Northwest Africa 10518 (NWA 10518)

(Northwest Africa)

Purchased: 2004 Feb

Classification: Ordinary chondrite (L3-7)

History: A single stone was obtained by Edwin Thompson from a Moroccan trader in Tucson, Arizona at the February 2004 Tucson Gem and Mineral Show. A portion (24 g) was donated and Patrick Thompson sold the main mass (85.6 g) to Cascadia.

Physical characteristics: Fusion crust, weathered patina, and dust partly cover exterior. Cut faces reveal light colored, partly reddish stained clasts and chondritic texture (chondrules, fragments, metal) with a grey matrix. One prominent large clast is ~2 × 1 cm across; a grey inclusion is also visible.

Petrography: (A. Ruzicka and K. Farley, Cascadia) In thin section, a breccia composed of an intimate mixture of mineral and chondrule fragments with diverse (type 3-6) characteristics and some larger clasts; areas between chondrules and clasts composed largely of smaller clasts. BSE imaging shows that most of the rock ("host") is composed of type 4-6 lithologies, with equilibrated olivine, and plagioclase commonly ~35-100 μm across, intermixed with well-defined chondrules and clasts with magnesian silicates. Prominent large clast contains granoblastic silicates, metal, relict chondrules, and feldspar which is especially coarse (length 75-326 μm, average 143±59 μm, N=35). Gray inclusion is ~8 mm long in thin section and composed chiefly of fine-grained zoned olivine set in glass.

Geochemistry: (K. Farley and A. Ruzicka) Host dominated by equilibrated olivine (median Fa_24.8, N=72) and low-Ca pyroxene (median Fs_21.0, N=53) but containing more varied olivine (Fa_0-39.5) and low-Ca pyroxene compositions (Fs_1.8-26.5). Overall average host including mixture of equilibrated and more varied compositions is olivine (Fa_21.3±7.8, Fe/Mn = 42.9±18.6 at., N = 72), low-Ca pyroxene (Fs_18.6±5.3Wo_1.6±1.4En_79.8±5.9, Fe/Mn = 23.5±9.3 at., N = 53), high-Ca pyroxene (Fs_9.6±2.4Wo_43.5±2.5En_46.9±1.6, Fe/Mn = 11.1±7.3 at., N= 29), feldspar (Ab_82.8±3.4An_11.7±2.5Or_5.4±2.1, N = 34). Prominent large clast has olivine (Fa_25.7±0.7, Fe/Mn = 53.9±20.5 at., N = 10), low-Ca pyroxene (Fs_22.0±0.9Wo_1.9±0.5En_76.0±1.0, Fe/Mn = 23.4±10.0 at., N = 7), high-Ca pyroxene (Fs_9.5±2.2Wo_43.8±2.3En_46.7±1.5 at., N = 8), feldspar (Ab_82.2±1.3An_11.2±1.0Or_6.6±1.0 at., N = 10).

Classification: L3-7 finely intermixed genomict breccia. Type 5-6 lithology implied by abundant equilibrated olivine and pyroxene and common coarse feldspar grain sizes; type 3 lithology implied by presence of magnesian olivine in chondrules and clasts; large prominent clast with unusually coarse feldspar can be described as type 7. Gray inclusion resembles shock melt observed in other ordinary chondrites.

Specimens: Cascadia holds the entire sample, which consists of 104.0 g in two pieces (85.6 and 18.4 g), additional small fragments (0.8 g), a polished thin section, and a potted butt.