The next sections of the course are about the Geosphere. As with the other spheres, we will begin with a look at what elements go in to the Geosphere. However, unlike the other spheres, the list of elements that are found in the Geosphere is long, and the composition of the Geosphere is very varied. We will therefore just focus on what is most abundant. Bear in mind that there are about 90 naturally occurring elements in the periodic table, and every one of them is found in the Geosphere!

Elements in the Geosphere

Our knowledge of the abundance of elements in the Geosphere, the solid Earth, is somewhat limited, because the deepest drilling by humans penetrates only just over 12 km into the Earth, a tiny fraction (0.2%) of the Earth’s total radius (6370 km). If we look at samples brought up from the mantle by volcanic eruptions of kimberlite, we can perhaps ‘see’ material from a few hundred (perhaps even 450) kilometres down, but this is still well under 10% of the Earth’s radius, and the samples we get are probably not from very typical mantle rocks. Our information about the deep interior has to be based on remote sensing methods, particularly the Earth’s gravity, magnetic field, and the behaviour of seismic (earthquake) waves as they pass through the interior. Based on these properties, it is possible to infer that the Earth’s interior, like that of the other terrestrial planets, can be divided into

• a central core, consisting mainly of iron;
• a mantle, the largest part by volume, consisting of magnesium and iron silicate minerals;
• a very diverse, but relatively thin crust.

Because of the impossibility of sampling the deep interior of the Earth, our best estimates of the overall composition of our planet are actually based mainly on chondrite meteorites, thought to represent material left over from the origin of the solar system, and therefore representative of what went into the terrestrial planets Mercury, Venus, Earth, Mars, and the Moon.

Based on all these lines of evidence, the most abundant elements in our planet are probably as follows:

• Iron (Fe, ~35%)
• Oxygen (O, ~30%)
• Silicon (Si, ~15%)
• Magnesium (Mg, ~13%)
• Nickel (Ni, ~2.4%)
• Sulphur (S, ~1.9%)
• Calcium (Ca, ~1.1%)
• Aluminum^[2] (Al, ~1.1%)
• Everything else (<1%)

However, these elements are unevenly distributed within the Geosphere. The Earth’s core is mostly iron, with nickel as the second most abundant element, so much of the Earth’s iron and nickel are locked up in the core. The mantle also contains some iron, but it also has abundant oxygen, silicon, and magnesium, with all the other elements listed in the table in lesser amounts. However, over the Earth’s 4.6 Ga history, silicon and oxygen, toether with calcium, aluminum, and many of the elements included in “everything else” have been preferentially concentrated in the crust, in a process of planetary differentiation.

The same overall distributions probably occur in the other terrestrial planets (though our knowledge of their interiors is even more sketchy, of course). However, each of these planets has an iron-dominated core, and a mantle with a composition similar to Earth’s, their crusts are wildly different from one another, because each planet has its own rock cycle, and it is the rock cycle that has formed the crust.

When we look at the Earth’s crust we see that there’s much less iron, magnesium, nickel and sulfur, and much more of almost everything else.

• Oxygen (O, ~46%)
• Silicon (Si, ~26%)
• Aluminum (Al, 8%)
• Iron (Fe, ~6%)
• Magnesium (Mg, ~4%)
• Calcium (Ca, ~2.4%)
• Potassium (K, 2.3%)
• Sodium (Na 2.1%)
• Everything else (~1%)

Minerals

Most of these elements occur in solid chemical substances called minerals. Each mineral has either a fixed composition, or varies between certain fixed limits. Most parts of the Geosphere are made of mixtures of minerals, called rocks. In this section we will look at the properties of minerals. Often, recognizing minerals is an important step towards understanding how a rock formed.

Scientific definition of a mineral

To be classified as a mineral in Earth science, a substance must satisfy the following criteria.

• Solid
• Naturally occurring
• Inorganic
• Fixed (or limited range of) chemical composition
• Crystalline structure: atoms are arranged in a regularly repeating 3-dimensional pattern called a crystal lattice

By this scientific definition, about 6000 discrete minerals are known. This definition has some unexpected consequences!

• Ice is a mineral (fits all 5 criteria)
• Coal is not a mineral because it is organic, and has no crystalline structure.

