Mineralogy, Mineral-Chemistry, and Composition of the Murchison (C2) Meteorite

Abstract

The Murchison meteorite shower, September 28, 1969, occurred in and around Murchison, Victoria, Australia. Chemical and mineralogical analyses established it as a type II carbonaceous chondrite (C2). Murchison consists largely of fine-grained black matrix which has been identified as primarily a mixture of two iron-rich, low-aluminum chamosite polytypes. Contained in the matrix are four main types of inclusions: (1) single crystals and crystal fragments, (2) loosely aggregated clusters of crystals (&quot;white inclusions&quot;), (3) discrete true chondrules, (4) xenolithic fragments of two other meteorite types (mostly a unique kind of C3 chondrite).<br/>The first type of inclusions consists of unzoned and highly zoned olivines, unzoned (disordered and ordered) orthopyroxenes, clinoenstatite, and rare diopside. Prominent minor phases are calcite, chromite, metal (with occasional traces of schreibersite), troilite, pentlandite, and two phases that could not be fully characterized.<br/>The second type of inclusions consists primarily of grains of olivine (Fa 0 to Fa 40), lesser low-Ca pyroxenes, and minor spinel, calcite, whewellite, hibonite, perovskite, chromite, pentlandite, and rare Ca-pyroxene.<br/>The true chondrules consist of olivine, Ca-poor pyroxene, occasional metal, and, in rare instances, one of the poorly characterized phases. The chondrules are not texturally typical of the ordinary chondrites, but resemble more closely those chondrules seen in C3 and C4 chondrites.<br/>The fourth type of inclusion consists mainly of distinct xenolithic fragments of a light blue-gray chondrite type that resembles certain C3 chondrites (like Vigarano), though not in all aspects. These xenolithic fragments consist of disequilbrated olivines and pyroxenes, abundant pentlandite and troilite, and virtually no metal. In addition, a single xenolithic fragment was found of an unknown meteorite type.<br/>Ca- and Al-rich glasses (of varying compositions) are found as blebs, with or without gas bubbles, contained within olivine crystals. The average Ca/Al ratio of these glasses approximates that for all meteoritic matter. They may represent early (nonequilibrium) subcooled condensates from the solar nebula. This nonequilibrium stage was apparently followed by equilibrium condensation through intermediate to low temperatures at which the layer-lattice phases condensed in abundance and incorporated crystals and fragments of the higher temperature phases.