20999-59 - NWA Possible Achondrite Meteorite Porphyritic Basalt. In study. 192 g

Petrographic description

Under petrographic microscope, the sample is a fine-grained volcanic rock with olivine and pyroxene phenocrysts, surrounded by a fine-grained matrix (Figs. 2 and 3). Phenocrysts show deformation, developing twins and cracks. Olivine grains have a thin rim (Figs. 2 and 3) of iddingsite (reddish border). Pyroxene is pale brown and presents some plagioclase inclusions. The matrix is formed by tabular euhedral plagioclase and anhedral pyroxene crystals, and ilmenite
grains. Volcanic glass do not appears in the sample. Accessory minerals are apatite and magnetite (Fig. 4).

The crystal size of olivine is 0.5-1.3 mm and 1.0-2.0 mm for pyroxene. In the matrix, plagioclase, pyroxene and ilmenite microcrysts present a crystal size in the interval 50-140 μm. The percentage of olivine and pyroxene phenocrysts in the sample is respectively 12 and 10%. The matrix contains 50% of plagioclase, 35 % of plagioclase and 15 % of ilmenite.

The sample has an olivine and pyroxene chondrule. Olivines are totally transformed to iddingsite. The sample has porphyritic texture.

Figure 4 shows olivine and pyroxene phenocrysts surrounded by plagioclase, pyroxene and ilmenite microcrysts. Olivine and pyroxene do not show chemical variation, except the transformation of the former to iddingsite.

Figure 5 displays the EDS analyses indicates pyroxene is augite, with Ca = 0.76 apfu (atoms per formula unit), proportion of magnesium Mg# = 0.87, and low Al = 0.29 apfu. Olivine is magnesium rich (Fo = 0.90).

The rock is porphyritic basalt (Le Maître et al., 2005). Possible achondrite meteorite.

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: USB UI-1490SE
o Image capture Software: uEye Cockpit (IDS).

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.