| Nodules (Click below to go to other pages.) |
The preferential crystallisation of minerals during diagenesis can lead to nodules or concretions. These can be based on any of the minerals which occur as cements in sedimentary rocks, such as calcite, siderite, haematite or silica. Our local Mid Wales concretions are largely siliceous and occur within the softer mudstones. They weather out readily and can be found in rivers and on beaches. |
| Home | The following pictures show a selection. |
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| This bipolar nodule was found in the Severn and is about 12 inches across. It shows the "beef" top surface. It probably grew between two sandstone beds. | A broken section of the same nodule showing cone in cone structure both top and bottom. |
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| Cut section of radial nodule also found in the Severn, at Llanidloes. It has been cut, etched in dil HCl and dusted with white pigment to show the cone boundaries. It is about 4 inches across. The structure shows it grew from one point, like a cabbage - or indeed a coral. (It is of course entirely inorganic). | Thin section (not very thin!) of the same nodule in transmitted light. Field about 25mm) The transparent areas are largely silica with clay minerals concentrated at the cone boundaries. The cone in cone, riffled or crenulated nature of the boundaries is shown at all scales. Pic taken with QX3 Toy Microscope! |
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| Cliffs at Clarach in the Aberystwyth Grits, showing mudstones and turbiditic sandstones. Nodules occur in bands within the mudstones. | This nodule is 10 inches across and is eroding out on the beach. It shows the cone in cone structures and the "beef" top surface. c.f. Top left. |
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| This nodule is about 12 inches across. It is from our own brook at Dolfor. It is siliceous and shows quartz to the handlens. | This is a big specimen again from our brook. We needed two people and a sacktruck to retrieve it. I have not investigated it yet as A. I don't want to damage it and B. I am not sure that I am strong enough! |