This is a picture of a mineral taken through a microscope. You may find it hard to believe that this is a mineral, but it is! This piece of orthopyroxene was cut very thin, mounted on a slide, and viewed in a polarizing light microscope. The image contains features you wouldn’t be able to see by just looking at that piece of orthopyroxene with the unaided eye. A trained mineralogist can see that the orthopyroxene crystal formed first, then partly dissolved, and that augite crystals formed around the original crystals. Minerals are valuable resources for just about every aspect of our lives. When and where different minerals form are also important clues in telling the history of Earth.
Minerals are made up of different chemical elements bound together. Understanding mineral chemistry aids in understanding how minerals form and why they have certain properties.
A chemical element is a substance that cannot be made into a simpler form by ordinary chemical means. The smallest unit of a chemical element is an atom . An atom has all the properties of that element. These are the parts of an atom:
An introduction to the atom is seen on this Kahn Academy video: http://www.khanacademy.org/video/introduction-to-the-atom .
Figure 3.1
Major parts of an atom. What chemical element is this? (Hint: 3 protons, 3 electrons)
Because electrons are minuscule compared with protons and neutrons, the number of protons plus neutrons gives the atom its atomic mass . All atoms of a given element always have the same number of protons but may differ in the number of neutrons found in its nucleus. Atoms of an element with differing numbers of neutrons are called isotopes . For example, carbon always has 6 protons but may have 6, 7, or 8 neutrons. This means there are three isotopes of carbon: carbon-12, carbon-13, and carbon-14. How many protons and neutrons make up carbon-12? Carbon-13? Carbon-14?
For a funny view of the chemical elements, check out this Tom Lehrer song: http://www.youtube.com/watch?v=GFIvXVMbII0&feature=related .
Atoms are stable when they have a full outermost electron energy level. To fill its outermost shell, an atom will give, take, or share electrons. When an atom either gains or loses electrons, this creates an ion . Ions have either a positive or a negative electrical charge. What is the charge of an ion if the atom loses an electron? An atom with the same number of protons and electrons has no overall charge, so if an atom loses the negatively charged electron, it has a positive charge. What is the charge of an ion if the atom gains an electron? If the atom gains an electron, it has a negative charge.
Electron orbitals are described in this Kahn Academy video: http://www.khanacademy.org/video/orbitals .
When atoms chemically bond, they form compounds. The smallest unit of a compound with all the properties of that compound is a molecule . When two or more atoms share electrons to form a chemical bond, they form a molecule. The molecular mass is the sum of the masses of all the atoms in the molecule.
Ions come together to create a molecule so that electrical charges are balanced; the positive charges balance the negative charges and the molecule has no electrical charge. To balance electrical charge, an atom may share its electron with another atom, give it away, or receive an electron from another atom.
The joining of ions to make molecules is chemical bonding . There are three main types of chemical bonds:
A video about chemical bonding, also from Kahn Academy: http://www.khanacademy.org/video/ionic--covalent--and-metallic-bonds .
Hydrogen and oxygen share electrons to form water, which is a covalently bonded, polar molecule. Watch this animation to see how it forms: http://www.youtube.com/watch?v=qmgE0w6E6ZI .
Figure 3.2
Water is a polar molecule. Because the oxygen atom has the electrons most of the time, the hydrogen side (blue) of the molecule has a slightly positive charge while the oxygen side (red) has a slightly negative charge.
Minerals are categorized based on their chemical composition. Owing to similarities in composition, minerals within a same group may have similar characteristics.
Minerals are everywhere! Figure below shows some common household items and the minerals used to make them. The salt you sprinkle on food is the mineral halite. Silver in jewelry is also a mineral. Baseball bats and bicycle frames both contain minerals. Although glass is not a mineral, it is produced from the mineral quartz. Scientists have identified more than 4,000 minerals in Earth’s crust. A few are common, but many are uncommon.
Figure 3.3
Silver and halite are minerals; the mineral quartz is used to make glass.
