Both granites and Larvikite are course grained igneous rocks, but whilst granites by definition, contain a large amount of free quartz, Larvikite only contains a small amount, and is therefore a "Syenite". The most popular use for this rock is as you see here, as a decorative cladding for shops.
With luck you will be visiting this earthcache on a sunny day where you will be able to see the pearlescent sheen produced as the sun reflects on it. This is called the Schiller effect. The crystals producing this effect are formed of k-feldspar, by far the most common constiutuent of Larvikite.
These crystals formed slowly as magma cooled 30km below the earths surface almost 300 million years ago, as the original supercontinent Pangea broke up. Over time the rock was brought to the surface where conditions are very different and as a result the large crystals which formed so far below the surface are no longer stable. As a result there are microscopic changes in the feldspar which allow for the beautiful shimmering. This effect does not happen by chipping off a random chunk. Experts must analyse each block to decide in which direction it should be cut to best produce the shimmering effect. Because of the nature of the cooling, the crystals formed on similar planes, and as a result the nature of the cut can either allow for shimmering from most crystals, or from very few.
To log this earthcache, please send me a message or email with answer to the following questions;
1) What is another name for the Schiller Effect?
2) Where is Larvikite found and quarried?
3) Look closely at the feldspar crystals. How might be we be able to tell that they cooled slowly?
4) Describe the appearance of the feldspar crystals. How would the appearance change if the block was cut differently and why?
5) Roughly how large are the feldspar crystals?
6) Finally, to prove that you have been here, how thick is the cladding?
As with all earthcaches, a photo from the site is appreciated.