4 - Enstatite chondrites

Summary

Introduction

Enstatite (E) chondrites are highly reduced meteorites, and as their group name implies, have a mineralogy dominated by iron-free orthopyroxene (enstatite) as the main silicate phase. Because of their reduced nature, iron occurs within enstatite chondrites mainly as metal and sulphides, rather than silicates, and enstatite chondrites have a high abundance of unusual opaque minerals. Not accounting for pairing, as of June 2014, there were 547 records for enstatite chondrites in the Meteoritical Bulletin, of which 17 are observed falls [4.1]. Around 50% have been classified in terms of subgroups (Table 1.2).

Enstatite chondrites are divided into two subgroups on the basis of their bulk iron content [4.2, 4.3]: the EH group (∼30 wt.% total iron) and the EL group (∼25 wt.% total iron). They are also distinguished by metal and sulphide compositions [4.3]. Like the ordinary chondrites the enstatite chondrite group is subdivided by petrologic type, from 3 to 7, but the distribution of petrologic types is not the same between EH and EL chondrites (Figure 4.1). Although there are several unequilibrated (type 3) enstatite chondrites, it is difficult to classify them into petrologic subtypes because of their thermal and shock histories [4.4, 4.5].

Most of the EH chondrites have been heavily shocked, generally to S4, whereas EL chondrites have a lower mean shock level of S2. Several E chondrites are impact melt breccias [4.6], and based on sulphide composition, EH4, EH5 and EH melt rocks can be divided into high- and lowtemperature classes [4.7]. An intermediate grouplet, currently composed of the meteorites Y-793225 (EH6-an) and QUE 94204 (EH7-an), has been suggested [4.8].

Mineralogy and texture

The relative abundance of different components in enstatite chondrites is given in Table 4.1. The chondrule-to-matrix ratio in each group is very similar, but the average size of chondrules in EH chondrites is ∼200 μm; that of EL chondrites is ∼600 μm [4.9, 4.10]. In both groups chondrules are bigger than metal grains [4.10].

Calcium, aluminium-rich inclusions

Calcium, aluminium-rich inclusions (CAI) are much less abundant in enstatite chondrites than in carbonaceous or ordinary chondrites, comprising <1 vol.% of the meteorites (Table 4.1).