Strunz Classification of Minerals

The Strunz classification is the universal standard for mineral classification and is based on their chemical composition. It was introduced by the German mineralogist Karl Hugo Strunz in his Mineralogische Tabellen 1941. In 2004, this classification was adjusted by the International Mineralogical Association (IMA).

Until its 8th revisión, minerals were classified into nine groups as follows.

I. Native elements

These are minerals found as single elements in their natural state rather than as compounds with other elements . About 20 elements are found in this native state and are not very abundant in the earth's crust. They are further sub-divided according to their chemical character into: metallic, semi-metallic and non-metals.

For example: Gold (Au), Sulfur (S), Diamond (C).

II. Sulfides and Sulfosalts

In this group are those minerals formed by combinations of metals or non-metals with sulfur. Also included are arsenides, antimonides, selenides and tellurides, as well as sulfosalts, which are double sulfides of a metal and a non-metal.

Most minerals in this group are opaque and have a high specific gravity. They are usually good conductors of heat and electricity. We find sphalerite, pyrite, proustite or galena, among others.

III. Halides

These anions are the halogens Fluorine (F), Chlorine (Cl), Bromine (Br) & Iodine (I), which are combined with metal cations (Sodium (Na), Potassium (K), Calcium (Ca) & Magnesium (Mg). Depending on the halogen atom forming the halide it will create a fluoride, chloride, bromide or iodide.

They have a relatively low hardness and many are soluble in water. Minerals belonging to this group are Sylvite, Halite and Fluorite.

IV. Oxides and Hydroxides

In these minerals the oxygen and hydroxyl group, respectively, occur in combination with one or more metals.

In general, they usually have high specific gravities and some of them have high economic value. In this group, we find Corundum, Magnetite, Pyrolusite, amongst others.

V. Nitrates, Carbonates and Borates

Minerals in this class are composed of anionic groups of the form XO3.

The Nitrate anion is NO3. Nitrates decompose more easily in acidic media than carbonates. They are very soluble in water and originate by precipitation in continental basins with strong evaporation.

Carbonates are composed of anions containing carbonate (CO3). Carbonates dissolve in acids and release CO2, which makes them effervescent. They are usually colorless, except those containing transition metals, and have a glassy appearance.

Borates take the form of a BO3 anion. Many borates are easy to hydrate. They are relatively soft and have colors that can be white, gray or yellowish. The colorless ones turn chalky white when exposed to air for a long time and have a bitter taste.

In this group we find Saltpeter, Calcite & Boracite, amongst many other minerals.

VI. Sulfates, Chromates, Molybdates and Wolframates

Sulfates are minerals that form when anionic groups of the form SO4 bind to metal cations. The anionic groups and cations are joined by electrostatic forces. There are two types of sulfates: anhydrous and hydrated sulfates.

Sulfates are soft minerals and of different molecular weights, depending on the type of cation they contain. Most sulfates dissolve in water, except those with Ba, Sr or Pb cations. Alkaline and alkaline earth sulfates are usually formed by sedimentary processes. The remaining sulfates come from surface alteration of metal sulfides.

There are two types of supergene or enriched minerals: chromates and molybdates (Crocoite and Wulfenite, are the most common, respectively). These minerals form in the enrichment zone below the oxidation zone.

The most common wolframates (Wolframite and Scheelite) are usually found in granitic pegmatites and hydrothermal seams.

VII. Phosphates, Arsenates and Vanadates

These consist of anionic groups joined by covalent bonds, while the anionic groups and cations are held together by electrostatic forces. Phosphates are common, but there are some that stand out, such as apatite. This is abundant and may contain additional anions of fluorine, chlorine and hydroxide. The most common arsenites and vanadates (Mimetite, Vanadinite) are formed from the oxidation of metallic minerals.

VIII. Silicates

This is the most abundant group of rock forming minerals, making up 90% of the Earth’s crust. Every silicate contains oxygen and silicon, in basic form with Silicon at the centre of four Oxygen atoms to form a tetrahedral anion stucture. Substitution of Silicon by other elements forms many different minerals. The properties of these minerals depend on the other elements present. The difference between the various groups lies in how these tetrahedra are joined together. Apart from the basic 4-fold tetrahedron, there are also 6-fold, 8-fold & 12-fold sites with substitution of Silicon by many metals.

The most common Silicates are Quartz, Feldspar, Mica, Amphibole, Pyroxene & Olivine

IX. Organic substances

In this last category of Strunz's classification are the following types of minerals: petroleum, coal, anthracite, lignites and others.

Amber is a mineral formed from fossilized plant resin. It usually comes from the remains of coniferous trees.

The current IMA (International Mineralogical Association) / CNMNC (Commission on New Minerals, Nomenclature and Classification) scheme (2009 onwards) is more systematic and divides minerals into ten classes, which are further divided into divisions, families and groups according to chemical composition and crystal structure