Northwest Africa 8707 (NWA 8707)

Morocco

Purchased: Feb 2012

Classification: Ordinary chondrite (L, melt rock)

History: Purchased by T. Ono in Tucson in Feb. 2012

Physical characteristics: A single stone of 64.5 g, ~70% covered with fresh black fusion crust. Broken surfaces reveal a light gray interior with abundant shock melt veins.

Petrography: (T. Mikouchi, UTok). Faintly brecciated (cm-to-mm scale) igneous rock mainly composed of olivine, pyroxene and feldspathic glass with variable grain sizes ranging from 0.05 mm to 1 mm. Most pyroxenes are larger than 0.5 mm and poikilitically enclose olivine chadacrysts. Olivine is rounded in shape and its size ranges 0.05-0.5 mm. Feldspathic glass is interstitial to olivine and pyroxene. There are some areas where feldspar is birefringent under petrographic microscope. Both olivine and pyroxene exhibit undulatory extinction and shock veins (~ 0.2 mm wide) cut through the section, suggesting severe shock metamorphism. In shock melt veins some olivine grains are transformed to ringwoodite. Although the shock metamorphism is extensive, the original igneous texture is well preserved. Mineral abundances are 53 olivine, 18 feldspathic glass, 15 low-Ca pyroxene, 4 high-Ca pyroxene, 4 Fe-Ni metal, 4 troilite, 1 chromite and 1 apatite (in vol%).

Geochemistry: (T. Mikouchi, UTok). All phases show fairly homogeneous compositions. Olivine is Fa_23.6±0.9 and its FeO/MnO ratio is 35-60 (N=256). CaO and Cr₂O₃ are <0.05 wt%, respectively. Low-Ca pyroxene is Fs_19.7±0.9Wo_3.3±1.0 with 0.3-0.4wt% Al₂O₃ and 0.3-0.6 wt% Cr₂O₃ (N=89). Its FeO/MnO ratio (wt%) is 25-40. High-Ca pyroxene is Fs_9.0±1.0Wo_42.0±1.9 with 0.5-1.0 wt% Al₂O₃, 0.3-0.4 wt% TiO₂, 0.7-1.0 wt% Na₂O and 1.1-1.7 wt% Cr₂O₃ (N=34). Feldspathic glass is Si-rich (71±2.2 wt% SiO₂, 23.2±1.1 wt% Al₂O₃) and does not show a stoichiometric composition of feldspar (N=144). CaO, Na₂O and K₂O are 1.5-3.5, 1.0-1.5 and 0.4-0.9 wt%, respectively. There is no compositional difference between optically isotropic and anisotropic areas. Chromite contains 58-59 wt% Cr₂O₃, 28-29 wt% FeO, 4.5 wt% Al₂O₃, 3 wt% MgO and 2 wt% TiO₂ (N=18). Kamacite has 6 wt% Ni and 0.9 wt% Co. Taenite contains 18 wt% Ni and 0.7 wt% Co.

Classification: L melt rock (strongly shocked). Texturally similar to acapulcoite/lodranite, but mineral compositions are in the range of equilibrated L chondrites.

Specimens: 15.4 g at UTok. Main mass with T. Ono