Which minerals that make up a rock depend on how the rock was formed, for example which geological processes acted on it and in what part of the earth it was formed. The rocks in the seabed have a certain mineral composition while the continents are made of different rock types with widely varying composition.

Supracrustal rocks

Rock types that are composed of various layers have in many cases originally been formed near the surface. You can often see the layers with the naked eye. These rocks are called supracrustal rocks. Even today these rocks are formed, for example together with volcanic eruptions as well as when sediments are deposited in water and get lithified to rocks. Supracrustal rocks from volcanoes are called volcanites, while the rocks created by deposited sediments are called sedimentary rocks.

Volcanites

There is often a division between silica poor (mafic) and silica rich (felsic) volcanites. The felsic volcanites are generally light colored with a dense mass of quartz and feldspar. Sometimes the rocks have grains of feldspar and quartz making the rock look spotted, these are called porphyritic rocks. Porphyries from Älvdalen are used as ornaments for example. Some ore types, such as the copper ore in Falun, are linked to metamorphic felsic volcanites.

Volcanites which are in between the felsic and mafic rocks are called intermediate volcanites. Felsic rocks have over 65 per cent silica (SiO2) content, intermediate has between 52 and 65 per cent and mafic rocks have less than 52 per cent. Rocks with less than 45 per cent silica content are called ultramafic.

Sedimentary rocks

There are a lot of different types of sedimentary rocks, for example sandstone, shale, limestone and dolomite. Sandstone has, as the name implies, been formed from sand, and with the help of a magnifying glass you often can see the original grains. Sandstone is created through different lithifying processes, such as increases of pressure from overhead sediments. As the pressure increases further these rocks can turn into quartzite, which are harder and have a higher proportion of quartz.

The quartzite can be considered valuable if a larger part of it is pure quartz and lacks, or has a small proportion of, dark micas, feldspar or calcite. Quartzite is used in, for instance; silicon production, glass or porcelain production, refractory stones and in the steel industry.

Shale, phyllites and schists are formed out of clay deposits and are very common in some areas. These rocks are often dark, graphite-bearing and rusty on the surface. The Viscaria copper ore west of Kiruna has large proportions of chalcopyrite that sits in a fine grained mix in black shale and slate. The ore is further surrounded by felsic volcanic rocks.

Plutonic rocks

Another large rock type group is the so called plutonic rocks. These rocks have solidified at a great depth in the crust. The magma has cooled off slowly so mineral grains have had the time to grow before they are locked in a massive texture that is typical for plutonic rocks. Minerals are often easy to see and identify due to grains being so large distinct.

Plutonic rocks have a similar composition as the volcanites and are divided in the same way between felsic, intermediate and mafic. The felsic rocks are, like the volcanites, often light colored. An example of this is granite, which is primarily composed of quartz, feldspar and micas.

Economically interesting minerals can occur together with granitic rocks. These include among others: molybdenite, scheelite, cassiterite, wolframite, beryl and fluorite. Pegmatite is a rock with close to the same composition as a granite, but with much larger mineral grains, where minerals such as quartz can grow to be several meters long, weighing thousands of tons. Rare minerals are sometimes found in pegmatites. Some of the interesting minerals you can find in a pegmatite include the violet lithium mica called lepidolite and the red, green and blue tourmalines.

An example of an intermediate plutonic rock is diorite. A rock that is close to diorite is the mafic rock gabbro. Nickel-bearing pyrrhotite is sometimes found in gabbro, often together with chalcopyrite. There are also plutonic rocks where the dark or green minerals are in majority, and light minerals such as feldspar is lacking or only occur in small proportions. These are called ultramafic rocks, and can have mineable quantities of chromium, nickel and platinum.