4. Minerals: rock’s elementary building block4.1. What is a mineral?
A mineral is a naturally occurring solid inorganic substance composed of a regular 3D
arrangement of atoms repeating in all directions (crystal lattice). Each mineral can be identified
with a chemical formula. For example:
Quartz: SiO2
Olivine: (Fe, Mg)2SiO4Halite: NaCl
Calcite: CaCO3
Pyrite: FeS2
Hematite: Fe2O3
A mineral growing freely form crystals whose external geometrical shape reflects the internal
regular arrangement of atoms. The crystal structure of a mineral － or crystal lattice － depends
on its chemical composition. It also depends on the temperature and pressure at which it formed.
For example, graphite and diamond have the same chemical composition (carbon) but they formed
in very different conditions of pressure and temperature, and therefore are characterized by very
different crystal structures. Since diamond forms at very high pressure deep inside the Earth’s
crust, the atoms of carbon in diamond are more densely packed than in graphite. Minerals with thesame chemical composition but with different crystal structures, like diamond and graphite, arecalled polymorphs.

4.2. Structure of matter and chemical bonds
Minerals are made of atoms. The simplest representation of an atom is a nucleus composed ofprotons and neutrons surrounded by electrons occupying different orbits, or shells. There are a
specific number of electrons that can occupy each orbit. Since atoms are electrically neutral, thenumber of protons in the nucleus is equal to the number of electrons. Each element ischaracterized by a specific number of proton(s)/electron(s) called the atomic number, e.g. H (1),C (6), N (7), and O (8). Atoms of a given element characterized by different numbers of neutrons
are different isotopes of that element. For example, the notation for an isotope of carbon that has
6 protons and 6 neutrons is carbon-12, or 12C. Two other isotopes of carbon are 13C (6 protons and7 neutrons) and 14C (6 protons and 8 neutrons). There are two types of isotopes: stable isotopes
and radioactive isotopes (or radioisotopes). The latter decays into stable or radioactive isotopes
over time while emitting radiations. For example, 14C is radioactive and decays into the stable
isotope 14N.
Since a neutron has a certain mass, different isotopes of a same element have a different mass.
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The mass of an atom can be approximated to the sum of masses of the protons and neutrons in
the nucleus. By convention, the mass of the isotope of carbon 12C (6 protons and 6 neutrons) is 12
atomic mass units. The atomic mass of an isotope is the sum of the number of protons andneutrons forming the nucleus. The atomic mass of an element depends on the relative
abundances of its isotopes (e.g. the atomic mass of carbon is 12.011 because 12C is by far themost abundant isotope of carbon, i.e. 98.9% of all carbon).
The electrons participating in chemical bonds are usually those of the outermost shell. Chemicalbonds allow the atoms to achieve a more stable electronic configuration. There is an optimal
number of electrons on the outermost shell which can be achieved through chemical bonds. For
example, the optimal configuration for C, N and O is to have 8 electrons occupying the outermostshell. In minerals, three types of chemical bonds exist: the covalent bond, the ionic bond and themetallic bond.
In Covalent bonds, an atom shares one or more electrons with another atom. The electrons of
one atom are attracted by the positively charged nucleus of the other atom and vice versa. Thisconfiguration results in very strong chemical bonds.
Example: diamond in which each atom of carbon is surrounded by 4 other atoms of carbon andeach atom shares 4 electrons with its neighbors. Note that another mineral has the same chemicalcomposition: graphite. However, graphite has a very different crystal structure. In graphite, theatoms of carbon are arranged in sheets. Within each sheet atoms are held together by covalentbonds but what holds these sheets together is a much weaker type of chemical bond (similar to themetallic bond described below). This makes graphite much less hard than diamond. The softnessof graphite can be observed every time one uses a pencil to write or to draw. Pencil cores are
made of graphite.
In Ionic bonds, an atom gains one or more electrons from another atom (transfer of electron(s)).
An atom that has lost one or more electrons is called a cation (positively charged ion). An atomthat has gained one or more electrons is called an anion (negatively charged ion). The ionic bondresults from the attraction between ions of opposite charges.
Example: NaCl (halite or table salt) in which the atom of chlorine that has gained one electron isnegatively charged (anion Cl-) and attracted by the positively charged sodium (cation Na+).
In Metallic bonds, atoms which have a strong tendency to lose electrons (and become cations)
are surrounded by free-moving electrons (delocalized electrons). The cations are held together by
their attraction to the surrounding electrons.
Example: copper which has one electron on the outermost shell that is only weakly attracted by thenucleus. In the metal copper, these outer electrons are not bounded to the nuclei and move freely.
The cations Cu+ are held together by their attraction to the free electrons surrounding them.