In other realms (e.g. law) the term mineral has different definitions. For example,

• Coal is a legal mineral in most jurisdictions;
• Ice is not legally a mineral.

Rocks vs. minerals

In contrast with minerals, rocks are mostly naturally occurring mixtures of minerals. Unlike minerals, rocks don’t always naturally fall into discrete categories. Although Earth scientists classify rocks using a variety of names, their compositions are continuously variable, and borderline cases are quite common.

For example, a typical rock such as granite contains several minerals. In the case of granite these are commonly:

• Feldspar
• Quartz
• Mica

Rocks may also contain non-mineral materials, for example:

• Organic matter such as carbon-based polymers. Coal is a rock made largely of non-mineral organic matter.
• Volcanic glass – effectively a supercooled liquid in which the atoms occur in a chaotic arrangement that has no crystal lattice.

Crystal lattices

Minerals are dstinguished by having a structure in which atoms are linked in 3 dimensions forming a crystal lattice. Although the term ‘crystal’ is used in common speech to describe a mineral grain with flat outside faces, all minerals are crystalline. The large flat surfaces of ‘crystals’ certainly reflect the symmetry of the internal lattice, but the are not necessary for the mineral to be called crystalline. Typically, the flat outside faces develop in relatively rare circumstances when mineral crystals grow part-way into fluid-filled spaces within the Geosphere. In most cases, crystals grow until they collide with other crystals. Under these circumstances, smooth outside faces don’t usually form, but the internal arrangement of the atoms still reflects the crystalline structure that is characteristic of each mineral.

It’s important to understand that there aren’t really molecules in most crystals. Although we can write a formula like SiO₂ for the mineral quartz, there are no SiO₂ molecules in solid quartz; the silicon and oxygen atoms are bonded in a continuous crystal lattice. The formula merely expresses the fact that there are two oxygen atoms for every on silicon atom.

Three main types of bonding between atoms occur in natural crystals.

Ionic bonds

The quantum mechanics of electrons makes certain configurations stable. For most of the elements we are interested in, 8 electrons in the outer shell surrounding an atom is a stable configuration. As a result, elements like sodium and potassium, with a single electron in the outer shell, can easily lose that electron to become a positively charged ion. Conversely, atoms like chlorine, that have 7 electrons in the outer shell, can easily achieve stability by gaining an electron to form a negatively charged chloride ion.

Hence, if a sodium atom is brought in contact with a chlorine atom, an electron tends to be transferred from sodium to chlorine, producing two ions, each with a stable eight electrons in their outer shell. The sodium ion carries a positive electric charge and the chloride ion has a negative charge. Because unlike charges attract each other, the two atoms tend to stick together. If many sodium and chloride ions are available, they assemble into a cubic lattice, in which the geometry is determined by the sizes of the ions.

The sizes of ions are partly determined by the number of electrons in the atom, but the electric charge is much more important. Negative ions have extra negatively charged electrons, so those electrons tend to be held more loosely by the positively charged nucleus. Negative ions, mostly of non-metallic elements, are therefore large, whereas positively charged ions (which are mostly metals) tend to be small.

Metallic bonds

A second class of bonding occurs in crystals of metals that are native elements:- minerals that contain only one kind of atom. Examples are copper and gold.

In these minerals, the outer, loosely held electrons associated with many atoms can merge to form a continuous electron cloud, effectively a single, merged electron shell, that holds the atoms together in a densely packed arrangement, usually with cubic or hexagonal symmetry.

Covalent bonds

A third type of bonding involves sharing of electrons between adjacent atoms. A common example occurs in silicate minerals. Silicon atoms, like carbon atoms, have four electrons in their outer shell, whereas eight electrons is the stable configuration. In many silicate minerals, silicon atoms share electrons from adjacent oxygen atoms so as to fill that outer shell. The oxygen atoms may aquire extra electrons to fill their outer shells too, to produce SiO₄^2- groups known as silicate tetrahedra.

The bonds in diamond are entirely covalent, contributing to the great hardness of diamond, the hardest mineral known.

