Each statement represents a primary learning objective for parts of this unit. After you complete the unit, you should be able to:

• Define mineral.

• Describe the basic structure of the atom.

• Derive basic atomic information from the periodic table of elements.

• Describe chemical bonding related to minerals.

• Describe the main ways minerals form.

• Describe the silicon-oxygen tetrahedron and how it forms common silicate minerals.

• List common nonsilicate minerals in oxide, sulfide, sulfate, and carbonate groups.

• Identify minerals using physical properties and identification tables.

The term “minerals” as used in nutrition labels and pharmaceutical products is not the same as a mineral in a geological sense. In geology, the classic definition of a mineral is: 1) naturally occurring, 2) inorganic, 3) solid at room temperature, 4) regular crystal structure, and 5) defined chemical composition. A naturally occurring substance that resembles a mineral but lacks a crystalline structure does not meet all the criteria to be classified as a mineral and is therefore referred to as a mineraloid. Common examples include coal, pearl, opal, and obsidian.

A rock is a substance that contains one or more minerals or mineraloids. As is discussed in later chapters, there are three types of rocks composed of minerals: igneous (rocks crystallizing from molten material), sedimentary (rocks composed of products of mechanical weathering, such as sand or gravel, and chemical weathering, i.e., precipitates from solution), and metamorphic (rocks produced by alteration of other rocks by heat and pressure).

2.1: Chemistry of Minerals

Rocks are composed of minerals that have a specific chemical composition. To understand mineral chemistry, it is essential to examine the fundamental unit of all matter, the atom.

Matter is made of atoms. Atoms consist of subatomic particles—protons, neutrons, and electrons. A simple model of the atom has a central nucleus composed of protons, which have positive charges, and neutrons, which have no charge. A cloud of negatively charged electrons surrounds the nucleus, with the number of electrons equaling the number of protons, thus balancing the positive charge of the protons for a neutral atom. Protons and neutrons each have a mass number of 1. The mass of an electron is less than 1/1000 that of a proton or neutron, meaning most of the atom’s mass is in the nucleus.

Periodic Table of the Elements

Matter is composed of elements, which are atoms that have a specific number of protons in the nucleus. This number of protons is called the atomic number for the element. For example, an oxygen atom has eight protons, and an iron atom has 26 protons. An element cannot be broken down chemically into a simpler form and retains unique chemical and physical properties. Each element behaves in a unique manner in nature. This uniqueness led scientists to develop a periodic table of the elements, a tabular arrangement of all known elements listed in order of their atomic numbers.

The first arrangement of elements into a periodic table was done by Dmitri Mendeleev in 1869 using the elements known at the time. In the periodic table, each element has a chemical symbol, name, atomic number, and atomic mass. The chemical symbol is an abbreviation for the element, often derived from a Latin or Greek name for the substance. The atomic number is the number of protons in the nucleus. The atomic mass is the number of protons and neutrons in the nucleus, each with a mass number of 1. Since the mass of electrons is so much less than the protons and neutrons, the atomic mass is effectively the number of protons plus neutrons.

The atomic mass of natural elements represents an average mass of the atoms comprising that substance in nature and is usually not a whole number as seen on the periodic table; this means that, in nature, the element's atoms have different amounts of neutrons. The differing number of neutrons affects the mass of an element in nature, and the atomic mass number represents this average. This gives rise to the concept of isotope. Isotopes are forms of an element with the same number of protons but different numbers of neutrons. There are usually several isotopes for a particular element. For example, 98.9% of carbon atoms have six protons and six neutrons. This isotope of carbon is called carbon-12 (12C). A few carbon atoms, carbon-13 (13C), have six protons and seven neutrons. A trace amount of carbon atoms, carbon-14 (14C), has six protons and eight neutrons.

Among the 118 known elements, the heaviest are fleeting human creations known only in high energy particle accelerators, and they decay rapidly. The heaviest naturally occurring element is uranium, atomic number 92. The eight most abundant elements in Earth’s continental crust are shown in Table 1. These elements are found in the most common rock-forming minerals.

Chemical Bonding

Most substances on Earth are compounds containing multiple elements. Chemical bonding describes how these atoms attach with each other to form compounds, such as sodium and chlorine combining to form NaCl, common table salt. Compounds that are held together by chemical bonds are called molecules. Water is a compound of hydrogen and oxygen in which two hydrogen atoms are covalently bonded with one oxygen, making the water molecule. The oxygen we breathe is formed when one oxygen atom covalently bonds with another oxygen atom to make the molecule O2. The subscript 2 in the chemical formula indicates the molecule contains two atoms of oxygen.

Most minerals are also compounds of more than one element. The common mineral calcite has the chemical formula CaCO3, indicating the molecule consists of one calcium, one carbon, and three oxygen atoms. In calcite, one carbon and three oxygen atoms are held together by covalent bonds to form a molecular ion, called carbonate, which has a negative charge. Calcium as an ion has a positive charge of plus two. The two oppositely charged ions attract each other and combine to form the mineral calcite, CaCO3. The name of the chemical compound is calcium carbonate, where calcium is Ca and carbonate refers to the molecular ion CO32-.

