NORTHWEST AFRICA 1235


Aubrite-anomalous
(possibly ungrouped)
standby for northwest africa 1235 photo
Found 2000
no coordinates recorded

An 80 g weathered meteorite was found at a location in Northwest Africa and subsequently purchased by a private collector. The analysis of Northwest Africa 1235 was conducted at the Vernadsky Institute in Moscow, and it was determined that the dark brown fusion crust and other features were consistent with a weathering grade of W2–W3 and that it had been shocked to stage S2/3 (Lorenz et al., 2003). The meteorite consists of 77 vol% essentially pure enstatite, with 8 vol% interstitial plagioclase. Enstatite contains micro-inclusions of glass, troilite, FeNi-metal, silica, schreibersite, niningerite, and a Mn-rich phase that might be a terrestrial decomposition product of alabandite.

The sulfides are predominantly Cr–Ti-bearing troilite (7 vol%), and occur throughout the various phases. The low Ti content in troilite indicates that NWA 1235 experienced more oxidizing conditions than all other enstatite meteorites. Oldhamite and sphalerite are also minor components, as well as sulfides of the FeS–MgS–MnS solid solution series. This sulfide solid solution normally serves as a reliable type indicator in E chondrites: alabandite (MnS) occurs only in EL chondrites, while niningerite (MgS) is present only in EH chondrites. The sulfides from this solid solution series identified in NWA 1235 fall within a range intermediate between alabandite and niningerite, a range that was previously observed only in some EH melt rocks and in the EH7-anomalous meteorite Itqiy.

Si-bearing FeNi-metal constitutes 4.8 vol% of this meteorite, and is present mostly as mm-sized aggregates. These aggregates enclose prismatic enstatite (a common feature among E chondrites), schreibersite, troilite, and graphite. Phlogopite, a rare F-bearing phyllosilicate associated with plagioclase, was identified in NWA 1235. This mineral is proposed to be a residual melt phase, and has previously been identified only in the EH melt rock Y-82189, and in the CV3 chondrite Grosnaja—in Grosnaja it occurs as sodium phlogopite replacing Ca-rich minerals in chondrules and CAIs.

The micro-inclusions (up to 20µm in size) of silica- and alkali-rich glass are ubiquitous in enstatite, with their shapes being controlled by the host enstatite crystal structure. These glass inclusions have also been discovered within enstatites in the Aubres and Norton County aubrites and the E-chondrite (EL6/7; IMR) Happy Canyon, as well as in other meteorite and terrestrial sources by Varela et al. (1998), and have undergone extensive study. The existence of these glass inclusions in NWA 1235 is consistent with their hypothesis supporting an initial origin in the solar nebula as a multi-phase precipitant from a fluid and/or vapor. Sometime thereafter, this pseudo-crystalline arrangement was affected by dehydration processes causing these ions to be exsolved and form glass inclusions within crystallized enstatites, exhibiting variable glass/bubble ratios.

Although NWA 1235 is a reduced achondrite with close similarities to other enstatite meteorites, it has many compositional features that are unique compared to the typical E chondrites and aubrites. It has a significantly higher abundance of FeNi-metal, and a higher content of Si within this FeNi-metal, compared to the aubrites; however, these abundances are similar to those of EL chondrites. NWA 1235 has a Ni/Co ratio that is distinct from all E chondrites, and all aubrites except Mayo Belwa. Indications are that this is a unique achondrite that is genetically related to the enstatite meteorite clan. The photo of NWA 1235 shown above is a 1.05 g fusion crusted partial slice.