine, pyroxene,
plagioclase, sulfide.
|
R
|
|
|
Group
|
Origin
|
Characteristic Minerals
|
|
|
Achondrites
Stony Meteorites without
chondrules. Scientists believe that some of these meteorites originated on
the surface of the Moon or Mars. 7.8 percent of meteorite falls
are achondrites.
Note on Groupings:
Howardites, Eucrites, and Diogenites have
been grouped as HED meteorites. These types may originate on the asteroid
Vesta. Chassignites, Shergottites, and Nakhlites are grouped as SNC
meteorites. These types are believed to originate on Mars.
See H. McSween,
Meteorites and Their
Parent Planets for and excellent explanation.
|
Howardites
|
Vesta regolith
|
Eucrite-diogenite mix
|
HOW
|
|
Eucrites
|
Vesta basaltic crust
|
Anorthite-pigeonite
|
EUC
|
|
Diogenite
|
Vesta deeper/plutonic
|
Hypersthene
|
DIO
|
|
Shergottites
|
Marian Basalt--shocked
|
Basaltic
|
SHE
|
|
Nakhlites
|
Martian plutonic rock
|
Diopside-olivine
|
NAK
|
|
Chassignite
|
Martian plutonic rock
|
Olivine
|
CHA
|
|
Lunar
|
The Moon
|
Basalt and Regolith
|
LUN
|
|
Aubrites
|
Melted E Chondrite
|
Enstatite
|
AUB
|
|
Acapulcoite
|
Remelted chondrite
|
Olivine,
Pyroxene
|
ACAP
|
|
Lodranite
|
Same as ACAP--more melt
|
Olivine,
Pyroxene
|
LOD
|
|
Ureilites
|
Melted C-chondrite body
|
Olivine-pigeonite
|
URE
|
|
Angrite
|
Non-HED Basalt
|
Olvn, Pyrx.,
Plagioclase
|
ANGR
|
|
Brachinite
|
A or S type Asteroids
|
Olivine
|
BRACH
|
|
Winonaite
|
Like IAB & IIICD incl.
|
|
WIN
|
|
|
Widmanstatten Bandwidth
|
|
|
Irons (structural classification)
These meteorites are made of a
crystalline iron-nickel alloy. Scientists believe that they resemble the outer core
of the Earth. 4.8 percent of meteorite falls are irons.
|
Hexahedrites
<6% Ni. Contains
kamacite, but
not taenite
|
>50mm
|
H
|
|
Octahedrites
6% to 17% Ni. Contains both
kamacite
and taenite and
has Widmanstatten pattern.
|
Coarsest
|
3.3-50mm
|
Ogg
|
|
Coarse
|
1.3-3.3mm
|
Og
|
|
Medium
|
.5-1.3mm
|
Om
|
|
Fine
|
0.2-0.5mm
|
Of
|
|
Finest
|
0.2mm
|
Off
|
|
Plessitic
|
0.2mm
Kamacite spindles
|
Opl
|
|
Ataxites
High Ni content
|
(no structure)
|
D
|
|
|
Minerals
|
Structural Classes
|
|
|
Irons (Chemical Classification)
A second scheme for classifying iron
meteorites is by their chemistry. The determining factors are groupings of
meteorites with similar ratios of trace elements to nickel. Generally, the
higher the Roman numeral of the classification, the lower the concentration
of trace elements. The casual observer cannot see this as one can with the
Widmanstatten bandwidth that is the determining factor for structural
classification. Chemical classification is important because it suggests
that certain iron meteorites share a common origin or were formed under
similar conditions. The Handbook of Iron Meteorites by Vahn
Buchwald has a more complete description.
|
kamacite,
taenite,
silicates,
carbides
|
Om-Og
|
IAB
|
|
kamacite,
taenite,
silicates,
carbides
|
Om-Og
|
IC
|
|
kamacite,
taenite, (daubreelite)
|
Ogg, H
|
IIAB
|
|
kamacite,
aenite
|
Ogg
|
IIC
|
|
kamacite,
taenite
|
Of-Om
|
IID
|
|
kamacite,
taenite,
silicates
|
Off-Og
|
IIE
|
|
kamacite,
taenite,
|
Plessitic Oct., Atax.
|
IIF
|
|
kamacite,
taenite,
troilite,
phosphides
|
Om-Og
|
IIIAB
|
|
kamacite,
taenite,
carbides
|
Off-D
|
IIICD
|
|
kamacite,
taenite,
carbides,
graphite
|
Og
|
IIIE
|
|
kamacite,
taenite
|
Om-Og
|
IIIF
|
|
kamacite,
taenite
|
Of
|
IVA
|
|
kamacite,
taenite
|
D
|
IVB
|
|
kamacite,
taenite,
silicates,
graphite
|
All
|
Anom
|
|
|
Primary Minerals
|
|
|
Stony Irons
These meteorites are mixtures of
iron-nickel alloy
and non-metallic mineral matter. Scientists believe that they are like the
material that would be found where the Earth's core meets the mantle.
1.2 percent of meteorite falls are stony irons.
|
Pallasites
|
Main-group Pallasites
|
iron, olivine
|
PAL
|
|
Eagle Station grouplet
|
iron, olivine,
pyroxene
|
|
|
Pyroxene Pallasite grouplet
|
iron, pyroxene
|
|
|
Mesosiderites (MES)
Mesosiderites have been divided into a
classification grid that resembles that for chondrites. There are three
broad petrologic groups designated A, B, and C. Within these groups there is
a numeric classification of metamorphic grade. Grade 1 is fine grained and
fragmental, 2 and 3 show progressive recrystallization, and 4 is a melt
breccia.
|
Class A (Basaltic)
|
iron, Ca pyroxene,
plagioclase
|
1A
|
|
2A
|
|
3A
|
|
4A
|
|
Class B (Ultramafic)
|
iron, Ca pyroxene,
plagioclase,
Orthopyroxene
|
1B
|
|
2B
|
|
3B
|
|
Class C (Orthopyroxene)
|
orthopyroxene
|
2C
|