Geologists have a very specific definition for minerals. A material is characterized as a mineral if it meets all of the following traits. A mineral is an inorganic, crystalline solid. A mineral is formed through natural processes and has a definite chemical composition. Minerals can be identified by their characteristic physical properties such as crystalline structure, hardness, density, flammability, and color.
Minerals are crystalline solids. A crystal is a solid in which the atoms are arranged in a regular, repeating pattern ( Figure below ). The pattern of atoms in different samples of the same mineral is the same. Is glass a mineral? Without a crystalline structure, even natural glass is not a mineral.
Figure 3.4
Sodium ions (purple balls) bond with chloride ions (green balls) to make table salt (halite). All of the grains of salt that are in a salt shaker have this crystalline structure.
Organic substances are the carbon-based compounds made by living creatures and include proteins, carbohydrates, and oils. Inorganic substances have a structure that is not characteristic of living bodies. Coal is made of plant and animal remains. Is it a mineral? Coal is a classified as a sedimentary rock but is not a mineral.
Minerals are made by natural processes, those that occur in or on Earth. A diamond created deep in Earth’s crust is a mineral. Is a diamond created in a laboratory by placing carbon under high pressures a mineral? No. Do not buy a laboratory-made “diamond” for jewelry without realizing it is not technically a mineral.
Nearly all (98.5%) of Earth’s crust is made up of only eight elements – oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium – and these are the elements that make up most minerals.
All minerals have a specific chemical composition. The mineral silver is made up of only silver atoms and diamond is made only of carbon atoms, but most minerals are made up of chemical compounds . Each mineral has its own chemical formula. Halite, pictured above, is NaCl (sodium chloride). Quartz is always made of two oxygen atoms bonded to a silicon atom, SiO 2 . If a mineral contains any other elements in its crystal structure, it's not quartz.
A hard mineral containing covalently bonded carbon is diamond, but a softer mineral that also contains calcium and oxygen along with carbon is calcite ( Figure below ).
Figure 3.5
The structure of calcite shows the relationship of calcium (Ca), carbon (C), and oxygen (O).
Some minerals have a range of chemical composition. Olivine always has silicon and oxygen as well as iron or magnesium or both, (Mg, Fe) 2 SiO 4 .
The physical properties of minerals include:
How physical properties are used to identify minerals is described in the lesson on Mineral Formation .
Minerals are divided into groups based on chemical composition. Most minerals fit into one of eight mineral groups.
The roughly 1,000 silicate minerals make up over 90% of Earth's crust. Silicates are by far the largest mineral group. Feldspar and quartz are the two most common silicate minerals. Both are extremely common rock-forming minerals.
The basic building block for all silicate minerals is the silica tetrahedron, which is illustrated in Figure below . To create the wide variety of silicate minerals, this pyramid-shaped structure is often bound to other elements, such as calcium, iron, and magnesium.
Figure 3.6
One silicon atom bonds to four oxygen atoms to form a silica tetrahedron.
Silica tetrahedrons combine together in six different ways to create different types of silicates ( Figure below ). Tetrahedrons can stand alone, form connected circles called rings, link into single and double chains, form large flat sheets of pyramids, or join in three dimensions.
Figure 3.7
The different ways that silica tetrahedrons can join together cause these two minerals to look very different.
Native elements contain atoms of only one type of element. Only a small number of minerals are found in this category. Some of the minerals in this group are rare and valuable. Gold, silver, sulfur, and diamond are examples of native elements.
The basic carbonate structure is one carbon atom bonded to three oxygen atoms. Carbonates include other elements, such as calcium, iron, and copper. Calcite (CaCO 3 ) is the most common carbonate mineral ( Figure below ).
Figure 3.8
The most common carbonate mineral, calcite, can be found naturally in a bivalve shell.
Azurite and malachite, shown in the Figure below , are carbonates that contain copper instead of calcium.
