Geologists view Earth as an operational system of interacting spheres. These spheres include the Geosphere (the solid body of the Earth), the Atmosphere (the gas envelope surrounding the Earth), the Hydrosphere (water in all its forms at and near the surface of the Earth), the Cryosphere (frozen water part of Earth), and the Biosphere (life on Earth in all its forms and interactions, including humankind).
Earth materials are the foundational building blocks of the Geosphere and are generally thought of as the minerals and rock that compose the solid Earth. However, soil and loose fragments of rocks and minerals that exist at the surface are also considered Earth materials. Fossil fuels and water also exist as Earth materials in solid, liquid and gaseous states. Additionally, the envelope of gases that encircle the Earth is also considered Earth material. Therefore, Earth Materials encompass the entire Earth System where each of Earth’s spheres interact. This chapter examines the origin and properties of materials largely found in the outermost layer of the solid Earth, the lithosphere. The lithosphere is the outermost rigid sphere of the Geosphere which includes both the crust and the solid upper mantle. These bound layers compose Earth’s tectonic plates.
What is a mineral?
The term “minerals” as used in nutrition labels and pharmaceutical products is not the same as what “mineral” means in a geological sense. In geology, the classic definition of a mineral is a substance that is: 1) naturally occurring, 2) inorganic, 3) solid at room temperature, 4) has an orderly and repeating internal crystalline structure, and 5) a chemical composition that can be defined by a chemical formula. Some natural substances technically should not be considered minerals, but are included by exception. For example, water and mercury are liquid at room temperature. Both are considered minerals because they were classified before the room-temperature rule was accepted as part of the definition. Although the mineral calcite, with the chemical formula CaCO3, is quite often formed by organic processes, it is considered a mineral because it is widely found and geologically important. Because of these discrepancies, in 1985, the International Mineralogical Association amended the definition to: “A mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes.” Typically, substances like amber, pearl, opal, or obsidian do not fit the definition of mineral because they do not have a crystalline structure. They are referred to as “mineraloids.”
The Building Blocks of Rock
A rock is a solid substance that is made of one or more minerals or mineraloids. As discussed elsewhere, there are three families of rock composed of minerals: igneous (rock crystallizing from molten material), sedimentary (rock composed of the products of mechanical weathering, [sand, gravel, etc.] and/or chemical weathering [minerals and mineraloids precipitated from solution]), and metamorphic (rock produced by the chemical and physical reorganization of other rock under conditions induced by elevated heat and/or pressure).
Mineral identification is the first step in understanding the formation of a rock and its history. Geologists learn to “read the rock” to understand Earth’s history at any given location where a rock is found in an outcrop. This allows geologists to understand what the environment was like at the moment the rock formed. Was there a volcano erupting or does the rock tell us that it formed deep inside a magma chamber? Was the rock formed by burial of an ancient beach? Was the rock formed by compressive forces deep within the crust as continents collided and new mountains were forming? The clues to these widely different environments of formation are “written in the rock.” The first step in understanding the rock’s history is being able to identify, characterize and quantify the minerals that compose the rock. Rocks are fascinating to a geologist because every rock has a story to tell. As we read the rock from one location to the next, it helps us piece together the fascinating story of the Earth.
As of 2018, there were over 5,000 minerals officially recognized by the International Mineralogical Association (https://www.ima-mineralogy.org/Minlist.htm). Many of these formed under very specific chemical and geological conditions and may only occur in one location on Earth. Fortunately for geology students, only a small subset of these minerals are common and truly necessary for identifying Earth’s most common rocks. These minerals have been dubbed, “the Big Ten” by the prominent American mineralogist Mickey Gunter (Dyar, M.D., Gunter, M.E. and Tasa D. Mineralogy and Optical Mineralogy: Mineralogical Society of America, Chantilly, Virginia, USA). These ten minerals, plus a handful of others, will be our focus and will recur in many discussions in this text.
“The Big Ten” minerals are: olivine, augite, hornblende, biotite, calcium-rich plagioclase (anorthite), sodium-rich plagioclase (albite), potassium-rich feldspar (commonly orthoclase), muscovite, quartz, and calcite.
The 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 a positive charge, 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, balancing with 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 1/1000th that of a proton or neutron, meaning most of the atom’s mass is in the nucleus. For our purposes, we can simplify the math by rounding the electron’s mass down to zero.
