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Tuesday 11th February 2003 Notes: R Coleman |
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A welcome return by Dr Bill French, who on his last visit gave an talk on the growth of the mineral Thaumasite in concrete. This time Bill set out to educate us about "Clay!" He started by saying that certain concepts are perpetuated blindly in many text books and that of clays, formed by sedimentary processes, are often described as the result of the erosion of rocks into ever smaller particles, and transported by fluvial systems, where they are further broken down into smaller and smaller particles, eventually reaching a marine environment. As the energy of the transport mechanism decreases, the heavier particles drop out of suspension, giving rise to first sandstones, mudstones and clays. Clays are often classified as having a particle size of < 2µm. However, he said that many of us, who have come into contact with the London Clay, appreciate that it would not be described as having a make-up limited by this particle size. This accepted, it still acts as a Clay! Bill went on to name a host of other marine clays that do not meet the often quoted particle size classification! He stated that these marine clays had a similar composition, one that closely parallels igneous rocks (chemical analysis not mineral content, so don't through your hands up in horror). Bill also described the formation and alteration of clays at various burial depths, and covered a host of other data, before zeroing in on his main objective. Bill pointed out the rather larger particles of some of the clay forming minerals seen in photographs of many "thin sections." He paid particular attention to the irregular shape and size of many of the quartz particles, most were in the order of 5 to 10µm, and were present in all the marine clays. Having made his point about the particle size, Bill then went on to look exclusively at the quartz grains. Electron microscope pictures showed these to have the typical concordial fractures of quartz. This accounted for their irregular shape. We were challenged to consider the mechanism for fracturing quartz to this size (and quantity). This point was the real challenge to the concept of the every decreasing particle size (sandstones, mudstones, clay). One would expect the quartz to be at least round and uniform! The real problem is the energy required to fracture quartz into such small particles. Bill said that his work showed that it was impossible with the energy in the normal environment (river systems, waterfalls etc), to fracture quartz to below a certain size! (A concept worth giving thought to, how do you break it smaller, how would you test the theory?). Closer views of the actual concordial fractures, displayed a multiple ridge, or wave pattern, leading up to the edge of the fracture. Bill said, that the wavelength of this pattern was consistent with microwave frequencies, which is indicative of a high temperature source for the fracture mechanism! He rather teasingly left us to decided upon this heat source? Bill also described some personal experience of dust storms on a coastal resort, and the inversion of the resulting mud layer in the harbour when sand was deposited over the resulting mess. This seemed like Bill teasing us again to think about other depositional mechanisms in play, in addition to the accepted, and often unchallenged, commonly held view on the formation of marine clays!
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We had an interesting and lively question time, more so than usual! Once again an excellent talk, the type that appeals to those of us that question everything! The very thought provoking observation of grain size and shape, especially that of the quartz grains deserves an answer on two points. What were the mechanisms for transport of the quartz grains and how did they get fractured into small particles! (One obviously thinks; volcanism! Underwater volcanism, mid-ocean ridges, or on land, whatever the answer, the next time a volcano erupts explosively, perhaps one ought to have a look at the ash particles a little closer? And perhaps, a closer look at the frosted appearance of desert sands? OK one extreme to the other, but I haven't got a clue what to expect, just an open mind — as always! RC).
Sorry no pictures of member's finds this month. Nothing on show, most were too busy pawing over the Field Trip sheets and planing this years campaign while drinking tea! © R Coleman 8/3/2003 |