Re: [meteorite-list] highly weathered chondrites: Reduction Spheres

[E.L. Jones]

Hello Listees,

I was impressed with the quality of the photos and the discussion/questions Mohamed posed. We who are fortunate to have a fair amount of reference materials/specimens/ experience/etc., immediately recognized that the specimen was not likely meteoritic. However , if one were to consider the literature alone regarding the definitions of chondrites, I , myself, could call this a chondrite. So I can see how this rock could be easily mistaken for a meteorite on first glance.

But having had a few years of experience, I thought this was a probably a sediment product; either what is essentially a concretion on the micro scale, or silica gel--(not quartz) or what I settled on is a "reduction sphere"*-- possibly a sulfate or carbonate akin to Alum, for example. In any case this is an interesting rock. I'd like to know how hard the pellets are and are they easily dislodged from the matrix. I suspect also that the soil is highly alkaline where this was found.

If one is going to hunt for meteorites, this situation emphasises the value in putting together a small collection of micro specimens for gaining experience and having something to compare unknown specimens to. Fortunately, the availability of African meteorites make this within most budgets.

Regards,
Elton

* Reduction Sphere: A white, leached, spheroidal mass produced in reddish or brownish sandstone by a localized reducing environment, commonly surrounding an organic nucleus or a pebble and ranging in size from poorly defined specks to a large perfectly round sphere more then 10 inches (25 cm) in diameter--McGraw-Hill "Dictionary of Geology and Mineralogy" pg. 237

In red-ox chemistry, reduction is a flow of molecules (electrons actually) to form new compounds. In geochemistry in this case it is a "post depositional" clumping of non crystaline compounds which are marginally soluible in water (hence, leeched ) in an oxygen- deprived environment. Like the mica mineral, glacounite, the molecules in solution tend to clump around detris such as fish or krill fecal pellets as they precipitate from the surface into the cold, oxygen-deprived depths. In this example the clumping occurs long after deposition in a weathering process.







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