Northwest Africa 4590 meteorite (NWA 4590), Sidi Ali Caïdat, Er-Rissani Cercle, Errachidia Province, Drâa-Tafilalet Region, Moroccoi

Regional Level Types
Northwest Africa 4590 meteorite (NWA 4590)	Meteorite Fall Location
Sidi Ali Caïdat	Caïdat
Er-Rissani Cercle	Cercle
Errachidia Province	Province
Drâa-Tafilalet Region	Region
Morocco	Country

Angrite, Cumulate
213 g — in small fragments, minor weathering

In 2006 scattered fragments from an apparently recent fall were found within several meters of each other near the border between Morocco and Algeria. The coarse-grained stone is composed of Clinopyroxene (33 vol%), Anorthite (28 vol%), Olivine (14 vol%), Kirschsteinite (5 vol%), Ulvöspinel (18 vol%) with minor accessories accounting for ~ 5 vol%. Exsolution lamellae in several phases plus grain boundary glasses indicate an original mafic or ultramafic rock which was subject to very rapid cooling — perhaps after an unusually violent impact ejection from deep within a very ancient homeworld.

Northwest Africa 4590 (or, NWA 4590) is an angrite — a small class of 21 listed stones (probably representing 13 or 14 separate falls). Angrites are basaltic achondrites — products of differentiating processes on a putative parent body. They are characterized by their low alkali contents, high Ca/Al ratios, and a varying, but distinctive mineral assemblage marked by several Al-Ca-Ti-rich phases which may include Ca-rich olivine with exsolved Kirschsteinite. Angrites have oxygen isotope ratios similar to those of the HED meteorites (likely fragments of asteroid Vesta). At the present time, however, on geochemical grounds, any close association between the HED achondrites and the angrites does not seem likely to most meteoriticists.

One thing is agreed upon, however. Angrites are derived from a very ancient homeworld. Formation of the NWA 4590 mineral assemblage occurred approximately 4.558 billion years ago. Several other angrites, including prototype Angra dos Reis, have similar ages (±1-2 million years). Other angrites apparently formed ~5 million years earlier. One hypothesis is that the younger Angrites originated from deeper realms within the putative parent body and cooled more slowly than the older angrites formed near the surface. Eventually, impacts of varying violence ejected both young and old rocks into space where happenstance and gravity have brought a few of them to rest upon the planet earth where an even smaller subset have been viewed by very interested eyes.

Angrite (Differentiated Achondrite)
Found 2006, 213 g

Northwest Africa 4590 (NWA 4590), a small purchased meteorite find with a fresh crust and mild weathering stains, appears to be a separate and unpaired meteorite — consisting only of impact-shattered fragments of a single meteorite fall. NWA 4590's principle constituents — fassaite [33 vol%], anorthite [28 vol%], fayalitic olivine [14 vol%], and kirschsteinite [5 vol%] — are the usual major constituent of almost all angrites. Its overall inventory of minor constituents is also quite similar to those reported for other angrites. However, unlike almost all recovered angrites, NWA 4590 appears to lack ferric iron. It also appears to have been more completely characterized than several other more massive and/or earlier recovered angrites. Rhönite and pigeonite, for example, have been reported and, more recently, a previously unidentified 'Ca-rich Silico-Phosphate' has been identified as a Si-rich apatite.

NWA 4590 is one of only 23 recorded angrites — a group of differentiated and quite ancient meteorites with high-temperature melt products and isotopic ratios similar to their prototype, Angra dos Reis (stone). While Angra dos Reis is the only angrite fall, it is somewhat unclear how many separate angrite falls are in our collections. 17 angrites from Northwest Africa have been listed at the Meteoritical Bulletin Database (March 2015), but whether they represent only a few falls or, say, 10-12 separate falls is not evident at this time. [Many NWA meteorite fragments have been distributed as purported 'separate' meteorites to enhance sales].

A word of caution. Some reports of apparent Fe-Ni metal in angrites have been labelled as 'kamacite' in published abstracts without accompanying compositional data. As at least some of the metal in angrites is Ni-poor (<2 wt%), it is not clear that such uncharacteristically Ni-poor iron should be given the conventional label for meteoritic iron. Meteoritic iron with an invariable higher Ni-component is found in almost all chondrites and irons. Rare instances of Ni-poor iron in meteorites are reported almost exclusively as tiny secondary phases in some differentiated achondrites.

Other Regions, Features and Areas containing this locality

References