Crystalline Structures

Topic: Rocks and Minerals

Made by Mr. Foodieman

Defining Minerals

BY ANNE E. EGGER, PH.D.

Terms you should know

• inorganic: not originating from a living organism; not a compound that contains hydrocarbons

• pigment: a substance that gives color

Introduction

Humans have always used materials from the earth selectively. Early human artists who painted on rock walls made their paints from red and yellow pigments present in soils, pigments we now know as the minerals hematite and ochre. Countries have fought wars and trade companies battled over deposits of table salt, also called halite, in the East Indies. Today, we build our houses out of drywall, made of gypsum; we make cement out of lime, a calcium oxide mineral; and we extract aluminum from the mineral bauxite to make aluminum foil and soda cans. Hematite, halite, gypsum, lime, and bauxite are all minerals, naturally formed materials that have a specific chemical composition and crystal structure.

Introduction (Continued)

Minerals are the building blocks of rocks, which can be composed of one or more minerals in varying amounts. Granite, for example, contains quartz, mica, feldspar, and other minerals. Marble, on the other hand, consists entirely of the mineral calcite. Although minerals combine to form rocks, they retain their characteristics, much like the ingredients in a salad. You can make a salad that contains a variety of vegetables, like lettuce, carrots, bell peppers, and sprouts, or you can make a salad that consists solely of lettuce. In either case, the individual components are still identifiable, the way minerals can be identified within a rock. Fortunately, most minerals form only under certain conditions, so by identifying the minerals present in a rock, scientists can start to understand how, where, and maybe even when that rock formed. The understanding of mineral formation also means that scientists can predict where to find economically important minerals like bauxite and gemstones like diamonds.

Early study of minerals

Initially, most miners knew little about how minerals formed, but a lot about extracting the materials they found valuable. Georgius Agricola, a German physician who was much more enthusiastic about mining than medicine, documented mining practices and mineral descriptions in his book De Re Metallica, published in 1556. The title translates as "On the Nature of Metals," but at that time the word "metal" was widely used to describe any material from the earth. Agricola describes every aspect of mining, from how to identify minerals to 16th-century techniques for crushing ore to the uses of minerals and the diseases that they could cause (see the Classics link under the Resources tab to see original woodcuts from De Re Metallica). Agricola's book remained a mining standard for nearly two hundred years and is considered the first major contribution to the science of mineralogy. Despite the comprehensive nature of the book, Agricola had little understanding of the fundamental composition of minerals – in other words, he had no way of knowing their chemical formulas. Though much thought had been devoted to the concept of atoms, the experiments that would allow scientists to define the nature of atoms, and thus the chemical composition of minerals, were more than 200 years away when Agricola began writing. Thus, early on, the science of mineralogy advanced based on describing the shape of minerals and their defining properties, like hardness, instead of their atomic structure.

Multiple Choice

Comprehension Checkpoint: The classic 1556 book De Re Metallica ("On the Nature of Metals") was about

• minerals.

• precious metals.

Defining a mineral

The word "mineral" means something very specific to Earth scientists. By definition, a mineral:

• Is naturally formed

• Is solid

• Is formed by inorganic processes

• Has a specific chemical composition

• Has a characteristic crystal structure

Defining a mineral (Continued)

Though each of these aspects of a mineral may seem simple, they have important implications when considered together.

• 1. Naturally formed: Minerals form through natural processes, including volcanic eruptions, precipitation of a solid out of a liquid, and weathering of pre-existing minerals. Today, scientists, engineers, and manufacturers synthesize many ceramics, plastics, and other substances with specific chemical composition and structure, but none of these synthetic substances is considered a true mineral.

• 2. Solid: Liquids and gases aren't considered minerals, in large part because their structure is constantly changing, which means they do not have a characteristic crystal structure. A true mineral must be solid.

