What Are Minerals?

What are minerals, how do they form, and which properties can be used to distinguish them?

The Origin of the Word

The word “mineral” can be traced back to Medieval Latin. The expression “aes minerale” roughly means “ore from a mine.” The terms mina, meaning “mine shaft,” and minare, in the sense of “to engage in mining,” are also connected with the origin of the word.

From an early stage, people paid particular attention to unusual rocks and gemstones because of their color, luster, or hardness. The modern mineralogical concept developed much later, however, and dates from a time when minerals were viewed primarily in connection with mining and the extraction of raw materials.

In German, both Minerale and Mineralien are used as plural forms. The science concerned with minerals is called mineralogy.

Definition – What Is a Mineral?

There are several ways to define a mineral:

Minerals are chemically homogeneous, naturally occurring components of the Earth and other celestial bodies (the Moon, meteorites, terrestrial planets, our own and other solar systems). With only a few exceptions, they are inorganic, solid, and crystalline.

Mineralogy, Martin Okrusch and Siegfried Matthes, 8th edition

The following points are particularly important in this definition:

• Minerals form naturally through geological processes.
• Most minerals are solids; a well-known exception is native mercury.
• Minerals consist of one or more chemical elements and have a characteristic chemical composition.
• Most minerals are inorganic.
• Minerals generally have an ordered crystal structure.
• Chemical composition and crystal structure give rise to characteristic physical and chemical properties.

Amorphe (nicht kristalline) Substanzen, die alle anderen Eigenschaften erfüllen, werden Mineraloide genannt.

The Difference Between a Mineral and a Rock

The term “mineral” is often confused with “rock.” A rock, however, is a natural aggregate composed of one or more minerals. Many rocks consist of several different mineral species, while some may be made almost entirely of a single mineral. The mineral components of a rock are commonly classified as major constituents, minor constituents, and accessory minerals.

In general, rocks are divided into igneous, sedimentary, and metamorphic rocks.

The Natural Formation of Minerals

Minerals form naturally under specific physical and chemical conditions. They may form, for example, during the cooling of magma, from hydrothermal fluids, through evaporation, sedimentary processes, or during metamorphism. Some mineral-forming processes occur relatively quickly, while others take place over very long geological periods.

Wie bereits aus der Definition hervorgeht, müssen Minerale nicht zwingend auf der Erde entstanden sein. Sie kommen beispielsweise auch in Meteoriten und in Gesteinen anderer Himmelskörper vor.

Artificially produced substances such as industrial diamonds or bismuth crystals grown in a laboratory are therefore not considered naturally formed minerals. Quartz produced in a laboratory is accordingly referred to as synthetic quartz.

Minerals Are Homogeneous Solids with a Defined Chemical Composition

Minerals are generally solids; only liquid mercury is recognized as a mineral by the IMA. The building blocks of minerals are the chemical elements of the periodic table. Based on their chemical composition, minerals can be systematically classified into different groups. In the German-speaking world, the classification system developed by Strunz is commonly used; according to this system, minerals are divided into a total of 10 classes.

Some chemical elements occur in nature directly as minerals and are therefore included in the class of native elements. Well-known examples include gold, copper, sulfur, and diamond. Most minerals, however, are chemical compounds composed of several elements.

It is also important that minerals are homogeneous with respect to their chemical composition. Naturally, their composition may vary within certain limits, but they are structurally homogeneous, meaning that they are uniform in their physical and chemical properties. As a result, if a mineral is mechanically divided into separate pieces, the individual fragments can exhibit the same chemical and physical properties.

Most minerals are inorganic compounds. Minerals belonging to the group of organic compounds are comparatively rare.

Minerals Are Crystalline

In a crystal, the building blocks are arranged regularly within a crystal structure. These building blocks may be ions, atoms, or molecules. They are arranged periodically in three dimensions.

A total of seven crystal systems are recognized, which are used to classify the symmetry properties of crystals. The cubic crystal system, for example, has a very high degree of symmetry; cubes and octahedra are typical forms of this system. In the triclinic system, by contrast, symmetry is much lower.

Minerals Have Mineral-Specific Properties

Minerals are crystalline solids with a clearly defined chemical composition. The periodic arrangement of their building blocks in the form of a crystal lattice also gives rise to characteristic, mineral-specific properties.

Homogeneity does not necessarily mean that the chemical and physical properties are identical in every direction. In particular, physical properties such as electrical conductivity, birefringence, or light refraction often vary depending on direction and are therefore vector-dependent. In this context, homogeneous simply means that a crystal behaves in the same way along all parallel directions. This phenomenon of direction-dependent properties is also referred to as anisotropy.

Mineral properties can be divided into optical, mechanical, magnetic, electrical, and chemical properties. Some minerals are also distinguished by characteristic odors or tastes. Sulfides, for example, may release a characteristic sulfurous odor when struck, while halite (rock salt) tastes salty and sylvite has a slightly bitter taste.

Discussing every possible property would go beyond the scope of this article. The most important mineral properties are usually listed in a mineral description. These include:

• chemical formula
• mineral class
• crystal system and, where applicable, crystal class
• Mohs hardness
• Density
• Cleavage
• Fracture
• Color
• Streak
• Transparency
• Luster

How Many Minerals Are There?

Die Zahl of known and recognized mineral species continues to grow. The International Mineralogical Association (IMA) reviews and approves newly described minerals through its Commission on New Minerals, Nomenclature and Classification (CNMNC). Numerous new mineral species are recognized each year, so the total number changes continuously.

A distinction must be made between mineral species and mineral varieties. Amethyst, for example, is not a separate mineral species, but a violet variety of quartz. The same applies to many other well-known gem and collector minerals.

The discovery of new minerals is made possible in part by increasingly sensitive analytical methods. Modern techniques such as X-ray diffraction, electron microprobe analysis, and spectroscopic methods now allow even very small mineral phases to be characterized chemically and structurally. As a result, even microscopic mineral species or species occurring in only a few samples can be described unambiguously.

More than 6,000 mineral species are now recognized. The exact number changes regularly as new minerals continue to be described and approved by the IMA.