Northwest Africa 16719 (NWA 16719)

(Northwest Africa)

Purchased: 2019

Classification: HED achondrite (Eucrite, polymict)

History: The meteorite was purchased from a nomad in Mali in the region of El Guettara in 2019. A 22.7 g piece was sent to UAb, where a thin section was prepared. The thin section and 20.7 g were sent to Cascadia for classification. Mr. Bourki sent an additional 5.2 g piece to Cascadia. A 0.2 g piece was sent to UNM for oxygen isotope analysis.

Physical characteristics: Physical Characteristics: The exterior of the end cut is covered by a bumpy, dark gray weathering rind. The cut face consists of a medium gray, fine-grained material surrounding angular, small (? 1mm across) white clasts. Subtle changes in texture and color indicate the presence of cm-scale clasts. A diffuse, darker-gray band cuts across the surface and partially rims some of the cm-scale clasts.

Petrography: (M. Hutson, A. Ruzicka, Cascadia): Much of the thin section is composed of a groundmass of angular grains of pyroxene and feldspar surrounding clasts with poorly defined boundaries. There are a handful of small monomineralic pyroxene clasts rimmed and veined by fayalitic olivine. The section contains a wide variety of lithic clasts, including rare subophitic basaltic eucrite clasts; gabbroic and cumulate eucrite clasts; one diogenite clast; numerous melt breccias comprised of normal or reversely zoned pyroxene fragments set in microlitic pyroxene/feldspar groundmasses; numerous clasts of symplectitic fayalite, low- and high-Ca pyroxene and silica; one clast with a symplectitic intergrowth dominated by ferrous augite, silica and troilite, with small amounts of remnant ferrous pigeonite, plagioclase feldspar, fayalitic olivine, and a phosphate mineral. One clast contains a large (~250 × 200 µm area) ovoid intergrowth comprised of cellular intergrowth kamacite and troilite typical in texture to shock melted metal/troilite clasts found in ordinary chondrites attached to a silicate area with numerous small magnesian olivine grains and irregularly zoned pyroxene grains in a microlitic feldspar/pyroxene groundmass. Another angular clast has a devitrified glassy mesostasis with ferromagnesian grains that grade across the clast from zoned Mg-rich orthopyroxene to more ferrous pigeonite crystals that form dendrites and then to skeletal olivine crystals. Two thin glassy shock veins cutting across clasts and one small glassy melt pocket were observed. The sample is moderately weathered with calcite veins cutting across the section.

Geochemistry: (M. Hutson, A. Ruzicka, Cascadia; K. Ziegler, UNM): The thin section as a whole is highly unequilibrated. The clast containing the cellular intergrowth of kamacite and troilite appears to be a fragment of an H-chondrite shock melt that has begun to homogenize with surrounding eucritic material, based on olivine (Fa_15.0±1.9, N=5) and orthopyroxene (Fs_20.2±2.5, N=5) compositions and bulk composition of metal (Ni/(Ni+Fe) = 8.1±0.6 wt%, N=8). Excluding this chondritic clast, the compositions of the major silicate phases in the section are: olivine Fa_75.8±11.4 (range Fa_48.4-85.0), Fe/Mn=40.9±3.6, N=19; low-Ca pyroxene Fs_45.6±15.4Wo_3.6±0.9 (range Fs_18.0-64.2), Fe/Mn=29.0±2.4, N=50; pigeonite Fs_47.8±11.4Wo_7.6±2.7 (range Fs_28.7-61.1Wo_5.1-18.8), N=62; augite Fs_32.9±6.9Wo_39.3±3.8 (rangeFs_15.7-56.2Wo_20.5-43.1), N=38; plagioclase feldspar An_90.0±3.2Ab_9.3±2.3Or_0.9±.0.4, N=34. The angular clast with a devitrified glassy matrix has pyroxene grains which grade from Fs_18.0Wo_1.9 to Fs_33.1Wo_18.3 (N=4) as they approach a change in mineralogy within the clast to olivine with Fa_49.1±1.1 (N=2). The one identified diogenite clast has pyroxene grains with the composition Fs_28.8±1.9Wo_3.8±0.2, Fe/Mn=29.9±0.9, Mg#=70.1±2.1, N=6. A typical cumulate eucrite clast has low-Ca pyroxene: Fs_34.5±0.6Wo_4.7±0.8, Fe/Mn=29.7±0.7, Mg#=63.8±0.5, N=3; plagioclase feldspar: An_90.5±0.8Ab_9.0±0.7Or_0.4±0.1, N=2. A typical basaltic eucrite clast with subophitic texture has low-Ca pyroxene: Fs_61.3±1.5Wo_5.3±1.7, Fe/Mn=30.4±0.8, Mg#=35.3±0.6, N=6, plagioclase feldspar: An_90.2Ab_9.4Or_0.4, N=1. Oxygen isotopic compositions of three acid treated aliquots with masses 2.10 mg, 2.60 mg, 1.10 mg: δ¹⁷O = 1.956,2.000,1.765, δ¹⁸O = 4.281,4.340,3.915, (all linearized, slope of 0.528).

Classification: HED achondrite (Eucrite, polymict), based on mineral chemistries and Oxygen isotopes.

Specimens: Cascadia holds 25.7 g in three pieces, as well as a polished thin section; Ahmed Salek and Omar Bourki hold the main mass.