Enstatite Chondrite Guidlines
Enstatite chondrites Chart
These chondrites were heated, but not to the 1200° C melting point. Composed of the minerals pyroxene and plagioclase feldspar, they are graded into high iron content (EH) and low iron content (EL) groups. These chondrites can have the highest content of free metal and sulphides found in meteorites.
These chondrites are highly reduced with all of the iron visible as metal or troilite (FeS). The silicate consists mainly of the iron-free pyroxene, enstatite. As with the ordinary chondrites, a subdivision is made based on the bulk iron content; the EH group contains ~30% total iron, while the EL group contains only ~25%. Each subgroup comprises a complete range of thermally metamorphosed types, from 3 through 6 (or 7), with the Si and Ni content in kamacite increasing with respect to increasing petrographic type of each series.
The following mineralogical relationships have been found to differentiate the subgroups:
Northwest Africa 1840
A 132.8 g complete and partially crusted stone was purchased in Erfoud, Morocco in August 2003. Description and classification (T. Bunch and J. Wittke, NAU): The meteorite has an igneous texture; it is a brecciated enstatite orthopyroxenite with a series of prominent, parallel compression fractures (0.1 to 0.8 mm spacing) that span the entire sample. These fractures acted as conduits for weathering; most weathering effects are limited to areas that border these fractures. Mineral modes: enstatite, 92 vol. %; Si-bearing metal, 4 %; plagioclase (maskelynite), 2 %; high Cr-troilite, 2 %; graphite, < 1%. Enstatite, FeO = 0.03 to 0.64 wt. %; CaO = 0.06 to 0.38 wt %.; Cr-rich troilite, Cr = 10.2 to 14.7 wt. % with Mn < 1.15 % and Ti < 1.76 wt. %. Largest metal grains (0.6 – 1.0 mm dia.) have graphite inclusions < 0.1 mm in size; metal, Ni = 6.5 wt. % (range = 6.2 – 6.9), Si = 1.25 wt. % (range = 0.9 – 1.81); plagioclase, An42. None of the exotic sulfides common to aubrites and enstatite chondrites were found. Shock level, S4 (parallel fractures, reduced birefringence, and mottled extinction in enstatite; all plagioclase is maskelynite); weathering grade is W2/3. Specimens: type sample, 21.1 g (NAU); main mass, Oakes.