2The most common mineral family on Earth is the family of silicate minerals. Silicate minerals areoften characterized by a combination of covalent and ionic bonds. The basic structural unit ofsilicate minerals is the silicon-oxygen tetrahedron or [SiO4]4- (silicate anion) in which one atomof silicon is surrounded by 4 atoms of oxygen and shares one electron with each oxygen. Eachatom of oxygen can share one electron with another atom of silicon (covalent bond) or can bindwith a positively charged ion (ionic bond).
Examples:(1) Quartz (SiO2) is one of the most common silicate minerals in the continental crust. Quartz iscomposed of a juxtaposition of silicon-oxygen tetrahedra. Each atom of oxygen is linked to 2
silicon atoms by electron sharing (covalent bonds). There is only one type of chemical bond in thismineral, i.e. covalent bond.
(2) Olivine is a silicate rich in Mg and/or Fe with the formula (Fe, Mg)2SiO4. It is very abundant in
the upper mantle and also common in the oceanic crust. In olivine, the silicon-oxygen tetrahedra
are linked to the metallic cations Fe2+ or Mg2+ by ionic bonds. The 4- charge of each silicate anion
is balanced by two metallic cations of 2+ charges each. Therefore, there are two types of chemicalbond in this mineral: covalent and ionic bonds.
There are many other combinations of silicate anions and other elements which produce a broad
variety of crystal structures in the family of silicate minerals (e.g. single-chain silicates,double-chain silicates, sheet silicates…).

4.3. Formation of minerals
Crystallization is the process by which atoms in a gas or a liquid assemble in an orderly 3D
pattern (crystal lattice) to form a solid substance (mineral crystal).
The formation of minerals can occur in the following five contexts:(1) Saturation: When dissolved anions and cations in an aqueous solution reach a certain
concentration, they become saturated, and if water is removed by evaporation or if more ions areadded to the solution, the anions and cations combine to form crystals. One example is theprecipitation of salt NaCl (halite = “table salt”) when seawater evaporates in a pond, lake orembayment isolated from the open sea. The rock produced by accumulation of minerals
precipitated from saturated brines is called evaporite (see chapter on sedimentary rocks).
(2) Phase change (liquid-solid, gas-solid): Crystals can form when the temperature of a liquiddrops and the liquid solidifies into a solid － solidification. This happens when water freezes toform ice. It also happens when magma (molten rock) cools and minerals crystallize to form a solidrock, i.e. igneous rock (see chapter on igneous rocks). The transformation of a gas into a solid
without transiting by the liquid phase is called deposition.
3(3) Biomineralization: Many organisms are capable of precipitating minerals to form a skeleton orshell. Our bones and teeth are mineralized structures. In the marine world, the most common
biomineral is calcium carbonate (CaCO3). It is produced by diverse organisms, such as reef corals,coralline algae, mollusks, urchins, sea stars, and foraminifera (one kind of large single-celledorganism). Another common biomineral is silica (SiO2) and is produced by siliceous sponges andseveral groups of marine microorganisms such as radiolarians and diatoms. Tooth enamel andbones are composed of hydroxyapatite (Ca5(PO4)3(OH)).
(4) Chemical weathering: change in rock mineralogy at/near Earth’s surface (e.g., reactionsinvolving interactions between rocks and rainwater, such as the transformation of orthoclase(K-feldspar), common in granite, into kaolinite, a clay mineral: 2 KAlSi3O8 + 2 H2CO3 + H2O →Al2Si2O5(OH)4 + 4 SiO2 + 2 K+ + 2 HCO3-).
(5) Metamorphism: recrystallization of a preexisting rock, when the rock in question is subjected
to variations in temperature and pressure (for example, as the rock becomes buried deeper anddeeper into the crust) or when the rock is in contact with hot hydrothermal fluid. The transformation
occurs at the solid state (see chapter on metamorphic rocks).

4.4. Common families (or classes) of minerals
Silicate minerals: They are the most common minerals. They form most of the Earth’s mantle andcrust. Some details about their structure have already been provided above (see also the slides).
Carbonates: The most common carbonate mineral is CaCO3, already mentioned above in relation
with biological mineralization. The basic structural unit of carbonates is the carbonate anion (CO3)2-.
In carbonate minerals, carbonate anions are arranged in sheets and are bound to cations locatedbetween the sheets. In the case of CaCO3, the cations are Ca2+. Calcium carbonate can alsoprecipitate abiologically in seawater saturated in Ca2+ and (CO3)2- or even (in relatively rare cases)
crystallize out of a cooling magma (e.g., the igneous rock called carbonatite).
Halides and sulfates: Minerals that often form in the context of saturated brines leading to theformation of evaporites. The most common halide mineral is halite (NaCl). Another common
mineral that can form during evaporation of seawater is gypsum. The basic structural unit ofsulfates is the sulfur-oxygen tetrahedron, i.e. an atom of sulfur surrounded by four atoms of oxygen(SO42-). In the case of gypsum, water is also present in the crystal lattice and the chemical formulaof gypsum is CaSO4.2H2O. When gypsum is buried under younger layers of sedimentary rocks,
the increasing pressure and temperature “squeeze” water molecules out of the mineral, andgypsum becomes a mineral called anhydrite (CaSO4).

