Trace elements

The map below shows the mean concentrations of barium (Ba) in the mineral soil at a depth of 50 cm in forest land.

Barium is a metallic element belonging to the group of alkaline earth metals. It is the 16th most abundant element and is a silvery-white, soft metal with an appearance similar to lead.

The average concentration of barium in rocks in the accessible part of the Earth’s crust is 0.04–0.05 weight percent, i.e. 400–500 ppm. In its biological role, barium is considered a trace element.

Barium occurs in nature mainly as the sulphate mineral barite (heavy spar; BaSO₄), and to some extent as the carbonate witherite (BaCO₃) or as alstonite, a mixed carbonate of calcium and barium. Barium sulphate is extremely insoluble in water and has, among other uses, been applied in medical radiology as a contrast agent. Most barium compounds are toxic and have caused numerous cases of poisoning.

The barium content in some common rock types is about 0.06 weight percent BaO in gneiss granite, 0.08% in fine-grained younger granites, 0.16% in red Växjö granite, and 0.05% in mafic rocks. Relative concentrations are about 0.025% Ba in volcanic rocks and 0.003% Ba in living plants.

Based on the 1,892 mineral soil samples at a depth of 50 cm on which the map is based, the mean barium concentration is 608 ppm, with a range of 32–1609 ppm.

The ionic radius of barium is too large for it to closely follow calcium, but it can instead substitute for and be associated with potassium.

Read more about barium on Wikipedia.

The map below shows the mean concentrations of lead (Pb) in the mineral soil at a depth of 50 cm in forest land.

Lead is the softest of the common so-called heavy metals. It is bluish-grey and lustrous, but quickly becomes coated in air with a dull oxide film. Lead is one of the oldest metals used by humans; it was known in Egypt as early as 5000 BC and is described in very ancient records. Lead and its compounds are toxic.

The average concentration of lead in rocks in the accessible part of the Earth’s crust is 13–15 ppm.
The most common lead mineral is galena (PbS), which is also the dominant lead ore. Galena may occur as thin veins in the crystalline basement, but it also occurs as impregnation in sandstone.

In soils, lead is very strongly bound to organic matter as well as to Fe-, Al-, and Mn-oxides. The transport of lead in soils therefore occurs largely as dissolved humus complexes or in colloidal form associated with humic substances and iron oxides.

Based on the 1,892 mineral soil samples taken at a depth of 50 cm in forest land, on which the map above is based, the mean lead concentration was 18 ppm. However, the variation was large: 8.5–389 ppm.

Read more about lead on Wikipedia.

The map below shows the mean concentrations of copper (Cu) in the mineral soil at a depth of 50 cm in forest land.

Copper is a metallic element which, in its pure state, is salmon-coloured. It is stable in dry air but develops a green surface film of basic carbonate (patina/verdigris) in moist air. Copper is highly malleable and can be rolled into foil as thin as 0.02 mm. It has excellent thermal and electrical conductivity. Copper has been known since prehistoric times, possibly for as long as 9,000 years.

The average concentration of copper in rocks in the accessible part of the Earth’s crust is, according to WebElements, 68 ppm.

Copper commonly occurs in association with sulphur, although usually in relatively small amounts. Such compounds include chalcocite (Cu₂S) and covellite (CuS). The most common ores are chalcopyrite (CuFeS₂) and bornite (Cu₅FeS₄).

Copper(I) oxide (Cu₂O) occurs as cuprite (red copper ore), while copper(II) oxide (CuO) occurs in small amounts as tenorite (melaconite). The minerals malachite, Cu₂(OH)₂(CO₃) (light green), and azurite, Cu₃(OH)₂(CO₃)₂ (light blue), have been found in the Ural Mountains and are used in decorative arts.

When exposed over long periods to sulphur-bearing air and moisture, copper develops a green coating of basic sulphate (patina). This patina is most commonly a basic copper carbonate, but near marine environments it may instead be a basic chloride. The term is also used for basic copper acetate, which is used as a pigment.

Copper is naturally present in both plant and animal organisms, although its physiological function is not fully understood in all cases.

Copper forms part of certain proteins with enzymatic functions. A copper-containing blue globulin, ceruloplasmin, is present in the blood plasma of all mammals. Hemocyanin, which in crustaceans and molluscs performs the same function as haemoglobin in vertebrates, contains copper as the active transition metal. Insufficient copper supply causes deficiency diseases in both plants and animals. However, higher concentrations of copper are toxic, and lower organisms are particularly sensitive. For humans, copper and its compounds are moderately toxic; severe poisoning is rare, partly because vomiting is easily induced.

The average copper concentration based on the 1,892 mineral soil samples from 50 cm depth on which the map above is based was 31 ppm. This can be compared with the median copper concentration in the fine fraction (<0.063 mm) from 12,815 sampling points in Swedish glacial tills (C horizon), reported by SGU (2007) as 12.1 ppm. In SGU’s map service in Swedish (“Kartvisaren”), a soil geochemical map for copper can be generated, showing the total copper content in the C horizon fine fraction (<0.063 mm) of tills (select Geochemistry under Theme and Soil geochemical map, copper under Map).

