Purbeck Mineral and Mining Museum
After the closure of Eldon sidings all clay exports were via lorry. For several years the clay was carried from the mines across the A351 by trains to the "Lorry Drop". It was realised that the lorries could take the clay directly from the mines and the cost of maintaining a railway system avoided. The transhipment buildings were raised to accommodate the lorries beneath them and in the early 1970s the railway system sold partly into preservation and the rest scrapped. Use of narrow gauge railways continued underground at Norden No 6 & 7 mines until all mining operations ceased in 1999.
Clay mineral X-ray diffraction
Clay minerals are one of the most diverse minerals but all have a commonalty of crystal or grain sizes below 2 µm. Chemically clays are defined by crystal structure and chemical composition. Sometimes fine grain sediments are mistakenly described as clays, this is actually a description of the “clay-size fraction” rather than the mineralogy of the sediment. There are three crystallographic clay groups: platy clays (phyllosilicates), fibrous clay minerals, and amorphous clay. Phyllosilicates are the most abundant clays and are categorized based on the layering of a tetrahedral and an octahedral layer. For most clays, the octahedral layer is centered with Al, Fe, or Mg(OH), but sometimes Zn, Li, and Cr can substitute as well. Si is normally the center of the tetrahedral layer but Al will often partially substitute and create a charge imbalance. Two-layer clays are composed of a tetrahedral layer and an octahedral layer (T-O) while three-layer clays contain an octahedral layer sandwiched by two tetrahedral layers (T-O-T). When substitution of Al for Si creates a charge imbalance, an interlayer cation will fill in between tetrahedral layers to balance the charge of the clay.