HED Meteorite Guidelines
Eucrites
Diogenites
Howardites
Chart
O/I chart
Howardites are polymict
breccias composed of angular clasts of eucrites and diogenites mixed together in
a
crushed matrix .
They do not have unique mineral characteristics like other meteorite
classes
But one defining parameter is that the composition mean
average must contain more
than 10% Diogenitic material .
Monomict Goup
The monomict subgroup containing eucrites, cumulate
eucrites, and diogenites is further subdivided .
A brecciated
B unbrecciated
Polymict Group
The polymict subgroup samples a compositional and textural
continuum of regolith and surface breccias
consisting of eucrites, cumulate eucrites, diogenites, and howardites.
Those meteorites containing more than 90% of a single component are given the
prefix "polymict"
attached to their present description
Polymict eucrites contain less than 10%
non-eucritic material; polymict diogenites contain more
than 90% orthopyroxenite or hypersthene). Those meteorites that contain less
than 90% of any single
component are defined as howardites. While this 10% level is still an arbitrary
dividing line based simply
on mineral proportions, it represents an amount of orthopyroxene that can easily
be detected by X-ray
diffraction techniques. An additional tool to distinguish between polymict
eucrites and howardites involves
pyroxenes in the basaltic clasts; within howardites they are mostly unzoned
whereas in polymict eucrites
they are usually zoned.
Classic modern write up
Northwest Africa 1929
Morocco
Find: May 2003
Achondrite (howardite)
History: A 922.2 g
partially crusted, complete stone was purchased in Erfoud, Morocco in 2003 May.
Petrography: (T. Bunch and J. Wittke, NAU) A breccia of 72 vol. %
cumulate eucrite clasts, 8% subophitic clasts, 14% diogenites and 6% melt clasts.
Pervasive solid-state recrystallization of plagioclase and pyroxenes with
localized melt pockets and veins in clasts. Geochemistry: Coarse-grained
gabbroic eucrite pyroxene, Fs45-40Wo7-20; plagioclase, An91.2-95.3;
Metal inclusions Ni = 0.97; Cr = 0.87 (both wt %); Diogenite pyroxene, Fs43-54Wo2.5-3.6.
Classification: Achondrite (howardite), heavily shocked. Specimens: A 22.02 g
specimen is on deposit at NAU. Farmer holds the main mass.
Northwest Africa 2060
Morocco
Found 2003
Achondrite (howardite)
A 985 g, very fresh stone with shiny fusion crust, was
purchased in Rissani, July 2003. Classification and mineralogy (T. Bunch and
J. Wittke, NAU): a howardite of unusual characteristics. Fine-grained
and powdery, average clast and mineral grain size is < 2 mm with few clasts
> 3 mm. Clast modes: diogenites, 74 vol. %; eucrites 20 vol. % (12 %
cumulate, 8 % basaltic); anorthositic-like clasts, 3 %; shock melt clasts. 2
%, and opaques, 1%. Diogenite orthopyroxene, Fs35.6-38.8Wo2.3-4.2;
plagioclase, An 87; chromite, Cr/(Cr+Al) = 0.81; metal, Ni = 0.67
wt %. Melt inclusions in opx contain Ca-rich pyroxene, Fs33.3Wo37.5;
phosphate, silica phase, and K-rich glass. Plagioclase-pyroxene cumulates,
Fs38.2Wo21; plagioclase, An86.Other eucrite
textures include recrystallized, granular, ophitic, subophitic, and
variolitic (Fs31.6Wo7 and Fs61.8Wo33).
Plagioclase-rich clasts have 95 vol.% plagioclase (An96.1).
Mildly shocked; no apparent oxidation or Fe staining. Specimens: 26 g,
NAU: main mass. Boswell.
Eucurites Chart
O/I chart
Eucrites are composed of orthopyroxene, pigeonite, Ca-rich
pyroxenes and plagioclase with minor amounts of chromite, ilmenite, troilite and
Ni-Fe metal. A few eucrites, such as
Moore County
are similar to terrestrial gabbros (plutonic rocks) in having cumulate
textures (preferred orientation of major minerals) that indicate crystal
layering by settling-out in a magma chamber, analogous to the accumulation of
fallen tree leaves into layered piles.
But one defining
parameter is that the composition mean average must contain
Less
than 10% Diogenitic material .
