Minerals

Click the links below to view maps showing the concentrations of different minerals in Swedish forest soils.

• Quartz
• Feldspar (potassium feldspar and muscovite)
• Plagioclase
• Apatite
• Mafic minerals

Quartz is a mineral composed exclusively of silicon dioxide (SiO₂). It is one of the most common minerals in the continental crust. In terms of mineralogical composition, quartz accounts for about 11 vol.% of the continental crust.

The mineral is often grey‑white and semi‑transparent, but it occurs in a wide range of colours. Quartz may appear as clear rock crystal, purple amethyst, yellow citrine, white milky quartz, pink rose quartz, or black‑brown smoky quartz (“smoky topaz”).

Because its colour varies, identification based on colour is unreliable. Instead, other properties are used, such as hardness. Quartz is an index mineral with a hardness of 7 on the Mohs scale and cannot be scratched with a knife. However, it is hard enough to scratch window glass.

The crystal structure of quartz means that it does not exhibit cleavage along defined crystal planes when struck. Instead, it fractures into hard, sharp, splintery fragments characterised by a conchoidal fracture.

In systematic mineral classification, minerals are commonly grouped according to their chemical composition. These groups include: (i) native elements, (ii) sulfides, (iii) oxides and hydroxides, (iv) halides, (v) carbonates, nitrates and borates, (vi) sulfates, chromates, molybdates and tungstates, (vii) phosphates, arsenates and vanadates, and (viii) silicates, to which quartz belongs. The silicate group is the most diverse.

From a more practical perspective, where minerals are regarded as building blocks of rocks, a simpler classification into two groups is often used: major minerals and accessory minerals. Major minerals are quantitatively dominant and important for rock classification. Accessory minerals, which typically make up less than 5 vol.% of a rock, are too minor to determine rock type but may still be of economic importance if locally concentrated.

Quartz is one of the minerals that characterise the rocks of the Swedish crystalline bedrock, which largely consists of granites, porphyries, gneisses and leptites.

Quartz rarely weathers under the climatic conditions normally prevailing at the Earth’s surface. It therefore remains when other rock‑forming minerals are broken down by thermal stress, abrasion, or dissolution by acidic agents in air and water. As a result, quartz is often the dominant, sometimes nearly the only, constituent of sedimentary rocks such as sandstones and metamorphic equivalents such as quartzites.

Most quartz found in rocks and soils originally formed through crystallisation from hot silica‑rich fluids derived from magmas. Quartz can also form from gel‑like precipitates in silica‑rich water, producing very fine‑grained varieties such as chalcedony, agate, opal and jasper.

To counter the common misconception that so‑called “acidic” rocks produce acidic soils, it should be emphasised that the geological term “acidic” refers to a high silica content (SiO₂ > 65%), not to soil acidity expressed as pH < 7. Soil acidity refers to hydrogen ion activity, which can be measured with a pH electrode. For example, crushing pegmatite (a coarse‑grained granite) and suspending it in water can yield pH values above 9, even without the presence of mafic minerals.

When the silica content falls below 65 wt.%, conditions are less favourable for free quartz to crystallise from cooling magma. Instead, mafic (basic) rocks form, which lack free quartz.

The quartz content of Swedish till soils depends on the composition of weathering residues and rock fragments that covered the region prior to the Quaternary glaciations, which reworked and deposited the material as till and glaciofluvial sediments.

Quartz in Swedish bedrock

While the continental crust contains about 11 vol.% quartz, the Swedish bedrock consists to roughly 75% of quartz‑bearing rocks, with quartz contents ranging from 20 to 45%. Since the densities of rock‑forming minerals vary only slightly (from about 2.65 g/cm³ for quartz to about 2.8 g/cm³ for calcium‑rich plagioclase), volume percent and weight percent can be regarded as approximately equivalent.

The quartz content of Swedish bedrock can be estimated at 15–35%. Compared with the average quartz content of about 40% in Swedish till soils, quartz is thus overrepresented in the till relative to even the most quartz‑rich igneous rocks.

