Feldspar is the most important group of minerals that we have, and make up 60% of the earth's crust. It is the most common mineral group we have on earth. Colour can vary from light, grey to reds, greens through to dark, blue/black. It is found throughout the world and typically distinctive as the pink veins in granite. In most deposits you find sodium feldspar, potassium feldspar and mixed feldspars. Feldspars are primarily used in industry because of their alumina and alkali content, most of the products we use on a daily basis are made with feldspar, from drinking glasses, floor tiles to fibreglass.
Feldspar minerals are found in all three rock groups, igneous, metamorphic and sedimentary rocks.
The mineralogical composition of most feldspars can be expressed in terms of the ternary system, Orthoclase (KAlSi3O8), Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8).
Chemically feldspars are silicates of aluminium containing sodium, potassium, iron, calcium or barium or a combination of all these.
The minerals of which the composition is comprised between Albite and Anorthite are known as the plagioclase feldspars due to the presence of alkali metals sodium and potassium.. The alkali feldspars are of particular interest in terms of industrial use. Though found in numerous rocks, feldspars are particularly abundant in igneous rocks like granite which can contain up to 70% of alkaline feldspar.
In front of you at GZ you see a large lump of granite, looking closely you will notice the individual crystals. The minerals that are found in granite are primarily quartz, plagioclase feldspars, potassium or K-feldspars, and micas.
Quartz is the milky white or grey colour, potassium feldspar or K-feldspar is the salmon pink colour and mica is the black specs.
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1a) How can you easily identify the feldspar here at GZ?
b) Granite is composed by two types of feldspar,plagioclase and orthoclase (K-feldspar). At gz you can typically only see one of the two varieties, which one? Explain how you got to your answer
2a) What is the average size of the feldspar crystals you see here at GZ?
b) How would you describe the shape and colour of the feldspar here?
3) What percentage of feldspar would you say the stone contains?
4) There is a brass plaque on the rock, what is it commemorating?
5) While not compulsory, it is always nice to see photos of your visit