Imperial College Rock Library

RLR5369- Sandstone (weathered)

Locality: Darley Dale, Derbyshire, UK view map
Age: Upper Carboniferous,320 Ma

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This sample is a fine-grained sub-arkose dominated by sub-angular grains up to 0.2 mm in size. The clastic grains comprise quartz (70%), feldspar (>25%) and mica (<5%) and are moderately well sorted. Quartz grains contain abundant crytocrystalline inclusions of which the largest are apatite and zircon. Most grains are single and monocrystalline, however, some are,polycrystalline quartz, with granular textures. A small proportion (<5%) of quartz grains exhibit undulose extinction. Feldspars include plagioclase, microcline and microperthite in approximately equal abundance. Some feldspars are heavily altered to sericite. Muscovite is the most abundant mica with <30% of biotite mica. Many mica grains are kinked between surrounding quartz and feldspar. The matrix contains goethite and probably hematite. In places a poorly developed overgrowth cement of quartz occurs, however, a primary intergranular porosity is preserved.
Minerals: quartz, plagioclase, microcline, perthite, muscovite, biotite, zircon, apatite


This sample is part of the millstone grit group of the Upper Carboniferous. It is a poorly sorted and relativiely immature fluvial sandstone. Post deposition diagenesis produced a siliceous cement, however, some of the original porosity remains.
Classification: sedimentary rock, sandstone

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