Enstatite Chondrite

Type of meteorite high in the mineral enstatite and also referred to as E-chondrites. Although they contain substantial amounts of Fe, it is in the form of Ni-Fe metal or sulfide rather than as oxides in silicates. Their highly reduced nature indicates that they formed in an area of the solar nebula that was very poor in oxygen (low oxygen fugacity). The oxygen isotope composition plots close to the terrestrial fractionation line (TFL). Previous research suggests that E-chondrites formed in the inner regions of the protoplanetary disk, possibly inside the orbit of Mercury, and they are therefore considered to be candidates for the building blocks of the terrestrial planets.

Important note: Many websites incorrectly identify the difference between EH and EL chondrites as being based on amount of bulk iron: EH chondrites contain ~30% Fe (high bulk iron), whereas EL chondrites have ~25% Fe (low bulk iron). This determination was based on a low number of E-chondrites available at the time and no longer correct.

The reduced character of E-chondrites is reflected in their mineralogy. They consist of nearly pure end member enstatite, Si-bearing kamacite, Ti-Cr-bearing troilite(FeS), and Mg-, Ca-, Mn-bearing sulfides. The latter sulfides are rare minerals and include niningerite (MgS), alabandite(MnS), and oldhamite (CaS), mostly unobserved in any terrestrial rocks. Unlike other chondrite classes, troilite (typically 5–17 wt%) and Si-bearing Fe,Ni metal (17–28 wt% ) are major constituents. Due to their bulk composition, enstatite chondrites have traditionally been subdivided into EH (high bulk iron) and EL (low bulk iron) chondrites.

Updated Enstatite Chondrite Group Characteristics

Chondrule Size0.2 mm0.6 mm
Metal Abundance~10 vol%~10 vol%
Si Content in Metal~3 wt%~1 wt%
Mineral Assemblages in Metalniningerite (MgS) &
perryite (Fe-Ni silicide)
ferroan alabandite ((Fe,Mn)S)

However, recent research based on the systematic evaluation of 80 different enstatite chondrites has revealed new subgroups.

New enstatite chondrite subgroups:

TroiliteCr < 2 wt%Cr > 2 wt%Cr < 2 wt%Cr > 2 wt%
(Mn,Mg,Fe)SFe < 20 wt%Fe > 20 wt%Fe < 20 wt%Fe > 20 wt%
KamaciteNi < 6.5 wt%Ni > 6.5 wt%Ni < 6.5 wt%Ni > 6.5 wt%

Since mineral chemistry does not correlate with the petrologic type, the degree of recrystallization is the only fundamental feature for defining the petrologic types of E chondrites. As it is not possible to distinguish type 4 from type 3 by the characteristics of opaque phases, it is suggested that olivine can only be a mineral of the type 3 E chondrite subgroups.

Significant portions of the above definition were derived from the 2018 paper, “Chemical variations of sulfides and metal in enstatite chondrites—Introduction of anew classification scheme.” M. Weyrauch, M. Horstmann, and A.  Bischoff.

Another paper with interesting information is the 2009 paper, “ENSTATITE CHONDRITE PHYSICAL PROPERTIES: DENSITY, POROSITY AND MAGNETIC
SUSCEPTIBILITY.”  Robert J. Macke SJ, et. al.