Rock types in which quartz is a major mineral:

Granite is an equigranular, massive rock in which the mineral grains are approximately the same size and lack preferred orientation. Granite belongs to the intrusive igneous rocks (plutonic rocks). Together with gneiss, granite is one of the most common rocks in the Earth’s crust and occurs in many forms. It displays a wide range of colours and is often named after a type locality where it is found.

Pegmatite can be described as granite occurring in dykes that cut through the bedrock. Pegmatite is characterised by very large mineral crystals. Grey‑blue or white‑reddish varieties are most common, depending on the composition and colour of the feldspars.

Porphyries are characterised by larger mineral grains (phenocrysts) embedded in a finer‑grained to dense groundmass. They often occur as dykes but nevertheless form extensive bedrock surfaces. Colours vary between grey, brown, and red, with red being the most common. Porphyries are classified as quartz porphyries or feldspar porphyries depending on their formation conditions and mineral composition. They belong to igneous dyke or volcanic rocks.

Porphyries occur in several parts of Sweden, but especially in western Sweden, from Dalsland in the south to Lapland in the north. Well‑known types include Dala porphyry, Åmål porphyry, Arvidsjaur porphyry, and Kiruna porphyry. Porphyry is also common in southeastern Östergötland and eastern Småland (Småland porphyry), as well as on the floor of the Baltic Sea (Baltic porphyry).

Quartzite consists of hard, quartz‑rich rocks formed from sandy sedimentary deposits. This rock type is highly resistant to chemical weathering. It may also occur as mica‑bearing quartzite. Quartzites belong to the metamorphic rocks.

Mica schist is a mica‑rich, quartz‑rich, banded or mottled rock, often containing other coloured minerals depending on the chemical composition of the original material. It belongs to the metamorphic rocks.

When clay shale is transported deep into the Earth’s crust through tectonic processes (movements in the crust), it is transformed by heat and pressure. The minerals in clay shale originally formed at the Earth’s surface and are stable only there. They have loose layered structures and contain large amounts of water. Therefore, they cannot withstand high pressures that tend to reorganise them into more compact minerals, nor strong heat that drives water out of their crystal structures. As a result, they are first replaced by quartz and mica, then by quartz, mica, and feldspar. With even stronger metamorphism, minerals such as garnet, cordierite, and sillimanite are formed.

Typical mica schist is dominated by quartz and muscovite, but the presence of magnesium and iron in clay minerals also leads to some biotite. Clay is always rich in aluminium and often so enriched that specific aluminium minerals form, such as the aluminium silicate andalusite.

Mica schist can also form from foliated granitic rocks through the influence of silica‑rich magmatic fluids.

Gneiss may be a coarse‑grained, white‑black or red‑white‑black, banded, folded, or foliated rock. Grain size ranges from coarse‑grained vein gneisses or augen gneisses to fine‑grained leptite gneisses. It belongs to the metamorphic rocks.

Gneiss and granitic gneiss are the most common rock types in Sweden. While granitic gneisses are always derived from granitic rocks (granite, granodiorite, tonalite) through deformation and recrystallisation, gneisses in general have varied origins. Types include homogeneous gneiss, banded gneiss, migmatitic gneiss, veined gneiss, and garnet‑bearing gneiss.

Sandstone is a granular rock composed of sand particles that have been consolidated into rock. Colours vary, but red, yellow, red‑yellow mottled, yellowish, and grey‑blue types are most common. Sandstones are sedimentary rocks but may undergo metamorphism, in which case they take on a quartzitic appearance and greater strength than the original sedimentary rock.

Most sandstones are dominated by quartz. They are formed from what are called mature sediments, produced through a combination of mechanical and chemical weathering. However, there are also many sandstones rich in feldspar and containing mica. In these cases, mechanical breakdown of the parent rock has dominated at the expense of chemical weathering, forming so‑called immature sediments. Examples include arkoses or sparagmites.

