SUMMARY. Mayo Belwa, a highly shocked enstatite achondrite, fell during the evening of 3 August I974 in the Adamawa District, NE. Nigeria (8 ~ 58' N., 12 ~ o5' E.). The stone weighed 4.85 kg and was deposited in the Geological Survey of Nigeria and loaned for study to the British Museum (Natural History). It is composed principally of enstatite (o.02 FeO%), with some olivine (Fo10o), diopside, feldspar (An 9 Abs8 Or3), and minor Fe-Ni metal and the sulphides oldhamite, daubr6elite, ferroan alabandine, and troilite. The kamacite contains between o-15 % and I-2 % Si and the troilite contains lq ~ Ti. The meteorite possesses vuggy cavities, which contain fluor-amphibole needles projecting from their walls, suggesting the presence at some time of a volatile-rich phase. The titanium-rich troilite, the nearly ironfree enstatite and the Mg/Si ratio are typical of the E-achondrites.THE Mayo Belwa meteorite fell in the Adamawa district of north-eastern Nigeria (8 ~ 58' N., 12 ~ o5' E.) during the evening of 3 August 1974. A fireball was seen by herdsmen and sounds were heard over an area extending to 25 km from the impact site. Only one stone, weighing 4"85 kg, was reported to have fallen and this was taken to the Geological Survey of Nigeria; it was reported that the stone smelt strongly of sulphur when first received. Subsequently the stone was loaned to the British Museum (Natural History) for study. On arrival in London it weighed 4"272 kg and was partly covered by a thin translucent fusion crust. It was ellipsoidal in shape, with a maximum circumference of about 4o cm, and composed of angular fragments of a milkywhite material set in a pale grey, sometimes glassy, matrix. The white fragments are of enstatite, and generally about 0"5 cm across though a few are much larger, up to 4 cm across. Also set in the microcrystalline matrix are rounded fragments of blackish, glassy material, up to o'5 cm across, many of which are disordered enstatites but some are shock-blackened olivines. Like most other aubrites, Mayo Belwa is a breccia, but it differs in having a number of irregular and sometimes interconnecting vuggy cavities, generally about t cm across, and extending inwards from the surface for a centimetre or so. The interior surfaces of these vugs are coated with white, sugary enstatite and are in part formed of 'bundles' of clear acicular diopside crystals with a coating of enstatite. Projecting from the walls of the rugs are numerous acicular crystals up to 3 mm long and approximately 20/~m in cross-section; some have been identified as a fluoramphibole by X-ray methods. The vugs and their minerals are described in detail by Bevan et al. (i977). The abundance of sulphide and metal phases is very low and these were not seen in the hand specimen.Microscopic examination. In thin section the fine-grained nature of the matrix makes optical determinations difficult, but it is composed almost entirely of a cloudy mixture of enstatite and minor feldspar crystals, each about 2/~m across. Within this matrix are set large clo...

The size distribution and morphology of metal grains have been examined in 11 sections of types I and II E‐chondrites. The changes in the grain‐size distribution and morphology of metal grains correspond with the petrologic types and define a series that reflects increase in thermal metamorphism in the following order: type I, Kota Kota‐Indarch‐South Oman‐St. Mark's; and type II, Jajh deh Kot Lalu‐Atlanta‐Daniel's Kuil‐Hvittis‐Pillistfer‐Khairpur‐Blithfield. Concentrations of metal grains adjacent to the perimeters of chondrules are observable throughout the sequence and delineate relic chondritic structure in six of the seven type II E‐chondrites; relic structures are absent from Blithfield.

The chondrites Bremervorde and Tieschitz (both H3), Beaver Creek, Menow, Monroe and Quenggouk (all H4), Allegan and Ambapur Nagla (both H5), and Butsura and Kernouve (both H6) were studied. Ca-poor pyroxenes almost invariably show enrichment in Mg relative to coexisting olivines; this was established in Tieschitz chondrules and in Bremervorde by crystal-liquid equilibria. In each chondrite, Ca-poor pyroxenes typically have less than 2 mol % wollastonite. In petrologic types 3 and 4, chondrule mesostases frequently show enrichment in normative diopside, which occurs as a resolvable modal phase in the type 6 chondrites. Partition of Ca between the two pyroxenes was initiated by crystal-liquid fractionation. Various chemical inhomogeneities may be traced from type 3 meteorites to those of type 6. Polycrystalline taenite occurs in Tieschitz, Bremervörde, Menow, Quenggouk and Butsura; it is rarely present in Kernouve. This textural form of taenite is interpreted as a relict solidification structure produced from a quenched liquid, which must have cooled rapidly to below about 700 °C. We argue that equilibration of the H-group chondrites took place during cooling, mainly from about 700 °C, which is compatible with several lines of evidence. If metamorphism is defined as change induced by an increase in temperature, then in the H-group chondrites studied we recognize it only in transient reheating probably produced by shock.

The bulk composition of metal (kamacite plus perryite) was determined in eleven E-chondrites and eight aubrites. The data are compatible with the subdivision ofthe E-chondrites into two groups (Yavnel, 1963; Anders, 1964), St Mark's and St Sauveur belonging to type 1 (Easton, 1985). The Ni content of kamacite plus perryite in Kota Kota (5.49%) is within the range covered by the remaining E-chondrites. Normative perryite, (Fe,NiJjSi,P)y constitutes 2.1% of Kota Kota and 2.7% of South Oman. The Ni content in the bulk metal ofAubres, Bishopville, Norton County and Peiia Blanca Spring is about half the average Ni content in the metal of E-chondrites or the remaining aubrites (Bustee, Khor Temiki, Mayo Belwa and Shallowater). High Ga/Ni and Ge/Ni ratios distinguish the metal in E-chondrites and aubrites from that in ordinary chondrites.In a large metal grain from Aubres perryite formed on reheating, whereas in one from Khor Temiki there is evidence of shock and displacement of fragmented schreibersite (rhabdite). Thirty-eight metal grains « 1.5 mm diameter) from Khor Temiki have a wide compositional range like that in Mayo Belwa (Graham, 1978). In Shallowater the distribution of Ni in the metal is bimodal (5.2 and 11.6%) and there is evidence of rapid cooling.The composition of both bulk metal and individual grains in aubrites makes it unlikely that they represent residual metal trapped during magmatic differentiation and/or fractional crystallization of E6 material. Compositional differences between metal grains strongly indicate that the aubrites are polymict breccias.

P and Ti) between silicate and sulphide groups ofminerals.