Eric Hunt photo, CC-BY, via Flickr
LEARNING OBJECTIVES
After reading this chapter, students should be able to:
- Explain the criteria that identify an Earth material as a mineral
- Recognize the basic chemistry that applies to minerals and their internal structure
- Summarize the ways a mineral can form
- Describe the importance of silicate minerals
- Summarize other common rock-forming minerals
Introduction
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).
U.S. Geological Survey image
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?
Rob Lavinsky photo, licensed under CC-BY-SA-3.0
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).
Ralph and Cheryl Dawes photo, CC-BY.
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. 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.
Atoms
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.
Public domain from Wikimedia , public domain (CC-0)
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 (for example, lead is Pb from the Latin plumbum) [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 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.
Isotopes
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 useful to historical geologists. Applications of the use of isotopes in geology are discussed elsewhere in this text.
Common Elements of the Earth’s Crust
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.
Chemical Bonding
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– .
OpenStax image, CC BY 4.0
Public domain by Benjah-bmm27. CC-0 via Wikimedia
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.
Now, take a look at a 3D representation of perfect mineral crystals of halite.
3D model of halite crystals
Malopolska`s Virtual Museums (Public domain) on Sketchfab
Do you notice the similarity of the molecular structure with the outward appearance of the crystals? The outward physical appearance of a mineral will reflect its internal atomic structure. How cool is it that the atomic level structure of a mineral is revealed in its macroscopic (hand-sample sized) shape! We will look at this in more detail in the upcoming section on diagnostic mineral characteristics.
Another type of chemical bonding can occur between elements in the formation of minerals that involves electron sharing. This is most easily explained in looking at a simple molecular model of H2O, water. Oxygen has 6 electrons in its valence shell and hydrogen has only one. By sharing these electrons, the valence shells will contain 8 electrons for oxygen and 2 for hydrogen. Both configurations are very stable.
The Silica Tetrahedron
The most common covalent bond in the formation of minerals is the bond that occurs between silicon and oxygen. The pie chart in the previous section shows us that the two most common elements in the Earth’s crust are oxygen and silicon. It makes sense that the most common minerals in the crust will therefore contain a lot of these two elements. The first nine of “The Big Ten” minerals are indeed from the largest mineral group, the silicate minerals. The silicates are named for the silicon and oxygen foundation in their composition. The basic framework of the silicate minerals is the silicon – oxygen (or silica) tetrahedron where four oxygen atoms pack very tightly around one silicon atom. The resulting molecular structure is shaped like a tetrahedron.
Callan Bentley image
The silica tetrahedron is referred to as an “complex molecule.” This means that the silicon and oxygen atoms are very tightly bonded however the overall charge is negative making this complex molecule a complex ion. The silica molecules acts as an anion. The silicon atom has four valence electrons while the oxygen atoms each have six valence electrons. To fill the outermost electron shell, each atom would need to share a total of eight electrons. This cannot happen since silicon has only four electrons to share. The result is a complex molecule with a net negative charge of –4. As a result, this complex silica molecule is ready to bond with other available elements or complex molecules. This is how all of the silicate minerals form.
Let’s look at one of our “Big Ten” silicate minerals, olivine, to see how this works.
Callan Bentley image
Olivine is largely an “igneous mineral” meaning that olivine most commonly forms as a magma cools and minerals begin to crystallize. The chemical formula for the mineral olivine is: (Mg,Fe)2SiO4 The elements of Mg (magnesium) and Fe (iron) appear together in parentheses because they can readily substitute for each other in the mineral’s crystalline structure depending on which of the elements are available in the cooling magma.
From: S Mahendran et al 2017, licensed under Creative Commons Attribution 3.0
Both Mg and Fe have a valence charge of +2; both Mg and Fe have two electrons in their outermost shells, e.g. electrons that they are looking to “give away.” They are swimming around in that magma looking for partners! As the magma cools, it will reach a temperature at which bonding can occur. As discussed, the complex silica molecule forms with the overall -4 charge. The available Fe+2 and Mg+2 will form an ionic bond with the complex silica molecule which will balance out the overall charge. However, as indicated above with the example of the mineral halite, one molecule does not constitute a mineral. The smallest representation of a mineral is one unit cell which appears in the diagram (right) for olivine.