Sandstone is often distinctly layered, especially when composed of different minerals. The layering may be straight and even, indicating deposition in calm water. It may also be irregular, with layers inclined relative to each other. In such cases, the water was flowing, and the structure is referred to as cross‑bedding or current bedding.

The map shown above of the quartz content in forest soils is based on a mathematical calculation or model in which the silica content remaining after the other silicate minerals have been determined is assumed to belong to quartz. The abundant occurrence of quartz in western Dalarna can be explained by the significant contribution of Dala sandstone and porphyries.

Feldspar is the most abundant mineral in the Earth’s crust and typically forms blocky (tabular) crystals.

There are potassium feldspars (alkali feldspar, e.g. microcline/orthoclase) and calcium–sodium feldspars (plagioclase), both belonging to the silicate minerals containing aluminium along with potassium and, respectively, sodium and/or calcium.

Potassium feldspar occurs in two forms, where the crystal structure determines the name (microcline and orthoclase). It has a vitreous lustre and is usually light brown‑red, red, grey, or white in colour. A distinctly grey variety is called amazonite.

Feldspar can be distinguished from quartz by its slightly lower hardness (6 on the Mohs scale) and, above all, by its tendency to cleave along well‑defined crystal planes or flat surfaces that reflect light like a mirror. The angle between the two perfect cleavage directions is nearly right‑angled. Large crystals are abundant in pegmatite.

Plagioclase is usually white or grey and has properties similar to potassium feldspar, although red varieties are very rare. It contains sodium and calcium in varying proportions, forming a continuous series from pure sodium plagioclase (albite) to pure calcium plagioclase (anorthite). Sodium‑rich plagioclase is the most common and is an important constituent of most gneisses and granites. A collective term for potassium and sodium feldspars is alkali feldspars.

The most sodium‑rich plagioclases are called albite and oligoclase.

Plagioclase with higher calcium content occurs as a rock‑forming mineral in diorite, gabbro, diabase, hyperite, porphyrite, and basalt. The most calcium‑rich plagioclases are called bytownite and anorthite. Intermediate members include andesine and labradorite. Calcium‑rich plagioclase in particular is easily altered and then acquires a yellowish or greyish colour.

Alkali feldspars occur largely in the same rock types as quartz, except for pure quartzite. Calcium‑rich plagioclase, however, occurs in darker rocks with elevated calcium content.

In Swedish till soils and in water‑sorted sediments derived from them, feldspars account on average for about 50%. The corresponding value for the mineralogical composition of the continental crust is as high as 58 vol.%. Feldspars thus belong to the group of major minerals, which also includes quartz and a few other mineral types.

Mica is a hydrous silicate mineral that occurs in stacks of thin, flat, strongly lustrous and light‑reflecting sheets. These sheets can very easily be split apart and are flexible or elastic. Its hardness is significantly lower than that of feldspar and it can almost be scratched with a fingernail.

Two common types of mica occur: the pale yellowish or brownish, silvery‑shining muscovite (pure potassium mica), and the black or brownish‑black biotite (iron‑magnesium mica). Biotite is less resistant than muscovite and is often altered to greenish‑black chlorite (“brittle mica”), which is softer and less flexible than typical mica. Biotite also contains potassium and is very likely the most important mineral source for the release of plant‑available potassium through chemical weathering. Its dark colour is due to extremely finely dispersed iron oxide (magnetite), and it is more widespread than muscovite.

Both of these mica minerals occur in rocks such as gneiss, granite, pegmatite, and mica schist. Biotite also occurs in diorite and sometimes in porphyrite.

The mica that predominantly occurs in mica schist is muscovite, which has very high resistance to chemical weathering. This limits the importance of mica schists as suppliers of plant‑available potassium, and such rock types are therefore considered only moderate sources of plant nutrients.

