Aubrites


Aubrites are enstatite achondrites that consist principally of nearly Fe-free enstatite with minor amounts of NiFe metal, troilite, plagioclase, olivine or diopside. Aubrites are named after the type example, Abres, that fell in France in 1836. They are igneous in origin and crystallized in an extremely reducing (oxygen-poor) magma that promoted the formation of small amounts of rare to unique minerals, for example, oldhamite (CaS), which is not found in terrestrial rocks.

Shallowater aubrite. Large crystals are enstatite with small olivine and augite grains. Height, 1.8 mm. Imagw © 2005 T. E. Bunch.

Nearly all of the aubrites are breccias, similar to ALH84007 above. Breccia clasts are remnants of huge enstatite crystals that may have been as large as 10-15 cm and are set in a matrix of crushed enstatite and the minor minerals. The Shallowater aubrite is unbrecciated which is unique among classic aubrites. There is a weak to moderate preferred orientation of enstatite crystals that suggests a cumulate origin or crystal settling in a magma chamber. This implies that the parent body was of sufficient size (>100 km in diameter) to have at least a weak gravitational field.

Summary of Aubrite Charactersitics

Minerals and compositions:

Mineral modes (vol%): Enstatite >75, plagioclase 1-16, diopside 0.5-3, olivine 1-10

Texture: Igneous, brecciated, unbrecciated; grain size: fine- to coarse-grained

Aubrite Poster PDF (large file) - presented at MetSoc 2008


Larned 38°11.97'N, 99°9.72'W

Kansas
Find: 1977
Achondrite (aubrite, anomalous)

History: Found by an anonymous landowner on his farm in 1977, about 1 mile north and 4 miles west of Larned, Kansas. The mass was brought to the Big Well in Greensburg, Kansas in April 2007 and purchased by Dr. Donald Stimpson.
Physical characteristics: A single mass weighing 28.1 kg surrounded by grayish to brownish, shaley weathering rind. The interior is mostly gray with metal visible in places, and it is transected by anastomosing, thin, dark brown veinlets.
Petrography (A. Irving and S. Kuehner, UWS; T. Bunch and J. Wittke, NAU): Polymict breccia composed mostly of large metal-poor clasts, but with locally abundant metal in the matrix. The major phase is near-pure enstatite (up to 1.2 cm, many exhibiting abundant fractures, mosaicism, and planar deformation features), with accessory diopside, schreibersite, sodic plagioclase glass, silica, Cr-bearing troilite, daubreelite, niningerite, Si-bearing kamacite, and rare taenite. One 1.4 × 1.0 mm clast is a websterite composed of 52 vol% enstatite, 40 vol% diopside, and 8 vol% schreibersite and sulfides (including an iron sulfide phase compositionally similar to greigite). Silica exhibits ballen structures and occurs as inclusions in enstatite and is locally abundant in the matrix. Secondary veinlets are composed of a hydrous iron silicate compositionally similar to greenalite.
Geochemistry: Enstatite (Fs0.1-0.2Wo0.4-1.1), diopside (Fs0.2Wo47), plagioclase glass (~Ab79Or21), kamacite (Si 1.352.2 wt%, Ni 5.96.6 wt%), taenite (Si 1.1 wt%, Ni 12.6 wt%).
Classification: Achondrite (aubrite, anomalous). This large specimen contains more metal and more silica polymorph than most aubrites, and has experienced a high degree of shock (producing fractured enstatite, ballen silica and feldspathic glass).
Specimens: A total of 190 g of sample and two polished thin sections are on deposit at UWS. The main mass is held by Dr. D. Stimpson (Donald Stimpson, Kansas Museum and Nature Center, 21255 K Street, Haviland, KS 67059).