Formation of Minerals
Minerals form when atoms bond together in a crystalline arrangement. In order for a mineral crystal to grow, the elements needed to make it must be present in the appropriate proportions, the physical and chemical conditions must be favorable, and there must be sufficient time for the atoms to become arranged.
Physical and chemical conditions include factors such as temperature, pressure, presence of water, pH, and the amount of oxygen available. Time is one of the most important factors because it takes time for atoms to move from place to place and become ordered. If time is limited, the mineral grains (with crystalline structure) will remain very small. The presence of water enhances the mobility of ions and can lead to the formation of larger crystals over shorter time periods.
Common Processes of Mineral Formation
Callan Bentley image
- Crystallization from molten rock material (magma or lava). The majority of minerals in the crust have formed this way.
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- Organic formation: formation of minerals by organisms within shells (primarily calcite) and teeth and bones (primarily apatite)
- Precipitation from aqueous solution (i.e., from hot water flowing underground, from evaporation of a lake or inland sea, or in some cases, directly from seawater).
- Weathering: during which minerals unstable at Earth’s surface (conditions of low temperature, low pressure, high moisture, and high oxygen levels) may be altered to other minerals.
- Metamorphism: formation of new minerals directly from the elements within existing minerals under conditions of elevated temperature and/or pressure.
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These processes are responsible for the formation of the various mineral groups that compose the Earth’s crust. See the diagram below which illustrates the composition of the Earth’s crust by the minerals that compose it.
Crystallization from Magma and the Formation of the Silicate Minerals
Most of the minerals of Earth’s crust formed through the cooling of molten rock (magma or lava). Molten rock is very hot, typically on the order of 1000 °C (1800 °F) or more. Heat is energy and temperature is a measure of that energy. Heat causes atoms to vibrate and temperature measures the intensity of the vibration. If vibrations are very strong, chemical bonds will break and the pre-existing minerals in Earth’s crust and mantle will melt. Melting releases ions into a pool, forming a magma chamber. Magma is simply molten rock with freely moving ions. If magma is allowed to cool at depth or erupted onto the surface (then called lava), mineral crystals will form as temperature decreases.
Callan Bentley image
The chemical composition of Earth’s crust is critical to the discussion of the dominant minerals to form from magma or lava, the silicate minerals. It is important to understand that not every location in the crust will have this composition. All rock is different and virtually no two rocks within the crust will have the exact same composition. The same also goes for magma. No two magma bodies will have the exact same composition. The magma will however, contain these elements in somewhat varying proportions depending on where exactly within the crust (or mantle) the magma was derived. What we can say is that the magma will be largely composed of silicon and oxygen with varying proportions of the remaining six elements plus other elements in trace amounts. The combination of these elements will form the different silicate minerals as the magma cools either deep within the Earth or near/at the surface.
Bowen’s Reaction Series
Crystallization of minerals from magma, or cooling lava, follows a very specific sequence. This sequence was first determined experimentally by the scientist Norman L. Bowen (1887-1956) at the Carnegie Institute in Washington, D.C. in the early 1900’s. This order is referred to as “Bowen’s Reaction Series” and represents the formation of the largest group of minerals, silicate minerals (see diagram below). This specific process of crystallization leads us to the formation of nine of “The Big Ten” minerals: olivine, pyroxene (augite), amphibole (hornblende), biotite, calcium-rich plagioclase (anorthite), sodium-rich plagioclase (albite), potassium-rich feldspar (commonly orthoclase), muscovite, and quartz.
Modified after an original by Steven Earle, licensed Creative Commons Attribution 4.0 International License
The left side of Bowen’s Reaction Series Diagram is labeled the “Discontinuous Branch” while the right side is labeled the “Continuous Branch.” On the left side, the minerals are most iron- and magnesium-rich at the top. Their crystal structure changes and iron and magnesium content decrease as temperatures drop. On the right side, the plagioclase minerals form in one continuous flow from calcium-rich at the top to sodium-rich at the bottom.
Beginning with the “Discontinuous Branch,” olivine forms from single silica tetrahedron ionically bonded to iron and/or magnesium for the crystalline structure. As the magma temperature cools and crystallization proceeds, the silica tetrahedron will link together (“polymerize”) to form chains, sheets and frameworks. This allows for the formation of the different minerals in progression along the Discontinuous Branch.
