Orbicular Rock and Granite

6. Orbicular Rock
Orbicular rocks have orbicules, i.e. spheroidal structures, within the even-grained matrix. The orbicules have a core made of country rock fragments or concentrations of mineral grains. The diameter of the orbicules varies from one centimetre to almost half a metre. The orbicules are unique and often flattened or otherwise irregular. Orbicular rocks were formed from magma deep underground. Conditions must be exceptional at the rock’s moment of creation. They may entail the fast cooling of molten rock, a fast-flowing lava stream, or a lack of oxygen. A competing theory suggests that orbicular rocks formed during the solidification phase of the rock due to the seepage of potassium. The majority of orbicular rock occurrences are boulders whose source, the bedrock they broke away from, is unknown. Finland has around twenty known bedrock occurrences; for instance, the orbicular rock deposits in Viitasaari and Maaninka, which are now protected.

7. Granite
Granite is Finland’s most common rock type, and it is composed primarily of three minerals: quartz, plagioclase (feldspar) and potassium feldspar. Granite is an intrusive igneous rock. Granite colour varies according to the colour and amount of the feldspar it contains. There are many colour variants, one of the most curious ones being orbicular granite.

These three are among the most common rock-forming minerals. Their shapes and colours are highly diverse. Quartz is the second most common mineral on Earth, behind feldspar. It has a hexagonal crystal structure and a white band. Quartz occurs in several different-coloured variants. Quartzes can be categorised into two different groups based on grain size: crystalline and cryptocrystalline quartzes. Ideal, pure quartz, i.e. rock crystal, is clear and translucent. Quartz is durable and has diverse uses: in ceramics and optical fibres as raw material, in the production of foundry sand, and in gemstones. Finnish quartz is mined in various places, including Nilsiä. Feldspar and quartz together comprise around 60 per cent of the Earth’s crust by volume. Feldspar is a silicate mineral. It is white, grey or reddish, occasionally colourless. It is mined in Finland for the production of ceramics and glass. Significant deposits include Kimito, Haapaluoma, Kaatiala and Eräjärvi. Potassium feldspar is a brittle, easily breaking mineral. The colours range from pink and reddish brown to yellowish grey, white, or green and translucent.