Polymorphs

Carbon forms the mineral diamond by covalent bonding. By sharing electrons with four adjacent carbon atoms, each carbon atom fills its outer shell with eight electrons, and the carbon becomes bonded into a strong 3-D lattice where each atom is surrounded by four others. The resulting structure produces the hardest mineral known. Carbon can exist in other forms too. In an alternative arrangement of atoms, carbon atoms are linked together in 2-dimensional sheets of hexagons. Only very weak bonds join the sheets to one another, producing a very soft mineral: graphite.

Both graphite and diamond have the same chemical formula — just C — but they are different minerals because they have different atomic structure. They are known as polymorphs^[3].

Polymorphs of minerals can provide information about conditions within the Earth, because each polymorph is stable under different conditions. The diagram is an example of a phase diagram, The axes of the graph are temperature and pressure. The diagram shows the stability regions of diamond and graphite, as well as liquid carbon and carbon gas. Each of these four forms of carbon is known as a phase. Phases are parts of a system that have different structure and are separated from one another by distinct boundaries.

Diamond is stable under conditions of very high pressure, more than 1 GPa (one gigapascal), such as are found in the mantle. Graphite is stable at lower pressures, including at the Earth’s surface where pressure is measured in just kilopascals. As you might expect, the liquid and vapour forms of carbon occur at high temperature.

How then is it possible for diamonds to survive at the Earth’s surface? The answer is that diamonds are metastable – given enough input energy to break the covalent bonds, the carbon atoms would reassemble into graphite, but starting the reaction takes a lot of energy, so diamonds are able to survive outside their field of stability.

In greater depth: Polymorphs in metamorphic rocks.

Three minerals, andalusite, kyanite, and sillimanite are polymorphs with the formula Al₂SiO5. These minerals form from mudstone and other aluminum-rich rocks, when subjected to heat and/or pressure in a range or conditions between the breakdown of water-bearing clay minerals of comparable composition and the onset of melting. Each is formed within a particular range of temperature and pressure. (Andalusite and sillimanite are also components of fired pottery, formed by the heating of clay.)

Identifying minerals: mineral properties

In common rock types that form the majority of the Earth’s crust, it’s possible to identify minerals using simple physical properties, if the texture of the rock is coarse enough for the mineral grains to be clearly distinguishable.

Optical properties

Colour

Colour^[4] is perhaps the most obvious of mineral properties, but it is not always the most helpful. Some minerals do have a distinctive colour, but in many other cases colour is determined by tiny amounts of impurities. Hence, the common mineral quartz can take on many different colours, some of which have their own names and are valued as semi-precious gemstones.

Varieties of quartz, named by colour

Despite these many names, all have the same crystal properties and almost exactly the same chemical composition. To a mineralogist, they are all quartz.

Lustre

Lustre of mineral surfaces

Lustre (Luster in the United States) describes the quality of light that reflects from the surface of a mineral. Many mineral surfaces reflect part of the light that strikes them, somewhat like glass. Such minerals are said to show glassy or vitreous lustre. Quartz is a common mineral that shows glassy lustre. Another common type of reflectivity is that shown by metals, which reflect most of the light striking their very opaque surfaces. Such lustre is described as metallic. The minerals pyrite and galena, sulfides respectively of iron and lead, show metallic lustre. Unreflective minerals are said to have earthy or dull lustre.

Streak

Although colour is often misleading as a mineral property, for some minerals the colour of a fine powder is distinctive. The iron oxide hematite (formula Fe₂O₃) ranges in colour and lustre from black and metallic to red and dull. However, finely powedered hematite always appears reddish in colour. The easiest way to test the colour of a powdered mineral is to draw a line with it on an unglazed porcelain tile, known as a streak plate. The colour is known as the streak of the mineral.

Other optical properties

Several other optical properties characterize particular minerals. Many are used for the identification of minerals under the microscope in the study of optical mineralogy. Here we mention just one, birefringence. The mineral calcite, the most common form of calcium carbonate CaCO₃, when in clear crystals (sometimes known as iceland spar), calcite has the property of splitting light into two polarized beams that are refracted (bent) by different amounts, producing a double image of anything viewed through the crystal.

Crystal properties

Because each mineral has its own distinctive crystal structure, properties related to that structure can be very useful for distinguishing minerals.

