Brachinites


Summary of Brachinite Characteristics

Minerals and compositions:

Mineral modes (vol%): Olivine >80, Ca-pyroxene 4-15, chromite up to 3

Texture: Equigranular metamorphic (polygonal); grain size: mean <2 mm


Northwest Africa 4872

Algeria
Find: 2007
Achondrite (brachinite)

History and physical characteristics: A 3000 g partially crusted, complete stone was purchased in Rissani in 2007. This stone is very dark in both crusted and wind eroded surfaces and has a low weathering grade.
Petrography (T. Bunch and J. Wittke, NAU): Protogranular texture with a strong bimodal olivine grain size: medium-sized grains range from 0.3 to 1.1 mm (mean = 0.6 mm) and the smaller grains range from 0.02 to 0.3 mm (mean = 0.18 mm). Weathering grade is low to moderate; most of the tiny metal grains are oxidized. The shock level is S2. Mineral modes (in vol. %): olivine, 85; Ca-pyroxene, 3; chromite, 5; Cl-apatite. 3; metal, 2 and sulfides, 2.
Mineral compositions: Olivine is Fa35.1 (FeO/MnO = 81); Ca-pyroxene is Fs10.3Wo47; chromite Cr/(Cr+Al) = 0.74, metal, Ni = 3.5 wt %, with pentlandite and troilite.
Oxygen isotopes (D. Rumble, CIW): Replicate analyses of acid-washed material by laser fluorination gave, respectively: Δ17O, -0.229 and -0.254; δ18O, 2.061 and 2.012; and δ18O, 4.354 and 4.308.
Classification: Achondrite (brachinite).
Type specimen: A total of 20.4 g are on deposit at NAU. Mr. Adam Aaronson is the main mass holder.

NWA 4872. Base width = 9 mm. Image © T. E. Bunch, 2008.


Northwest Africa 4874

Algeria
Find: 2007
Achondrite (brachinite)

History and physical characteristics: A 28g nearly complete single stone was purchased in Erfoud in 2007. The stone is partially crusted and has a low degree of surface weathering with limonitic staining.
Petrography (T. Bunch and J. Wittke, NAU): Dunitic rock with a medium-grained (mean size = 0.6 mm) protogranular texture. Mineral modes (vol. %): olivine, 90; Ca-pyroxene, 4; chromite, 3; plagioclase, 2; merrillite, 1; and metal 1. Shock level is S2.
Mineral compositions: Olivine is very homogeneous at Fa34.1 (FeO/MnO = 80). Augite is Fs12Wo48.3, plagioclase is An40.7, metal has 3.15 wt % Ni and chromite Cr/(Cr+Al) = 0.73.
Classification: Achondrite (brachinite).
Type specimen: A total of 6 g are on deposit at NAU. Mr. Adam Aaronson holds the main mass.

NWA 4872. Base width = 12 mm. Image © T. E. Bunch, 2008.


Northwest Africa 4876

Northwest Africa
Find: 2007
Achondrite (brachinite)

History and physical characteristics: a 130 g stone was purchased in Tagounit, Morocco in June 2007. Surface is well polished by desert wind ablation with very little oxidation.
Petrography (J. Wittke and T. Bunch, NAU): Dunitic rock with very fine to medium-grained (0.02 to 1.35 mm) protogranular texture. Mineral modes (vol. %): olivine, 90; Ca-pyroxene, 5, plagioclase, 2, chromite; metal, and FeS, 3. No phosphates were found. Very low weathering grade and the shock level is S2.
Mineral compositions: Olivine is Fa33.4 (FeO/MnO = 68), Ca-pyroxene is Fs13Wo43.8, plagioclase is An38.1, chromite Cr/(Cr + Al) = 0.73, and metal Ni = 4.1-6.4 wt %.
Classification: Achondrite (brachinite).
Type specimen: A total of 20.2 g is on deposit at NAU. The main mass holder is anonymous.

NWA 4872. Base width = 9 mm. Image © T. E. Bunch, 2008.


Northwest Africa 4969

Northwest Africa
Find: 2007
Achondrite (brachinite)

History and physical characteristics: A 180g incomplete stone was purchased in Erfoud Morocco by M. Cimala in 2007. Partially covered by a thin transparent ablation crust.
Petrography (J. Wittke and T. Bunch, NAU): Fine to medium-grained (0.1 to 1.2 mm, mean = 0.75 mm) with protogranular texture. Mineral modes (in vol. %): olivine, 89: Ca-pyroxene, 6; plagioclase, 2.5; chromite, 1.5; metal and FeS, 1. Weathering grade is very low, with minor Fe staining. Shock level is S2.
Mineral compositions: Olivine is Fa34.7, FeO/MnO = 72-75; diopside is Fs10Wo46.4 (Cr2O3 = 0.88-1.2 wt %; Al2O3 = 1.24 wt %), FeO/MnO = 42; plagioclase is An34.6 and chromite Cr/(Cr + Al) = 0.71. Metal is predominately taenite (Ni = 20.1 wt %).
Classification: Achondrite (brachinite).
Type specimen: A total of 20.6 g are on deposit at NAU. The main mass holder is M. Cimala.

NWA 4872. Base width = 6 mm. Image © T. E. Bunch, 2008.