Figure 3.9
Two carbonate minerals: (a) deep blue azurite and (b) opaque green malachite.
Halide minerals are salts that form when salt water evaporates. Halite is a halide mineral, but table salt is not the only halide. The chemical elements known as the halogens (fluorine, chlorine, bromine, or iodine) bond with various metallic atoms to make halide minerals (see Figure below ).
Figure 3.10
Fluorite is a halide containing calcium and fluorine.
Oxides contain one or two metal elements combined with oxygen. Many important metals are found as oxides. Hematite (Fe 2 O 3 ), with two iron atoms to three oxygen atoms, and magnetite (Fe 3 O 4 ) ( Figure below ), with three iron atoms to four oxygen atoms, are both iron oxides.
Figure 3.11
Magnetite is the most magnetic mineral. Magnetite attracts or repels other magnets.
Phosphate minerals are similar in atomic structure to the silicate minerals. In the phosphates, phosphorus, arsenic, or vanadium bond to oxygen to form a tetrahedra. There are many different minerals in the phosphate group, but most are rare ( Figure below ).
Figure 3.12
Turquoise is a phosphate mineral containing copper, aluminum, and phosphorus.
Sulfate minerals contain sulfur atoms bonded to oxygen atoms. Like halides, they form where salt water evaporates. The sulfate group contains many different minerals, but only a few are common.
Gypsum is a common sulfate with a variety of appearances ( Figure below ). Some gigantic 11-meter gypsum crystals have been found. That is about as long as a school bus!
Figure 3.13
Although the orange crystals on the left looks nothing like the white sands on the right, both the crystals and sands are gypsum.
Sulfides are formed when metallic elements combine with sulfur. Unlike sulfates, sulfides do not contain oxygen. Pyrite, or iron sulfide, is a common sulfide mineral known as fool’s gold . People may mistake pyrite for gold because the two minerals are shiny, metallic, and yellow in color.
Minerals can be identified by their physical characteristics. The physical properties of minerals are related to their chemical composition and bonding. Some characteristics, such as a mineral's hardness, are more useful for mineral identification. Color is readily observable and certainly obvious, but it is usually less reliable than other physical properties.
Mineralogists are scientists who study minerals. One of the things mineralogists must do is identify and categorize minerals. While a mineralogist might use a high-powered microscope to identify some minerals, most are recognizable using physical properties.
Check out the mineral in Figure below . What is the mineral’s color? What is its shape? Are the individual crystals shiny or dull? Are there lines (striations) running across the minerals? In this lesson, the properties used to identify minerals are described in more detail.
Figure 3.14
This mineral has shiny, gold, cubic crystals with striations, so it is pyrite.
Diamonds are popular gemstones because the way they reflect light makes them very sparkly. Turquoise is prized for its striking greenish-blue color. Notice that specific terms are being used to describe the appearance of minerals.
Color is rarely very useful for identifying a mineral. Different minerals may be the same color. Real gold, as seen in Figure below , is very similar in color to the pyrite in Figure above .
Figure 3.15
This mineral is shiny, very soft, heavy, and gold in color, and is actually gold.
The same mineral may also be found in different colors. Figure below shows one sample of quartz that is colorless and another quartz that is purple. A tiny amount of iron makes the quartz purple. Many minerals are colored by chemical impurities.
Figure 3.16
Purple quartz, known as amethyst, and clear quartz are the same mineral despite the different colors.
Streak is the color of a mineral’s powder. Streak is a more reliable property than color because streak does not vary. Minerals that are the same color may have a different colored streak. Many minerals, such as the quartz above, do not have streak.
To check streak, scrape the mineral across an unglazed porcelain plate ( Figure below ). Yellow-gold pyrite has a blackish streak, another indicator that pyrite is not gold, which has a golden yellow streak.
Figure 3.17
The streak of hematite across an unglazed porcelain plate is red-brown.