Defining a mineral (Continued)

• 3. Formed by inorganic processes: Any material produced through organic activity – such as leaves, bones, peat, shell, or soft animal tissue – is not considered a mineral. Most fossils, although they were once living, have generally had their living tissues completely replaced by inorganic processes after burial; thus, they are considered to be composed of minerals as well.

Defining a mineral (Continued)

4. Specific chemical composition: Most minerals exist as chemical compounds whose compositions can be expressed using a chemical formula. The chemical formula of salt, or halite, is NaCl, meaning each molecule of salt consists of one sodium atom (Na) and one chlorine atom (Cl). Other common minerals have much more complicated formulas, such as muscovite (KAl2(AlSi3O10)(OH)2). A few minerals, such as graphite, consist of only one type of atom (carbon, in this case); therefore, the chemical formula for graphite is written simply as C. All minerals are defined by their chemical composition. If we tried to change the composition of muscovite by replacing aluminum with iron and magnesium, for instance, we would end up with a new and different mineral called biotite. On the other hand, many minerals do contain impurities, and these impurities can vary. Quartz, for example, has the chemical formula SiO2 and generally does not have any color in its pure form. The presence of a minute amount of titanium (Ti), however, causes the slight pinkish coloration present in rose quartz, as seen in Figure 1. The amount of titanium relative to the amount of silicon and oxygen is on the order of parts per million, however, so this is considered an impurity rather than a change in the chemical composition. In other words, rose quartz is still quartz. Similarly, the gemstone amethyst is a form of quartz that is colored pale to deep purple by the presence of the impurity iron (Fe). It was not until the 1900s, 350 years after Agricola's book, that scientists were able to determine the specific chemical composition of minerals. The invention of the mass spectrometer, ever more powerful microscopes, and the use of diffraction techniques allowed the kind of highly detailed analysis that caused the science of mineralogy to flourish.

Figure 1: An example of rose quartz, colored by trace amounts of titanium.

Defining a mineral (Continued)

5. Characteristic crystal structure: Nicolaus Steno, a Dutch contemporary of Isaac Newton, made an important contribution to mineralogy in 1669 when he noted that the angles between faces (or sides) of quartz crystals were constant, no matter how big the crystals were or where they had formed. Today, we know that Steno's Law of Interfacial Angles concerning the external appearance of crystals reflects a regular, internal arrangement of atoms. The angles are constant between faces on quartz crystals because every single quartz crystal is made of the same atoms: one atom of silicon for every two atoms of oxygen, written with the molecular formula SiO2. The chemical composition of a mineral is reflected internally in a regular, repeating arrangement of atoms, called the crystal structure of the mineral. The crystal structure of halite is shown in Figure 2. The internal structure (shown on the left) is reflected in a generally consistent external crystal form (shown on the right), as noted by Steno. The cubic shape of salt crystals very clearly reflects the right-angle bonds between the Na and Cl atoms in its atomic structure (see our Chemical Bonding module). Most importantly, this structure repeats itself. As the halite crystal is broken into smaller and smaller pieces, it retains its cubic structure. Take a look at a dash of table salt under a microscope and you will confirm that this is the case.

Figure 2a: A sodium chloride crystal.

Figure 2b: The cubic shape of salt crystals results from the regular arrangement of atoms forming the crystal.

Multiple Choice

Comprehension Checkpoint: Minerals can be found in nature or made in a laboratory.

• True

• False

The importance of crystal structure

The graphite-diamond mineral pair is an extreme example of the importance of crystal structure. These two very different minerals have the same chemical formula (C), but the crystal structure of the two minerals is very different. In graphite, carbon atoms are bonded together along a flat plane, as shown in Figure 3. These sheets of carbon are loosely held together by weak attractive forces. However, the attractive forces between sheets can be easily broken, allowing them to slide past one another. Thus graphite is a soft, slippery mineral that is often used as a lubricant in machines (see Figure 4). When graphite is rubbed against another material, such as a piece of paper, it leaves a trail of small sheets that have broken free, thus it is also used in pencils.