Callan Bentley image
Shelley Jaye photo
As the temperature drops, and assuming that some silica remains in the magma, the olivine crystals react (combine) with some of the silica in the magma to form pyroxene (commonly augite). As long as there is silica remaining and the rate of cooling is slow, this process continues down the discontinuous branch: olivine to pyroxene, pyroxene to amphibole (commonly hornblende), and amphibole to biotite.
At about the point where pyroxene (augite) begins to crystallize from the cooling magma, plagioclase feldspar also begins to crystallize on the continuous (right) side of Bowen’s Reaction Series Diagram. As cooling continues, plagioclase minerals retain their original framework silicate crystal structure. The core of the crystal begins calcium-rich and, once crystallization is complete, ends with a rim of sodium-rich plagioclase. See the photo below. Finally, if the magma is quite silica-rich to begin with, there will still be some left at around 750° to 800°C, and from this last magma, potassium feldspar, quartz, and maybe muscovite mica will form.
The animation below links to an online model that simulates how silicate minerals form from a cooling magma. Click through and then use the sliders to change the overall silica content (ultramafic to felsic) and cooling time. This is a very good visual representation of how the ions of the different common elements exist in magma and combine to form the different minerals found on Bowen’s Reaction Series as temperature decreases and the magma crystallizes.
Watch the following video which will introduce you to the silicate minerals:
The Silicate Minerals: Nine of “The Big Ten”
Olivine
Chemically, olivine is mostly silica, iron, and magnesium and typically green in color. Olivine is the primary mineral component in mantle rock (called peridotite) and ocean floor rock (called basalt). It is characteristically green when not weathered. The chemical formula is (Fe,Mg)2SiO4. The comma between iron (Fe) and magnesium (Mg) indicates these two elements occur in a solid solution. Not to be confused with a liquid solution, a solid solution occurs when two or more elements have similar properties and can freely substitute for each other in the same location in the crystal structure. The crystal structure of olivine is built from independent silica tetrahedra.
The iron- and magnesium-rich silicate mineral olivine (green).
Credit: Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. CC BY Attribution 3.0
The Pyroxene Family
Augite is the most common mineral of the pyroxene family and one of our Big Ten Minerals. Augite is iron and/or magnesium-rich forming a complex structure of elements bonded to polymerized single chains of silica tetrahedra. Augite is typically black or dark green in color. The chemical formula for augite is complex, indicating that different elements may substitute in the structure depending on what is available in the cooling magma: (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6.
The iron- (Fe) and magnesium- (Mg) rich silicate mineral augite is a member of the pyroxene family.
Robin Rohrback GIGAmacro CC BY
The Amphibole Family
As we move down Bowen’s Reaction Series, the internal crystal structure of each mineral becomes increasingly more complex. Amphibole minerals are built from polymerized double silica chains. The most common amphibole, hornblende, is usually black. The chemical formula is very complex with several solid solution opportunities and generally written as (Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH)2.
The iron- (Fe) and magnesium- (Mg) rich silicate mineral hornblende is a member of the pyroxene family.
Robin Rohrback GIGAmacro CC BY
The Sheet Silicates
Biotite and muscovite are varieties of mica. The silica tetrahedra in the micas are arranged in continuous sheets. Bonding of elements between sheets is relatively weak which allows these minerals to be split easily along the sheets (this refers to the mineral’s characteristic pattern of breaking, or cleavage).
The difference between the two micas is that biotite will contain iron and/or magnesium while muscovite mica, the location of the iron/magnesium is replaced by potassium. Therefore, biotite is dark while muscovite is light in color.
The sheet silicate mineral biotite. The color indicates that Fe and/or Mg exists in the mineral’s chemistry. Note that one thin sheet of dark colored biotite can be virtually colorless. The ability to separate into thin sheets is described as the mineral’s cleavage.
Robin Rohrback GIGAmacro CC BY
The sheet silicate mineral muscovite. This mineral belongs to the same family as biotite (mica minerals) however it does not contain any Fe/Mg therefore it is clear in color.
