CompuServe Messages

Hi, Al and Joseph– I am back in OK now, and looked up ureilites in "The Meteorite & Tektite Collector's Handbook" (Bagnall). Ureilites are coarse-grained olivine-pyroxene achondrites. The large grain size suggests slow cooling and the grains tend to be oriented, leading Bagnall to remark "In any event the mixture is difficult to achieve by known mechanisms." So far, a planetary origin sounds like a viable theory. They have no chondrites, like Earth rocks. Bagnall seems to say they include kamacite (nickel-iron), unlike Earth rocks. I would sort of expect Venus rocks to resemble Earth rocks in these regards (pending the arrival of samples)(native iron would be more expected under Venus' atmosphere, but I am somewhat surprised if it is disseminated in olivine; part of the Venus-impacter, or an orebody mixed in by impact?) I see ureilites include diamond and lonsdaleite (an "ultra-high-pressure carbon polymorph"), which are taken as indicators of a terrific impact that knocked the ureilites loose from their parent body. That is what it would take to bust them loose from Venus… Joseph's remarks suggest the diamonds could be interstellar in origin (I assume lonsdaleite would continue to suggest major impact) – I do not see how they would be at all common on Venus (no such diamonds are found on Earth, are there?). I see no mention of datings – it was the youth of SNC meteorites that best rules out planetesimals as their source and leave Mars the best bet; without a too-young date, I would regard planetesimals as equal to Venus as candidates. Thus far, Venus origin seems possible to me, but with problems. If this is all there is to go on, I would rate it at best one among many competitive theories. Hmm.. lonsdaleite .. I had thought buckyballs were the third polymorph of carbon, but Fleischer's says lonsdaleite and chaoite were there first. I do not expect to make it to Tucson this year (next year for sure!!), but things could change… –Doug