Figure 3a: The internal structure of graphite shows strong bonds within planes and weak forces between them.

Figure 3b: Graphite has a metallic sheen, is soft, and can be easily broken into thin sheets.

The importance of crystal structure (Continued)

In diamond, by comparison, every single carbon atom is bonded strongly to four surrounding carbon atoms in a 3-dimensional structure (see Figure 5). This structure results in one of the hardest natural substances on the planet (see Figure 6), a property that contributes to its value. The structure of each of these minerals is crucial to determining their physical properties.

Figure 4a: The internal structure of diamond shows equally strong bonds in all directions.

Figure 4b: An uncut diamond crystal is clear and is the hardest substance known.

The importance of crystal structure (Continued)

Chemical composition and crystal structure are the most important factors in determining the properties of a mineral, including shape, density, hardness, and color. Geologists use these properties to identify which minerals are present in rocks. Hardness and fracture characteristics can be easily determined in the field with a small magnifying lens and a hammer, allowing for rapid identification of the mineral. The internal atomic structure of graphite and diamond, shown in Figures 3 and 5, explains the properties of the two minerals.

Multiple Choice

Comprehension Checkpoint: Graphite and diamond have very different properties because their __________ is very different.

• chemical formula

• atomic structure

Why do we care?

By identifying the minerals present in a given rock, geologists can begin to understand the history of that rock. Some minerals form only when magma erupts out of a volcano and cools, others form only deep within Earth's crust under tremendous heat and pressure, and still, others form only at the surface through evaporation. The basalt that erupts out of the volcanoes in Hawaii, for example, contains olivine, a mineral that forms only within Earth's mantle at depths greater than 70 km. This tells us that the source of magma in the Hawaiian Islands is very deep. Sediment cores from the bottom of the Mediterranean Sea contain layers of gypsum and halite, two minerals that form only when water evaporates; this discovery led geologists to the conclusion that the Mediterranean Sea had dried up repeatedly in the past. Identifying minerals on other planets has also led to a greater understanding of our solar system. Hematite is a mineral that forms most commonly on Earth's surface in the presence of water. It is, essentially, rust, and it forms during weathering of iron-bearing minerals. The discovery of hematite "blueberries" on Mars was part of the evidence that led geologists to conclude that there once was liquid water on the planet (see the News and Events links under the Resources tab). The study of minerals began with mining, and we still use our knowledge of minerals to find important economic deposits. But our understanding of mineral composition and structure has become essential to many other areas of study as well. The environmental remediation of mines, the exploration of other planets and search for extraterrestrial life, and the study of the geologic history on our planet are all areas that require knowledge of minerals and their sources.

Did you know?

Did you know that identifying minerals is what led scientists to conclude that there was water on Mars? Understanding the specific conditions necessary for different minerals to form helps scientists understand the history of Earth and can even shed light on the search for extraterrestrial life.

Summary

The study of minerals provides a window into the history of Earth and other planets in our solar system. This first module in a three-part series describes the history of our understanding of minerals and then defines a mineral, focusing on chemical composition and structure.

Key Concepts

• Minerals have specific chemical compositions, with a characteristic chemical structure.

• Minerals are solids that are formed naturally through inorganic processes.

• Chemical composition and crystal structure determine a mineral's properties, including density, shape, hardness, and color.

• Because each mineral forms under specific conditions, examining minerals helps scientists understand the history of the earth and the other planets within our solar system.

Properties of Minerals

BY ANNE E. EGGER, PH.D.

Terms you should know

• luster: the way an object reflects light; sheen, gloss

• ore: rock from which a valuable product such as a metal can be extracted

• quartz: a common mineral made of silicon and oxygen

• rover: a vehicle used for exploring remote regions

Introduction

Geologists have recently determined that the minerals goethite and hematite exist in abundance on Mars, sure signs of the presence of water (see Figure 1 for a picture). None of those geologists have been to Mars, of course, but the unmanned rovers Spirit and Opportunity have. These rovers are equipped with three mass spectrometers, each of which is capable of determining the chemical composition of a solid with a high degree of accuracy. With such a precise chemical analysis in hand, geologists on Earth had no problem identifying the minerals.