In the map above, of feldspar content in forest soils, the presentation has been constructed such that potassium feldspar is combined with the occurrence of the mica variety muscovite. This is because both minerals contain similar amounts of potassium and are relatively resistant to weathering under our climatic conditions. Their relative proportions cannot be resolved without detailed mineralogical analysis, for example using X‑ray diffraction. The map showing the distribution of potassium feldspar and mica is based on the potassium content of the soils, and the potassium content of potassium feldspar has been approximated using the potassium content of mica.

In the maps above, the distribution of plagioclase has been separated from the potassium feldspar group (including mica). Although both feldspar types belong to the so‑called alkali feldspars, their geochemical characteristics justify this distinction. The map of soil plagioclase content is based on sodium content, and the use of oligoclase is based on the assumption that oligoclase is the dominant plagioclase type in material derived from granitic and gneissic bedrock, where a granitic composition predominates. Notably high plagioclase contents occur in northeastern Norrbotten. Other minerals that may contribute to the sodium content of soils include amphiboles, of which hornblende is the most common representative.

Apatite belongs to the group of accessory minerals, often referred to as minor minerals, where the concentration in the rock is less than 5 vol.%.

Apatite is a moderately hard calcium phosphate (hardness 5 on the Mohs scale) containing fluorine (most commonly) or chlorine. The colour is usually white, blue‑green, or blue‑violet, and the mineral commonly crystallises as hexagonal prisms. Small grains occur in most rocks, whereas larger crystals are very rare.

owever, apatite represents the natural source of phosphorus in our forest soils. Geochemically, phosphorus is often bound to precipitated secondary minerals that characterise the illuvial horizon (B horizon or enrichment horizon) in podzol soil profiles in forest land.

Other minerals that occur as accessory minerals in our rocks include fluorite, garnet, titanite, cordierite, epidote, magnetite, pyrite, and calcite.

In the calculation of apatite content in soils, all phosphorus has been allocated to the mineral apatite, even though other minerals may also contain phosphorus.

Under the heading mafic minerals, a number of minerals are grouped together that are characterised by their content of magnesium and iron. Due to the presence of iron, they often have a dark colour and are therefore also referred to as dark minerals.

This group includes minerals such as hornblende (belonging to the amphibole group), augite and diopside (belonging to the pyroxene group), chlorite, and epidote. Hornblende crystallises in the form of “boxes”, prisms, or needle‑like shapes.

In all likelihood, hornblende is the most common dark mineral in our forest soils. Hornblende is a black or greenish‑black, more rarely brownish‑black silicate mineral that, like mica, contains water but is nearly as hard as feldspar (hardness 5–6 on the Mohs scale). The higher the iron content in hornblende, the darker its colour.

In addition to iron, hornblende contains magnesium, calcium, aluminium, and sometimes sodium. Given the relative ease with which hornblende undergoes chemical weathering, it may be one of the sources of plant‑available sodium in our forest soils.

Hornblende occurs in many types of gneiss and granitic rocks, especially in varieties with lower silica content. It is a principal constituent of rocks such as diorite and amphibolite, as well as in altered gabbro and diabase, and often also in porphyrite. Hornblende is rare in pegmatite, although when present it may form large crystals. Hornblende and biotite are the most common dark (mafic) silicate minerals in the Earth’s crust.

In the mineral assemblages (skarns) surrounding most of our ore deposits (e.g. in Bergslagen), hornblende is an important mineral, as are other amphiboles. Hornblende may be altered and then transformed into epidote or chlorite.

Many amphiboles, including common hornblende, have needle‑like or radiating forms. This is due to their crystal structure, which forms long chains of atoms. The most strongly fibrous is actinolite, a calcium‑rich amphibole that is grey‑green to grey‑white or green to green‑black in colour. When mineral needles are arranged in parallel aggregates, the rock is termed actinolitic (radiating stone). In amphibole asbestos, actinolite has crystallised into fibres, with the finest fibres occurring in silky‑lustrous asbestos (amianthus).

