2.1: Minerals

Defining "Mineral"

A mineral is a naturally occurring inorganic element or compound that is solid at room temperature (25º C), has a regular internal structure, and has a defined chemical composition. Let’s break down the five aspects of this definition:

"Naturally occurring" implies that minerals are not artificially made. Many minerals (e.g., diamond) can be made in laboratories, but if they can also occur naturally, they still qualify as minerals.

Organic chemicals are made of the element carbon (C), so generally minerals are not composed of carbon. There are a handful of important minerals that include carbon so the term "organic” in this case means compounds of carbon, oxygen, and hydrogen. Sugar crystals, therefore, are not minerals. "Inorganic" with respect to minerals means not consisting of or deriving from living matter.

At 25º C minerals must be in the solid state. There are a few exceptions to this rule made for substances defined as minerals before 1959 when strict procedures were established for defining what a mineral is. For example, water ice is considered a mineral which is only solid at or below 0º C. Mercury is also considered a mineral even though it is only solid below -39º C. When found in rocks above -39º C, mercury forms silvery liquid blobs (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Drops of native mercury (pure mercury, Hg), also known as quicksilver, from California, USA. This sample is on public display at the Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA. "Mercury (California, USA) 1" by James St. John via Flickr is shared under CC BY.

A mineral has a specific "repeating three-dimensional structure" or "lattice," which is the way in which the atoms are arranged. The elements sodium (Na) and chlorine (Cl) atoms bond in a regular repeating pattern to form the mineral halite (Figure \(\PageIndex{2}\)). That happens to be about the simplest mineral lattice of all; most mineral lattices are much more complicated.

Figure \(\PageIndex{2}\): Halite crystal lattice. The purple spheres represent the cation sodium (Na⁺) and the green spheres represent the anion chlorine (Cl^-). Halite is the mineral commonly known as table salt. This work by Steven Earle is licensed under CC BY 4.0.

Some substances that we think of as minerals are actually not minerals by definition because they lack this repeating three-dimensional structure. Volcanic glass is an example, as are pearl, and opal. Figure \(\PageIndex{3}\) illustrates the regular crystalline structure of SiO2 in two-dimensions. Compare that to Figure \(\PageIndex{4}\) which illustrates silica glass. While some bonds exist, they are not consistent throughout the material.

Figure \(\PageIndex{3}\): The regular internal crystalline structure of SiO₂ in two-dimensions. Smaller blue spheres represent silicon atoms which are bonded in a regular repeating pattern with oxygen, represented by larger blue spheres. This is a slice through the crystal lattice of quartz wherein each silicon atom is actually bonded to four oxygen atoms. The fourth oxygen atom is not visible in this plane of view. "SiO2 Quartz" by Wimmel via Wikimedia Commons is in the public domain.

Figure \(\PageIndex{4}\): The random network structure of silica glass in two-dimensions. Note that, as in the crystal, each silicon atom is bonded to 4 oxygen atoms, where the fourth oxygen atom is obscured from view in this plane. "Silica.jpg" by Jdrewitt via Wikimedia Commons, a derivative of the original work, is in the public domain.

"Defined chemical composition" means that most minerals have a specific chemical formula or composition. The mineral pyrite, for example, is FeS₂ (two atoms of sulfur for each atom of iron), and any significant departure from that would make it a different mineral. But many minerals can have variable compositions within a specific range. The mineral olivine, for example, can range from Fe₂SiO₄ to FeMgSiO₄ to Mg₂SiO₄. Intervening compositions are written as (Fe,Mg)₂SiO₄ meaning that Fe and Mg can be present in any proportion, and that there are two of them for each Si present. This type of substitution is known as a solid solution. Other common minerals such as plagioclase feldspar also occur in solid solution.

Query \(\PageIndex{1}\)

Atoms

Minerals are made up of atoms, and all atoms are made up of three main subatomic particles known as protons, neutrons and electrons. As summarized in Table 2.1.1, protons are positively charged, neutrons are uncharged, and electrons are negatively charged. The negative charge of one electron balances the positive charge of one proton. Both protons and neutrons have a mass of 1 amu, while electrons have almost no mass.

Table 2.1.1: Charges and masses of atomic particles

Particle	Charge	Mass (amu)
Electron	-1	~0
Proton	1	1
Neutron	0	1

The simplest atom is that of hydrogen (atomic number 1) which has one proton and one electron. The proton forms the nucleus, while the electron spins around it. All other elements have neutrons as well as protons in their nucleus, such as helium (atomic number 2). The positively-charged protons tend to repel each other, and the neutrons help to hold the nucleus together.

The atomic number is the number of protons. The mass number is equal to the total number of protons and neutrons in an atom. Hydrogen (H) for example, has an atomic number of 1 because it has 1 proton. Helium (He) has an atomic number of 2 because it has 2 protons. Figure \(\PageIndex{5}\) provides a simplified view of the elements up to number 36.