Robin Rohrback GIGAmacro CC BY
The Framework Silicates
Quartz and feldspar are the two most abundant minerals in the continental crust. In fact, feldspar itself is the single most abundant mineral in the Earth’s crust (see pie chart above) . The main feldspar minerals are potassium feldspar, (a.k.a. K-feldspar or K-spar) and the continuum of sodium- to calcium-rich plagioclase feldspars where albite is most sodium-rich and anorthite is most calcium-rich. Potassium feldspar and sodium-rich plagioclase feldspar are abundant in the rock of the continental crust while calcium-rich plagioclase feldspar is abundant in the rock of oceanic crust. Together with quartz, these minerals are classified as framework silicates because they are built with a three-dimensional framework of silica tetrahedra. Within the framework of the feldspar minerals are holes and spaces into which potassium, sodium, and calcium can fit giving rise to a variety of compositions.
Calcium-rich plagioclase feldspar, a framework silicate mineral.
Robin Rohrback GIGAmacro CC BY
Sodium-rich plagioclase feldspar, a framework silicate mineral. Use your trackpad as a zoom.
Robin Rohrback GIGAmacro CC BY
Potassium-rich feldspar, a framework silicate mineral. Use your trackpad as a zoom.
Robin Rohrback GIGAmacro CC BY
Quartz is composed of pure silica, SiO2 with the tetrahedra arranged in a three dimensional framework. Quartz is the final mineral on Bowen’s Reaction Series. It is the last mineral to crystallize from a silica rich magma. In quartz, the silica tetrahedra are bonded in a “perfect” three-dimensional framework. Pure quartz is composed entirely of SiO2 however, impurities consisting of atoms within this framework give rise to many varieties of quartz among which are gemstones like amethyst, rose quartz, and citrine.
Quartz, a framework silicate mineral composed of almost entirely SiO2. This is typically how we find quartz in common rocks, both clear and smoky grey in color.
Robin Rohrback GIGAmacro CC BY
Other Important Rock Forming Minerals
Calcite
Public domain by Benjah-bmm27 licensed CC0 1.0 via Wikimedia
We’ve made it through the formation of nine of “The Big Ten” minerals. At this point, you’re probably wondering, “what about calcite? As the final ‘Big Ten’ mineral, how does calcite form?” Calcite formation is most typically associated with sedimentary processes. Let’s take a look at calcite and some of the other “non-Big Ten” minerals that are important in historical geology.
Calcite is a “non-silicate” mineral (see the pie chart of the minerals of the crust). Calcite belongs to another family, or group of minerals, called the carbonates. The basic chemical and crystalline foundation for calcite and other minerals of the carbonate group is the complex carbonate ion (CO3)-2 where one carbon cation is surrounded by three oxygen anions in a triangular pattern.
Calcite forms by the ionic bonding of the complex carbonate ion with a calcium anion. The diagram above represents one unit cell of the mineral calcite.
Crystallization of Calcite
Image by Temp4psu, CC BY
Calcite can form by:
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- Biomineralization, where organisms like clams and corals extract calcium and carbonate ions from seawater or fresh water and then combine them inside their tissues to form their skeletal material.
- Precipitation, where calcium and carbonate ions combine to precipitate micro-crystals of calcite from supersaturated seawater or circulating groundwater.
- Evaporation, where inland lakes and epicontinental seas become isolated and slowly dry up leaving crystalline calcite plus other minerals.
- In rare circumstances, calcite can occur through igneous processes.
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One of the most common sedimentary rocks found on Earth is limestone. Most limestone is biogenic in origin, meaning, it is formed through the lithification of the skeletal remains of organisms. These organisms, both macroscopic and microscopic, have shells or exoskeletons composed of calcite. When these organisms die, their remains accumulate and their soft body parts are scavenged or decompose. The remaining calcite hard parts become trapped in layers of sediment and eventually are cemented together by additional calcite that precipitates in the pore spaces between the skeletal remains. The resulting sedimentary rock is called fossil limestone. This accounts for the majority of calcite formation on Earth. Other processes mentioned above require special environmental conditions to allow for the formation of calcite.
Dolomite is another common carbonate mineral closely related to calcite often occurring in similar environments of formation, making it another important mineral in the study of Historical Geology. Dolomite has many similar characteristics to calcite including its chemical formula CaMg(CO3)2.
All of these samples are of the mineral calcite. Calcite is typically clear to white to yellow in color and displays a characteristic rhombohedral cleavage. All calcite will readily effervesce in dilute hydrochloric acid.
