Penrose Odd Rock EarthCache
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This rock caught my eye soon after the NT placed it on the verge to stop irresponsible parking of vehicles. I did some investigative work and tried to find its origin; the local NT ranger eventually came back and said it had been knocking about the Penrose Estate for some time and they just utilised it to stop irresponsible parking before the wooden posts were erected. The car park is just 20m down the turning.
The rock in question is made up of quartz which is common around Cornwall because it is often found in igneous rocks like granite (which is usually the local bedrock). In granite quartz forms when cracks and fissures are caused by the movement and folding of the granite due to tectonic forces. Molten magma or hot hydro thermal vents rise up from deep in the Earth’s crust into the fissures in the granite. If the hydrothermal veins lack the base element of silica in solution it can be leached out of the surrounding granite which has a high silica content, binding with the hydrogen and oxygen molecules in the water, hence the reason why it is common in granite. When the magma or water temperature and pressures reduce crystals grow and are formed. Quartz is a hard material and when graduated by the Mohs scale (created in 1812 by German geologist and mineralogist Friedrich Mohs) it has a hardness of 7 and cannot be scratched by steel.
This particular rock is a coloured variety of quartz and is the most common variety of quartz which often becomes a part of other kinds of rocks through various geological processes such as erosion, deposition and compression.
Quartz in its purest form is clear but it also comes in various colours caused by impurities or flaws in the crystals: these crystals are often used as semi precious stones: The colour of this particular rock is caused by minute fluid inclusions of gas, liquid, or both, trapped during crystal formation which can give an attractive greasy lustre, it is however of little value for optical and quality gemstone applications but has been used for ornamental carvings.
Below are brief descriptions of the various common types of quartz.
• Clear, colourless quartz is called rock crystal and is the purest form.
• Translucent/opaque white quartz is called milky quartz.
• Milky pink quartz is called rose quartz. Its colour is thought to be due to various impurities (titanium, iron,
manganese) or microscopic inclusions of other minerals.
• Purple quartz is called amethyst. Its colour is due to "holes" of missing electrons in the crystal structure in
combination with iron impurities.
• Yellow quartz is called citrine. Its colour is due to iron impurities.
• Green quartz is called praseolite. Iron impurities account for its colour, too.
• Grey quartz is called smoky quartz. Its colour is due to "holes" of missing electrons in combination with
aluminium impurities.
• Brown smoky quartz is called cairngorm and black smoky quartz is called morion.
To claim this cache you will need to inspect the rock closely and perhaps do a little research and then email the cache owner with the answers to the following questions:
1. Describe the surface of the rock in question and suggest which type of quartz you think it is.
2. Give the chemical formula of quartz ( a quick internet search should turn it up).
3. On the top left of the rock (your back to the cottage) are some fine filament like coloured lines. What colour
are these and explain what you think may cause them.
Email or message your answers to the cache owner, do not wait for a response to log the eathcache, however logs without an email/message to the owner are likely to be deleted.
Additional Hints
(Decrypt)
Gur ovt bar
Treasures
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