The members of the achondrite classification can be
placed into two specific subgroups:
Classic Modern write up
Northwest Africa 1836
Morocco
Found 2002
Achondrite (monomict eucrite)
A 1102 g partially crusted, single stone was purchased in Rissani, Morocco
in 2003. Classification and mineralogy (T. Bunch and J. Wittke, NAU):
highly contorted mass of eucrite clasts with cataclastic mantles that are in
turn covered by partially melted to recrystallized masses. Cumulate texture
with subophitic enclaves. Clast orthopyroxene, Fs56.4–59Wo2.5–4;
exsolution lamellae, Fs43.4-46.3Wo23-25.6;
recrystallized zone, Fs54.5–57.5Wo8.4–13.5. Chromite,
Cr/(Cr+Al) = 0.85, TiO2 = 14.6 wt. % with exsolution lamellae of
ilmenite; plagioclase, An91–93.6. Shock level, S1 to S5. The
meteorite is very fresh. Specimens, 25.5 g, NAU; main mass, Oakes.
Orlando 28° 32' 51²N
81° 21' 44²W
Orange County, Florida, U. S. A.
Fell 2004 November 8
Achondrite (eucrite)
History: On Monday, November 8, 2004, around 6:15
p.m., Ms Donna Shuford was startled by the noise of something hitting
the side of her house. She discovered that something had hit the top of
her car and ricocheted onto the side of her house. A single stone, mass
~180 g, had fragmented on impact. Petrography and Geochemistry: (D.
Mittlefehldt and M. Zolensky, NASA JSC) Major phases are low-Ca
pyroxene, Wo3En35Fs62, Fe/Mn ~30 with
lamellae of high-Ca pyroxene Wo45En29Fs26
and calcic plagioclase An71-83Ab16-28Or~1.
Minor phases include titanian chromite (TiO2 = 16-20; Al2O3
= 2-3; MgO = 0.4; MnO = 0.8; all wt%); Ilmenite (MgO = 0.5; MnO =
0.9 (both wt%); with silica, iron sulfide, and Fe-Ni metal. The rock is
largely unbrecciated, but has shock veins with granular texture and
containing some glass. Remnant ophitic/sub-ophitic igneous texture is
preserved with plagioclase laths ~1 mm by ~30 µm, and ~2 mm blocky
pyroxene grains. In much of the rock pyroxene has been recrystallized
to ~20-50 µm equant grains while plagioclase retains its original
shape. Classification: Achondrite (monomict basaltic eucrite).
Specimens: A 20 g specimen on deposit at SI. The finder holds the
main mass.
Diogenites
Chart
O/I chart
Diogenites are composed essentially of orthopyroxene
and hypersthene, with minor amounts of plagioclase, olivine, troilite, and
chromite.
Like eucrites, diogenites contain little NiFe metal , most diogenites have been
crushed to some degrees.
The remaining hypersthene crystal fragments are
as large as 6 cm .
With the extreme severity of crushed: individual crystals it
is not readily identified unless viewed in crossed-polarized light.
In this light component fragments show some rotation and small pockets of
crushed material.
Diogenite Groups
1. Monomict Diogenite Group
Example Johnstown
2. Polymict diogenite Group Example NWA 1239
3. Olivine Diogenite Group Example NWA
2115
Polymict Group
The polymict subgroup samples a compositional and textural
continuum of regolith and surface breccias
consisting of eucrites, cumulate eucrites, diogenites, and howardites.
Those meteorites containing more than 90% of a single component are given the
prefix "polymict"
attached to their present description
Polymict eucrites contain less than 10%
non-eucritic material; polymict diogenites contain more
than 90% orthopyroxenite or hypersthene). Those meteorites that contain less
than 90% of any single
component are defined as howardites. While this 10% level is still an arbitrary
dividing line based simply
on mineral proportions, it represents an amount of orthopyroxene that can easily
be detected by X-ray
diffraction techniques. An additional tool to distinguish between polymict
eucrites and howardites involves
pyroxenes in the basaltic clasts; within howardites they are mostly unzoned
whereas in polymict eucrites
they are usually zoned.
Classic write up
Northwest Africa
1648
Morocco
Purchased 2002 October
Achondrite (diogenite, polymict)
A complete fusion-crusted stone (803 g) was purchased from a Moroccan dealer
by A. and G. Hupé (Hupé). Classification and mineralogy (A. Irving
and S. Kuehner, UWS): Heterogeneous breccia composed of multiple
clast types of varying size. Angular mineral clasts are predominantly
orthopyroxene with subordinate anorthite (Ab5), silica, exsolved pigeonite,
ilmenite and troilite. The orthopyroxene clasts are homogeneous and have low
Ca contents, but range widely in Fe/Mg ratio (FeO = 12.6–22.5 wt%, FeO/MnO =
28.2–34.8), suggesting multiple diogenitic parent rocks. Sparse
polycrystalline clasts include cumulate eucrites, basaltic eucrites, a
quench-textured clast consisting of dendritic olivine grains in glass, and
fine grained ferroan inter-growths (breakdown of former pyroxferroite)
composed of hedenbergite + fayalite + silica ± ilmenite ± troilite attached
to anorthite laths. Breccia consisting of various diogenites (>80% by
volume), sparse cumulate eucrites and rare basaltic eucrites. Specimens:
type specimen, 22 g, and thin section, UWS; main mass, Hupé.