Robin Rohrback GIGAmacro CC BY
Clay Minerals
You might be surprised to learn that clay is a mineral – and not just one mineral – but a whole group of minerals. Clay minerals are also silicate minerals (see previous pie-chart) that form through the processes involved with weathering of pre-existing silicate minerals in the presence of weak acid and water. As rain forms in the atmosphere, it combines with carbon dioxide to form a weak acid called carbonic acid with the chemical formula, H2CO3. This weak acid rains down on Earth’s surface and immediately attacks minerals exposed at the surface. The pre-existing minerals become altered in their chemical composition through a reaction called hydrolysis. Hydrolysis is a type of chemical weathering that forms a wide range of clay minerals.
ACEMAC Nano Scale Electron Microscopy and Analysis Facility, University of Aberdeen by GSoil , CC BY
Clay minerals are sheet silicates similar to micas. Their chemical formula is dependent upon the minerals from which they were derived however all clay minerals contain a great deal of aluminum and silica plus varying amounts of water. Individual clay crystals tend to be very small, and often cannot be identified without a microscope or other equipment such as a scanning electron microscope (see photo below). Clays are important to the study of Historical Geology because they are the main components of fine-grained sediment (mud) and form another very common sedimentary rock, shale. The following is a hydrolysis reaction that occurs when silicate minerals encounter carbonic acid to produce clay and other ions:
Silicate mineral + carbonic acid (H2CO3) + H2O → clay + metal cations (Fe+2, Mg+2, Ca+2, Na+, etc.) + bicarbonate anions (HCO3)-1 + silica (SiO2)
Other Rock-Forming Silicates
The silicate minerals that fall into the group of “other” on the pie chart above (link to it) are largely formed through metamorphic processes. Metamorphism most commonly occurs deep within the Earth’s crust where rocks are buried and experience intense changes in temperature and pressure during periods of mountain building (orogeny). Some of the pre-existing minerals found within these deeply buried rocks can change in physical form and chemical composition as they recrystallize in response to these altered conditions.
One common metamorphic mineral that most students are familiar with is garnet. You might be surprised to learn that garnet is not just one mineral but also a part of a larger group. The most common garnet minerals are dark red in color (like the birthstone) and have the general chemical formula of:
(Ca, Mg, Fe)Al2(SiO4)3
Garnets are largely formed through the metamorphism of clay minerals which are rich in aluminum and silica. These clay minerals would be found in the common sedimentary rock shale that is subjected to varying degrees of pressure and temperature changes during the collision of tectonic plates and the formation of mountains.
Very large garnet crystals (porphyroblasts) formed by changes in temperature and pressure deep within the crust during mountain building accompanied by regional metamorphism. This schist is from Wrangell, Alaska.
Robin Rohrback GIGAmacro CC BY
Minerals Important to Historical Geology (“Big Ten” in bold)
| Mineral Groups | Common Rock Forming Mineral |
| Silicates | Olivine Pyroxene (Augite) Amphibole (Hornblende) Biotite Muscovite Calcium-rich plagioclase feldspar (anorthite) Sodium-rich plagioclase feldspar (albite) Potassium-rich Feldspar (orthoclase) Quartz Clay Garnet |
| Carbonates | Calcite, Dolomite |
| Oxides | Hematite, Magnetite |
| Halides | Halite |
| Sulfides | Pyrite, Chalcopyrite, Galena, Sphalerite |
| Sulfates | Gypsum |
| Phosphates | Apatite |
| Native Elements | Gold, silver, copper |
Oxides, Halides, and Sulfides
After carbonates, the next most common non-silicate minerals are the oxides, halides, and sulfides.
Oxides
Oxides consist of metal ions covalently bonded with oxygen. Iron oxides are important for producing iron ore deposits from which we extract metallic iron. The formation of Earth’s major iron ore deposits are extremely important in studying Earth’s history. Prior to about two billion years ago (2 Ga), the early oceans were saturated with soluble iron due to the lack of free oxygen. So much iron existed in the water that our early oceans were thought to have been green in color from the dissolved iron. As oxygen levels slowly increased in the atmosphere and dissolved in the oceans, the oxygen immediately bonded with the iron forming the common ore minerals of hematite (Fe2O3) and magnetite (Fe3O4). (See the case study on the Great Oxidation Event).
Banded iron formation. The greyish color is the iron oxide mineral, hematite, the red is the sedimentary rock, chert (when red, it is called jasper), and the gold is tiger’s eye. The sample is 26 cm across.
