Northwest Africa 16532 (NWA 16532)

(Northwest Africa)

Purchased: 2013 Oct

Classification: HED achondrite (Eucrite)

History: A 48.2 g end cut lacking the remainder of the stone was included in a lot of northwest African stones shipped to ET meteorites. Cascadia acquired this end cut on October 18, 2013.

Physical characteristics: Physical Characteristics: The exterior of the end cut is covered by light tan-orange weathering product; patches of dark-brown fusion crust are present. The cut face shows a coarse basaltic texture with white elongate feldspar grains and pale yellowish pyroxene grains. Fractures and light gray veins cut across the face.

Petrography: (M. Hutson, A. Ruzicka, Cascadia): In thin section, the sample is composed predominantly of exsolved high- and low-Ca pyroxene and calcic plagioclase feldspar grains. Plagioclase grains are clouded by inclusions and have undulose to mosaic extinction when viewed in cross-polarized light; some grains show offset twins. Textures vary from fine- to medium-grained granoblastic to coarse-grained sub-ophitic, with many fractures and faults cutting and offsetting sections of the thin section. BSE imaging shows numerous examples of pyroxene lamellae that are deformed or offset by microfaults, with apparent incipient melting adjacent to abundant thin, glassy veins. Of those veins that were analyzed, some are close to pyroxene normative, while others are feldspathic. Examples of schlieren were observed at the boundary between plagioclase glass and pyroxene grains. A silica polymorph is abundant throughout the section. Additional phases observed include ilmenite, an Al-Ti-bearing chromite, fluor-apatite, merrillite, and troilite. No olivine or metal grains were observed.

Geochemistry (M. Hutson, A. Morrelli, A. Ruzicka, Cascadia): Pyroxene and plagioclase feldspar compositions are relatively equilibrated. Low-Ca pyroxene: Fs_64.4±1.0 Wo_2.3±0.6 (range Fs_62.6-66.1Wo_1.6-3.6), molar Fe/Mn=30.5±1.0, N=21; augite (grains and lamellae): Fs_28.8±0.9Wo_43.1±0.9 (range Fs_27.8-30.8Wo_40.4-44.1), molar Fe/Mn=30.9±1.6, N=19; plagioclase feldspar: Ab_10.7±0.8Or_0.6±0.1An_88.7±0.8 (range Ab_9.3-12.1Or_0.4-0.8An_87.3-90.2), N=16.

Classification: HED achondrite (eucrite) based on mineralogy, texture, and pyroxene and feldspar compositions.

Specimens: Cascadia holds 42.9 g in one piece, as well as a polished thin section and material in an epoxy butt.