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Erg Chech 002 Achondrite-ung

Erg Chech 002 Achondrite-ung was found in the sand dunes of southwest Algeria in May 2020. Stones range in size from pebbles to a few weighing in the 4 kilogram range. Total weight could be about 45 kilograms. Classification was done by Tony Irving, University of Washington (Seattle), Paul Carpenter at Washington University (St. Louis) and Karen Ziegler at University of New Mexico (Albuquerque).

This is a lightly shocked igneous rock with a groundmass of pyroxenes, feldspars and minor minerals in which are included large crystals of pyroxene. The megacryst compositions differ considerably from each other and are generally more magnesian and chromium rich than diogenite pyroxene. The feldspar composition is quite sodic for a meteorite. That is, it is higher in sodium (and lower in calcium) than expected. This difference and its oxygen isotopes distinguish it from regular HED meteorites and also from a certain small group of anomalous eucrites.

In thin section this meteorite is visually striking with the megacrysts, their rims and the prismatic pyroxene grains embedded in the pyroxene / plagioclase feldspar groundmass.

 

 

Several pyroxene megacrysts stand out in the largely pyroxene groundmass. Plagioclase appears white to grey in cross-polarized light photos. This image and the next were made on a scanner. Sample is about 33mm long.

 

 

This sample is also about 33mm long. Note the patches of brown from terrestrial weathering.

 

 

A cluster of opaque grains of metal and chromite. Weathering products have spread among the surrounding mineral grains. Plane polarized light. Field of view is 3.1mm wide.

 

 

The opaque cubic grains are chromite. PPL, FOV=1.5mm.

 

 

In the same field, metal is white and chromite is light gray in this reflected light.

 

 

An 8.7mm wide field. Tan megacryst with decorated rim.

 

 

5.1mm wide field crossed by long prisms of pyroxene with a megacryst in lower right.

 

 

FOV=3.1mm.

 

 

Dark green object in center is a cross section of a pyroxene crystal. FOV=3.6mm.

 

 

Two cross sections. FOV=3.1mm.

 

 

Four cross sections. FOV=3.1mm.

 

 

Several megacrysts. FOV=7.2mm.

 

 

FOV=3.1mm.

 

 

FOV=3.1mm.

 

 

FOV=3.1mm.

 

 

Field of view is 3.7mm wide.

 

 

The same field but with the crystal on the right not in optical extinction. The grain does have a void.

 

 

 

 

A portion of the 7mm long megacryst above. FOV=3.1mm.

 

 

Space object passing through atmosphere.

 

 

Longitudinal sections through long pyroxene crystals. FOV=3.1mm.

 

 

Upper portion of the central crystal above. FOV=0.34mm.

 

 

FOV=9.0mm. Note alignment of fine grained overgrowth surrounding the megacryst.

 

 

A 1mm long portion of the crystal in the center is shown below. FOV=3.1mm.

 

 

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