Luster describes the reflection of light off a mineral’s surface. Mineralogists have special terms to describe luster. One simple way to classify luster is based on whether the mineral is metallic or non-metallic. Minerals that are opaque and shiny, such as pyrite, have a metallic luster. Minerals such as quartz have a non-metallic luster. Different types of non-metallic luster are described in Table below .
Table 3.1
| Luster | Appearance |
| Adamantine | Sparkly |
| Earthy | Dull, clay-like |
| Pearly | Pearl-like |
| Resinous | Like resins, such as tree sap |
| Silky | Soft-looking with long fibers |
| Vitreous | Glassy |
Can you match the minerals in Figure below with the correct luster from Table above ?
Figure 3.18
(a) Diamond has an adamantine luster. (b) Quartz is not sparkly and has a vitreous, or glassy, luster. (b) Sulfur reflects less light than quartz, so it has a resinous luster.
Density describes how much matter is in a certain amount of space: density = mass/volume.
Mass is a measure of the amount of matter in an object. The amount of space an object takes up is described by its volume. The density of an object depends on its mass and its volume. For example, the water in a drinking glass has the same density as the water in the same volume of a swimming pool.
Gold has a density of about 19 g/cm 3 ; pyrite has a density of about 5 g/cm 3 - that’s another way to tell pyrite from gold. Quartz is even less dense than pyrite and has a density of 2.7 g/cm 3 .
The specific gravity of a substance compares its density to that of water. Substances that are more dense have higher specific gravity.
Hardness is a measure of whether a mineral will scratch or be scratched. Mohs Hardness Scale, shown in Table below , is a reference for mineral hardness.
Table 3.2
| Hardness | Mineral |
| 1 | Talc |
| 2 | Gypsum |
| 3 | Calcite |
| 4 | Fluorite |
| 5 | Apatite |
| 6 | Feldspar |
| 7 | Quartz |
| 8 | Topaz |
| 9 | Corundum |
| 10 | Diamond |
(Source: http://en.wikipedia.org/wiki/Mohs_scale , Adapted by: Rebecca Calhoun, License: Public Domain)
With a Mohs scale, anyone can test an unknown mineral for its hardness. Imagine you have an unknown mineral. You find that it can scratch fluorite or even apatite, but feldspar scratches it. You know then that the mineral’s hardness is between 5 and 6. Note that no other mineral can scratch diamond.
Breaking a mineral breaks its chemical bonds. Since some bonds are weaker than other bonds, each type of mineral is likely to break where the bonds between the atoms are weaker. For that reason, minerals break apart in characteristic ways.
Cleavage is the tendency of a mineral to break along certain planes to make smooth surfaces. Halite breaks between layers of sodium and chlorine to form cubes with smooth surfaces ( Figure below ).
Figure 3.19
A close-up view of sodium chloride in a water bubble aboard the International Space Station.
Mica has cleavage in one direction and forms sheets ( Figure below ).
Figure 3.20
Sheets of mica.
Minerals can cleave into polygons. Fluorite forms octahedrons ( Figure below ).
Figure 3.21
Fluorite has octahedral cleavage.
One reason gemstones are beautiful is that the cleavage planes make an attractive crystal shape with smooth faces.
Fracture is a break in a mineral that is not along a cleavage plane. Fracture is not always the same in the same mineral because fracture is not determined by the structure of the mineral.
Minerals may have characteristic fractures ( Figure below ). Metals usually fracture into jagged edges. If a mineral splinters like wood, it may be fibrous. Some minerals, such as quartz, form smooth curved surfaces when they fracture.
Figure 3.22
Chrysotile has splintery fracture.
Some minerals have other unique properties, some of which are listed in Table below . Can you name a unique property that would allow you to instantly identify a mineral that’s been described quite a bit in this chapter? (Hint: It is most likely found on your dinner table.)