Figure \(\PageIndex{5}\): The first four rows of the Periodic Table Showing Element Symbols (e.g., Cr), Names (e.g., chromium), Atomic Numbers (upper left of each element symbol) and Atomic Weights (upper right of each element symbol). Access an interactive version of the Periodic Table of Elements. This work by Steven Earle modified from the original is licensed under CC BY.

Electron orbits around the nucleus of an atom are arranged in what we call shells—also known as energy levels. The first shell can hold only two electrons, while the next shell will hold up to eight electrons. Subsequent shells can hold more electrons, but the outermost shell of an atom will hold no more than eight electrons. These outermost shells are important in bonding between atoms, and bonding takes place between atoms that do not have full outer shells.

Isotopes

Isotopes are atoms that have the same atomic number (number of protons), but different mass numbers due to a difference in the number of neutrons.

The atomic mass (or atomic weight) of an element is a weighted average of all naturally occurring isotopes of that atom. Let's consider oxygen. There are three naturally occurring stable isotopes of oxygen on Earth: oxygen-16 (denoted ¹⁶O), oxygen-17 (¹⁷O), and oxygen-18 (¹⁸O). Table \(\PageIndex{2}\) summarizes the characteristics of these three stable isotopes. Oxygen's atomic mass (Figure \(\PageIndex{5}\)) is ~15.999 amu, illustrating that oxygen-16 is more abundant than oxygen's other two stable isotopes.

Table \(\PageIndex{2}\): Stable oxygen isotopes

Nuclide	Protons (Z)	Neutrons (N)	Natural abundance
^16O	8	8	99.738%
^17O	8	9	0.0367%
^18O	8	10	0.187%

This is an example of a stable isotope, an isotope which does not decay (or change) over time. Oxygen-18, which is noted in the table above and is referred to as one of the environmental isotopes. It is important in paleoclimatology, the study of past climates, for example, because we can use the ratio between Oxygen-18 and Oxygen-16 in ice cores to determine the temperature of precipitation over time.

Radioactive, or unstable, isotopes are essential for dating minerals and rocks. For more on how isotopes are used to study the ages of Earth materials, see Geologic Time.

Bonding and Lattices

An atom seeks to have a full outer shell (i.e. 8 electrons for most elements, or 2 electrons for hydrogen, helium, lithium, and beryllium) to be atomically stable and thus, nonreactive. This is accomplished by lending, borrowing, or sharing electrons with other atoms. The noble gasses, such as helium, neon, argon etc., already have completed outer orbits so they don’t need to lose or gain any electrons.

Sodium has 11 electrons, 2 in the first shell, 8 in the second, and 1 in the third (Figure \(\PageIndex{6}\)). Sodium readily gives up this third shell electron, and when it does it loses one negative charge and becomes positively charged. Chlorine, on the other hand, has 17 electrons, 2 in the first shell, 8 in the second, and 7 in the third. Chlorine readily accepts an eighth electron for its third shell, and therefore becomes negatively charged. In changing their number of electrons these atoms become ions—the sodium a positive ion or cation, the chlorine a negative ion or anion. The resulting electric attraction between these ions is known as an ionic bond. Electrons can be thought of as being transferred from one atom to another in an ionic bond.

Figure \(\PageIndex{6}\): Electron configuration of sodium and chlorine atoms (top). Sodium gives up an electron to become a cation (bottom left) and chlorine accepts an electron to become an anion (bottom right). This work by Steven Earle is licensed under CC BY 4.0.

Common table salt (NaCl) is a mineral composed of chlorine and sodium linked together with ionic bonds that are depicted as grey line in (Figure \(\PageIndex{2}\)). The mineral name for NaCl is halite and its structure is cubic, meaning that all of the bonds are at right angles to each other. This is why halite grows as cubic crystals (Figure \(\PageIndex{7}\)), and it is also why it breaks along three planes at right angles to each other (see Mineral Properties).

Figure \(\PageIndex{7}\): The mineral halite is composed of sodium and cholorine bonded together with ionic bonds. It has a cubic crystal lattice which is why it grows in cubic crystals. This work by Hans-Joachim Engelhardt is licensed under CC BY-SA.

Video \(\PageIndex{1}\): How Atoms Bond

The following video illustrates these different ways in which atoms bond to one another:

The elements silicon and oxygen are the most abundant elements in Earth’s crust and mantle and are present in the minerals that make up most rocks. Silicon and oxygen bond together to create a four-sided pyramid shape with an oxygen at each corner and a silicon in the middle, known as a silicon-oxygen tetrahedron (Figure \(\PageIndex{8}\)). This is the building block of the many important silicate minerals. The bonds in a silicon-oxygen tetrahedron have some of the properties of covalent bonds and some of the properties of ionic bonds. As a result of the ionic character, silicon becomes a cation (with a charge of +4) and oxygen becomes an anion (with a charge of -2). The net charge of a silicon-oxygen tetrahedron (SiO₄) is -4. As we will see later, silicon-oxygen tetrahedra are linked together in a variety of ways to form most of the common minerals of the crust.