External crystal form

Most commonly, the term “crystal” describes a mineral that shows external flat surfaces determined by its internal crystal structure. In more advanced courses, you will learn to classify minerals into crystal systems, such as cubic, hexagonal, monoclinic, based on the symmetry of both the internal crystal structure and the external crystal form.

External crystal form is most commonly shown by minerals that have grown into a large fluid-filled space within the Earth, known as a geode.

Cleavage

In some crystals, atoms are only weakly bonded across certain surfaces that have a regular orientation in the crystal lattice. Such minerals are easily split along these flat surfaces. Such a property — breaking along flat surfaces controlled by crystal structure — is known as cleavage.

Calcite, the most common form of calcium carbonate CaCO₃ shows three different orientations of cleavage, causing the crystals to break into rhombohedra — six-sided blocks each side of which is a parallelogram.

Fracture

Minerals that don’t show cleavage break along irregular surfaces. This property is known a fracture. Although several types of fracture can be distinguished, we will mention only one here: conchoidalfracture describes a curving style of fracture with concentric lines shown by broken glass and the mineral quartz (SiO₂). The curving surfaces can be likened to shell of a mollusc or chonch, giving rise to the name conchoidal.

Hardness

The overall strength of bonding between atoms in a mineral determines its hardness — the ease with which it can be scratched. Minerals can be tested by scratching one against the other, and against common materials of known hardness. The Mohs scale ranks common minerals in order of hardness from 1 (softest) to 10 (hardest).

1. 

   Talc

2. Gypsum^[5]
3. Calcite
4. Fluorite
5. Apatite
6. Feldspar
7. Quartz
8. Topaz
9. Corundum
10. Diamond

Some common materials can be placed on this scale: Fingernail 2.5; Copper coin^[6] 3.5; Knife blade 5.5.

Therefore, if you can scratch a mineral with your fingernail, the hardness is below 2.5. Conversely, if a knife will not scratch the surface of the mineral, the hardness is above 5.5. When scratching minerals it’s important to try each object against the other. Sometimes a softer mineral will draw a line of powder on a harder mineral, mimicking a scratch!

Other useful properties for mineral identification

Density

Minerals vary greatly in density^[7]. Carefully measured, density may be given in kg/m³ or in g/cm³ (or g/cc). For example quartz has a density of 2650 kg/m³ or 2.65 g/cm³, a pretty average value for rock-forming minerals. People who handle rocks habitually get used to this density; when they heft a sample containing (for example) hematite (7.9 g/cm³) or (if they are lucky) gold (19.3 g/cm³) they immediately notice the extra density. To make accurate determinations of density, it’s necessary to weigh a sample in and out of water, using Archimedes' principle.

Magnetic properties

A few minerals are magnetic, in the sense that a magnet will stick to a sample of the mineral. The most important example is the iron oxide mineral magnetite Fe₃O₄. The magnetic properties of magnetite become important for the study of paleomagnetism, used to track the motion of plates in tectonics.

Reaction with acid

Carbonate minerals, including particularly calcite (CaCO₃) effervesce (fizz) when a drop of dilute hydrochloric acid is applied to the surface. Geologists often carry a small bottle of dilute hydrochloric acid when working in the field, for the purpose of identifying carbonates.

Silicate minerals

Because oxygen is the most abundant element in the Earth’s crust, and silicon is the second most abundant element, it’s not surprising that the most abundant rock-forming minerals belong to a group called silicates, in which silicon and oxygen are combined with other metals in a variety of mineral structures.

Silicate tetrahedron

Silicate minerals have certain parts of their atomic structure in common. At the heart of all silicate mineral structures is a group of atoms called a a silica tetrahedron (silica is just a name for silicon oxide), a silicate tetrahedron, or an SiO₄ tetrahedron. These three terms mean the same thing and can be used interchangeably. This structure arises because silicon, like carbon, has four electrons and four spaces in its outer electron shell, and silicon can form covalent bonds sharing one electron with each of with four oxygen atoms.

Each oxygen in the silica tetrahedron is left with one vacant space in the outer shell, and it can acquire electrons either in covalent bonds (often to other tetrahedra) or in ionic bonds to other metals, and we see both of those in silicate minerals.