Read more about copper on Wikipedia.

References

SGU (2007). Extract from SGU’s Regional Soil Geochemical Database, September 2007 (from the Copper data sheet).

The map below shows the mean concentrations of chromium (Cr) in forest soils, in the mineral soil at a depth of 50 cm.

The element chromium is a lustrous blue‑white metal and is resistant in air. Chromium does not occur in native form. In the Earth’s crust, the average concentration of chromium is about 140 ppm. The average concentration in forest soils, at 50 cm depth, is 59 ppm. The most important mineral is chromite (FeCr₂O₄). Chromite occurs in the Scandinavian Caledonides (the mountain chain), but is not economically exploitable there under normal conditions. The mineral occurs in serpentine rocks.

Chromium commonly exists either in the trivalent (Cr³⁺) or hexavalent (Cr⁶⁺) state. Hexavalent chromium can cause allergic reactions, is carcinogenic, and is harmful to the environment. Trivalent chromium is not considered allergenic, but may under certain conditions be transformed into the hexavalent form.

Cr³⁺ is strongly retained in the soil through complexation with organic matter and co‑precipitation with iron oxides, particularly at low pH and under anaerobic conditions. Cr⁶⁺ generally occurs as chromate (CrO₄²⁻), adsorbed onto Fe and Al oxides. Chromate is relatively mobile at high pH and in dry soils.

More about chromium on Wikipedia.

The map below shows the mean concentrations of molybdenum (Mo) in forest soils, in the mineral soil at a depth of 50 cm.

Molybdenum is a metallic element belonging to Group 6 in the periodic table. It is a light, lustrous, typically grey powder. It is part of the chromium group together with chromium and tungsten.

Molybdenum is a relatively rare metal, with an average concentration in rocks in the accessible part of the Earth’s crust of about 1.1 ppm.

Most of the 1,892 mineral soil samples from 50 cm depth on which the map above is based had molybdenum concentrations below the detection limit of 4 ppm.

The most important mineral is molybdenite (MoS₂), a soft, platy, bluish material that resembles graphite in appearance. Molybdenite occurs widely in the bedrock of Scandinavia. Swedish deposits are normally not economically viable. The relative content of molybdenum is about 15 ppm in volcanic rocks and 0.5 ppm in living plants.

In soils, molybdenum occurs as primary minerals, such as molybdenite, or as molybdate (MoO₄²⁻), which is strongly adsorbed to Fe and Al oxides when pH < 7. At high pH values, molybdenum is relatively mobile. Under reducing conditions, molybdenum is often very strongly bound to sulfides and organic matter.

More information about molybdenum is available on Wikipedia.

The map below shows the mean concentrations of nickel (Ni) in forest soils, in the mineral soil at a depth of 50 cm.

Nickel is a hard, silvery‑white metal that belongs to the so‑called iron metals together with iron and cobalt. It is resistant in air and is weakly magnetic. At high concentrations, nickel is toxic to most forms of life.

The average concentration of nickel in rocks in the accessible part of the Earth’s crust is about 90 ppm. The arithmetic mean concentration of nickel in the mineral soil at 50 cm depth in Swedish forest land is 22 ppm.

The most important nickel minerals are sulfides and arsenides. Some of these belong to the nickel arsenide type, for example nickeline (NiAs). Others belong to the pyrite type, where sulfur is partly replaced by arsenic or antimony, and where iron, cobalt, and nickel can substitute for one another.

Relatively pure sulfides include millerite (NiS) and pentlandite ((Ni,Fe)₉S₈). Nickel is also obtained from garnierite, which can be regarded as the silicate chrysotile with magnesium partly replaced by nickel. Other nickel minerals include chloanthite and nickeline.

The element commonly occurs in nickel-bearing pyrrhotite, which is particularly associated with the sulfide ores of central Sweden.

In soils, nickel occurs bound to organic matter, and at high pH values it can also be associated with Fe, Al, and Mn oxides. Under reducing conditions, nickel is also bound in sulfides. Nickel is relatively mobile at low pH values but is strongly retained in soils at high pH values.

More information about nickel is available on Wikipedia.

The map below shows the mean concentrations of strontium (Sr) in forest soils, in the mineral soil at a depth of 50 cm.

Strontium is one of the alkaline earth metals and is a malleable, lustrous, silvery‑white metal that is unstable in moist air. In nature, it occurs as the carbonate strontianite (SrCO₃) and as the sulfate celestine (SrSO₄).

The average concentration of strontium in rocks in the accessible part of the Earth’s crust is 370–450 ppm. The relative content in volcanic rocks is about 150 ppm and in living plants about 20 ppm. The mean concentration of strontium in the 1,892 samples from the mineral soil at 50 cm depth in Swedish forest land was 193 ppm.

Strontium is readily taken up by organisms in place of calcium, and in vertebrates it is mainly stored in the skeleton. The radioactive isotope ⁹⁰Sr is dispersed in the atmosphere during nuclear weapons tests and was present in the radioactive plume from the Chernobyl accident in 1986. It can therefore become a highly dangerous source of radiation. The isotope ⁹⁰Sr has a half‑life of 29 years.

