21001-149 - NWA Possible Achondrite Meteorite Hypabyssal Igneous Rock. In study. 6.1 g

Petrographic description

The sample is mainly composed of euhedral to subhedral clinopyroxene and olivine phenocrysts in a fine-grained groundmass. Clinopyroxene (augite) crystals are zoned, occasionally showing green non-pleochroic core. The outer rim of augite crystals show light brown color, probably produced by transformation to pigeonite. Olivines have oxidized edges and mostly transformed to iddingsite.

Under polarized-light microscope (crossed light), orthopyroxenes show a complex extension patterns. Phenocrysts have fractures filled by glassy material. Clinopyroxenes have 1-2 mm in size, although some grains reach size values higher than 4 mm. Orthopyroxene phenocrysts are also zoned although less abundant (Figs. 2 and 3).

Considering the minerals in the phenocrysts, augite represents the 22 % of the total volume, olivine the 13 % and orthopyroxene 5 %. The matrix of the sample is fine-grained, composed by augite, plagioclase and abundant ilmenite and magnetite as metallic phases (Fig. 4). Chlorite and barite are scarcer and appear as alteration, filling cracks. Chondrules of plagioclase and pyroxene crystals are also observed. The texture is porphyritic and holocrystalline, similar to basalt or hypabyssal magmatic rock.

The scanning electron microscopy in backscattered electron mode (BSE) images highlight the abundance of augite in the matrix. The zonations of pyroxene crystals are remarkable. Fe-Ti oxides are common in the matrix (Fig. 4).

EDS analyses indicate a high aluminum (0.45-0.90 apfu, atoms per formula unit) and a low calcium (0.48-0.80) content in pyroxenes. Olivines have also moderate aluminum (up to 0.25 apfu) and high Mg (Fo= 0.86) composition.

According to texture features, the analysed sample is similar to a basalt or hypabyssal igneous rock (Le Maître et al., 2005), although rock-forming minerals are rare. Remarkably, the pyroxene zonations and the inverse formation of pigeonite to the augite-rim could be an indication of the extraterrestrial origin of this sample.

Mineralogy and textural properties of samples were studied on (30m-thick) polished thin sections using transmitted and reflected light microscopy at LPA (UA) and UGR.

Photomicrographs were performed by using:
o Petrographic microscope: ZEISS Assioskop (LPA-UA).
o Digital camera: Photometrics CoolSNAPcf
o Image capture Software: RS ImageTM v.1.8.6

Samples (polished thin-sections) also were studied under scanning electron microscope (Research Technical Services – UA). Minerals were located and identified using BSE mode on Hitachi S3000N SEM coupled with an X-ray detector Bruker XFlash 3001 for microanalysis (EDS) and mapping.

Semi-quantitative EDS X-ray microanalysis was performed using EDS spot analysis with acquisition
time 60s and maximum process time to achieve best resolution of peaks in spectra.

ANNEX II. REFERENCES
Le Maître, R.W. (Editor), Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., Bellieni, G., Dudek, A., Efremova, S., Keller, J., Lameyre, J., Sabine, P.A., Schmid, R., Sørensen, H. & Woolley, A.R. 2005. Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Cambridge University Press. Second Edition, revised, 236 p. Cambridge, UK.