Northwest Africa 4832

Morocco
Find: 2006
Achondrite (aubrite)

History and physical characteristics: Hundreds of platey fragments, the largest weighing 33.1 g, were found in the Western Sahara region of Morocco and purchased by A. Aaronson in 2007. The total known weight is 268g. Fragments tend to break along the preferred orientation plane of constituent enstatite grains. Most of the dark brown pieces are moderately to heavily weathered, a few show small, fresh metal nuggets.
Petrography (T. Bunch and J. Wittke, NAU): composed mostly of fine-to medium-grained (0.3 to 1.2 mm), euhedral to subhedral cumulate enstatite together with interstitial metal, schreibersite, graphite, a silica phase present as round inclusions in FeS, FeS, daubreelite, brezinaite and very fine-grained intercumulus plagioclase. Much of the metal is oxidized. The stone is unbrecciated with a low shock level.
Mineral chemistry: Enstatite is essentially Fe-free (En99.6Fs0.1Wo0.3). Kamacite is Si-enriched (Si = 4.5 to 5.6 wt. %) and FeS contains 2.6 wt. % Ti. The rare mineral brezinaite contains S, 45.2 wt. %; Cr, 36.9 to 46.6 wt. %; Fe, 18.7 to 9.0 wt. % and 1.1 wt. % Mn.
Classification: Achondrite (aubrite).
Type specimen: A total of 20.6 g is on deposit at NAU. Mr. A. Aaronson is the main mass holder.


Northwest Africa 4871

Morocco
Find: 2006
Achondrite (aubrite)

History and physical characteristics: Sixty-one pieces with a total weight of 906 g were purchased in Tagounite, Morocco in May 2007. Most of the stones are medium to dark brown with some remnant fusion crust.
Petrography: (T. Bunch and J. Wittke, NAU) A brecciated cumulate rock with clasts as large as two cm. The intact lithology is dominated by subhedral to anhedral polysynthetically twinned enstatite with fine-grained intercumulus intergrowths of feldspathic glasses and a silica phase. Other minerals include graphite, FeS, daubreelite, schreibersite, niningerite, perryite as exsolution lamellae in Si-bearing kamacite and Zn-bearing brezinaite. Large clasts show a multitude of sub-parallel, closely spaced compression fractures. Shock level is S3 and the weathering grade is W5 for small pieces (<3 cm) and W3 for the interiors of larger specimens.
Mineral compositions: Enstatite is essentially Fe-free (Fs0.1Wo0.3), kamacite Si content is 3.1 to 3.6 wt. % and perryite is (in wt. %): Si, 12.4; Fe, 4.5; Ni, 77.5 and P, 3.8. Zincian brezinaite is (in wt. %): Cr, 43.2; S, 44.1; Fe, 6.3; Mn, 1.2; Zn, 5.1.
Classification: Achondrite (aubrite), possibly paired with NWA 4799.
Type specimen: A total of 22.3 g are on deposit at NAU. Mr. A. Aaronson holds the main mass.

NWA 4871. Base width = 7 mm. Image © T. E. Bunch, 2008.

NWA 4871. BSE image of NiFe metal exsolving perryite, (NiFe)5(SiP)2. Image © J. H. Wittke, 2008.


Northwest Africa 5217

Morocco
Find: 2007
Achondrite (aubrite)

History and physical characteristics: A 39.1 g complete and fully crusted stone was found in Morocco in 2007 and given to the present owner at the Tucson Gem and Mineral Show in January, 2008.
Petrography (T. Bunch and J. Wittke, NAU): A fine to medium-grained (<1.2 mm) unbrecciated, cumulate-textured stone is composed essentially of enstatite with interstitial Si-bearing kamacite, troilite, daubreelite, brezinaite, oldhamite, alabandite, niningerite, caswellsilverite, graphite, plagioclase, a silica phase, and schöllhornite, a weathering product of caswellsilverite. Weathering grade is low as is the shock level (S2).
Mineral compositions: Enstatite is En99.4Fs0.2Wo0.4, metal is Si-rich kamacite (Ni = 5.8-6.6 wt. %; Si = 3.9-4.5 wt. %). Oldhamite (in wt. %): S = 38.7, Ca = 59.2, Cr = 1.4, Mn = 1.2; brezinaite (in wt. %): S = 46.0, Cr = 54.1; caswellsilverite (in wt. %): S = 45.3, Cr = 38.4, Na = 14.7, Fe = 0.7; troilite (in wt. %): Cr = 2.68, Ti = 3.38; plagioclase: An17Or3.7; and schöllhornite (in wt. %): S = 46.0, Cr = 37.3, Fe = 1.1 (remainder is oxygen and hydrogen).
Classification: Achondrite (aubrite).
Type specimen: A total of 8.5 g and one thin section are on deposit at NAU. The holder of the main mass is anonymous.

NWA 5217. Base width = 8 mm. Image © T. E. Bunch, 2008.