|MadSci Network: Earth Sciences|
Dear Shona, I can answer your question in a general way, though not perhaps as specifically as you may wish. Sedimentologists who specialize in the Oxford Clay could, I am sure, tell you exactly what minerals it contains and in what proportions. I can describe it in more general terms. If you want to know more than my answer provides, try contacting the Department of Earth Sciences at the Open University, which is located on the main campus in Milton Keynes. I am an Open University tutor, and not directly employed on the staff at Milton Keynes, and also not a specialist on clays! As you correctly state, the Oxford Clay is of Jurassic age, being a little less than 180 million years old. Its outcrop runs from southwest to northeast across the middle of England from Dorset to South Yorkshire, and is thickest (about 20 m thick) between Peterborough and Milton Keynes. A clay is a rock made of clay minerals, and in which the smallest sedimentary particles are less than 0.002 mm in size. The particles are so small that they are the last sedimentary particles to settle out of suspension in water, and so are carried the farthest. The Oxford Clay reflects this because it is was deposited in a deep marine basin far enough removed from shore that larger particles such as silt and sand were not transported there. Clay minerals usually belong to one of three main groups: kaolinite, illite and montmorillonite. These may all be present in the Oxford Clay in different proportions, but this is where you would need a real expert on the Oxford Clay to confirm. Kaolinite is formed by the breakdown through chemical weathering of the mineral potassium feldspar, which has the formula 2KAlSi3O8 (our apologies: the MadScientist word processor does not allow me to subscript numbers, but the 3 and the 8 in this formula are subscripted). Water combines with aluminium and silica in the feldspar to produce kaolinite, which has the formula Al2Si2O5(OH)4 (all the numbers are subscripted). Illite has the formula KAl2(Al,Si3)O10(OH)2 (all the numbers subscripted). It is formed by the breakdown of the minerals biotite and muscovite, otherwise known as mica. Montmorillonite has the complex formula (Al,Mg,Fe)2(Si,Al)4O10(OH) 2nH2O,Ca,Na (all numbers subscripted). It is formed by the breakdown of the iron/magnesium-rich minerals pyroxene and amphibole. The first part of the formula up to (OH)2 defines the crystal structure, and the water, calcium and sodium at the end of the formula are nestled in the crystal lattice but without being chemically bonded to it. It is also worth noting that the Oxford Clay contains a high proportion of reduced organic carbon in the form of finely dispersed petroleum hydrocarbons. Organic-rich clays like the Oxford Clay are the source rocks for much of the world’s deposits of oil and natural gas. The Oxford Clay was deposited as an ooze on the sea bed in a deep, quiet marine basin with poor water circulation and little oxygen (rather like the bottom waters of the Black Sea today), and the absence of oxygen inhibited the breakdown of organic material by microbial action. In such anoxic conditions, organic matter drifting down from the surface would settle on the bottom and not decompose, building up over millions of years. The Oxford Clay is noted for its beautiful ammonite fossils, often preserved by the mineral pyrite, FeS2, which crystallizes in such anoxic conditions. Millions of years after it was deposited, after the clay was buried to a depth of perhaps several kilometres and subjected to heat and pressure in the Earth’s crust, the organic carbon would be “cooked” to form petroleum hydrocarbons. The black colour of the Oxford Clay is the result of its hydrocarbon content. As you can appreciate, clays are very complex materials. To summarize, they are formed by the deposition, usually in very calm conditions such as enclosed marine basins, of the breakdown products (clay minerals) of rocks on land. They can also incorporate other substances, in particular organic carbon and its breakdown products in the form of hydrocarbons. I hope this answers your question. Best wishes, David Scarboro
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