3.1: 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.

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, 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^th that of a proton or neutron, meaning most of the atom’s mass is in the nucleus.

Figure \(\PageIndex{1}\): Electron cloud model of the atom

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 8 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 number.

Figure \(\PageIndex{2}\): The Periodic Table of the Elements

The first arrangement of elements into a periodic table was done by Dmitri Mendeleev in 1869 using the elements known at the time [1]. 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 [2]. 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 one. 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.

Figure \(\PageIndex{3}\): Formation of Carbon 14 from Nitrogen 14

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, meaning that an element exists in nature with atoms having different numbers 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 6 protons and 6 neutrons. This isotope of carbon is called carbon-12 (¹²C). A few carbon atoms, carbon-13 (¹³C), have 6 protons and 7 neutrons. A trace amount of carbon atoms, carbon-14 (¹⁴C), has 6 protons and 8 neutrons. The heaviest naturally occurring element is uranium, atomic number 92. The eight most abundant elements in Earth’s continental crust are shown in Table \(\PageIndex{1}\): [3; 4]. These elements are found in the most common rock-forming minerals.

Table \(\PageIndex{1}\):: Eight Most Abundant Elements in the Earth’s Continental Crust % by weight (Source: USGS). All other elements are less than 1%..

Element	Symbol	Abundance %
Oxygen	O	47%
Silicon	Si	28%
Aluminum	Al	8%
Iron	Fe	5%
Calcium	Ca	4%
Sodium	Na	3%
Potassium	K	3%
Magnesium	Mg	2%

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.

Figure \(\PageIndex{4}\): A model of a water molecule, showing the bonds between the hydrogen and oxygen.

Most minerals are also compounds of more than one element. The common mineral calcite has the chemical formula \(\ce{CaCO3}\) indicating the molecule consists of one calcium, one carbon, and three oxygen atoms.

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 substitute for one another. Some olivine has all magnesium, some has all iron, and some has a 50/50 mix or any combination in between.

Charge

The number of electrons, which carry a negative charge, are compared with the number of protons, which carry a positive charge, to determine an atom's overall charge. An atom with a positive or negative charge is called an ion.

Ionic Bonding

Ionic bonds, are formed by the electrostatic attraction between atoms having 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 which takes them up. Through this transfers both atoms thus achieve stability. 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. This is the weaker of the two bond types presented here.

Figure \(\PageIndex{6}\): Cubic arrangement of Na and Cl ions in Halite

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 achieve stability. For example, oxygen (atomic number 8) has 8 electrons. In the case of oxygen, two atoms attach to each other and share 2 electrons to become the common oxygen molecule we breathe (O₂). Methane (CH₄) is another covalently bonded gas. The carbon atom needs 4 electrons and each hydrogen needs 1 to become stable. Each hydrogen shares its electron with the carbon to form a molecule as shown in the figure below. This is the stronger of the two bond types presented here.

Figure \(\PageIndex{7}\): Methane molecule

References

• Mendeleev, D. The relation between the properties and atomic weights of the elements. Journal of the Russian Chemical Society 1, 60–77 (1869).
• Scerri, E. R. The Periodic Table: Its Story and Its Significance. (Oxford University Press, USA, 2007).
• Clarke, F. W. H. S. W. The Composition of the Earth’s Crust. 127, (United States Geological Survey, 1927).
• Hans Wedepohl, K. The composition of the continental crust. Geochim. Cosmochim. Acta 59, 1217–1232 (1995).
• Science literacy: concepts, contexts, and consequences. (National Academies Press (US), 2016).
• Bohr, N. On the constitution of atoms and molecules. Philos. Mag. 26, 1–24 (1913).