Hello list, Ted Bunch has given his okay to post the following information on Mike's NWA 2092, for all those interested, as I am. Also of interest, a nice thin section image of an L7 showing those features characteristic of type 7 chondrites can be seen on my NWA 1839 page of meteoritestudies.com (courtesy of NAU). When I get a piece of the LL6/7, I also have a nice TS photo of that one to show for comparison.
Regards, David __________________________________________________ Preface: the overall texture of 2092 is complicated by a shock metamorphism overlay; brecciation and thermal annealing along fractures, clast outlines and matrix, i. e., instead of an equigranular texture (grains of similar size), which we find in true and uncomplicated sevens, the grain size is highly variable from a shock recrystallization event. Dodd criteria: 1.the presence of poorly defined chondrules in type 6, but only relict chondrules in type 7 – There are relict chondrule fragments of mostly radial chondrules in 2092. I respect the Dodd criteria, although they were formulated ~ 24 years ago and nothing else has come along, even though many more sevens have been recorded. If we are to have a seven category, then we feel that there should be no chondrule remnants in a seven, otherwise why have the additional compartment. Regardless of whether they are very fuzzy or only partly fuzzy. 2.low-Ca pyroxenes in type 6 contain no more than 1.0 wt% CaO (1.0 wt% = ~1.9 mol% Wo), but more than 1.0 wt% in type 7; conversely, the CaO content of high-Ca pyroxenes decreases from type 6 to type 7. 2092 CaO contents average 1.0 wt % in orthopyroxene – the fence line on this issue. 3.feldspar grains gradually coarsen to reach a size of at least 0.1 mm in type 7. The 0.1 mm size is a good point, but is not all of the story if the size range of all minerals are considered. We feel that the similar grain size of all silicates is the critical issue. In addition, in true sevens, some plagioclase shows simple twinning. Not so in lesser grades. In addition to Dodd’s criteria, we have observed that the amount of metal (modal/vol. %) decreases significantly in sevens. NWA 3100 has <1 vol % metal, which is taenite. Apparently, during the last stages of metamorphism, low metal is consumed to make olivine. In fact, our modal analyses of sevens show that considerable amounts of olivine have been made at the expense of metal and pyroxenes, which leaves only small amounts of Ni-rich metal and lower amounts of orthopyroxene and clinopyroxene relative to those in lower metamorphic grades. There are other observations, but none of those given here (original to us) have been tested in a court of science review. Later, perhaps. Bottom line: We could go either way on the classification (6 or 7), but the shock metamorphic overlay and the Dodd margins suggested to us that the best classification was and is a transitional 6/7. We will stand by this until our future study or studies by other investigators suggest otherwise. Best regards, Ted ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
