Dala sandstone. Photo.

Photo: Jan-Olov Svedlund/SGU


Which minerals that make up a rock depend on how the rock was formed, for example which geological processes were responsible for its formation and in what part of the Earth it was formed.

The rocks in the oceanic crust, the bedrock beneath the sea floor, have a certain mineral composition, while the continents are made of other rock types of different composition.

Supracrustal rocks

Rock types that are composed of layers have in many cases originally been formed near the surface. The layers are often visible to the naked eye. These rocks are called supracrustal rocks. Such rocks are continuously formed over geological time, for example from volcanic eruptions, as well as when sediments are deposited in water and later lithified to rocks. Supracrustal rocks from volcanic eruptions are called volcanic rocks, while the rocks created by the deposition of sediments are called sedimentary rocks.

Volcanic rocks

Geologists often distinguish between silica-poor (basic) and silica-rich (acid) volcanic rocks. The acid volcanic rocks are generally light-coloured (felsic) and composed of a compact mass of quartz and feldspar. Sometimes felsic volcanic rocks contain larger crystals of feldspar or quartz which gives the rock spotted appearance, a texture called porphyritic. For example, porphyritic rocks from Älvdalen in central-west Sweden are used for ornamental applications. Some ore types, such as the copper ore in Falun, are hosted by metamorphosed felsic volcanic rocks.

Volcanic rocks with a moderate silica content in between the acid and basic rocks are called intermediate volcanic rocks. Acid rocks contain more than 65 per cent silica (SiO2), intermediate rocks between 52 and 65 per cent, and mafic rocks less than 52 per cent. Rocks with less than 45 per cent silica content are called ultrabasic.

Sedimentary rocks

There are many different types of sedimentary rocks, for example sandstone, shale, limestone and dolostone. Sandstone has, as the name implies, been formed from sand, and the individual grains are visible by aid of a magnifying glass or sometimes even to the naked eye. Sandstone is formed through different lithifying processes, such as increases in temperature and pressure due to burial by overlying sediments. In an orogenic (mountain-building) setting, in which the pressure and temperature may increase even further, sandstone may recrystallize into quartzite, which is a considerably harder rock.

Quartzite may be of economic interest if it mainly is composed of pure quartz and lacks, or has a only small proportion of, dark micas, feldspar or calcite. Quartzite is used in, silicon production, glass or porcelain production, refractory stones, and in the steel industry, to name a few applications.

Shale, slate, phyllite and schist are formed out of clay deposits . These rocks often show a dark-coloured interior and a rusty surface. They may also be graphite-bearing. The Viscaria copper ore west of Kiruna consists of large proportions of fine-grained chalcopyrite that are hosted by black schist. The shist is in turn surrounded by volcanic greenstones.

Igneous rocks

Another large rock type group is the so called igneous rocks. These rocks have crystallized from a magma at great depth in the crust. The magma has cooled off slowly enough for crystals to have time to grow large before they are interlocked in the massive texture that is typical for igneous rocks. Minerals are often easy to observe and identify due to the large crystal sizes.

Igneous rocks show the same range in composition as the volcanic rocks and are divided in the same way between acid, intermediate and basic. The acid rocks are, like the acid volcanic rocks, often light-coloured. An example of this is granite, which is primarily composed of quartz, feldspar and micas.

Minerals of economic interest may occur together with granitic rocks. These include among others: molybdenite, scheelite, cassiterite, wolframite, beryl and fluorite. Pegmatite is a rock of similar composition as a granite, but with much larger crystals, in which minerals such as quartz in extreme cases may grow to several metres in size, with single crystals weighing thousands of tons. Rare minerals are in some cases found in pegmatites, such as the violet lithium mica called lepidolite, as well as the red, green and blue tourmalines.

Intermediate igneous rocks have lower contents of silica than acid igneous rocks, at the same proportions as for intermediate volcanic rocks. An example of an intermediate igneous rock is diorite.

Basic igneous rocks, just like basic volcanic rocks, have even lower silica content. The perhaps most common basic igneous rock is called gabbro. Nickel-bearing pyrrhotite is sometimes found in gabbro, often together with chalcopyrite. There are also igneous rocks in which the dark or green mafic minerals are in majority, and felsic minerals such as feldspar is lacking or only occur in small proportions. These are called ultramafic rocks, and may contain mineable quantities of chromium, nickel and platinum.

Last reviewed 2021-02-01