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NWA 12774 Angrite

NWA 12774 angrite was purchased from a Mauritanian dealer in March 2019, changed hands in Ensisheim in June and we bought material in August to thin section. Angrites are rare ancient achondrites which crystallized on the parent body as early as only 4 million years after the condensation of CAI. Samples have a wide distribution of cosmic ray exposure ages suggesting that the parent body or bodies are large and might still exist. Candidates include the asteroids 289 Nenetta, 3819 Robinson, 5261 Eureka and the planet Mercury.

In early 2020 there are only 21 known unpaired angrites. With two exceptions they are considered either coarse-grained plutonic angrites or fine-grained quenched angrites. NWA 12774 is among the latter. Its texture is porphyritic with olivine and pyroxene crystals set in a fine groundmass.

 

 

Part slice 40mm long. Dark matrix and an olivine megacryst.

 

 

Thin section from half of the above partial slice.

 

 

Field of view (FOV) is 23mm wide. Cross-polarized light (XPL).

 

 

FOV=14mm.

 

 

FOV=8mm.

 

 

Compositionally zoned skeletal olivine in center surrounded by smaller zoned olivine crystals and unzoned pyroxene crystals all embedded in a dark matrix. FOV=3mm. XPL.

 

 

The quench textured matrix is parallel laths of feldspar (anorthite, the calcium end member), olivine (magnesium rich), kirschsteinite (calcium rich olivine), pyroxene (augite, a calcium rich solution) and minor other minerals.

 

 

FOV=3mm. XPL.

 

 

Spinel crystals are transparent brown here in PPL and black in XPL (as long as the spinel occupies the full thickness of the thin section). Because they crystallize in the isometric (cubic) system they are isotropic with respect to transmitted light. That is, their refractive index is the same in all directions. All polarized light that passes through is extinguished by a subsequent polarizing filter at 90 degrees to the first.

 

 

Spinel at lower right. Rounded opaque object to the left is probably metal, staining the adjacent minerals. FOV 3.5mm PPL.

 

 

Same field. XPL.

 

 

The parallel laths of the groundmass are seen more clearly here because when the thin section was cut only a thin layer of them remained overlying a transparent mineral grain. FOV=0.35mm. XPL.

 

 

Same view with brown spinel top right. PPL.

 

We asked Tony Irving about the spinel. He did the original classification of NWA 12774. From experience and an earlier examination he told us it is a chrome spinel. For more details Tony sent our uncovered polished thin section to Paul Carpenter, his collaborator at Washington University in St. Louis. Paul made a number of back-scattered electron (BSE) images and element maps for us. He also probed the cores and rims of four spinel grains to quantify compositional zoning.

The spinel group of minerals has many members and variants, among them spinel s.s., franklinite, magnetite, ulvöspinel and chromite. Here, our Cr-spinel grains lie in a range between
(Mg,Fe)Cr2O4 and (Mg,Fe)Al2O4.

For the probed spinel grains Paul sent extensive data that I’ve drastically condensed. Percentages do not total 100% due to rounding and my elimination of minor component contributions. The numbers show that cores contain more chromium than rims. As the crystal grew the rims became more aluminum rich as the chromium in the melt was depleted.

Back-scattered electron images depict minerals with higher mean atomic numbers in lighter shades.

Wavelength-dispersive X-ray spectrometry (WDS) element maps show detailed distribution and relative concentrations of individual elements.

This BSE image has two bright spots near the center. FOV= 2.3mm.

 

 

The corresponding iron map confirms our suspicion that these are blebs of metal.

 

 

We’ve seen that olivine in this angrite viewed in XPL appears clearly zoned. These element maps show the particular compositional differences. Magnesium content is high in the cores and reduced toward the rims (orange fading to yellow). Iron concentration is low in the cores and rises somewhat toward the rims (blue to lighter blue).

 

 

And back to Cr-spinel. The grain nestled into olivine in the center of this BSE image appears only vaguely zoned but the element maps below reinforce what the numbers have told us. FOV= 0.6mm.

 

 

The element maps show aluminum content is moderate in the core and higher at the rim (orange rising to red). Chromium is moderate in the core and diminishes toward the rim (yellow/ orange to blue). Iron might rise very slightly from core to rim.

 

Many thanks to Paul Carpenter and Tony Irving.

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