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Sulfides: In sulfides the sulfide anion (S2-) is bound to metallic cations (e.g. Fe2+, Mg2+, Cu2+). An
example of sulfide is pyrite (FeS2). Sulfides are important because they represent a major sourceof metals for the industry (metal ores). A geological setting in which sulfides commonly form is theocean floor near mid-ocean ridges. Seawater percolates through cracks in the ocean crust where itis heated near spreading centers (mid-ocean ridges). The hot seawater reacts with the rock of theocean crust (basalt) and dissolves iron, zinc, copper and sulfur. When this hot water returns to theocean and meets the cold seawater, the sudden drop of temperature triggers the precipitation ofsulfides which accumulate around the hot spring and builds a chimney called a hydrothermal vent(also known as “black smoker”). Hydrothermal vents are of particular interest, not only because
they are formed of metal sulfides, but also because they shelter a diverse ecosystem whose
primary source of energy is not sunlight (photosynthesis) but chemical reactions (chemosynthesis,e.g., CO2 + 4H2S + O2 → CH2O + 4S + 3H2O). Chemosynthesis can be performed by specializedbacteria and can sustain a diverse ecosystem in the deep ocean without any sunlight! One theory
about the origin of life on Earth suggests that the first unicellular organisms may have emerged
near hydrothermal vents billions of years ago.
Oxides: In oxides the oxygen is bound to other atoms, usually metallic cations, via ionic bonds.
One very common oxide and major source of iron for the industry (iron ore) is the iron oxide called
hematite (Fe2O3). If the anion is not O2- but OH-, the mineral is called a hydroxide. Another iron ore
is the iron hydroxide called goethite (FeO(OH)). Famous rock formations rich in iron oxide are thebanded iron formations. These rocks are composed of thin layers of sediments rich in iron oxide.
These rocks are usually Precambrian in age (older than 500 x 106 years). One hypothesis
suggests that they formed when the oxygen produced by photosynthetic activity of cyanobacteria
reacted with the dissolved iron in the ocean. Their study is important to understand the history of
the atmospheric oxygen of our planet (see chapter on the evolution of life).
Native elements: Minerals composed of a single element. Sometimes they can be combined inalloys like the iron-nickel alloy of metallic meteorites. Other examples of native element minerals
are native gold, native copper, sulfur, graphite and diamond.

4.5. Physical properties of minerals
Minerals can be characterized by their physical properties. Some of the properties that can beused to characterize and identify minerals are listed below.
Hardness: Harder minerals are able to scratch softer minerals. The hardest mineral is diamondwhich is able to scratch all the others. On the Mohs scale of hardness, diamond has a hardness of10. The softest mineral is talc and has a hardness of 1. The hardness of a mineral depends on its
structure and chemical bonds.

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Cleavage: Many minerals tend to split along specific planes which depend on their crystal
structure. Chemical bonds along these cleavage planes tend to be weaker and thus break moreeasily.
Fracture: a mineral may be broken along surfaces that are not cleavage planes. These surfaces
are called fractures. The quality (shape and texture) of the broken surface is another criterion usedto identify minerals (e.g. the conchoidal fracture of quartz, the splintery fracture of fibrous minerals
like asbestos).
Luster: This property of minerals refers to the way the surface of a mineral reflects light. Different
terms are used to qualify mineral luster (e.g. metallic, vitreous…).
Color: The color of a mineral is also a property that can be used in mineral identification. However,the color of a mineral may depend on the presence of tiny amounts of specific elements. For
instance, quartz is colorless, but amethyst, a variety of quartz with trace amounts of Fe3+, is violet.
Citrine, another variety of quartz also containing Fe3+, is yellow and can be obtain by heating
amethyst. Streak refers to the color of the mark left by a mineral when it is scratched on anabrasive surface such as a plate of unglazed porcelain (e.g. the reddish-brown streak of hematite).
Density: The density of a mineral (g/cm3) depends on how closely the atoms are packed and on
the mass of individual atoms. Diamond and graphite are both composed of carbon but diamond isdenser because the atoms of carbon in diamond are packed more closely. Fe-olivine (Fe2SiO4)
and Mg-olivine (Mg2SiO4) have a similar structure but Fe-olivine is denser than Mg-olivine because
the atomic mass of iron is much greater than that of magnesium. The density of minerals iscommonly expressed in terms of specific gravity which is the ratio of the weight of a volume of amineral to the weight of an equal volume of water at a temperature and pressure of reference. The
specific gravity of water is 1. The specific gravity of halite is 2.16 and that of pyrite is 5.
Crystal habit: It is the shape of an individual crystal or an aggregate of crystals of a given mineral.
The crystal habit of a mineral reflects the internal arrangement of atoms as well as the speed anddirection of crystal growth.

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