Northwest Africa 1877
Morocco
Purchased 2003 June
Achondrite (Olivine-rich diogenite)
A partly crusted stony meteorite (312 g) was
purchased in Zagora by a Moroccan dealer for A. and G. Hupé (Hupé)
in 2003 June and more of the same material (622 g) was purchased in
Tagounite in 2003 December. Classification and mineralogy (A. Irving and
S. Kuehner, UWS): Coarse grained, dense and somewhat friable,
with an overall yellow-green color. Harzburgitic peridotite composed of
subequal amounts of orthopyroxene (Fs22.4-23.3Wo1.5;
FeO/MnO = 25.2-29.7) and olivine (Fa27.8, FeO/MnO = 44-50)
with minor Al-poor chromite [Cr/(Cr+Al) = 84.8-88.0, Mg/(Mg+Fe) =
11.7-19.8], troilite (as abundant blebs within silicates and as
inclusions in chromite), and sparse Ni-free metal (partly altered to
limonite). The modal amount of olivine estimated from three serial thin
sections is approximately 45%, which is consistent with the moderately
high density of this meteorite. The relatively magnesian silicate
compositions are similar to those in Antarctic olivine diogenite
GRA98108, and quite different from those in another Moroccan olivine
diogenite NWA 1459 (Irving et al., 2003). Unlike both of these
examples, however, NWA 1877 does not contain plagioclase. Specimens:
type specimen, 24 g and three polished thin sections, UWS; main
mass, Hupé.
Northwest Africa
2434
Northwest Africa
Purchased 2004
Achondrite (diogenite, metal-rich)
A 441 g partially crusted stone was
purchased in Erfoud, Morocco. Description and classification (J.
Wittke and T. Bunch, NAU): highly brecciated, metal-rich
diogenite with orthopyroxene fragments up to 11 mm in size and
scattered recrystallized pockets. Modal content in vol. %:
orthopyroxene, 86.2; metal, 7.3 (heterogeneously distributed, 4.3 to
13.2 vol. %); plagioclase, 5.5, phosphate, sulfide and chromite < 2.
Orthopyroxene, Fs24.2Wo1.3 (FeO/MnO = 32.7);
Ca-rich pyroxene, Fs8Wo46.9; plagioclase, An95.7;
chromite, cr# = 69; metal, Ni = 6.5 wt. %. Very fresh; shock level,
S2. Specimens: type specimen, 27.6 g, NAU; main mass, Hall.
The following sections present specimen illustrations and descriptions of differentiated achondrites, which includes the HED clan (howardites, eucrites, and diogenites), angrites, aubrites, and ureilites. A summary of the diagnostic mineralogic characteristics of all achondrites is given in the table below (modified after Table 8.1 in Hutchison, 2004, Meteorites: A Petrologic, Chemical and Isotopic Synthesis, Cambridge University Press).
Differentiated Achondrites |
Acapulcoite & Lodranite | 1AB iron silicate & Winonaite | Ureilite | Brachinite | Aubrite | Eucrite | Diogenite | Howardite | Angrite |
Olivine/Pyroxene | <=1 | <1 | >1 | >>1 | <<1 | <<1 | <<1 | <<1 | 1 |
Olivine | Fa3-14 | Fa1-8 | Fa5-25 | Fa30-35 | Fa0 | - | Fa27-35 | Fa8-89 | Fa10-100 |
Olivine Fe/Mn | |||||||||
Orthopyroxene | En86-97 | En91-99 | En80-90 | tr | En100 | tr | En67-77 | variable | - |
Ca-pyroxene | En51Wo44 | min | En75Wo15 | min | En50Wo50 | variable | tr | variable | Wo>50 |
Plagioclase | An12-31 | An8-25 | - | An22-32 | tr: An2-8 | An60-98 | An60-91 | An60-98 | An100 |
Silica | - | - | - | - | - | tr | tr | tr | - |
Kamacite | min | min | min | - | min | tr-min | tr | tr | tr |
Taenite | min | min | - | tr | tr | - | - | tr | - |
Troilite | min | min | tr | min | tr | tr | tr | tr | tr |
Notes: min = minor, <5 vol. %; tr = trace, <0.5 vol. %