Figure \(\PageIndex{8}\): The silicon-oxygen tetrahedron, made of four oxygens bonded to one silicon, is the building block of all silicate minerals. "SiO4 4- Silicate tetrahedron" by Karla Panchuk, a derivative of the original work, is licensed under CC BY-SA 3.0.

3D Model of Silicon-Oxygen Tetrahedron

The following 3D model illustrates the silicon oxygen tetrahedron in three dimensions:

SiO₄ tetrahedron by Earth Sciences, University of Newcastle on Sketchfab

Box: Silicon (Si)

The element silicon (Si) is one of the most important geological elements and is the second-most abundant element in Earth’s crust (after oxygen). Silicon bonds readily with oxygen to form a silica tetrahedron (Figure \(\PageIndex{8}\)). Pure silicon crystals are used to make semi-conductive media in electronic devices called silicon wafers. A silicate mineral is one in which silicon and oxygen are present as silica tetrahedra (plural of tetrahedron). Silica also refers to a chemical component of a rock, and is expressed as % SiO₂. The mineral quartz is made up entirely of silica tetrahedra, and some forms of quartz are known as silica. Silicone is a synthetic product (e.g., silicone rubber, resin, or caulking) made from silicon-oxygen chains and various organic molecules.

The following video provides an overview of the ways we use the abundant element silicon:

Mineral Groups

Minerals are most often organized into groups according to their chemical composition, particularly their anion or anionic group (the negatively charged end of the chemical formula).

Elements bonded together in various configurations form minerals. A mineral is a naturally occurring, solid compound with a specific composition and a regular repeating lattice structure (like the halite in Figure \(\PageIndex{3}\)). Most minerals are made up of a cation (a positively charged ion) or several cations, and an anion (a negatively charged ion) or an anion group. For example, in the mineral hematite (Fe₂O₃) the cation is Fe (iron – Fe³⁺) and the anion is O (oxygen O^2-). (The two +3 iron ions contribute 6 positive charges and the three -2 oxygen ions contribute 6 negative charges, so the charge is balanced, which is a requirement of all minerals.) We group minerals into classes on the basis of their predominant anion or anion group. These include oxides, sulfides, carbonates and silicates, and others. Silicates are by far the predominant group in terms of their abundance within the crust and mantle, and they will be discussed in the next section. Some examples of minerals from the different mineral groups are given in Table \(\PageIndex{2}\).

Table \(\PageIndex{2}\): The primary mineral groups, their anions, and examples of minerals for each group

Group	Anion	Example Minerals
Oxides	O-	Hematite (iron-oxide – Fe2O3), corundum (aluminum-oxide Al2O3), water ice (H2O)
Sulfides	S-	Galena (lead-sulfide – PbS), pyrite (iron-sulfide – FeS2), chalcopyrite (copper-iron-sulfide – CuFeS2)
Carbonates	CO3^2^-	Calcite (calcium-carbonate – CaCO3), dolomite (calcium-magnesium-carbonate – (Ca,Mg)CO3
Phosphates	PO4^4^-	Apatite (Ca5(PO4)3(OH)), monazite (Ce, La, Y, Th)PO4, turquoise CuAl6(PO4)4(OH)8·4H2O
Silicates	SiO	Quartz (SiO2), feldspar (sodium-aluminum-silicate – NaAlSi3O8), olivine (iron or magnesium-silicate – (Mg,Fe)SiO4) Note that in quartz the anion is oxygen, and while it could be argued, therefore, that quartz is an oxide, it is always classed as a silicate.
Halides	Halogen element	Fluorite (calcium-fluoride – CaF2), halite (sodium-chloride – NaCl) Halide minerals have halogen elements as their anion – the minerals in the second last column on the right side of the Periodic Table, such as F, Cl, Br etc. (see Figure 2.1.5)
Sulfates	SO4^-2	Gypsum (calcium-sulfate – CaSO4·2H2O), barite (barium-sulfate – BaSO4) Note that sulfates are different from sulfides. Sulfates have the SO4^-2 ion while sulfides have the S^-2 ion)
Native elements	Single element	Gold (Au), diamond (C), graphite (C), sulfur (S), copper (Cu)

Oxide minerals have oxygen as their anion, but they exclude those with oxygen complexes such as carbonate (CO₃^2-), sulfate (SO₄^2-), or silicate (SiO₄^4-). If the oxygen is also combined with hydrogen to form the hydroxyl anion (OH–) the mineral is known as a hydroxide. Some important hydroxides are limonite (FeO(OH)) and gibbsite (Al(OH)₃), which are common within ores of iron and aluminum.