More information about strontium is available on Wikipedia.

The map below shows the mean concentrations of vanadium (V) in forest soils, in the mineral soil at a depth of 50 cm.

The average concentration of vanadium (V) in rocks in the accessible part of the Earth’s crust is 110–135 ppm.

The most important vanadium mineral is patronite, the polysulfide VS₄. Also of importance is vanadinite, an analogue of apatite with the formula Pb₅Cl(VO₄)₃. Very large amounts of vanadium, although at low concentrations, occur in iron ores, where vanadium is associated with titanium and phosphorus.

The relative amount of vanadium in volcanic rocks is about 150 ppm and in living plants about 0.2 ppm. The mean concentration of vanadium in the 1,892 samples from the mineral soil at 50 cm depth in Swedish forest land was 65 ppm.

In soils, vanadium occurs in a large number of primary minerals, but also as vanadate (H₂VO₄⁻), which is strongly adsorbed to Fe and Al oxides at pH values below 10. In acidic soils, vanadium occurs as vanadyl (VO²⁺), which is very strongly bound to organic matter. Only at very high pH values (>10) does vanadium become readily soluble. Inputs of vanadium inhibit the activity of soil microorganisms, which may lead to reduced mineralisation of phosphorus, making it less available for plant uptake.

More information about vanadium is available on Wikipedia.

The map below shows the mean concentrations of zinc (Zn) in forest soils, in the mineral soil at a depth of 50 cm.

Zinc is a bluish‑white metal, lustrous, crystalline, and relatively soft but with limited ductility. It dissolves in acids and alkalis and is attacked in moist air where it forms a whitish corrosion layer.

The average concentration of zinc in the Earth’s crust is relatively low, about 75 ppm. In soils, concentrations typically range from 10 to 300 ppm, and zinc occurs in a variety of minerals. To some extent, zinc can substitute for or replace iron and magnesium in mineral crystal lattices, particularly in ferromagnesian minerals such as augite, hornblende, and biotite. The arithmetic mean concentration in Swedish forest soils at 50 cm depth is close to 60 ppm.

Zinc concentrations in soils depend largely on the mineral composition of the parent material. Soils derived from basic igneous rocks tend to have high zinc concentrations, whereas soils derived from more silica-rich rocks are notably zinc-poor. Locally, zinc concentrations may be influenced by sulfide mineralisations, and elevated zinc levels are common in areas with current or former mining activities or where mining waste has been deposited.

The most important zinc mineral is sphalerite (ZnS), which has a characteristic resinous luster. Pure sphalerite can be yellow or brown due to iron [(Zn,Fe)S], and the higher the iron content, the darker the colour. Finely powdered sphalerite shows a distinctly brown streak, making streak colour a simple and characteristic identification test. Sphalerite commonly occurs together with other sulfide minerals, with galena being a particularly important associate. Other zinc minerals are considered to have formed through oxidation of primary sphalerite. Apart from sphalerite, which can occur under reducing conditions, most other zinc minerals are too soluble to persist in soils over long periods.

Zinc is essential in very small amounts for both plants and animals. The concentration of zinc in soil solution has been reported to range from 3×10⁻⁸ to 3×10⁻⁶ M. The solubility of Zn²⁺ increases markedly at low pH values. In the upper soil horizons, zinc occurs mainly bound to organic matter. Zinc deficiency in vegetation is rare but may occur in near-neutral or alkaline soils, as zinc compounds become less available with increasing pH and precipitate as hydroxides. Adsorption to goethite [α‑FeO(OH)], and likely also to other iron oxyhydroxide minerals such as ferrihydrite, increases with rising pH, which is one reason for the limited availability of zinc in neutral and alkaline soils. Similarly low solubility is observed in the presence of calcium carbonate (calcareous soils), where pH values can reach up to about 8.2. The optimal availability of zinc for plants is within the pH range 5.5 to 7.0. Zinc deficiency can cause, among other effects, chlorophyll deficiencies.

In mammals, zinc is a component of several enzymes, and in the pancreas it plays a role in the production of insulin. Zinc is also considered to enhance immunity against a number of diseases. Zinc deficiency can lead to impaired wound healing and hair loss.

More information about zinc is available on Wikipedia.
 

The map below shows the mean concentrations of zirconium (Zr) in forest soils, in the mineral soil at a depth of 50 cm.

The average concentration of the element zirconium in rocks in the accessible part of the Earth’s crust is 160–165 ppm. The most common mineral is zircon (ZrSiO₄), which occurs as an accessory mineral in silica‑rich igneous rocks such as granite and syenite. Zircon is highly resistant to weathering and occurs in the heavy mineral fraction of many sand and sediment deposits, so‑called placers. Another mineral is baddeleyite (ZrO₂).

The average concentration of zirconium in the 1,892 Swedish forest soil samples from the mineral soil at 50 cm depth was 322 ppm.

More information about zirconium is available on Wikipedia.