In order to log a find for this EarthCache you must visit the location given, make some observations and answer some questions. You may email me your answers (for a quick reply) or use the message system. You are invited to log your find at the same time as sending your answers. I will reply and be aware "find" logs not supported with answers within 10 days will be deleted.
This is located on the main shopping street in St Peter Port and as such can be a busy location, however since this is no longer a bank you shouldn't have any trouble looking at the facade for a few minutes.
The vast majority of the island of Guernsey is metamorphic rocks gneiss and igneous granite. This cache however concentrates on the granite used on the building at the location given.
Only two of the nine main rock types found in Guernsey are Granites, the rest being Gneiss.
Photo courtesy of Wikipedia
Gneiss is a metamorphic rocks which has been altered by being re-heated, but not fully melted. This changes the chemical make up of the rock but can retain some of the sedimentary layers. Granites however is an Igneous rock which has originated from magma, a fully liquid rock, which has then cooled allowing crystals to form. Magma is usually the uppermost layer of molten rock. It is the rock that is erupted from volcanoes and has solidified either on the surface or in large chambers many kilometres deep underground.
The hardness of granite and gneiss is very similar, they are both around 6 to 7 on Mohs hardness scale, meaning they are both pretty tough to quarry and cut when used in building. The best way to tell the difference between gneiss and granite is to look for the different bands of colour in gneiss, stripes. The metamorphic processes, such as heat and pressure, that form gneiss force the different mineral crystal to orient themselves in parallel sheets of colour but it could be due to layering of the donor rock (eg sandstone and shale) being metamorphosed or if the original rock was igneous (maybe granite) the existing crystals can get squeezed and partially melted and become flatter orientating themselves at right angles to the pressure. With large pre-existing crystals this would mean that they start to look layered. Put very simply, in granite the crystals are 3 dimensional and in gneiss they have been squeezed flat. Granite is likely to be speckled with the minerals forming crystals as it cools. When the stone is cut and dressed these crystals are cut through giving the finished stone a spotty look.
As you tour the island you will see houses are faced with white black and pink stone, this is most likely local quarried granite or gneiss which is used because it is hard wearing and cheap to source, being from the island and not needed to be shipped in.
The focus of this cache is the facade of the Old Bank, here the three stones have been used and I'd like you to study the stones in;
a) the columns
b) the plinths on which the columns are placed
Here are your questions
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Please describe each of the two stone types, in your description please include colour, texture, crystals and roughness of the surface. In your explanation please tell me why these are granites and not gneiss.
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Please tell me which minerals are prevalent in each type of granite and explain how you came to your conclusion.
You may email or message the answers to me, I tend to respond to email quicker, and you don't need to wait for a reply to log your find.
If you need some help to answer the questions above please read on.
The colour and consistency of the gneiss and granite is depended upon the minerals that it was originally made of . Here are some common colours of granite and gneiss rocks and the minerals that give them their colour.
Here are some general trends in the colour of a granite which has predominant mineral within its make-up.
Quartz - typically milky white colour
Feldspar - typically off-white colour
Potassium Feldspar - typically salmon pink colour
Biotite - typically black or dark brown colour
Muscovite - typically metallic gold or yellow colour
The combination of the minerals above make up most of the colours we typically see in a granite.
Granite is a made up of minerals and rocks, primarily quartz, potassium feldspar, mica, amphiboles, and trace other minerals. Granite usually consists of 20-60% quartz, 10-65% feldspar, and 5-15% micas (biotite or muscovite). The minerals that make up granite give it the unique colours we see in different types of granite at this location.
The relative proportion of different coloured minerals that make up a granite is largely due to the original source of molten rock that cooled to form the granite in the first place. If the molten rock was abundant in potassium feldspar, the granite is more likely to take on a salmon pink colour. On the other hand, if the molten rock is abundant in quartz and minerals that make up amphibole, you will likely get a black and white speckled granite commonly seen on counter-tops.
Here is a little more detail on some of the colours.
White granite is composed primarily of quartz (milky white) and feldspar (opaque white) minerals. The small black specks in the granite above are likely small amphibole grains. This could be due to a lack of chemical components needed to form amphibole, or the cooling process was not amenable to formation of amphiboles.
Pink coloured granite is a result of an abundance of potassium feldspar within the granite. You will see small specs of milky semi-transparent quartz, dark brown/black amphibole, and opaque white feldspar. However, in a granite that is mostly pink the primary mineral is potassium feldspar.
A black and white granite will have equal parts quartz, feldspar, and amphibole, making a speckled black and white granite. This is one of the most common types of granite and one that is most commonly seen used for granite countertops.
Red granite is a variation of the pink potassium feldspar abundant granite, where the k-feldspar takes on a redder than pinker colour. Also, you can get red colouring from iron oxide in hematite grains or inclusion within feldspar, essentially the same process that makes rusted metal ruby red coloured.
Hope you enjoyed the cache
treboR