by Ian Donaldson

The brothels of Crediton were thought by Cromwell to be a distraction to his troops, so to keep them busy he got them to dig a trench through a nearby steep hill. The pretext was that it is quicker to carry heavy artillery through a hill than over the top. To this day the 30 foot deep lane on the Hollacombe Road is known as Cromwell's Cutting. The soft rusty conglomerate is 280 million years old, and is formed of debris from the top of the Dartmoor granite.

WEGA members in Cromwell's Cutting

Cromwell's Cutting

Dr Richard Scrivener pointed out to us on a recent field trip that though there is little granite in the mass, there are many white rhombs from 6 to 12 mm long.. These are of Perthite, a calcium-rich feldspar.  His theory is that the final act in the placement of the Dartmoor granite was a gigantic explosion, when the caldera collapsed, shattering the friable carapace - the solid shell topping the batholith.

A discussion of feldspars followed, and some bookwork with Arthur Holmes and the net has resulted in the following simplified summary:-

The feldspars are the most important rock forming minerals, since they make up 60% of the outer 15 km of the earth's crust. The classification of igneous rocks depends to a large extent on the character of the feldspars. They are all aluminium silicates, with varying quantities of sodium, potassium, calcium, and much less commonly small amounts of other Group 2 and Group 1 elements such as barium, strontium, lithium, rubidium, and caesium. They can be classified chemically by the position they fall in an equilateral triangle. The corners of this represent high concentrations of calcium, or of sodium, or of potassium aluminium silicate. The key minerals are the sodium-rich Albite, NaAlSi₃O₈,   potassium - rich Orthoclase, KAlSi₃O₈ and calcium-rich Anorthite, Ca Al₂Si₂O₈ Sometimes a small portion of the sodium or potassium can be replaced by the chemically similar but much less common caesium and rubidium. Likewise calcium can be replaced to a minor extent by barium or strontium. Up to 5% of the aluminium can be replaced by iron.  The size of the ions is more important than their chemical similarity, so it is easier for sodium to be replaced by the Group 2 calcium which is of similar size, than by either potassium or rubidium which are Group 1 Elements like sodium, but are much larger than sodium. As rubidium and potassium have similar ionic sizes, they are occasionally found together in feldspars. In fact the only commercial source of rubidium is feldspar. This association is fortunate geologically, for both potassium and rubidium have radioactive isotopes, and their differing rates of decay is one important way of dating rocks.

Feldspar composition diagram. For a feldspar represented by a point within the triangle, the proportions of the three quantities Orthoclase, Albite, Anorthite are given by the lengths of the perpendiculars from the point onto the sides of the triangle. Each corner represents 100% of the component named there. With increasing Ca the plagioclases grade from Albite (0 - 10% CaAl₂₎ to Oligoclase, Andesine, Labradorite, Bytownite and Anorthite ( 90 - 100% CaAl₂).

Without having to resort to chemical analysis, the feldspars can often be identified visually by one of their planes of cleavage.

Orthoclase splits at exactly 90 degrees (Greek orthos=straight or perpendicular), calcium-rich plagioclases split at about 86 degrees (Greek plagio = slope, klasis = fracture) Optically  they can be divided inti two crystal types, of either triclinic symmetry or else of monoclinic symmetry Alkali feldspars are those rich in the alkali metals sodium and potassium, in any ratio of Or to Ab.

Common alkali feldspars are Microcline, the triclinic variety of the potash rich orthoclase, and its monoclinic equivalent, Sanidine, which is more stable above 500 degrees. Microcline can invert into sanidine by prolonged heating at high temperature, though the reverse, convertion of sanidine into microcline has not yet been achieved in the laboratory. The orthoclase of commerce, used in glass and ceramic glazes, is usually not true orthoclase but microcline. As its name suggests, this has a plane of cleavage not at a perfect 90 degrees, but at a tiny bit less, 89.7 degrees.

Orthoclases form at 500 to 1100 degrees Celsius, and the plagioclases form at higher temperatures - 1100 to 1544 degrees.  Plagioclases have been arbitrarily divided into 6 members, depending on the ratio of Albite (Ab) to Anorthite (An).  With increasing calcium, and decreasing sodium, the series is :- Albite, Oligoclase, Andesine, Labradorite, Bytownite, and Anorthite. (Can anyone suggest a mnemonic for these?)  Labradorite for instance is Ab 40% An 60%   plus or minus 10% of either component. This often has a beautiful blue shimmer. A similar attractive shimmer is seen in the well known decorative dark grey or black igneous rock Larvikite, from Norway, This rock contains 90% or more Anorthoclase (NaK)AlSi₃O₈. This triclinic feldspar was named in 1885 from the fact that it too does not cleave at a perfect right angle. The shimmer is known by various terms such as chatoyant-like, pearl-grey iridescence and sometimes labradorescence. Feldspars when pure are usually white or colourless, but can be coloured by the presence of small quantities of other elements or minerals.  For instance, the inclusion of haematite causes the golden shimmer of Aventurine, a form of oligoclase.  The cause  of the beautiful green of the microcline Amazonite is unknown, though  it  contains small amounts of rubidium and caesium, and the more rubidium, it is said, the darker the green. Also known as Amazonstone, the name is a misnomer, for the green mineral found along the Amazon is actually the unrelated Nephrite. Most Amazonite now comes from Pike's Peak, Colorado, or the Urals.

         Perthite, was discovered in 1843 in the town of Perth in Ontario. When seen under the microscope it is not a uniform crystal. Instead it consists of alternate bands of sodium - rich Albite and potassium-rich Orthoclase, Sanidine or Microcline. If the sodium-rich areas predominate, it is known as Antiperthite. The banding of Perthite is a consequence of minerals which are soluble in one another at high temperature separating out on cooling. This process is known as exsolution, and the minerals have been exsolved..

        Weathering of the feldspars dissolves out the potassium, sodium and calcium leaving an increasingly aluminium-rich residue. This eventually becomes a hydrated aluminiun silicate - a clay mineral.  In Cornwall this is kaolin, the white china clay. Weathering can also produce the related but somewhat more complicated montmorillite, bentonite and fullers' earth.  All these have sheets of silica tetrahedra alternating with sheets of alumina tetrahedra.  It is the ease which these flat sheets glide over one another which makes clays slippery.

          Under certain tropical conditions the silica can also dissolve, leaving .bauxite, the only important aluminium ore. The name comes from Les Baux, in Provence.

          Getting back to Cromwell, his soldiers associated rocking stones or logan rocks with superstition. Many of these interesting stones, the result of weathering or glaciation, were toppled over, or else tampered with to prevent them rocking. For instance in  Cornwall, at Stithney  a very sensitive logan rock known as Men Amber was rendered immobile because legend held that  Men Amber would stand until England had no king.