In some minerals — typically those with lower amounts of silica compared with other metals — the SiO₄ tetrahedra are isolated from one another, but in others they share oxygen atoms at their corners, building up strucutres in which tetrahedra are linked in rings, chains, sheets, or three-dimensional frameworks.

Common silicate structures. By Chris Schaller. CC BY-NC https://chem.libretexts.org/^[8]

In general, the structures with more sharing of oxygen atoms between tetrahedra have smaller amounts of other elements (because there are fewer spaces left for electrons to be accepted by oxygen). The most extreme case is quartz, in which every oxygen is shared with an adjacent tetrahedron in a three-dimensional framework. The resulting formula is pure silica SiO₂.

This simple picture is modified in some silicates, where aluminum (Al), with only 3 shareable electrons, sits in the middle of some of the tetrahedra. It is said to substitute for silicon. This frees up extra space for electrons in the outer shell of the oxygen atoms, which in turn allows ionic bonding with other metals. The feldspar group of minerals, the most abundant group in the Earth’s crust, has this type of structure, accommodating sodium, potassium, or calcium in a three-dimensional framework of SiO₄ and AlO₄ tetrahedra.

Minerals with isolated silicate tetrahedra

Our first mineral group is olivine, which is built from isolated silicate tetrahedra surrounded by ions of iron (Fe) and magnesium (Mg). This is a very important group because it consists of the four most abundant elements in the Earth, and especially in the Earth’s mantle. As you might expect this is most abundant mineral found in rocks from the mantle.

Olivine displays another important feature of silicate minerals. The ions of magnesium Mg²⁺ and Fe²⁺ have identical charge and are almost exactly the same size. They can easily substitute for one another, and in fact a continuous range of compositions is possible, from a pure magnesium end-member Mg₂SiO₄ to a pure iron end-member Fe₂SiO₄. This range is called a solid solution. It’s important to note that this variability affects only the iron and magnesium. The proportion of the total amount of Fe + Mg in the crystal is constant, so we can write a general formula:

(Mg,Fe)₂SiO₄

Most olivine in the Earth’s upper mantle contains much more magnesium than iron: about 90% of the crystal sites are occupied by magnesium and 10% are occupied by iron.

Solid solution between magnesium and iron is very common in silicate minerals. Because of the way light interacts with these ions, most such minerals have colours ranging from black to green. Olivine, as its name implies, is typically green. We refer to these minerals as ferromagnesian minerals.

Chain silicates

Minerals that contain larger amounts of silica typically contain oxygen atoms that are shared between adjacent tetrahedra. In simple chain silicates, each tetrahedron shares two oxygen atoms, leaving two to form ionic bonds with metals. The most important group is the pyroxene group, which forms the second most important component of the upper mantle.

Pyroxenes are also ferromagnesian minerals, and most have colours between green and black. A common example has the formula:

(Mg,Fe)SiO₃

However, some pyroxenes contain calcium too, occupying exactly half of the available ionic sites in the crystal lattice. A typical formula is:

(Mg,Fe)CaSi₂O₆

The calcium ions have the same charge as the magnesium and iron ions but their size is different. They therefore fit in a different place in the crystal lattice, and the amount of calcium is fixed; it does not participate in a solid solution. Notice that to avoid fractions in the formula, it’s necessary to double the amount of Si and O in the formula but the ratio remains unchanged, so the overall structure is similar.

Pyroxenes typically form rather rectangular-shaped crustals with two sets of cleavage planes almost at 90° to one another. Pyroxene, together with feldspar, is a very common component of igneous rocks formed from magma that rises from the mantle.

Sheet silicates

Micas

If three of the oxygen atoms are shared between silica tetrahedra, the result is a continuous sheet, characteristic of sheet silicates. Micas are the best-known group. In most cases there is some aluminum subsituted for silicon in the sheets, and also some oxygen atoms are bonded to hydrogen in hydroxyl groups, and other elements like Fluorine may get involved. This makes for complicated formulas!^[9]

K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂

As you can see from the above formula, some micas are ferromagnesian minerals with solid solution of iron and magnesium. Such micas tend to be dark in colour — black or greenish like most ferromagnesian minerals.

Other micas, known as white mica, have aluminum in the equivalent position in the crystal lattice. White mica is not a ferro-magnesian mineral.

