A SHORT INTRODUCTION TO MINERALOGY

The natural sciences geology and geophysics in first instance study the formation of the earth, but nowadays also extraterrestrial bodies such as planets in and outside our solar system. In general geology focuses on  large scale processes such the formation of mountain ranges and movements in the earth crust. In contrast, mineralogy focuses on the elemental parts that form the rocks and that determine their general appearance, the minerals. Nowadays, mineralogy is no longer confined to terrestrial material but also to materials of extraterrestrial origin. In the last decades the nature of mineralogy has drastically changed and can no longer easily be defined. In general one can say with certainty that, in addition to being a branch of geology,  it is a branch of inorganic chemistry, but the discipline focuses on the origin, description, and classification of minerals. Nowadays, four main categories can be described;

1) crystal chemistry (composition, atomic arrangements)

2) paragenic mineralogy (study of mineral associations and occurrences)

3) descriptive mineralogy (study of physical properties and means for identification)

4) taxonomic mineralogy (classification, systematization and nomenclature)

The word mineral come from the Latin word minare, which means mining. It expresses the strong historical relationship between mining and the history of studying the ore, the minerals. Nowadays research in mineralogy is much more oriented to the formation of minerals, under which physical and chemical conditions, their behavior under high pressure and temperature, their crystallography, their chemical composition and their possible uses.

Minerals are defined as inorganic solids consisting of a regular three-dimensional ordered structure of atoms. This ordering of the atoms determines not only the crystalline nature of minerals, but it also gives us the opportunity to give a mineral a characteristic chemical composition. When minerals can grow without any hindrance they will be terminated by crystal faces, which are ordered according to a regular system resulting in a characteristic relationship typical for each mineral. This is studied in the crystallography.

Classification of minerals

Minerals are classified firstly based on their chemical composition, and then so far as possible by isomorphism or similarity  of crystalline form. In general the cations are of less importance than the anions or anionic groups. Normally the following ten major groups are identified

Mineral names

Minerals usually have both mineral name and a chemical name. Thus for example silicon dioxide is generally known as quartz, or carbon is, depending on the crystal structure, either known as graphite or as diamond. Some old mineral names are of unknown origin, while many come from Latin words, e.g. orpiment (auri pigmentum) or from Greek words, e.g. chalcosite (chalcos = copper or bras) or saponite (sapoun = soap). Most modern minerals names end with "ite". Some have a chemical annotation, such as molybdenite (Molybdenum sulfide MoS₂) or zincite (zinc oxide ZnO). Minerals like tetrahedrite and hemimorphite have  names with a crystallographic origin. Specific physical properties also form the basis for mineral names, e.g. magnetite (magnetic), graphite (to write), rhodochrosite (rose color). Geographic origin forms sometimes the basis for mineral names, such as in labradorite and vesuvianite. Last but not least, a large number of minerals have been named after individuals, e.g. scheelite, smithsonite, goethite, etc.

Properties of minerals

Crystallographic properties

One of the most characteristic properties of crystals is the shape of their crystals (also known as morphology). When crystals can grow freely they will clearly show their specific crystal faces (idiomorphic crystals). However, in many instances crystals will be deformed, e.g. due to lack of space or interference with other growing crystals. Also, not always the same faces will be recognized on a crystal. For example in a cubic system one can observe the cubic faces, but also octaeder or rhombododecaeder faces or combinations of faces in a single crystal. This can result in large differences in morphology. Analysis of the symmetry relations of the faces enables a mineralogist to determine its crystal class.

Optical properties

Optical properties of minerals are related to the crystal structure, the kind of atoms present in the structure and their electronic arrangement. The optical properties are usually examined under a microscope by examining small fragments in transmitted plane-polarized light for non-opaque minerals and in reflected plane-polarized light for opaque minerals. Amorphous and cubic substances are isotropic, they have only one index of refraction for give monochromatic light. Hexagonal and tetragonal minerals are uniaxial and are also double refracting. All other minerals are both biaxial and double refracting. The optical properties of most minerals are well known and are extensively tabulated in books and other publications. In certain instances the optical properties are related to the chemical composition of a mineral series, e.g. the plagioclase series. The optical properties are also related to the color of the mineral.

Physical properties

in general the physical properties of a mineral are those properties that can be used for the identification by sight, touch, etc. These properties are directly related to the crystal structure, the chemical composition and the kinds of chemical bonds present in the crystal structure. The two best defined properties are hardness, cleavage and density. Color and luster are must more difficult to access.

The hardness of a mineral is basically related to the chemical bonds present in the crystal structure. So, minerals with stronger chemical bonds are harder than those with weaker bonds. Absolute hardness can be measured by a microhardness tester, which places an indentation on a surface of the surface of a crystal beyond a certain pressure value. Much easier to use, especially in the field, for mineralogist is a relative scale based on 10 minerals with increasing hardness. This scale is known as the Mohs scale.

Thus a mineral with  a hardness of 6.5 will scratch potash feldspar but will not scratch quartz.

Cleavage is the preferential breakage along certain crystallographic planes. Cleavage is easiest along those planes with the weakest chemical bonds in the crystal structure. If all bonds are similar in all three directions (e.g. in framework silicates) then the plane with the least bonds per unit area will be the preferred cleavage plane. Cleavage is generally classified based on the perfection of the cleavage: perfect, good, distinct, poor or no cleavage. Fracture is the way a mineral breaks other than cleavage. Typical terms used for fractures are splintery, hackly, conchoidal, rough and smooth.

Density plays a significant role in the crystallochemical calculations as it is directly related to the structure and the chemical formula. Density is normally expressed in units of grams per cubic centimeter. Closely related to density is the specific gravity. The specific gravity is the ratio of the weight of a mineral to the weight of an equal volume of water (theoretically at 4°C).

Color is more difficult to access. Quite a number of minerals may occur in one, two or many different colors, but usually pure end-members (which are hardly found in nature, but only in synthetic minerals) have a particular color. The color of a mineral is caused by the adsorption of certain wavelengths of light. The color corresponds to the least absorbed wavelengths. One can make a distinction between idiochromatic minerals, which have their own specific color, and allochromatic minerals, where the color is not determined by the mineral itself. Color is often caused by impurities or crystal imperfections. Careful description of the color of a mineral can be obtained by comparison to standard color charts. Streak is the color of finely powdered mineral particles and is usually obtained by scratching the mineral over an unglazed porcelain surface. Luster is very subjective and describes basically the appearance of the mineral surface in reflected light and is related to the index of refraction in  a direct way. Luster increases with increasing index of refraction. Two important types of luster are metallic and nonmetallic. Many elements such as silver and sulfides are metallic in appearance. Nonmetallic luster can be divided in vitreous (e.g. quartz), resinous (e.g. sphalerite) or adamantine (e.g. diamond). The luster of a mineral is also influenced by its physical appearance or morphology. Fibrous minerals may have a silky luster, while platy minerals often display a pearly luster.

Fluorescence is observed when a mineral that absorbs ultraviolet light, emits visible light as a result. The word fluorescence comes from the mineral fluorite, which shows this property very beautifully. Although interesting and sometimes spectacular, this property is only useful in very special occasions for identification purposes. Fluorescence is not a constant property and can vary from sample to sample and from locality to locality. Other properties that sometimes can be used for identification are magnet, radioactive or electrical properties.

special mineralogical topics

Introduction to clay mineralogy