Northwest Africa 2999 (NWA 2999)

NWA 2999 was discovered in the Sahara Desert during 2004 in the form of twelve stones totalling 392g. They were later classified as only the 10th member of the exceedingly rare Angrite group. NWA 2999 has caused significant excitement and controversy as it differs from all other known Angrites.

A very interesting abstract was written regarding the origin theories for NWA 2999. It is titled "Unique Angrite NWA 2999: The Case For Samples From Mercury" by A.J. Irving, S. M. Kuehner, D. Rumble, T. E. Bunch and J. H. Wittke. This study says that although most Angrites have textures indicative of rapid cooling from melts, NWA 2999 has an overall plutonic, polygonal-granular texture but with distinctive large anorthite, spinel and recrystallised olivine porphyroclasts and discontinuous anorthite coronas around spinel grains which is thought to be a unique feature among meteorites.

This study also asks one of the most captivating questions in meteoritical science... Are Angrites From Mercury? There is no definitive answer to this question but arguments for this theory of origin were made in this study and mention that "Angrites are mafic to ultramafic "igneous" or metamorphic rocks that probably derive from a large, differentiated planetary body; yet, in the absence of any "ground truth" the possibility that they are samples from Mercury rests only on circumstantial arguments." These include:

• The virtual lack of Sodium implies a highly refractory planet (near the Sun?).

• Oxygen isotopic compositions are close to and parallel to the TFL (Terrestrial Fractionation Line). (Like planetary rocks from Earth, Moon, Mars and Vesta).

• Preserved corona textures in NWA 2999 require a parent body capable of km-scale tectonic uplift of lithospheric material (by thrust faulting?).

• Each Angrite specimen is texturally different with a unique CRE age. (CRE - Cosmic Ray Exposure).

• The wide range in CRE ages (55 to <6.1 Ma) suggests that the parent body is large enough to be struck repeatedly and may still exist.

• Very ancient formation ages (>4.555 Ga) imply very rapid core segregation and cooling following parent body accretion (consistent with contraction?).

• Dynamical calculations predict that ~1% of material ejected from Mercury could reach Earth.

• The limited shock effects may mean that some Angrites, including NWA 2999, were ejected by spallation; others may be impact melts. (Could vesicles in some quenched specimens be trapped impact rock vapour?).

The specimen below is an endcut from one of the twelve stones and displays what appears to be shiny black fusion crust on the backside. This endcut is part of the Meteorites Australia Collection (MA.08.0023).

NWA 2999 (Angrite) - 1.35g Crusted Endcut.
Enlargement ---> 1500 x 986 (355KB)

NWA 2999 (Angrite) - 1.35g Crusted Endcut.
Enlargement ---> 1500 x 980 (325KB)