The mineral olivine has the chemical formula (Mg,Fe)2SiO4, in which one silicon and four oxygen atoms are bonded with two atoms of either magnesium or iron. The comma between iron (Fe) and magnesium (Mg) indicates the two elements can occupy the same location in the crystal structure and substitute for one another.

Valence and Charge

The electrons around the atom’s nucleus are located in shells representing different energy levels. The outermost shell is called the valence shell. Electrons in the valence shell are involved in chemical bonding. In 1913, Niels Bohr proposed a simple model of the atom that states atoms are more stable when their outermost shell is full. Atoms of most elements thus tend to gain or lose electrons so that the outermost shell is full. In Bohr’s model, the innermost shell can have a maximum of two electrons and the second and third shells can have a maximum of eight electrons. When the innermost shell is the valence shell, as in the case of hydrogen and helium, it obeys the octet rule when it is full with two electrons. For elements in higher rows, the octet rule of eight electrons in the valence shell applies.

The rows in the periodic table present the elements in order of atomic number, and the columns organize elements with similar characteristics, such as the same number of electrons in their valence shells. Columns are often labeled from left to right with Roman numerals I to VIII and Arabic numerals 1 through 18. The elements in Columns I and II have one and two electrons in their respective valence shells, and the elements in Columns VI and VII have six and seven electrons in their respective valence shells.

In Row 3 and Column I, sodium (Na) has 11 protons in the nucleus and 11 electrons in three shells—two electrons in the inner shell, eight electrons in the second shell, and one electron in the valence shell. To maintain a full outer shell of eight electrons per the octet rule, sodium readily gives up that one electron so there are ten total electrons. With 11 positively charged protons in the nucleus and ten negatively charged electrons in two shells, sodium is an ion with an overall net charge of +1 when forming chemical bonds.

All elements in Column I have a single electron in their valence shell and a valence of one. These other Column I elements also readily give up this single valence electron and thus become ions with a +1 charge. Elements in Column II readily give up two electrons and end up as ions with a charge of +2. Note that elements in Columns I and II, which readily give up their valence electrons, often form bonds with elements in Columns VI and VII, which readily take up these electrons. Elements in Columns III through XV are usually involved in covalent bonding. Column XVIII (18)—the last column—contains the noble gases. These elements are chemically inert because the valence shell is already full with eight electrons, so they do not gain or lose electrons. An example is the noble gas helium, which has two valence electrons in the first shell. Its valence shell is therefore full. All elements in Column VIII possess full valence shells and do not form bonds with other elements.

As seen above, an atom with a net positive or negative charge as a result of gaining or losing electrons is called an ion. In general, the elements on the left side of the table lose electrons and become positive ions, which are called cations because they are attracted to the cathode in an electrical device. The elements on the right side tend to gain electrons. These are called anions because they are attracted to the anode in an electrical device. The elements in the center of the periodic table, Columns III through XV, do not consistently follow the octet rule. These are called transition elements. A common example is iron, which has a +2 or +3 charge depending on the oxidation state of the element. Oxidized Fe+3 carries a +3 charge and reduced Fe+2 is +2. These two different oxidation states of iron often impart dramatic colors to rocks containing their minerals—the oxidized form producing red colors and the reduced form producing green.

Ionic Bonding

Ionic bonds, also called electron-transfer bonds, are formed by the electrostatic attraction between atoms that have opposite charges. Atoms of two opposite charges attract each other electrostatically and form an ionic bond in which the positive ion transfers its electron (or electrons) to the negative ion. Through this transfer, both atoms thus achieve a full valence shell. For example, one atom of sodium (Na+1) and one atom of chlorine (Cl-1) form an ionic bond to make the compound sodium chloride (NaCl). This is also known as the mineral halite, or common table salt. Another example is calcium (Ca+2) and chlorine (Cl-1) combining to make the compound calcium chloride (CaCl2). The subscript 2 indicates two atoms of chlorine are ionically bonded to one atom of calcium.

Covalent Bonding

Ionic bonds are usually formed between a metal and a nonmetal. Another type, called a covalent or electron-sharing bond, commonly occurs between nonmetals. Covalent bonds share electrons between ions to complete their valence shells. For example, oxygen (atomic number 8) has eight electrons—two in the inner shell and six in the valence shell. Gases like oxygen often form diatomic molecules by sharing valence electrons. In the case of oxygen, two atoms attach to each other and share two electrons to fill their valence shells, becoming the common oxygen molecule we breathe (O2). Methane (CH4) is another covalently bonded gas. The carbon atom needs four electrons and each hydrogen needs one. Each hydrogen shares its electron with the carbon to form a molecule, as shown in the figure.

2.2: Formation of Minerals

Minerals form when atoms bond together in a crystalline arrangement. Three main ways this occurs in nature are: (1) precipitation directly from an aqueous (water) solution with a temperature change, (2) crystallization from a magma with a temperature change, and (3) biological precipitation by the action of organisms.