Whereas the silicate sheets are held together by strong covalent bonds, the sheets are ionically bonded together by potassium ions, and these bonds are much weaker. This allows mica to split into astonishingly thin, elastic sheets. Large crystals of white mica have historically been used instead of window glass. Mica is also used in electrical components and as a source of “glitter” for greeting cards.

Micas, both light and dark, are common components of metamorphic rocks formed by the action of heat, pressure, and deformation, particularly on sedimentary rocks. They are also an important component of silica-rich igneous rocks produced by melting in the crust. (Most magma formed by melting of the mantle does not usually contain enough aluminum to form mica. An exception is kimberlite, the igneous rock from which diamonds are obtained, which typically contains magnesium-rich mica).

Clay minerals

Sheet silicates are also produced in abundance by the processes of chemical weathering. The mineral grains produced tend to be very small — only a few microns across — making identification with the naked eye very difficult. (To identify them, a technique called X-ray diffraction is commonly used.) These fine-grained minerals, mostly sheet silicates, are collectively known as clay minerals. As the name implies, they are the main component of clay, used to make pottery of all kinds.

One example you may meet if you are taking a lab course is kaolinite, which has a relatively simple formula (for a sheet silicate):

Al₂Si₂O₅(OH)₄

You can see that it’s an aluminum silicate with included hydroxyl ions. Kolinite is formed by the weathering of feldspars under tropical conditions, and is the principal component of china clay used to make porcelain and fine china pottery.

Framework silicates:

Quartz

If all the oxygen atoms in silicate tetrahedra are shared, the resulting three-dimensional structure is entirely covalently bonded; there are no empty spaces available in the outer electron shells of the oxygen atoms for ionic bonding. The resulting structure has the formula SiO₂ and represents the mineral quartz. Quartz is a common component of many kinds of rock: sedimentary, igneous and metamorphic. Quartz is the most abundant single mineral in the Earth’s continental crust, although the minerals of the feldspar group, taken as a whole, are more abundant. It is much less common in the oceanic crust, which contains less silica, and largely unknown in rocks derived from the mantle.

Quartz is recognizable by its hardness — it’s the hardest of the common rock-forming minerals — and lack of cleavage. Internally, the silicate tetrahedra are linked in a spiral arrangement without through-going planes of weakness. When broken, it displays conchoidal fracture. Colour is a poor guide to identification: quartz can occur in almost any colour. Some of those colours have particular names and are valued as semi-precious stones.

Feldspar group

The most common silicate group of all is the feldspar group. Examples are found in almost all igneous rocks. Feldspars are framework silicates in which every oxygen atom is shared between two tetrahedra, but a proportion of the tetrahedra (between one quarter and a half) contain aluminum atoms instead of silicon, leaving space in the outer shell of some of the oxygen atoms for either one or two electrons. These electrons may be derived from ionically bonded sodium Na⁺, potassium K⁺, or calcium Ca²⁺. A variety of solid solutions are possible between end-member compositions:

KAlSi₃O₈

NaAlSi₃O₈

CaAl₂Si₂O₈

Feldspars form blocky, typically box-shaped crystals, with two cleavages approximately at right angles. Sometimes they contain twin planes across which the mineral structure is reflected into mirror images. These are sometimes visible when rotating a crystal in a beam of light. Colours range from pink (common in potassium feldspar) through white to medium grey (common in sodium and calcium feldspar) although more detailed study of optical properties under a microscope is necessary to securely determine the composition of a feldspar.

In detail: Labradorite

Solid solution occurs more readily in feldspar at high temperature. Almost any composition between the Na and Ca members is possible. However, as feldspars cool down, they tend to “unmix”, forming sub-microscopically thin plates of different composition. In the variety labradorite, containing roughly 60% calcium and 40% sodium, these thin plates are just the right thickness (comparable to the wavelength of visible light) to cause a play of colour called schiller, a distinctive and prized property of labradorite.

Taken as a whole, feldspars are the most abundant group of minerals in the Earth’s crust. The Earth’s mantle contains much lower concentrations of the elements in feldspar, and so feldspar is rare in the mantle. It occurs where the mantle is present at shallow depths beneath oceanic crust, but at depths greater than 10–15 km, aluminum is taken up by another mineral group, spinel, and below 50 km by garnet, so feldspar is not present in deeper parts of the mantle.

