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.
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.
Many thanks to Paul Carpenter and Tony Irving.