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
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).
corona textures in NWA 2999 require a parent body capable of
km-scale tectonic uplift of lithospheric material (by thrust
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.
formation ages (>4.555 Ga) imply very rapid core segregation
and cooling following parent body accretion (consistent with
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
NWA 2999, A Unique Angrite with a large chondritic component.