Sulfides are minerals with the S^2- anion, and they include galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS₂) and molybdenite (MoS₂), which are the most important ores of lead, zinc, copper and molybdenum respectively. Another important sulfide mineral is pyrite (FeS₂).

The carbonates include minerals in which the anion is the CO₃^2- complex. The carbonate combines with +2 cations to form minerals such as calcite (CaCO₃), magnesite (MgCO₃), dolomite ((Ca,Mg)CO₃) and siderite (FeCO₃). The copper minerals malachite and azurite are also carbonates.

In phosphate minerals the anion is PO₄^4-. An important phosphate mineral is apatite (Ca₅(PO₄)₃(OH)), which is what your teeth are made of.

The silicate minerals include the elements silicon and oxygen in varying proportions ranging from SiO₂ to SiO₄. These are discussed in more detail in the next section.

The halides are so named because the anions include the halogen elements chlorine, fluorine, bromine etc. Examples are halite (NaCl) and fluorite (CaF₂).

Sulfates are minerals with the SO₄^-2 anion, and these include anhydrite (CaSO₄ and its cousin gypsum CaSO₄⋅2H₂O) and the sulfates of barium and strontium: barite (BaSO₄) and celestite (SrSO₄). In all of these cases the cation has a +2 charge which balances the -2 charge on the sulfate ion.

Native minerals include only one element (bonded to itself), such as gold, copper, sulfur, or carbon (which could be graphite or diamond).

Silicate Minerals

The vast majority of the minerals that make up the rocks of Earth’s crust are silicate minerals. These include minerals such as quartz, feldspar, mica, amphibole, pyroxene, olivine, and a great variety of clay minerals. The building block of all of these minerals is the silica tetrahedron,. As discussed in the section on bonding and lattices, it is a combination of four oxygen atoms and one silicon atom. In silicate minerals these tetrahedra are arranged and linked together by sharing oxygen atoms in a variety of ways, from single units to complex frameworks (Table \(\PageIndex{2}\)). The simplest silicate structure, that of the mineral olivine, is composed of isolated tetrahedra bonded to iron and/or magnesium ions. In olivine the -4 charge of each silica tetrahedron is balanced by the addition of two divalent (i.e., +2) iron or magnesium cations. Olivine can be either Mg₂SiO₄ or Fe₂SiO₄, or some combination of the two, which is written like this: (Mg,Fe)₂SiO₄, meaning that any proportions of Mg and Fe are possible.

Table 2.1.3: Silicate Mineral Subgroups

Silicate Subgroup	Tetrahedral Arrangement	Example Minerals
Isolated tetrahedra (nesosilicates)	This work by Allison Jones, a derivative of the original, is licensed under CC BY-NC-SA 4.0.	Olivine, garnet, zircon, kyanite
Sorosilicates	This work by Allison Jones, a derivative of the original, is licensed under CC BY-NC-SA 4.0.	Epidote
Single chain (inosilicates)	Illustration of a single chain of silica tetrahedra; each tetrahedron is connected to two other tetrahedra. This work by Jonathan R. Hendricks is licensed under CC BY-NC-SA 4.0.	Pyroxene group minerals (e.g. augite), wollastonite
Double chain (inosilicates)	Illustration of a double chain of silica tetrahedra. Note that every other silica tetrahedron shares a corner oxygen with the parallel chain. This work by Jonathan R. Hendricks is licensed under CC BY-NC-SA 4.0.	Amphibole group minerals (e.g. hornblende)
Cyclosilicates	This work by Allison Jones, a derivative of the original, is licensed under CC BY-NC-SA 4.0.	Beryl, benitoite
Sheet (phyllosilicates)	Illustration of a sheet of silica tetrahedra. Note that each silica tetrahedron shares a corner oxygen with three other tetrahedra. This work by Jonathan R. Hendricks is licensed under CC BY-NC-SA 4.0.	Mica, clay minerals, serpentine, chlorite
Framework (tectosilicates)	Illustration of 3D framework (tectosilicate) atomic structure. This is the simplest example: quartz. This work by Allison Jones, a derivative of the original, is licensed under CC BY-NC-SA 4.0. View a 3D model of quartz, a tectosilicate.	Feldspar group minerals (e.g. plagioclase feldspar and potassium feldspar), quartz

References

1. "Atomic Weight of Oxygen | Commission on Isotopic Abundances and Atomic Weights". ciaaw.org. Retrieved 2022-03-15.
2. Earle, S. Physical Geology
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4. Press, F., and R. Siever. 1994. Understanding Earth. W. H. Freeman and Company, New York, 593 pp.