The other day I was walking to an event in this part of London when the facade on the building in front of you made me stop and take a closer look. It was a sunny day and the various colours in the stone were beautifully illuminated. I later found out through online research that this material is called Larvikite. It is named after the town of Larvik in southern Norway, where it can be found naturally. It is also known as "blue pearl granite", although it isn't really granite
Larvikite is an intrusive igneous rock, i.e. it was formed during the slow cooling process of magma deep (about 30 kms) below the earth's surface. This occurred in the early Permian time, about 298-293 million years ago, in association with the break-up of the super continent Pangea and eventual formation of the Oslo Rift. Larvikite is comprised nearly entirely of feldspar, but can also contain small amounts of olivine, clinopyroxene, amphibole, biotite, magnetite and apatite. Larvikite comes in several varieties which differ slightly in colour and composition.
After its formation deep below ground the Larvikite was slowly transported to the surface by a variety of geological processes. The conditions on the surface led to microscopic changes within the feldspar crystals. They have a lamellar microstructure which creates the colours that you can see when looking at the stone from different angles. This optical effect is known as Schiller effect or labradorescence (Labradorite is another feldspar mineral that shows this effect very impressively).
Larvikite is highly valued as a dimension stone, i.e. natural stone that is cut and finished to specified sizes and shapes, which can be used for buildings, monuments, paving, furniture, and decorative objects. As a very durable material Larvikite is mostly used for facades on commercial buildings, but also for kitchen worktops, bar counters and gravestones.
Now here are your tasks. At the given coordinates look at the south side of the building which gets more natural light.
1. Describe the Larvikite that you can find on this building. What colours can you see? How are the crystals arranged?
2. Why do you think the crystals are the size they are?
3. Now look at the brightest crystals. What is their average size? Can you see any patterns in them?
4. Describe in your own words why the crystals display this iridescent optical effect.
5. On the south side of the building, near the corner, you can find a round brown tag. Look at the 4 wall slabs that frame this tag. Which of them do you think shows the highest density of bright crystals?
6. Optional: post a photo of yourself, your mascot or GPS at GZ, without revealing any of the answers.
Send your answers to me as a private message or email before you log this cache. Don't add them to your online log. You don't have to wait for a reply from me.
I recommend doing this cache during daytime to see the colours and patterns properly.
(Information is from www.geoparken.com and Wikipedia)