ALBERT

Description: A stony meteorite found in Antarctica in 1985, recently identified as a type of chondrite, probably represents material from a previously unsampled region of the solar system. Scientists are currently debating how the object formed. Preliminary results on Allan Hills 85085 (ALH85085) were presented.

Description: Kaidun is a remarkable chondrite breccia fall containing lithic clasts that span a wide range of chondrite groups including C and E chondrites, as well as having clasts with characteristics not yet found in existing chondrite samples. The dominant lithology in Kaidun appears to be CR chondritic, consonant with recent O isotope data. The carbonates in Kaidun are presented as one mineralogical basis for comparing it to the other hydrated chondrites and to better understand its relative alteration history. The four polished thin sections of Kaidun studied contained a variety of lithologies that we classified into four groups -- CR, E, CM-like, and dark inclusions (DIs). DIs contain sulfide and magnetite morphologies that superficially resemble CI chondrites, and some of the previously reported CI lithologies in Kaidun may be what we term DIs. Carbonates were found in all lithologies studied. Carbonates in Kaidun are similar in composition to those in CR chondrites. Some of the DIs in Kaidun, previously characterized as CI, have carbonates similar to those in CR chondrites and are unlike those in CI or CM chondrites. Most carbonates in Kaidun and CR chondrites are calcites, some of which formed at temperatures above 250 C. Dolomite is less common and some may be metastable. Alteration temperatures in the Renazzo CR chondrite were estimated to be approximately 300 C, based on O isotope fractionation between phyllosilicates and magnetite. Temperatures of up to 450 C were proposed for the alteration of a CR-like dark inclusion in Kaidun, based on the presence of hydrothermal pentlandite veins. The alteration temperatures for Kaidun and the other CR chondrites are considerably higher than those suggested for CI or CM parent bodies.

Description: Mineralogical examination of thin sections of non-Antarctic and Antarctic HED (howardite, eucrite, diogenite) achondrites indicates that they contain a variety of lithic components. Some of these components occur both as monomict meteorites and as clasts in polymict meteorites, whereas others occur only as clasts in some polymict breccias. The components may be classified by the degree of homogenization of the pyroxene present. In order of increasing homogeneity these are: (1) Y-75011-type basalt clasts; (2) Pasamonte; (3) Y-790266-type clasts; (4) Stannern and Nuevo Laredo; (5) Juvinas and Haraiya; and (6) Ibitira. Type 1 has been least modified by post-igneous thermal annealing, while types 5 and 6 were thoroughly metamorphosed. Three types of cumulate eucrites are recognized and are believed to represent (a) cumulates from thick lava flows or layered intrusions; (b) lunar highlands type crust; and (c) differentiation products of diogenitic magmas.

Description: Iron meteorites with oxygen-bearing phases can be classified in terms of their oxygen isotopic abundances. These iron meteorite classes are isotopically similar to various stony meteorite classes, which may indicate a common origin. The group IAB and IIICD irons may be related to the winonaites; group IIE irons may be related to H chondrites; group IVA irons may be related to L or LL chondrites.

Description: The paper presents a study of olivines and their surrounding coronas in mesosiderites texturally and compositionally using optical and microprobe methods. Olivine composition ranges from Fo(58-92) and shows no consistent pattern of distribution within and between mesosiderites; olivine occurs as large single crystals or as partially recrystallized mineral clasts, except for two lithic clasts. These are Emery and Vaca Muerta, and both are shock-modified olivine orthopyroxenites. Fine-grained coronas surround olivine, except for those in impact-melt group mesosiderites and those without tridymite in their matrices. Coronas consist largely of orthopyroxene, plagioclase, clinopyroxene, chromite, merillite, and ilmenite, and are similar to the matrix, but lack metal and tridymite. Texturally the innermost parts of the corona can be divided into three stages of development: (1) radiating acicular, (2) intermediate, and (3) granular.

Description: Eight of eleven Apollo 16 rake-sample anorthosites are very similar to each other, to hand-specimen Apollo 16 anorthosites, and to Apollo 15 anorthosites. They have feldspar An-96.6, both high- and low-Ca pyroxene with a restricted range of (low-magnesium) composition, minor olivine, traces of ilmenite and chromite, and originally coarse-grained, but now cataclastic texture. Such ferroan anorthosite is evidently a coherent, distinctive and widespread lunar rock type of cumulate origin which may not necessarily be very closely related genetically to other highland rock types.

Description: In light of a study of the Emery mesosiderite, it is determined that the high modal abundances of merrillite and tridymite in most mesosiderites are attributable to redox reactions between silicates and P-bearing Fe-Ni metal within a limited T-fO2 range at low pressure. The recalculated amounts of dissolved P and S in the metallic portion of Emery reduce the metal liquidus temperature to less than 1350 C, and the solidus to less than 800 C, so that the mixing of liquid metal with cold silicates would have resulted in silicate metamorphism rather than melting. This redox reaction and redistribution of components between metal and silicates illuminates the complexities of mesosiderite processing, with a view to the recalculation of their original components.

Description: A theory of the origin of ureilites is presented based on mineralogical and petrological investigation of eight species including Kenna, Novo Urei, and Goalpara. The theory suggests that ureilites are primarily olivine-pigeonite cumulates crystallized from a silicate liquid which also contained suspended solid carbon phases. The carbon induced reduction of the melt and affected the ureilite mineral compositions. Petrofabric analyses show that mafic silicates are oriented in lineated and foliated patterns similar to cumulate rocks; strain rate of silicates suggests that ureilites were deformed after lithification by a mild tectonic stress and a moderate to severe shock.

Description: The system of nomenclature for basaltic achondrite meteorites is discussed, and new classification criteria are proposed. Under the new system, all achondrites are divided intno the broad groupings 'monomict' and 'polymict' by the number of lithologies present. The monomicts are classified structurally as brecciated or unbreccciated and as eucrites, diogenites, or cumulate eucrites. The polymicts are classified using an arbitrary mineral-chemical standard based on the percentage content of diogenite (magnesium orthopyroxenite): diogenites have more than 90 percent, eucrites have less than 10 percent, and all other polymicts area howardites. Tables listing all known achondrites by classification are provided.

Description: Seven Yamato and six Allan Hills polymict eucrite specimens were compared by modal analysis. The analyses reveal differences of plagioclase and pyroxene content between the two groups. The Yamato suite has more 'pigeonitic' pyroxene and less plagioclase and low-calcium pyroxene than the Allan Hills suite. Variations within each suite are small and three sections of Allan Hills A78040 are more variable than the Allen Hills suite considered as a group. Modal data provides a basis for pairing polymict eucrite specimens when used together with mineralogical and petrographic criteria. Modal data furthermore confirms the presence of several rock types previously identified using pyroxene crystallography and hints at the presence of an augite-rich component.