Other fibrous amphiboles include anthophyllite (grey to brown, magnesium‑rich) and tremolite (grey to green, containing both magnesium and calcium as major components). These minerals often occur in bundles where the fibres are arranged fan‑like. Like actinolite, they commonly occur in skarns, an old mining term for mineral assemblages adjacent to ore bodies, formed through metasomatic alteration of the bedrock.

The pyroxenes constitute an entire mineral group that can also be classified as mafic minerals. In terms of frequency, they rank third after hornblende and biotite as representatives of dark (mafic) minerals in the Earth’s crust.

Pyroxenes resemble common hornblende in appearance but are consistently more “blocky” in form. Their hardness is the same or slightly higher (5–6 on the Mohs scale). In addition to silicon and oxygen, they contain magnesium, iron, calcium, and aluminium in varying proportions, but unlike amphiboles they do not contain structurally bound water.

The most common mineral in the pyroxene group is augite, which contains all of the mentioned elements. In hypersthene, calcium and aluminium are absent, while in diopside, aluminium and largely iron are absent. Augite and hypersthene are black or brownish‑black, whereas diopside is green or greenish‑black.

Augite and hypersthene are major minerals in rocks such as gabbro and dolerite, and they also commonly occur in basalt, and sometimes in syenite, diorite, and porphyrite.

Diopside is an important mineral in skarns surrounding several occurrences of crystalline limestone (marble), dolomite, and iron ore in the Precambrian basement. Near iron ores, augite and other pyroxenes are also present.

Manganese skarns contain the red manganese pyroxene rhodonite (e.g. at the Garpenberg Norra mine). In pegmatites, the grey‑white lithium pyroxene spodumene may occur, although rarely (e.g. Varuträsk and Utö).

Epidote is an accessory mineral in our bedrock and can to some extent also be included among the mafic minerals.

Epidote is one of the most common alteration minerals in the Earth’s crust. It usually occurs as very small crystals, often forming aggregates, although larger crystals also occur but are rare. These crystals have a stronger lustre than other forms of the mineral. Its hardness approaches that of quartz (6–7 on the Mohs scale). The colour is typically grass‑green, with shades of yellow, brown, or black.

Epidote is a hydrous silicate composed mainly of calcium and aluminium, but also iron. The higher the iron content, the darker the colour, with black epidote being richest in iron. The largest crystals and aggregates are found among alteration minerals (skarns) associated with ore deposits and limestones.

However, epidote is most widespread in common silicate rocks, particularly those rich in calcium (diorite, gabbro, dolerite, basalt, porphyrite). In these rocks, epidote has generally formed through alteration of plagioclase, hornblende, and pyroxene. Strong epidotisation is common in calcite‑bearing rocks, such as basalt with calcite‑filled vesicles and crystalline limestone (marble).

Epidote constitutes an entire mineral group with varying chemical compositions and physical structures. Examples include epidote (or pistacite), a calcium‑aluminium‑iron hydroxy‑silicate, and zoisite and clinozoisite, both calcium‑aluminium hydroxy‑silicates with different crystal structures.

Chlorite is another mineral that can be included in the group of mafic minerals. It is a green or green‑black silicate mineral formed through alteration of minerals such as hornblende and biotite. It has a greasy lustre and perfect cleavage, forming flexible but non‑elastic sheets. Its hardness ranges from 2 to 2.5 on the Mohs hardness scale.

Chlorite is an important component of soapstone in the Scandinavian mountain range. It is also common in, for example, central Swedish gneisses and basement schists. Fractures and shear zones in bedrock are often coated with chlorite, forming so‑called mica slickensides. During blasting of underground caverns and tunnels, chlorite‑filled slip zones with very low strength are sometimes encountered, greatly complicating and increasing the cost of excavation work.

The basis for calculating the distribution of mafic minerals has been magnesium and calcium, combined according to the elemental composition of the individual minerals.