7 - Brachinites

Summary

Introduction

Brachinites are achondrites that formed early in Solar System history. They consist almost exclusively of olivine; if they were terrestrial rocks, they would be classified, petrographically, as dunitic wehrlites [7.1]. It is not completely clear whether brachinites are primitive achondrites or more differentiated cumulate meteorites [7.2]. According to the Meteoritical Bulletin, not accounting for pairing, there are currently (June 2014) 37 brachinites, none of which was observed to fall (www.lpi.usra.edu/meteor/metbull.php). The type specimen, Brachina, was found in South Australia in 1974, and originally thought to be a second chassignite [7.3]. It was recognized as a unique meteorite almost a decade after it was found [7.4], and the brachinite group was proposed following the recovery of several additional meteorites [7.5–7.8]. The olivine-rich achondrite LEW 88763 is recorded as a brachinite in the Meteoritical Bulletin, but has characteristics that suggest it might be a unique achondrite [7.9]. The Tafassasset meteorite has also been grouped with the brachinites, but it too may be unique [7.10].

Mineralogy

The most complete descriptions of brachinites are given by [7.1, 7.11], from which much of the following information is derived.

The main mineralogical component of brachinites is olivine, with variable amounts of high-Ca pyroxene (augite or diopside), iron sulphides, chromite (0.5–2 vol.%), traces of orthopyroxene, minor phosphates (mainly chlorapatite and whitlockite) and Fe-Ni metal (Table 7.1). The composition of olivine and pyroxene is shown in Figure 7.1. Olivine compositions range from Fa₂₉ to Fa₃₆, with molar Fe/Mn ratio of 52–77 and CaO content generally around 0.10 wt.% [7.7, 7.11]. Clinopyroxene composition is Wo_38–45 with mg# of 79–84; orthopyroxene, when present, has Wo_2–4 and mg# of 71–74 [7.1, 7.9–7.12].

Chrome-rich spinels are Ti-poor in comparison with those of equilibrated ordinary chondrites [7.1, 7.11] and have molar Cr/(CrþAl) ratios ranging from 0.73 to 0.83 [7.11]. Sulphides are mainly troilite with 0.4–3% Ni, whilst the rare metal grains are taenite containing 19–55 wt.% Ni and 1–2 wt.% Co [7.1, 7.5, 7.11]. Brachina and EET 9940n both contain up to 10 vol.% plagioclase (An_14–40), whilst the other brachinites contain almost none. The same two meteorites have up to 0.26 wt.% CaO in olivine [7.5, 7.12, 7.14]. A plot of CaO vs. Cr₂O₃ in olivine distinguishes brachinites from other primitive achondrites, as well as from HEDs (Figure 7.2).