MadSci Network: Earth Sciences

Re: Can you tell me what minerals are in the clay of The Oxford Clay Belt

Date: Mon Nov 12 16:59:19 2001
Posted By: David Scarboro, Faculty, Earth Sciences, The Open University
Area of science: Earth Sciences
ID: 1005181285.Es

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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