MINERALS: the building blocks of rocks

Minerals are solid inorganic chemical compounds with a definite chemical structure (‘lattice’) and specific chemical composition. The physical properties listed on this page are all a function of chemical composition and structure. These physical properties are very useful for geologists, since many of them allow minerals to be identified in their natural setting without using specialized laboratory equipment.

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There are five principal classes of chemical bonds „ „

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Quality specimens of the mineral halite, or ‘table salt’. The atomic arrangement (crystal lattice) in halite places it in the ‘cubic symmetry’ class of minerals, which is expressed in the shape of the crystals.

Ionic- electrostatic attraction between charged atoms, i.e. ‘opposites attract’ Covalent- shared electrons between atoms without ionization (loss of electrons); usually involves 1-6 other atoms Metallic- electrons are shared by a large number of atoms, i.e. they ‘flow’ (electricity) Van der Waals- weak ‘ionic-like’ bond due to electron distribution; important in a small number of minerals Hydrogen- form between hydrogen and oxygen, nitrogen, or sulfur. Weak; not very important in minerals

Of the five principal classes of chemical bonds, the two most important classes are ionic and covalent. Metallic bonds are important in some ore deposits. Silicate and carbonate minerals—which are the two most important groups of rock-forming minerals—are good examples of how covalent and ionic bonds are often mixed. The anions (negatively charged portion) of silicate and carbonate minerals are held together tightly by covalent bonding. Anions are bonded to cations (positively charged atoms) by a combination of ionic and covalent bonding. Only a few minerals found at the Earth surface—most notably halite—are strongly ionic, primarily because ionic bonds are easily dissolved in water.

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Ionic bonds occur as the result of the attractive force between atoms with opposite electrical charge, i.e. ‘opposites attract’. They occur between ions, which are stable charged forms of atoms. Ions form because of the tendency of atoms to obey the ‘rule of octet’, which states that atoms are more stable when their outer shell of electrons is completely occupied. The atom sodium—which has a single electron in its outer orbital—is more stable when it loses this electron, resulting in a net positive charge (+1). Loss of this single electron means the second orbital becomes the ‘outer shell with electrons’, thereby satisfying the ‘rule of octet’. Chlorine has seven electrons in its outer orbital, and has a natural tendency to acquire a free electron in order to fill the outer orbital. As a result, chlorine (Cl) is more stable with a net negative charge of -1. The net electrical charges of these atoms allow them to participate in bonds with oppositely-charged ions.

The mineral structure of diamond is a good example of covalent bonding. Carbon has six electrons in its neutral (non-charged) state, as shown in the right-hand figure. Two of these electrons are in the innermost orbital shell, as they are for every atom except hydrogen. The remaining four are in the second orbital shell. In order to satisfy the rule of octet in this example, each neighboring carbon in the carbon tetrahedra is sharing one electron with the central carbon, giving all of the carbon atoms a net total of 8 electrons in the outer electron orbital. The term ‘covalent’ is derived from the term ‘valence’, which is used by some to describe an orbital ‘shell’ or level; the idea of sharing these valences between atoms led to the term ‘co-valency’.

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This figure shows the geometrical relationship between sodium and chloride ions in the mineral halite. This arrangement—where all bonds are perpendicular—leads to the cubic shape of visible halite crystals.

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In addition to the charge of an ion, another factor that governs how atoms fit together to form crystal structures is the size of the ion. One of the most important variables that controls the size of a cation is the net positive charge. As can be seen in the example above, there is a strong tendency for cations with high positive charge to be smaller than cations with smaller positive charge. Overall atomic mass is also a factor; assuming the ionic charge is equal, atoms with more protons and neutrons tend to have larger atomic radii than atoms with fewer protons and neutrons. Anions also show a variation in atomic radii with charge, but the relationship is generally more complex.

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There are six general classes of crystal symmetry, of which cubic and hexagonal are two. Notice how the different crystal symmetries lead to different crystal shapes.

Relatively small differences in atom bonding arrangements can have profound effects on mineral properties. For example, the mineral graphite is formed by each carbon atom sharing electrons with three adjacent carbon atoms, and weak electrical attractions between layers of carbon atoms (in part due to the odd way the ‘extra electron’ needs to be shared). Graphite is a very soft mineral, which is why it is used in pencils. Diamond is the hardest known mineral due to the central atom sharing electrons with four neighboring carbon atoms. Both of these minerals have the same chemical composition. Minerals that have identical chemical compositions but different bonding arrangements are referred to as polymorphs. Polymorphism is due to differences in the stability of atom arrangements formed at different temperatures and pressures.

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Limestone is one of the most important rock types, and is the most important formed by minerals in the carbonate group. The figure above shows how carbonate ions (CO32-)— which are formed by covalent bonding—are held in planar arrangements by ionic bonding with calcium atoms.

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The Mohs scale of hardness is useful for rapid evaluation of general mineral properties and is still in use by geologists who identify minerals in the field.

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Minerals like biotite (this example) have planar arrangements of silica tetrahedra where three of the four oxygen atoms are shared with adjacent silica tetrahedra. The resulting planar structure gives rise to ‘sheet-like’ mineral layers that are held together by ionic bonds with cations between the silica tetrahedra sheets.

The cleavage angles of pyroxenes and amphiboles are one of the most diagnostic properties of these mineral groups, which often have very similar (dark) colors.

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Mineral luster refers to the ‘shine’ of the mineral surface.

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Streak refers to the color of mineral powder formed as a mineral is dragged across a ceramic plate. Streak color can be quite different from the observec color of the mineral, and is diagnostic for a few minerals, especially the important ore-forming mineral hematite.

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Silicates are the most important rockforming minerals • Five classes of silicate minerals • Principal difference between the five classes is the number of ‘shared oxygens’ • Different number of shared oxygens leads to different degrees of covalent/ionic bonding behavior

Chemical classes of minerals are defined by the anions. By far the most important chemical class is the silicate minerals.

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Isolated tetrahedra are held together by cations in the spaces between the tetrahedra. Single chain structures are formed by the sharing of two corners of each silica tetrahedra. Double chain structures are formed when there is an alternation of sharing behavior between adjacent silica tetrahedra in the pattern 2-3-2-3-2. Double chains are essentially single chains bonded together by shared oxygens. Sheet structures are planes formed by sharing of three oxygens in every silica tetrahedra. Framework structures form by sharing all four oxygens in the tetrahedra.

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Amphibole- important rock-forming mineral for igneous and metamorphic rocks; double-chain silicate. Calcite- carbonate mineral. Primary mineral component of limestone, one of the most important sedimentary rocks. Composed of carbonate ion and calcium. Diamond- do you need my help on this one?!?!?! Dolomite- carbonate mineral with calcium and magnesium.

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Feldspar- one of the most important rock-forming minerals, especially in felsic igneous rocks. Framework silicate.

Hematite- iron oxide. Important economic mineral used to make steel. Kaolinite- a clay-like silicate mineral. Often found as the end product of weathering of pyroxene and amphibole in warm and humid climates.

Garnet- found primarily in metamorphic rocks. Gypsum- calcium sulfate. Important economic mineral.

Mica- sheet silicate with many different varieties. Variation in this silicate group is due to the differences in distances between the sheets when different cations are present.

Halite- sodium chloride. Table salt.

Olivine- important mineral that is abundant in the lower crust and mantle. Found primarily in basalt rock.

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Pyrite- iron sulfide mineral. Often found with other sulfide minerals containing copper, zinc and other economic metals. Pyroxene- important mineral constituent of igneous and metamorphic rocks. Quartz- the most important silicate mineral. Framework silicate class.

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