Figure 1: The small spheres in this picture were dubbed “berries” by geologists who 1st saw them. They sit on the surface of Mars and were photographed by the Mars rover Opportunity. A mass spectrometer on the rover was able to determine the chemical content of the berries and geologists recognized the chemical formula for hematite (Fe2O3).

Introduction (Continued)

A mineral is defined in part by specific chemical composition. In theory, therefore, it is always easy to identify a mineral, if you can determine the chemical composition with a mass spectrometer like the Mars rovers. In reality, however, even if you are looking at rocks on Earth, determining the exact chemical composition of a substance involves significant time preparing the sample and sophisticated laboratory equipment (and often significant money). Luckily, it is usually unnecessary to go to such lengths, because there are much easier ways that require little more than a magnifying lens and a penknife.

Identifying minerals by physical properties

The most common minerals in Earth's crust can often be identified in the field using basic physical properties such as color, shape, and hardness. The context of a mineral is important, too – some minerals can form under the same conditions, so you are likely to find them in the same rock, while others form under very different conditions and will never occur in the same rock. For this reason, context (the other surrounding minerals and type of rock) can often be used to rule out minerals that have similar colors, for example. Although there are many thousands of named minerals, only a dozen or so are common in Earth's crust. Testing a few physical properties, therefore, means that you can identify about 90% of what you are likely to encounter in the field. Because the physical properties of a mineral are determined by its chemical composition and internal atomic structure, they can be used diagnostically, the way a runny nose and sore throat can be used to diagnose a cold. There are many physical properties of minerals that are testable with varying degrees of ease, including color, crystal form (or shape), hardness, luster (or shine), density, and cleavage or fracture (how the mineral breaks). In addition, many minerals have unique properties, such as radioactivity, fluorescence under black light, or reaction to acid. In most cases, it is necessary to observe a few properties to identify a mineral; to extend the medical analogy even further, a runny nose is a symptom of a cold virus, allergies, or a sinus infection among other things, so we have to use other symptoms to diagnose the problem – a headache, fever, watery eyes, and so on.

Multiple Choice

Comprehension Checkpoint: The context in which a mineral is found

• can be useful in identifying the mineral.

• has little to do with identifying the mineral.

Color

The most obvious property of a mineral, its color, is unfortunately also the least diagnostic. In the same way that a headache is a symptom of a whole host of problems from the flu to a head injury, many minerals share the same color. For example, several minerals are green in color – olivine, epidote, and actinolite, just to name a few. On the other extreme, one mineral can take on several different colors if there are impurities in the chemical composition, such as quartz, which can be clear, smoky, pink, purple, or yellow. Part of the reason that the color of minerals is not uniquely diagnostic is that several components of the crystal compositions and structure can produce color. The presence of some elements, such as iron, always results in a colored mineral, but iron can produce a wide variety of colors depending on its state of oxidation – black, red, or green, most commonly. Some minerals have color-producing elements in their crystal structure, like olivine (Fe2SiO4), while others incorporate them as impurities, like quartz (SiO2). All of this variability makes it difficult to solely use color to identify a mineral. However, in combination with other properties such as crystal form, color can help narrow the possibilities. As an example, hornblende, biotite, and muscovite are all very commonly found in rocks such as granite. Hornblende and biotite are both black, but they can be easily distinguished by their crystal form because biotite occurs in sheets, while hornblende forms stout prisms (Figure 2). Muscovite and biotite both form in sheets, but they are different colors – muscovite is colorless.

Figure 2: These three minerals can be distinguished using both color and form. Hornblende (left) and biotite (middle) share the same color, but are different forms; muscovite (right) and biotite share form but not color.

Multiple Choice

Comprehension Checkpoint: Color is one of the best ways to identify a mineral.

