NWA 2092

standby for northwest africa 2092 photo
purchased 2003
no coordinates recorded A fresh (W1), mostly complete stone, weighing 320 g, was purchased in Morocco by an American dealer. A portion of the meteorite was sent to Northern Arizona University for classification (T. Bunch and J. Wittke), and it was determined that NWA 2092 is a transitional type LL6/7. Features of this chondrite, such as brecciation, thermal annealing along fractures and clast borders, and a variable grain size in the matrix are consistent with a shock metamorphism/recrystallization event; the shock stage is S3.

Northwest Africa 2092 is an ordinary chondrite which exhibits features that do not permit an unambiguous resolution of petrologic type, but instead, exhibits features intermediate between types 6 and 7. Type 7 ordinary chondrites were originally defined by Dodd et al. (1975) according to specific petrographic characteristics. They listed three metamorphic criteria to distinguish between petrologic types 6 and 7, though a definite hiatus is not readily resolved:

  1. the presence of poorly defined chondrules in type 6, but only relict chondrules in type 7
  2. low-Ca pyroxenes in type 6 contain no more than 1.0 wt% CaO (1.0 wt% = ~1.9 mol% Wo), but more than 1.0 wt% in type 7; conversely, the CaO content of high-Ca pyroxenes decreases from type 6 to type 7
  3. feldspar grains gradually coarsen to reach a size of at least 0.1 mm in type 7

In the intervening time since Dodd et al. proposed their classification parameters, additional type 7 chondrites have been found and studied. Following more recent studies at Northern Arizona University, it was proposed by T. Bunch (pers. comm., 2004) that a type 7 category should not comprise meteorites containing any relict chondrules (such as those present in low abundance in NWA 2092), but rather, should represent a metamorphic extreme in which no sign of chondrules remains. This would lump those meteorites containing ‘poorly defined’ chondrules, and those containing ‘relict’ chondrules, into the type 6 category. Following the present scheme of Dodd et al. however, the presence of relict chondrules in NWA 2092 is still consistent with an assignment as type 7.

The low-Ca pyroxene in NWA 2092 has an average CaO content of 1.0 wt%, which is consistent with a borderline, transitional 6/7 type (strictly defined, perhaps a Dodd type 6). Furthermore, the size of plagioclase grains was used by Dodd et al. to delimit a boundary between types 6 and 7: the coarsening of grains to at least 0.1 mm defines a type 7. However, Bunch and Wittke suggest that the relative size of all of the silicates would provide a better gauge—silicates attain an equigranular texture only under the highest metamorphism. They have also discovered that simple twinning of plagioclase occurs only in type 7, and suggest that this could be utilized as an additional parameter.

Furthermore, it was revealed by Bunch and Wittke that modal metal contents decrease significantly during late metamorphic stages; i.e., low-Ni metal, as well as pyroxenes, are consumed to produce olivine, resulting in only small amounts of Ni-rich metal and lower amounts of orthopyroxene and clinopyroxene compared to those amounts in lower metamorphic grades. Based on all of the data, it was concluded that NWA 2092 best fits a transitional 6/7 classification. The photo above shows a 2.38 g partial slice of NWA 2092. The photo below is a thin section in crossed polarized light which shows the variable grain size of the silicates.

standby for northwest africa 2092 photo
Photo courtesy of T. Bunch—NAU

Leave a Reply