The Periodic Table of 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 of 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 – they are ‘elemental’ for this reason! (Of course, nuclear reactions can break an atom down to a smaller, simpler form, but nuclear reactions are not chemical reactions). Down to the atom, each element retains unique chemical and physical properties, leading to particular behaviors 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 and chemical properties.
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 (for example, lead is Pb from the Latin ‘plumbum’) . 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 that of protons and neutrons, the atomic mass is effectively the number of protons plus the number of neutrons. For example, the element silicon (Si) is atomic number 14 on the periodic table. Silicon has 14 protons and 14 neutrons in its nucleus and 14 electrons located in its electron cloud. The atomic mass of silicon is 28.01 mass units.
The atomic mass of an element represents an average mass of all of the atoms of that element found in nature. The atomic mass, noted on the periodic table, is usually not a whole number. Atoms of the same element can have differing numbers of neutrons in their nucleus. These are referred to as isotopes of that element. Isotopes behave the same chemically however they are slightly different in mass.
Isotopes are very important to Historical Geology. Applications of the use of isotopes in geology is discussed elsewhere in this text.
Among the 118 known elements appearing on the periodic table, the heaviest are fleeting human creations known only in high energy particle accelerators, and they decay rapidly. The naturally occurring elements on the periodic table end with uranium, atomic number 92. Of these 92 elements, only eight are abundant in the Earth’s crust and are shown in the pie chart below. Therefore, it is of these eight elements that the most common rock-forming minerals are composed.
Minerals are formed by the chemical bonding that occurs between these elements. Most minerals are compounds containing multiple elements bonded together in a specific arrangement. Chemical bonding describes how these atoms attach with each other to form compounds, such as sodium (Na) and chlorine (Cl) combine to form the mineral halite with the chemical formula: NaCl. The mineral halite is common table salt. There are some minerals that are composed of only one element, such as native Copper (Cu), native Gold (Au), or native Silver (Ag). These single elements bond to each other to form molecules, however they are not compounds because they are made of only one element.
Valence, Charge and Ions
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 (1885-1962) proposed a simple model of the atom that states atoms are more stable when their outermost shell is full [3, 4]. Atoms of most elements thus tend to gain or lose electrons so the outermost or valence shell is full. Elements that appear on the left side of the periodic table have smaller numbers of electrons in their outermost (valence) shell while elements on the right side have increasingly greater numbers of electrons in the valence shell. It takes a great deal of energy for the nucleus of an atom to hang on to a small number of valence electrons. These elements tend to search out partners that need a few electrons to fill their valence shell. If an element gains or loses electrons in its valence shell, it is then referred to as an ion. Because the number of electrons and protons are no longer equal in an ion, it has a charge. The charge is positive if protons outnumber electrons, and negative if electrons outnumber protons.
Let’s use a familiar example to help illustrate. Sodium (Na) is located on the far left side of the Periodic Table. It has one lonely electron in its valence shell that it desperately wants to shed. Remember, one atom of sodium (Na) has the same number of protons in its nucleus as it has electrons in its cloud. Therefore, if it loses any electrons it will have an excess positive charge because the positive charge of the protons in the nucleus will now outnumber the electrons. As sodium (Na) relinquishes its one valence electron is becomes a positively charged ion called a cation and is designated Na+. Coincidentally, chlorine (Cl) has one spot left for an electron in its valence shell. If chlorine (Cl) gains an electron, it will have a net negative charge and is referred to as an anion and is designated Cl– .
A chemical bond will form between these oppositely charged ions which creates a molecule. This type of chemical bond is an ionic bond where electron transfer and a strong attraction between oppositely charged ions creates the bond. This molecule of NaCl (sodium chloride) is not a mineral. Remember, a mineral must have an “orderly and repeating internal crystalline structure.” The smallest representation of a mineral is one unit cell. The unit cell for the mineral halite (sodium chloride, NaCl) appears in the figure below where the sodium atoms are purple and the chlorine atoms are green.
Explore the following 3D molecular crystal structure of the mineral halite, below. Roll your cursor over to grab and rotate the structure.
- 1 Introduction
- 2 What is a mineral?
- 3 The Building Blocks of Rock
- 4 The Chemistry of Minerals
- 5 Formation of Minerals
- 6 The Silicate Minerals: Nine of “The Big Ten”
- 7 Other Important Rock Forming Minerals