Non-silicate minerals

Non-silicate minerals make up only about 5% of the Earth’s crust. Why then do we devote a whole section of this chapter to them? The reason is that non-silicate minerals, ranging from common limestone to extremely rare gold and diamonds, have been sought after by humans for milennia. Many of the minerals we encounter in this section have important uses in human technology, and are therefore mined in different parts of the world.

Carbonates

Carbonate ions figure prominently in the carbon cycle, because they are one of the forms of oxidized carbon that can occur in the Hydrosphere. Carbonate ions contain one carbon atom covalently bonded to three oxygen atoms; the overall ion carries a double negative charge.

CO₃^2-

There are a number of important rock-forming minerals that are combinations of positive metallic ions and negative carbonate ions. The most important is calcite, with the formula CaCO₃, in which Ca²⁺ ions are combined with carbonate ions in a structure that has three-fold rotational symmetry.

Calcite sometimes appears similar to quartz, but is much softer (hardness 3, so it can easily be scratched with a knife), and calcite effervesces when a drop of dilute hydrochloric acid is applied. Its cleavage is also distinctive, causing it to break into rhombohedral fragments.

Calcite is formed by many organisms, including cyanobacteria, microscopic planktonic plants called coccolithophores, many corals, crustaceans, molluscs (clams and snails), and echinoderms (starfish and sea urchins). All these organisms build their shells or other skeletal material using calcium and carbonate ions extracted from water.

Calcite is the principal mineral component in the sedimentary rock limestone, and in its metamorphosed equivalent marble. Calcite is also deposited from groundwater, forming stalagmites and stalactites of dripstone in caves, and calcite veins that fill fractures in the Earth’s crust. Calcite, in the form of limestone, is mined in large quantities for the production of cement, and in smaller amounts for agricultural lime, used to increase the pH of acid soils.

Halides

A second group of minerals consist of ionically bonded combinations of metals that have one free electron in the outer electron shell (alkali metals from Group I of the periodic table) with halogens from group VII— elements with one free space for an electron in the outer shell. We have already met the most common mineral in this group: halite or common salt, sodium chloride NaCl.

Halite is a soft mineral (hardness 2, so it can be scratched with a fingernail) that crystallizes in perfect cubes, with cleavage planes parallel to the faces. Halite dissolves in water readily, but does not fizz with hydrochloric acid. When pure it is white or transparent, but it is easily coloured by traces of impurities.

Halite is typically formed by the natural evaporation of sea-water, and is therefore one of a class of minerals known as evaporite minerals. A rock formed mainly of halite is known as rock salt.

Sulfates

In addition to chloride ions, sea-water contains abundant sulfate^[10] ions consisting of a sulfur atom oxidized by four oxygen atoms, carrying a net double negative charge. As a result, some sulfate minerals are also produced when sea-water is evaporated, and are included in the category of evaporites.

The most common is gypsum, hydrated calcium sulfate, which actually includes water molecules in its crystal lattice. Its formula is

CaSO₄.2H₂O

Gypsum is also soft (hardness 2) and crystallizes in a variety of forms including transparent diamond-shaped crystals called selenite, which displays perfect cleavage parallel to the large flat surfaces, and long thin fibres known as satin spar.

Gypsum is used by humans to make plaster. Heating gypsum drives off three quarters of the water, leaving a powder (Plaster of Paris) which turns solid when water is added. Plaster wall-board (“Gyp-rock”) is an important component of new homes in many parts of the world.

Sulfides

Not to be confused with sulfates are sulfides, minerals containing negative ions of reduced sulfur. Two of these are quite common, though many more occur as ores of various metals.

Pyrite

Pyrite FeS₂ is a common sulfide mineral, containing Fe²⁺ ions ionically bonded to ions consisting of two sulfur atoms covalently bonded together and carrying a 2- charge. It’s common in organic-rich sedimentary rocks, where sulfate ions from sea-water are reduced after deposition (during a stage in sedimentary rock evolution called diagenesis) by organic carbon also present in the sediment.

