Friday, 21 July 2017

tough stuff

I described the local building stone in my home town in New Red Sandstone, but there is another stone, obtained from a quarry about 12 miles south of Penrith, that is used around town for purely decorative purposes. This is Shap granite, which I propose to describe in detail and provide examples of where it has been used around town.

Igneous rocks—that is, rocks that started out as molten magma—are classified according to two criteria: their chemical composition and where they end up. Taking the second criterion first, a rock that has been extruded onto the surface is described as ‘volcanic’ or ‘extrusive’, while a rock that solidified before it reached the surface is described as ‘intrusive’. There are two types of intrusive igneous rock: hypabyssal and plutonic. The first of these terms describes shallow intrusions, usually along bedding planes (a ‘sill’) or up near-vertical faults, cracks and joints (a ‘dyke’), while granite is a typical plutonic rock and is often intruded at great depth, hence the reference to the Roman god of the underworld.

You might think it necessary to perform some kind of analysis to determine the chemical composition of a rock, but you can get an approximate idea of that composition from the minerals present. There are five main groups of rock-forming minerals, and the relative abundance of each is a pointer to a rock’s chemistry. The first group are the feldspars, in which the silicon atoms in a crystal are arranged in a three-dimensional lattice, which places severe constraints on what other elements can fit into the gaps. In practice, all feldspars contain aluminium in a fixed ratio with the silicon, and differences between minerals are determined by the relative proportions of sodium, potassium and calcium, all of whose atoms are similar in size to the silicon, aluminium and oxygen of the main crystal lattice.

In minerals classified as micas, the silicon atoms form flat sheets, meaning that there are obvious lines of cleavage through a crystal, and a typical specimen will appear flaky, like puff pastry. In pyroxenes and amphiboles, the silicon atoms are arranged in single and double chains, respectively, meaning that the structure can accommodate larger atoms such as iron and magnesium, but I don’t intend to discuss these minerals in more detail, because they rarely occur in granite. The fifth rock-forming mineral is olivine, which is an iron/magnesium silicate that never occurs in granite.

The relative abundance of these minerals reflects the amount of silica (silicon dioxide) in a rock. Rocks with the most silica are classed as ‘acid’, and with progressively less and less silica, rocks are classed as intermediate, basic or ultrabasic (there are no ultrabasic rocks in the Lake District). Granite is an acid igneous rock, with a silica content in excess of 66 percent, while the best-known example of a basic igneous rock is probably basalt, perhaps because most people have heard that the Giant’s Causeway in Northern Ireland and Fingal’s Cave on the island of Staffa in the Inner Hebrides are basalt.

There is a group of small islands in the east of Hong Kong, all of which are surrounded by impressive cliffs. One of these islands is called Basalt Island, presumably, by analogy with the Giant’s Causeway and Fingal’s Cave, because of the columnar jointing displayed on these cliffs. However, the rock here is rhyolite, the volcanic equivalent of granite, and what this shows is how easy it is to make mistakes when you jump to conclusions about a subject you know nothing about. Hexagonal columnar jointing is found in all volcanic rocks and is merely an artefact of shrinkage during cooling.

There is no basalt in Cumbria, but there is a small intrusion of gabbro, the plutonic equivalent of basalt, that forms a mountain—Carrock Fell—in the northeast of the Lake District; and there is a sill of dolerite, the hypabyssal equivalent of basalt, in the north Pennines, a dozen miles east of Penrith. This is the Whin Sill, which I believe to be the largest sill in Britain.

Returning to Shap granite, the first photograph shows the use of this stone to form four pillars framing the side entrance to Penrith’s town hall (the town’s library is no longer housed here).


Reflections on the pillars are an indication that Shap granite polishes up extremely well, mainly because of its crystalline nature. Notice too the black spot near the base of the left-hand pillar, indicated by a red circle. This is a xenolith (‘foreign stone’), a piece of the surrounding rock—probably an intermediate volcanic rock called andesite—that fell into the magma before it had cooled and was not completely assimilated. And this is a closer view:


This photo also provides a good indication of the rock’s structure. The large pinkish crystals scattered throughout the rock are orthoclase, which is a potassium aluminium silicate and a type of feldspar. As you can see, these crystals are much bigger than the crystals of minerals in the groundmass, and as such they are known as phenocrysts. If you look closely at the groundmass, you will see a white mineral. This is plagioclase, also a type of feldspar but with a varying composition in which either sodium or calcium replaces the potassium of orthoclase. Plagioclase is in fact what is known as a solid solution series, meaning that all possible ratios of sodium and calcium are theoretically possible, including 100:0 and 0:100.

There is also a semi-transparent mineral, quartz, which is a crystalline form of silica that occurs mostly in acid igneous rocks, reflecting the high silica content of the magma, but never in basic and ultrabasic rocks. Most of the black specks are biotite, a type of mica, but a small proportion are magnetite, an oxide of iron that was the basis of the original Chinese magnetic compass.

There are in fact two types of Shap granite—light and dark—both obtained from the same quarry, and the next two photos illustrate the difference. The first shows the offices of Banco Santander in Market Square, where Shap granite has been used for the pilasters on each side, while the second is a close-up of part of the right-hand pilaster. Note the small xenolith near the top of the second picture.



I’m not sure what causes the difference between the light and dark forms of the granite, but I’m inclined to think that it reflects localized variations in the iron content of the magma. This element is present in only trace quantities here, but the plagioclase in the dark variety is definitely reddish rather than white, and iron is the most likely culprit.

I used to think that there were many more places around town where Shap granite had been used, but I was able to find only three other locations during a recent survey. The first is the town’s main post office, which was rebuilt circa 1960 on the site of an earlier post office (I’ve been unable to find the exact date). The second is a close-up.



The next photo is of an optician’s shop in King Street, where Shap granite has been used for the pediment below the windows, followed by a close-up. This was a newsagent’s and tobacconist’s shop when I was growing up.



The final two photos are of a second optician’s shop. This one was a greengrocer’s shop when I was growing up. Not many shops around town retain their original use now, and many traditional shop fronts have been vandalized by rebuilding, but I’m pleased to see that both these shops retain the original design.



The Dayson Building, which I featured in Windows Ten, can be see beyond the shop in the first of the two previous photos.

Shap granite has another important use, although you wouldn’t necessarily be aware of it unless somebody told you. It is an extremely hard rock, so when it is crushed into an aggregate and mixed with bitumen, it is ideal for use as a wearing course on motorways and other major roads. Locally obtained Carboniferous limestone is often used on the area’s minor roads, but this rock is far too soft to use on any surface where the traffic density is high.

Finally, Shap granite was formed towards the end of the Caledonian orogeny, the mountain-building event that produced the mountains of the Lake District, which makes it approximately 400 million years old.

2 comments:

  1. I'll always remember the school trips to Castleton in Derbyshire when we would hunt around for bits of beautiful Blue John

    ReplyDelete
    Replies
    1. Blue John, otherwise known as fluorspar or fluorite, is indeed a beautiful mineral. Cubic symmetry, if I remember correctly.

      By the way Keith, I tried to email you a couple of weeks ago, but it was returned because the address was invalid. I wanted to ask if you’d seen A Rotten English Question, because nobody has submitted the correct answer yet, and it seems like the kind of puzzle you’d get immediately.

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