Mineral assemblages and the chemical history of chondritic meteorites
By: [by] Robert F. Mueller -- and Edward J. Olsen --.
Contributors: Olsen, Edward John, 1927- joint author.
Series: Fieldiana -- v. 16, no. 15; Field Museum of Natural History. [Publication] -- no. 1084; Publication (Field Museum of Natural History) -- 1084.
Publisher: Field Museum of Natural History
Published: 1969
Place of publication: [Chicago]
Subjects: Meteorites.; Chondrites (Meteorites)
Description: An attempt has been made to correlate the mineralogical and bulk chemical characteristics and the textural and structural features of chondritic meteorites. -- The thermodynamic basis for the observed mineral assemblages of the ordinary chondrites is discussed in some detail with respect to the temperature of crystallization, state of oxidation and the liquidus relations. The approximate oxidation fields of the ordinarey, enstatite and carbonaceous chondrites are delineated and compared with a gas of solar composition. It is shown that the field of the ordinary chondrites is quite well defined and that these chondrites are more oxidized than the solar gas so that direct condensation from this medium is excluded. The evidence for initial high temperature liquidus crystallization for most ordinary chondrites is discussed, and it is concluded that the chondrites of uniform composition probably represent rapid crystallization through liquidus temperatures, with perhaps some annealing to a near equilibrium distribution of Mg and Fe 2+ just below the liquidus. The nonuniform, disequilibrium chondrites appear to represent, in part, a lower rate of cooling and crystallization and show evidence that they originated under conditions of varying oxidation. -- The observed textural features of the metal grains of most ordinary chondrites with the coexisting silicates are interpreted as having resulted, not from any magmatic or mechanical processes but from transport and deposition from the vapor phase. This accounts well for nearly all the observed textural and structural features. -- The analytical relations are derived relating the normative olivine contents of the chondrites to the ratio of oxidized to reduced iron and consequently to the degree of oxidation. This derived relation is then compared with that observed for 43 chondrites and a good correspondence is found. This again tends to support the approximate validity of Prior's rules when account is taken of analytical errors and a certain amount of variation in the bulk composition. -- A critique is also presented of certain views on the origin of chondrites which depend primarily on mineral chemistry and little agreement with these is found.