Robin Rohrback GIGAmacro CC BY
The most familiar oxide of iron is rust, which is a combination of iron oxides (Fe2O3) and hydrated oxides. Hydrated oxides form when iron is exposed to oxygen and water. The red color that we often see in soil and rock is usually due to the presence of iron oxides. For example, the red sandstone cliffs in Zion National Park and throughout Southern Utah consist of white or colorless grains of quartz coated with iron oxide which serves as cement, holding the grains together.
Public domain by Daniel Mayer, via Wikimedia
Halides
The halide minerals, also important to the study of historical geology contain the element chlorine ionically bonded with sodium or other cations. These minerals include halite or sodium chloride (NaCl; common table salt) and sylvite or potassium chloride (KCl), commonly used in cooking as a salt substitute.
The mineral halite which can be processed into common table salt.
Robin Rohrback GIGAmacro CC BY
Halide minerals are extremely important to the study of Earth’s history because their occurrence indicates a period of evaporation of sea water or other isolated bodies of water. A well-known example of modern halide mineral deposits created by evaporation is the Bonneville Salt Flats, located west of the Great Salt Lake in Utah (need a good pic). Watch the following video on extraction of rock salt from the Cayuga Salt Mine in central New York state. The halite mineral of the rock salt represents evaporation of a great inland sea that existed some 385 million years ago in the Great Lakes region.
Watch the following video where at 2300 feet underground, the Cayuga Mine in Lansing, NY, processes approximately 10,000 tons of road salt daily that is shipped to more than 1,500 locations throughout the northeast United States.
Sulfides
Many important metal ores are sulfides, in which metals are bonded to sulfur. Significant examples include: pyrite (iron sulfide, sometimes called “fool’s gold”), and chalcopyrite (iron-copper sulfide), galena (lead sulfide), sphalerite (zinc sulfide). Sulfides are well known for being important ore minerals. For example, galena is the main source of lead, sphalerite is the main source of zinc, and chalcopyrite is the main copper ore mineral.
Caption: Common iron sulfide mineral often referred to as “fool’s gold.”
Robin Rohrback GIGAmacro CC BY
The iron sulfide mineral pyrite is of particular importance to Historical Geology. Pyrite is unstable in the presence of oxygen and will readily chemically decompose to form stable iron oxide. However, pyrite occurs in some of Earth’s early (older than 2.0 Ga) sedimentary rocks. Sediments are formed from the weathering of pre-existing rock exposed on Earth’s surface. Pyrite, existing as sedimentary clasts, or pieces, within these ancient rocks, indicates that surface conditions must have been lacking oxygen to allow the pyrite to remain unaltered and ultimately preserved.
Caption: Zoom in close to find the pyrite in this sample of Archean age (greater than 2.5 billion year old) metamorphosed conglomerate. The large grey-white clasts are composed of quartz.
Robin Rohrback GIGAmacro CC BY
Sulfate Minerals
Sulfate minerals contain a metal ion, such as calcium, bonded to a sulfate ion. The sulfate ion is a combination of sulfur and oxygen (SO4)-2. The sulfate mineral gypsum (CaSO4ᐧ2H2O) is important to Earth’s history because it indicates formation from evaporating water and usually contains water molecules in its crystalline structure.
Caption: A variety of the mineral gypsum called alabaster. Gypsum is a common sulfate mineral that is so soft, you can scratch it with your fingernail.
Robin Rohrback GIGAmacro CC BY
Phosphate Minerals
Phosphate minerals contain the complex cation (PO4)-2 combined with various anions and cations. The best known and important phosphate mineral to the study of historical geology is apatite, Ca5(PO4)3(F,Cl,OH). Variations of this mineral are found in teeth and bones.
Native Elements
Native element minerals, usually metals, occur in nature in a pure or nearly pure state. Gold, silver and copper are examples of common native element minerals. Carbon is often found as a native element in minerals such as graphite and diamond.
Robert Lavinsky photo via Wikimedia, CC BY
Works Cited
[1] Mendeleev, D., 1869. “The relation between the properties and atomic weights of the elements.” Journal of the Russian Chemical Society 1, 60–77.
[2] Scerri, E. R., 2007. The Periodic Table: Its Story and Its Significance. Oxford University Press, USA.
[3] Science literacy: concepts, contexts, and consequences. National Academies Press (US), 2016.
[4] Bohr, N., 1913. “On the constitution of atoms and molecules.” Philos. Mag. 26, 1–24.
