Northwest Africa 12605 (NWA 12605)

(Northwest Africa)

Purchased: 2017 Jun 18

Classification: HED achondrite (Eucrite)

History: Purchased 18 Jun 2017 at Ensisheim Show from a seller named Mr. Ismaily.

Physical characteristics: A slice with a thickness of 3 mm was availabe. Surface as well as interior are well preserved and appear fresh. Fusion crust mostly preserved, flowlines visible. Porous interior. Some pores are filled around the surface with secondary minerals.

Petrography: Macroscopic: The cut surface of the interior appears gray-brown, a somewhat darker and irregular rim of 1-5 mm is visible along the surface. Mineral and lithic clasts up to 9 mm are well visible. The sample is noticeably porous. The largest lithic clast is comparably coarse grained (plag. up to 2 mm) and could be a part of a cumulate (no analysis available). A narrow whitish vein transects the slice. Microscopic: The thin section consists of larger lithic and mineral clasts within a fine grained matrix that appears reequilibrated. The brecciated nature of the sample is revealed by the presence of lithic clasts with contrasting fabrics. The matrix minerals appear to have coalesced into larger grains, thereby hiding their primary clastic origin. The porosity is also observable on the microscopic scale. Locally a very fine grained rim of minerals covers the pore sufaces. Observed minerals are Ca-poor pyroxene of ferrosilitic to pigeonitic composition (with fine exsolution lamellae mainly below 10um) and Ca-rich pyroxene of augitic composition (larger grains in part poikilitic). A silica phase is common and locally shows evidence of subgrain formation. Some lager plagioclase grains show a slight patchy and undulous extinction. Several larger pyroxene grains contain irregular fractures that end at the borders of the grains. Larger, coarser grained lithic clasts have a gabbroic texture and could be derived from cumulates. Additionally observed phases are troilite, Ti-rich chromite, ilmenite, zircon and iron metal. Terrestrial calcite was observed in veins.

Geochemistry: Composition of Ca-poor pyroxene (pigeonite or othopyroxene) (n=47): Fs_52.90±3.09Wo_7.55±3.92 (range Fs_42.78-56.16Wo_3.75-18.80); Fe/Mg = 1.34, Fe/Mn 28.61. Composition of Ca-rich pyroxen (clinopyroxene) (n=21): Fs_29.79±4.08Wo_35.61±5.23 (range Fs_25.66-40.88Wo_21.22-40.26); Fe/Mg = 0.96, Fe/Mn 27.84 No olivine found in thinsection. Plagioclase composition (n=45): An_90.10±2.50Or_0.42±0.16 (range An_83.4-93.59Or_0.23-1.04). Larger plagioclase grains show a slight increase in xAn towards the core. Bulk analysis of cut surface by pXRF (wt%): Fe/Mg = 4.12±0.44, Fe/Mn 29.8±0.5 (n=10), Fe = 13.0 wt.% (n=10). Oxygen isotopes: (R. Greenwood, OU): δ¹⁸O=3.775‰, δ¹⁷O=1.746‰ Δ¹⁷O=-0.217‰.

Classification: This meteorite is a reequilibrated basaltic eucrite breccia based on oxygen isotopes, mineral compositions and whole-rock pXRF-analyses. There is evidence for a slight Stannern trend.

Specimens: A total of 25.1 g and one polished section at NMBE