Northwest Africa 16178 (NWA 16178)

(Northwest Africa)

Purchased: 2020 Aug

Classification: Ordinary chondrite (H3-5)

History: Mr. Jasper Spencer purchased six stones from a Mauritanian dealer in August 2020 via FB Messenger. Cascadia received six end cuts (one from each stone) totaling 21.2 g.

Physical characteristics: Physical Characteristics: Cut faces of all six stones showed that they are breccias. Abundant metal was visible in all six stones. Four of the end cuts displayed rounded or ovoid clasts (largest 2 cm × 1.3 cm) that were medium gray-brown separated by a black matrix. The remaining two end cuts displayed more angular clasts, also medium gray-brown, separated by a black matrix. Two thin sections, one with rounded clasts and one with angular clasts, were prepared for analysis.

Petrography: (M. Hutson, G. Nelson, A. Ruzicka, Cascadia) Both thin sections are comprised of clasts with discernable chondrules that grade into surrounding matrix. Plagioclase feldspar grains up to 50 µm are present. Olivine grains predominantly show undulose extinction and irregular fractures, suggesting that the medium gray-brown chondritic clasts have not been heavily shocked. Clasts are separated by a mix of opaque veins entraining blackened clasts in which individual small grains can be seen. BSE imaging shows that these clasts mainly contain chondrules and chondrule fragments containing zoned Mg- and Fe-rich olivine grains. Chondrules and fragments in these chondrule-bearing unequilibrated clasts are both crosscut and surrounded by a network of metal/sulfide veins. BSE images show the presence of silicate melt rock and metal-troilite grains with a cellular texture in one well-studied patch. Additionally, there are four opaque angular clasts in one thin section, and a fifth in the second section which lack a chondritic texture, and are comprised of small aligned rounded silicate grains crosscut by abundant small parallel metal and sulfide veins. The network of parallel veins stops abruptly at the clast edge. The largest of these clasts is 700 × 500 µm across. Chemical analysis showed that olivine and pyroxene grains in one of the five heavily veined clasts are unequilibrated, while olivine and pyroxene grains in the remaining four heavily-veined clasts are equilibrated.

Geochemistry: Olivine: average for the medium gray-brown equilibrated chondritic clasts: Fa_19.8±0.7 (N=46); the largest chondrule-bearing unequilibrated clast: Fa_17.4±10.9 (N=27). The heavily-veined angular clasts were consistently slightly more iron rich than either the equilibrated or unequilibrated chondritic clasts, with the average for three equilibrated heavily-veined clasts: Fa_20.5±0.5 (N=19), and one unequilibrated heavily-veined clast: Fa_19.5±13.7 (N=12). Pyroxene: average for the medium gray-brown equilibrated chondritic clasts: Fs_17.4±0.6Wo_1.3±0.2 (N=49); the largest unequilibrated chondritic clast: Fs_10.3±7.3Wo_1.1±0.9 (N=16); three equilibrated heavily-veined patches: Fs_18.5±0.8Wo_1.5±0.5 (N=17); unequilibrated heavily-veined patch: Fs_12.1±9.2Wo_1.5±1.1 (N=11).

Classification: Ordinary Chondrite (H3-5). Chemistry and texture of the medium gray-brown clasts best fits with an H5 designation, whereas the unequilibrated clasts with chondrules best fit with H3. This breccia contains clasts with varying shock stages. Olivine deformation in the medium gray-brown equilibrated clasts is consistent with a shock stage of S2. The unequilibrated chondritic clasts are substantially opaque in plane-polarized light, due to the presence of silicate and metal-sulfide melts which are visible in BSE images. The presence of melt suggests a shock stage for these clasts of S4 or S5.

Specimens: Cascadia holds 17.3 g in six pieces, as well as two polished thin sections and material in epoxy stubs. Mr. Jasper Spencer holds the main mass.