3 - Ordinary chondrites

Summary

Introduction

The ordinary chondrites (OC) are the largest class of meteorites: from a total of 49,407 records in the Meteoritical Bulletin (update of June 2014), 43,101 (87%) are ordinary chondrites [3.1]; see Table 1.2. Of course, this does not take pairing of desert meteorites into account. Nonetheless, a similar percentage obtains when only records for meteorites observed to fall are considered: of the 1264 observed falls, 870 are OC, some 70% [3.2].

The OC class is subdivided on the basis of chemistry into three groups, the H, L and LL chondrites, for high total iron, low total iron and low total iron plus low metallic iron [3.3–3.7]. On the basis of their mineralogy, a small group of about five OC (including Burnwell) has been recognized as being extremely low in FeO, but with total iron and iron metal even higher than H-group chondrites (‘HH chondrites’) [3.8]. There are also several meteorites (e.g., Tieschitz) that are transitional between groups [3.9]. There are two groups of iron meteorites with silicate inclusions associated with OC: silicates in IIE irons are related to H-group chondrites [3.10, 3.11], whilst silicates in IVA irons have affinities with LL-group meteorites [3.12, 3.13].

As outlined in Chapter 1, the three main ordinary chondrite groups are subclassified into petrologic types that are a measure of the extent of thermal processing experienced by a meteorite. All three groups exhibit the full spectrum of petrologic types from 3 to 6 (Figure 3.1 and see Table 1.2). The most unequilibrated chondrites of petrologic type 3 are known as the UOC (for unequilibrated ordinary chondrites) [3.14, 3.15], and these can be further divided into petrologic subtypes from 3.0 to 3.9 [3.16]. The UOC are the least abundant of all the OC. Even subtracting the effect of large pairing groups of desert OC by considering only meteorites observed to fall, the most abundant meteorites are still H5 and L6 (Figure 3.1) with UOC being ∼3% of all falls [3.2].

The three chemical groups of OC have similar mineralogies, but different mineral chemistries. Although each group is an assortment of olivine, pyroxene and plagioclase, with accompanying Fe-Ni-metal and metal sulphides (Table 3.1), each group also exhibits a specific range of silicate and metal compositions [3.9, 3.17–3.19].