Table 3.3
| Property | Description | Example of Mineral |
| Fluorescence | Mineral glows under ultraviolet light | Fluorite |
| Magnetism | Mineral is attracted to a magnet | Magnetite |
| Radioactivity | Mineral gives off radiation that can be measured with Geiger counter | Uraninite |
| Reactivity | Bubbles form when mineral is exposed to a weak acid | Calcite |
| Smell | Some minerals have a distinctive smell | Sulfur (smells like rotten eggs) |
| Taste | Some minerals taste salty | Halite |
(Adapted by: Rebecca Calhoun, License: CC-BY-SA)
A simple lesson on how to identify minerals is seen in this video: http://www.youtube.com/watch?v=JeFVwqBuYl4&feature=channel .
Minerals form under an enormous range of geologic conditions. There are probably more ways to form minerals than there are types of minerals themselves. Minerals can form from volcanic gases, sediment formation, oxidation, crystallization from magma, or deposition from a saline fluid, to list a few. Some of these methods of mineral formation will be discussed below.
A rock is a collection of minerals. Imagine a rock that becomes so hot it melts. Many minerals start out in liquids that are hot enough to melt rocks. Magma is melted rock inside Earth, a molten mixture of substances that can be hotter than 1,000 o C. Magma cools slowly inside Earth, which gives mineral crystals time to grow large enough to be seen clearly ( Figure below ).
Figure 3.23
Granite is rock that forms from slowly cooled magma, containing the minerals quartz (clear), plagioclase feldspar (shiny white), potassium feldspar (pink), and biotite (black).
When magma erupts onto Earth's surface, it is called lava . Lava cools much more rapidly than magma. Mineral crystals do not have time to form and are very small. The chemical composition will be the same as if the magma cooled slowly.
Existing rocks may be heated enough so that the molecules are released from their structure and can move around. The molecules may match up with different molecules to form new minerals as the rock cools. This occurs during metamorphism, which will be discussed in the chapter “Rocks.”
Water on Earth, such as the water in the oceans, contains chemical elements mixed into a solution. Various processes can cause these elements to combine to form solid mineral deposits.
When water evaporates, it leaves behind a solid precipitate of minerals, as shown in Figure below .
Figure 3.24
When the water in glass A evaporates, the dissolved mineral particles are left behind.
Water can only hold a certain amount of dissolved minerals and salts. When the amount is too great to stay dissolved in the water, the particles come together to form mineral solids, which sink. Halite easily precipitates out of water, as does calcite. Some lakes, such as Mono Lake in California ( Figure below ) or The Great Salt Lake in Utah, contain many mineral precipitates.
Figure 3.25
Tufa towers form when calcium-rich spring water at the bottom of Mono Lake bubbles up into the alkaline lake. The tufa towers appear when lake level drops.
Magma heats nearby underground water, which reacts with the rocks around it to pick up dissolved particles. As the water flows through open spaces in the rock and cools, it deposits solid minerals. The mineral deposits that form when a mineral fills cracks in rocks are called veins ( Figure below ).
Figure 3.26
Quartz veins formed in this rock.
When minerals are deposited in open spaces, large crystals form ( Figure below ).
Figure 3.27
Amethyst formed when large crystals grew in open spaces inside the rock. These special rocks are called geodes.
Some minerals are very useful. An ore is a rock that contains minerals with useful elements. Aluminum in bauxite ore ( Figure below ) is extracted from the ground and refined to be used in aluminum foil and many other products. The cost of creating a product from a mineral depends on how abundant the mineral is and how much the extraction and refining processes cost. Environmental damage from these processes is often not figured into a product’s cost. It is important to use mineral resources wisely.
Figure 3.28
Aluminum is made from the aluminum-bearing minerals in bauxite.
Geologic processes create and concentrate minerals that are valuable natural resources. Geologists study geological formations and then test the physical and chemical properties of soil and rocks to locate possible ores and determine their size and concentration.
A mineral deposit will only be mined if it is profitable. A concentration of minerals is only called an ore deposit if it is profitable to mine. There are many ways to mine ores.