Pyrite has a distinctive yellow colour and metallic lustre. It is sometimes known as fool’s gold, but its hardness is 6–6.5, much harder than gold. Well formed crystals are commonly cubic, but sometimes form dodecahedra with twelve pentagonal faces. Pyrite shows no cleavage, breaking instead along irregular rough surfaces.

Pyrite is not a commercial ore of iron, because removing the sulfur is a difficult and environmentally damaging process compared with extrating iron from its oxides.

Galena

A second sulfide mineral is lead sulfide galena with the formula PbS₂. Galena also crystallizes in cubes (though not dodecahedra), but its hardness is about 2.5 (much softer than pyrite) and it’s grey and metallic in colour and lustre. Galena shows perfect cubic cleavage, breaking into small cubes when struck with a hammer.

Galena is an important ore of lead, occuring in vein systems particularly in carbonate rocks. In some cases, galena contains silver as an impurity; the silver may be more valuable than the lead when galena is mined.

Oxides

Oxides are also an important class of minerals, forming ores of several metals that are mined by humans. We will look at two, both ores of iron. Iron can exist in two oxidation states. In iron II, a reduced form, two electrons are lost to produce Fe²⁺ ions as are common in ferro-magnesian minerals. However, oxygen, when present, can remove an extra electron, forming iron III, the oxidized form, which produces Fe³⁺ ions. Minerals containing iron III are characteristically red or orange in colour, in contrast with the green to black colours characteristic of iron II compounds.

Magnetite

Magnetite contains a mixture of iron II and iron III producing the overall formula Fe₃O₄. Magnetite is a hard (hardness ~6) black mineral with metallic lustre. It is sometimes found in cubic crystals that lack cleavage. Its most distinctive property is that it is attracted to a magnet, and some magnetite (lodestone) is naturally magnetic. Magnetite is a major component of banded iron formations which are mined for the production of iron and steel. The extraction of pure iron from magnetite is carried out by reacting it with carbon (typically in the form of coal) producing iron or steel and carbon dioxide as a byproduct.

Hematite

Hematite^[11] is a purely iron III oxide Fe₂O₃. It may also be black and metallic (specular hematite) but is distinguished from magnetite by its lack of magnetism and its red streak. In fine-grained form it is an earthy red colour and has dull or earthy lustre. Very small amounts of hematite occur in many red sedimentary rocks, where it’s an indicator that the rock was oxidized at some point in its history. However, red colour does not indicate an abundance of iron; the colour of fine-grained hematite is so intense that only a tiny amount is required to make a very red rock.

Like magnetite, hematite is a major component of banded iron formations from which iron is mined in many parts of the world.

Native elements

Minerals are described as native elements when they consist one kind of atom, bonded only to other atoms of the same element. Only some elements in the periodic table occur naturally in this way. Two are particularly valued by humans for their appearance and durability.

Gold

Gold (Au) is one of the densest minerals known, at 19300 kg/m³ or 19.3 g/cm³.^[12]. It is over seven times denser than common rock-forming minerals like quartz, feldspar, and calcite.

Gold shows metallic bonding and is distinguished, if you are lucky enough to find it, by its softness (hardness 2.5) and the ease with which it can be worked into shapes with hand tools. Gold is principally found in veins where it was deposited, often with other minerals, from hot water flowing through cracks within the crust. However, it’s chemically very unreactive, and so tends to survive weathering and erosion of the rocks around it. Because of its density, it tends to be transported differently from more common minerals in streams, and gold particles may be concentrated in certain places, known as placer deposits. The density of gold enables other minerals to be separated from it by moving water when panning for gold.

Diamond

Diamond (C) is entirely covalently bonded, and as a result is the hardest mineral known. Diamond is a high-pressure polymorph of carbon and is only stable at great depth within the mantle. As far as we know, with the exception of small diamonds formed in meteorite impacts, all natural diamonds formed over 100 km down in the mantle where pressures are sufficient to stabilize diamond. Diamonds reach the surface through the eruption of kimberlite magmas in explosive eruptions that occur very rarely – no kimberlite eruptions have been recorded in human history, but many examples are known from the geological record.

Diamond typically forms octahedral crystals. For jewelry, these are cut so as to maximize the play of colour. The optical properties of diamond cause different colours of light to be refracted differently as they pass through diamonds, giving it a special type of lustre called adamantine.