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Northwest Africa 4925 (NWA 4925)

NWA 4925 was discovered in the Sahara Desert in Northwest Africa during 2007 and purchased in Erfoud, Morocco by a private collector. It was in the form of a single 282.3g fragment which was partly covered by fusion crust. The stone was classified as a moderately weathered Martian meteorite; specifically an Olivine-Phyric Shergottite. The NWA 4925 classification mentions that it is severely shocked with melt pockets. It also mentions that the "meteorite displays a porphyritic texture [Large-grained crystals (phenocrysts) in a fine-grained matrix.] with large chemically zoned olivine megacrysts set into a fine-grained groundmass composed of pyroxene and maskelynite. The olivine megacrysts often contain melt inclusions and small chromites." This meteorite displays a light weathering rind around the edge. The olivine megacrysts (phenocrysts) are weathered (oxidised) to a bright pink/orange colour but darken to a grey/black colour in the unweathered centre. This can be seen in the first photograph below where the outer edge is on the left has the brightly coloured phenocrysts which darken towards the right side of the slice. The specimen below is part of the Meteorites Australia Collection (MA.08.0009).

Planetary meteorite collector, Norbert Classen, has made some very interesting observations regarding NWA 4925 which I will share below with his permission.

"NWA 4925 exhibits a pronounced light colored weathering rind, typical for desert finds with long terrestrial residence ages. Olivine phenocrysts near to or within the weathering rind do often show a bright red appearance, a sign that most of the iron within these olivines has been oxidized in the terrestrial environment. Overall, the matrix color within or near to the rind is more reddish - reminding us of the same process that gives Mars its red appearance although most of the Martian rocks are originally of grey, or grey to green color. The interior of NWA 4925 is actually dark green, with shock altered dark-brown olivine phenocrysts set in a matrix of more fine grained greenish pyroxenes and dark maskelynite, both bearing witness for the fact that the interior of the rock is more or less pristine and fresh, just as if it left Mars only yesterday. It's that interesting contrast of the more light-colored weathering rind with its neat reddish olivines, and the dark-green, very fresh interior that makes NWA 4925 visually most attractive, and scientifically interesting because it models the surface weathering on the Red Planet. From the samples of NWA 4925 that I could study I would further suspect that the olivine and orthopyroxene phenocrysts show a preferred orientation like in NWA 1195, and other ol-opx-phyric shergottites, something which is indicative of magmatic flow prior to the cooling and solidification of the sample. Other most interesting features are melt inclusions, and small chromites that are enclosed into the large olivine phenocrysts."

NWA 4925 (Shergottite) - 0.364g Partslice
NWA 4925 (Shergottite) - 0.364g Partslice.
Enlargement ---> 1000 x 759 (265KB)

NWA 4925 (Shergottite) - 0.364g Partslice
NWA 4925 (Shergottite) - 0.364g Partslice.
Enlargement ---> 1000 x 770 (335KB)


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