Surface mining allows extraction of ores that are close to Earth’s surface. Overlying rock is blasted and the rock that contains the valuable minerals is placed in a truck and taken to a refinery. As pictured in Figure below , surface mining includes open-pit mining and mountaintop removal. Other methods of surface mining include strip mining, placer mining, and dredging. Strip mining is like open pit mining but with material removed along a strip.
Figure 3.29
These different forms of surface mining are methods of extracting ores close to Earth's surface.
Placers are valuable minerals found in stream gravels. California’s nickname, the Golden State, can be traced back to the discovery of placer deposits of gold in 1848. The gold weathered out of hard metamorphic rock in the western Sierra Nevada, which also contains deposits of copper, lead, zinc, silver, chromite, and other valuable minerals. The gold traveled down rivers and then settled in gravel deposits. Currently, California has active mines for gold and silver and for non-metal minerals such as sand and gravel, which are used for construction.
Underground mining is used to recover ores that are deeper into Earth’s surface. Miners blast and tunnel into rock to gain access to the ores. How underground mining is approached - from above, below, or sideways - depends on the placement of the ore body, its depth, concentration of ore, and the strength of the surrounding rock.
Underground mining is very expensive and dangerous. Fresh air and lights must also be brought into the tunnels for the miners, and accidents are far too common.
The ore’s journey to becoming a useable material is only just beginning when the ore leaves the mine ( Figure below ). Rocks are crushed so that the valuable minerals can be separated from the waste rock. Then the minerals are separated out of the ore. A few methods for extracting ore are:
Figure 3.30
The de Young Museum in San Francisco is covered in copper panels. Copper is mined and extracted from copper ores.
To extract the metal from the ore, the rock is melted at a temperature greater than 900 o C, which requires a lot of energy. Extracting metal from rock is so energy intensive that if you recycle just 40 aluminum cans, you will save the energy equivalent of one gallon of gasoline.
Although mining provides people with many needed resources, the environmental costs can be high. Surface mining clears the landscape of trees and soil, and nearby streams and lakes are inundated with sediment. Pollutants from the mined rock, such as heavy metals, enter the sediment and water system. Acids flow from some mine sites, changing the composition of nearby waterways ( Figure below ).
Figure 3.31
Acid drainage from a surface coal mine in Missouri.
U.S. law has changed so that in recent decades a mine region must be restored to its natural state, a process called reclamation . This is not true of older mines. Pits may be refilled or reshaped and vegetation planted. Pits may be allowed to fill with water and become lakes or may be turned into landfills. Underground mines may be sealed off or left open as homes for bats.
Some minerals are valuable because they are beautiful. Jade has been used for thousands of years in China. Diamonds sparkle on many engagement rings. Minerals like jade, turquoise, diamonds, and emeralds are gemstones. A gemstone , or gem, is a material that is cut and polished for jewelry. Many gemstones, including many in Figure below , are minerals.
Figure 3.32
Gemstones come in many colors.
Gemstones are usually rare and do not break or scratch easily. Most are cut along cleavage faces and then polished so that light bounces back off the cleavage planes ( Figure below ). Light does not pass through gemstones that are opaque, such as turquoise.
Figure 3.33
Uncut (left) and cut (right) ruby. The cut and polished ruby sparkles.
Gemstones are not just used in jewelry. Diamonds are used to cut and polish other materials, such as glass and metals, because they are so hard. The mineral corundum, of which ruby and sapphire are varieties, is used in products such as sandpaper.
Minerals are used in much less obvious places. The mineral gypsum is used for the sheetrock in homes. Window glass is made from sand, which is mostly quartz. Halite is mined for rock salt. Copper is used in electrical wiring, and bauxite is the source for the aluminum used in soda cans.
Opening image courtesy of Omphacite, http://en.wikipedia.org/wiki/File:PyroxeneExsol_0.5mmm.jpg , and is in the public domain.
