SOLAR SYSTEM/Meteorites 231 Table 1b Differentiated meteoritesà Table 1a Undifferentiated meteorites Class Symbol Example Class Symbol Example Carbonaceous chondrites CI CM CO CV CK CR CH R K LL L H EL EH Orgueil Murchison Ornans Allende Karounda Renazzo ALH 85085 Rumuruti Kakangari Saint Mesmin L’Aigle Wiluna Eagle Saint Sauveur Irons I AB Campo del Cielo Stony irons IC II AB II C II D II E II F III AB III CD III E III F IV A IV B Mesosiderites Rumurutiites Kakangari type chondrites Ordinary chondrites Enstatite chondrites Carbonaceous chondrites: characterized by sparse to abundant chondrules set in a dark, friable matrix of carbon rich compounds, phyllosilicates, mafic silicates, Ni Fe metal, and glass The letter symbols separate groups based on different mineralogy, relative abundance of different lithophile and siderophile elements, relative abundance and size of chondrules, and oxygen isotope signatures Numerical suffixes 3, 2, and denote progressive aqueous alteration and 3, 4, 5, and progressive thermal alteration Rumurutiites: a new rare group of chondrites Kakangari type chondrites: a small group of chondrites now separately defined Ordinary chondrites: chondrules are embedded in a finely crystalline matrix of mafic minerals, pyroxene, and olivine, together with NI Fe metal and glass Some are recystallised thermally and lose the definition of chondrules and the glass The H, L, and LL groups differ in the magnesian/iron ratio in the ferromagnesian silicate minerals The number suffixes denote degree of thermal alteration (loss of original texture and recystallization) Enstatite chondrites: these are chondrites with the Mg rich pyroxene enstatite The EL and EH groups have different relative abundances of silicates and metals The numerical suffixes above (3 6) may be applied Age There are four periods of time that are significant in the history of any meteorite: Terrestrial age: the time spent on Earth since fall Obviously, the material from an observed fall has an immediately known terrestrial age Cosmic-ray induced isotopes are used to obtain this age from such finds We know from observed fall meteorites how much of these isotopes are in a meteorite when it arrives A meteorite found later will have less isotopes because the Earth’s atmosphere protected it after arrival, and unstable products of cosmic radiation, such as 14C will decay, so that the difference between the normal content on arrival and that Stones (Achondrites) Pallasites Eucrites Diogenites Howardites Angrites Primitive achondritesà à Ureilites Aubrites SNC Meteorites (Mars sourced?) Shergottites Naklites Chassignite (Orthopyroxenite) Basaltic and anorthositic achondrites (Lunar sourced) Brachinites Winonaites Sikhot Alin Cape York Gibeon Mount Padbury Krasnojarsk Camel Donga Johnstown Kapoeta Angra dos Rios Novo Urei Aubres Shergotty Nakhla Chassigny ALH 84001 ALH 85085 Brachina Winona The primitive achondrites have igneous textures with no chondrules, but their mineralogy and bulk chemistry shows little difference from ordinary chondrites They are supposed to have undergone igneous processes but with no fractional crystallization, but partial melting and segregation of the phases to varying degrees The irons were formerly separated into octahedrites (kamacite plus taenite; on etching yield criss crossing Windmanstatten patterns) (Figure 4): hexahedrites (mostly kamacite, yield only narrow thin Neumann lines on etching) and ataxites (no etch pattern) The Symbol classification above which replaced this metallurgical classification is still being modified and I AB and III CD have recently been grouped as I AB III CD These symbols reflect the differences in chemistry (nickel, gold, iridium content, etc.) The eucrites have basaltic textures Many meteorites defy classification and are listed as unclassified For example, the Bencubbin (find, Australia) meteorite appears to be a stony iron but is in fact a mixture of four types, an iron, an enstatite achondrite, and two chondrites, one carbonaceous It would seem to be the result of more than one collision, the first mixing occurring very early in its history (ca 4500 Ma) and causing heating and melting