Precipitation from Aqueous Solution

Solutions consist of ions or molecules, known as solutes, dissolved in a medium or solvent. In nature, this solvent is usually water. Many minerals can be dissolved in water, such as halite or table salt, in which the Na+1 and Cl-1 ions separate and disperse into the solution.

Precipitation is the reverse process, in which ions in solution come together to form solid minerals. Precipitation is dependent on the concentration of ions in solution and other factors such as temperature and pressure. The point at which a solvent cannot hold any more solute is called saturation. Precipitation can occur when the temperature of the solution falls, when the solute evaporates, or when chemical conditions change in a solution. An example of precipitation occurs in our homes when water evaporates and leaves behind a rind of minerals on faucets, shower heads, and drinking glasses.

In nature, changes in environmental conditions may cause the minerals dissolved in water to form bonds and grow into crystals or to cement grains of sediment together. In Utah, deposits of tufa formed from mineral-rich springs that emerged into Lake Bonneville in the last ice age. Now exposed in dry valleys, this porous tufa was a natural insulation used by pioneers to build their homes with a natural protection against summer heat and winter cold. The travertine terraces at Mammoth Hot Springs in Yellowstone Park are another example formed by calcite precipitation at the edges of the shallow spring-fed ponds.

Another example of precipitation occurs in the Great Salt Lake, Utah, where the concentration of sodium chloride and other salts is nearly eight times greater than in the world’s oceans. Streams carry salt ions into the lake from the surrounding mountains. With no other outlet, the water in the lake evaporates and the concentration of salt increases until saturation is reached and the minerals precipitate out as sediments. Similar salt deposits include halite and other precipitates, occurring in other lakes like Mono Lake in California and the Dead Sea.

Crystallization From Magma

Heat is energy that causes atoms in substances to vibrate. Temperature is a measure of the intensity of the vibration. If the vibrations are violent enough, chemical bonds are broken and the crystals melt, releasing the ions into the melt. Magma is molten rock with freely moving ions. When magma is emplaced at depth or extruded onto the surface (then called lava), it starts to cool and mineral crystals can form.

Precipitation by Organisms

Many organisms build bones, shells, and body coverings by extracting ions from water and precipitating minerals biologically. The most common mineral precipitated by organisms is calcite, or calcium carbonate (CaCO3). Calcite is often precipitated by organisms as a polymorph called aragonite. Polymorphs are crystals with the same chemical formula but different crystal structures. Marine invertebrates such as corals and clams precipitate aragonite or calcite for their shells and structures. Upon death, their hard parts accumulate on the ocean floor as sediments and may eventually become the sedimentary rock limestone. Though limestone can form inorganically, the vast majority is formed by this biological process. Another example is marine organisms called radiolaria, which are zooplankton that precipitate silica for their microscopic external shells. When the organisms die, the shells accumulate on the ocean floor and can form the sedimentary rock chert. An example of biologic precipitation from the vertebrate world is bone, which is composed mostly of a type of apatite, a mineral in the phosphate group. The apatite found in bones contains calcium and water in its structure and is called hydroxycarbonate apatite, Ca5(PO4)3(OH). As mentioned above, such substances are not technically minerals until the organism dies and these hard parts become fossils.

2.3: Silicate Minerals

Minerals are categorized based on their composition and structure. Silicate minerals are built around a molecular ion called the silicon-oxygen tetrahedron. A tetrahedron has a pyramid-like shape with four sides and four corners. Silicate minerals form the largest group of minerals on Earth, comprising the vast majority of the Earth’s mantle and crust. Of the nearly four thousand known minerals on Earth, most are rare. There are only a few that make up most of the rocks likely to be encountered by surface dwelling creatures like us. These are generally called the rock-forming minerals.

The silicon-oxygen tetrahedron (SiO4) consists of a single silicon atom at the center and four oxygen atoms located at the four corners of the tetrahedron. Each oxygen ion has a -2 charge, and the silicon ion has a +4 charge. The silicon ion shares one of its four valence electrons with each of the four oxygen ions in a covalent bond to create a symmetrical, four-sided geometric pyramid figure. Only half of the oxygen’s valence electrons are shared, giving the silicon-oxygen tetrahedron an ionic charge of -4. This silicon-oxygen tetrahedron forms bonds with many other combinations of ions to form the large group of silicate minerals.

The silicon ion is much smaller than the oxygen ions (see the figures above) and fits into a small space in the center of the four large oxygen ions, visible if the top ball is removed (as shown in the figure above). Because only one of the valence electrons of the corner oxygens is shared, the silicon-oxygen tetrahedron has chemically active corners available to form bonds with other silica tetrahedra or other positively charged ions such as Al+3, Fe+2,Fe+3, Mg+2, K+1, Na+1, and Ca+2. Depending on many factors, such as the original magma chemistry, silica-oxygen tetrahedra can combine with other tetrahedra in several different configurations. For example, tetrahedra can be isolated, attached in chains, sheets, or three-dimensional structures. These combinations and others create the chemical structure in which positively charged ions can be inserted for unique chemical compositions, forming silicate mineral groups.

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