Chemical Evolution of the Carbonaceous Chondrites

X-ray diffraction microtechnique has been applied to nine carbonaceous chondrites, so that a definite knowledge of their mineral composition has been obtained. The existence of magnesium sulphate, elemental sulphur and a magnetic spinel containing Fe3+ ion, has been confirmed. However, troilite is rare, and absent in many cases. Dolomite has been observed in meteorites for the first time, and pentlandite has also been found. Hydrous silicates, apparently alteration products of olivine, occur in several varieties which are distinct from chlorite and serpentine. Two unknown minerals have been found in Mokoia. Evidence is presented to show that these meteorites and their peculiar minerals can be accounted for by the preterrestrial action of water on olivine, troilite, iron, etc. The crystal perfection of the dolomite is evidence that the aqueous phase acted over a prolonged period certainly longer than 1000 years. Equilibrium studies make it clear that no special oxidizing agent is required to produce the sulphate or Fe3+. It is shown that one need not assume a planetary origin to account for the long-term presence of water. Our calculations show, rather, that an “internal atmosphere”, sealed by an ice layer from a low pressure exterior, could have persisted for at least several million years if the parent body were at an asteroidal distance from the sun.