Hello Graham, Bernd, list While there is no technical definition of a "shock vein" so far as I know, it is in wide use and I hold it to be a version of a "healed" fracture; healed by the 1) injection or accumulation of adjacent wall melt where the filling material has an origin in high pressure, high temperature phase change or 2) formed by shock front passage: compression melting followed by decompression solidification. In the later case, material goes from crystalline solid to non crystalline liquid to non crystalline solid aka "glass" within a nano to micro to a second of time. Feldspars form masklynite glass but since fledspar in meteorites are usually limited to small blebs it isn't a major shock vein component.
Other silicates such as olivine go through a phase change and melt only to recrystallize into the mineral ringwoodite. In this case the high pressure squeezes all the extra space out of the crystal matrix on an atomic level by repacking them but maintaining molecular bonds There are other minerals that are formed by other pyroxenes resulting in similar phase shifts (majorite, akimotoite, NaAlSi3O8<--a felspar, hollandite and jadeite). Oxides such as chromite also morph. there presence of these minerals provide information about the shock history of the parent. Fractures occur when there is a physical rupture of adjacent material. It may be "unhealed"-- meaning a void filled only by liberated gases or it may be "healed" by any one of a several processes. If that process is shock melting, it is a high pressure, high temperature event where a rapid cycling of solid-melt- solid, expands to cement the adjacent sides and strengthening the fracture. As such, meteorites almost never fragment along shock veins but across them, from personal observation. When vewing under a hand lens or microscope, Ringwoodite has a dark almost black yet translucent appearance, with a slight purple hue. Maskylenite is transparent and clear. A fracture can be a slickenside filled with smectite: a low temperature,low to mid pressure, mechanical weathering mixture produced as adjacent surfaces mill and grind each other down. Separation of surfaces along these is more common as they are natural lines of weakness. Slickensides are striated and the striations generally orientated in the same direction; shock veins are smooth and may be branched. Slickensides are opaque and look like dark shale, graphite, or fusion crust in color and texture. I believe that formation of slickensides and shock veins at the same location in the same event are mutually exclusive. So as to your question. I believe that all the veins seen in this photo represent mid to low pressure, low temperature fractures of which slickensides are a subset. Without examination under a microscope, one would not be able to positively determine if any shock veins are present. Given the clear presence of slickensides and the appearance of the matrix of this meteorite to others which have an easily crumbled consistency (i.e. friable), the occurrence of shock veins is unlikely--Furthermore, it is extremely remote that any of these features are from terrestrial impact. This meteorite over all appears to have had a really brutal cosmic history. EPILOGUE: That said there is a report that slickensides in meteorites might be better interpreted as "shock fractures" as opposed to analogues of terrestrial slickensides. <http://adsabs.harvard.edu/abs/1966Metic...3...31D> Elton For more reading: <http://adsabs.harvard.edu/abs/2004AGUFMMR11A0925X> <http://www.nsls.bnl.gov/newsroom/science/2004/06-Chen.htm> <http://www3.sympatico.ca/jeff.rowell/Shock.htm> <http://www.meteorites.com.au/features/sahara99898.html> <http://adsabs.harvard.edu/abs/1966Metic...3...31D> <http://books.google.com/books?id=ZnUPAAAAIAAJ&pg=PA182&lpg=PA182&dq=slickensides+in+meteorites&source=web&ots=YfWcqNTll-&sig=kIPuKfITF2U4paiP_RBNB36db9g> --- ensoramanda <[EMAIL PROTECTED]> wrote: > Hi Elton, > > I thought these looked like shock veins...still > having trouble knowing > the difference between shock veins and slickenslides > in this one? > <http://i117.photobucket.com/albums/o43/LaburnumStudio/DSCN6902.jpg> > > Graham ______________________________________________ Meteorite-list mailing list Meteoritefirstname.lastname@example.org http://six.pairlist.net/mailman/listinfo/meteorite-list