• true

• false

Crystal form

The external shape of a mineral crystal (or its crystal form) is determined largely by its internal atomic structure, which means that this property can be highly diagnostic. Specifically, the form of a crystal is defined by the angular relationships between crystal faces (recall Steno's Law of Interfacial Angles as discussed in our Minerals I module). Some minerals, like halite (NaCl, or salt) and pyrite (FeS) have a cubic form (see Figure 3, left); others like tourmaline (see Figure 3, middle) are prismatic. Some minerals, like azurite and malachite, which are both copper ores, don't form regular crystals and are amorphous (Figure 3).

Figure 3: Examples of different types of crystal forms. On the left, pyrite has a cubic form; tourmaline (middle) is prismatic; azurite and malachite (on the right) are often amorphous.

Crystal form (Continued)

Unfortunately, we don't always get to see the crystal form. We see perfect crystals only when they have had a chance to grow into a cavity, such as in a geode. When crystals grow in the context of cooling magma, however, they are competing for space with all of the other crystals that are trying to grow and they tend to fill in whatever space they can. The shape of the crystal can vary quite a bit depending on the amount of space available, but the angle between the crystal faces will always be the same.

Multiple Choice

Comprehension Checkpoint: Which is more helpful in identifying a mineral?

• The shape of its crystals.

• The angle between its crystal faces.

Hardness

The hardness of a mineral can be tested in several ways. Most commonly, minerals are compared to an object of known hardness using a scratch test – if a nail, for example, can scratch a crystal, then the nail is harder than that mineral. In the early 1800s, Friedrich Mohs, an Austrian mineralogist, developed a relative hardness scale based on the scratch test. He assigned integer numbers to each mineral, where 1 is the softest and 10 is the hardest. This scale is shown in Figure 4.

Figure 4: Mohs' scale of mineral hardness, where 1 is the softest and 10 is the hardest.

Hardness (Continued)

The scale isn’t linear (corundum is 4 times as hard as quartz), and other methods have now provided more rigorous measurements of hardness. Despite the lack of precision in the Mohs scale, it remains useful because it is simple, easy to remember, and easy to test. The steel of a pocketknife (a common tool for geologists to carry in the field) falls almost right in the middle, so it is easy to distinguish the upper half from the lower half. For example, quartz and calcite can look the same – both are colorless and translucent, and occur in a wide variety of rocks. But a simple scratch test can tell them apart; calcite will be scratched by a pocketknife or rock hammer and quartz will not. Gypsum can also look a lot like calcite but is so soft that it can be scratched by a fingernail. Variations in hardness make minerals useful for different purposes. The softness of calcite makes it a popular material for sculpture (marble is made up entirely of calcite), whereas the hardness of diamond means that it is used as an abrasive to polish rock.

Multiple Choice

Comprehension Checkpoint: The hardness of a mineral can be determined by attempting to scratch it with a knife.

• true

• false

Luster

The luster of a mineral is the way that it reflects light. This may seem like a difficult distinction to make, but picture the difference between the way light reflects off a glass window and the way it reflects off of a shiny chrome car bumper. A mineral that reflects light the way glass does has a vitreous (or glassy) luster; a mineral that reflects light like chrome has a metallic luster. There are a variety of additional possibilities for luster, including pearly, waxy, and resinous (see pictures in Figure 5). Minerals that are as brilliantly reflective as diamonds have an adamantine luster. With a little practice, luster is as easily recognized as color and can be quite distinctive, particularly for minerals that occur in multiple colors like quartz.

Figure 5: Examples of only a few of the different clusters that can be seen in minerals. Galena (left) has a metallic luster, amber (middle) is resinous, and quartz (right) is glassy.

Density

The density of minerals varies widely from about 1.01 g/cm3 to about 17.5 g/cm3. The density of water is 1 g/cm3, pure iron has a density of 7.6 g/cm3, pure gold, 17.65 g/cm3. Minerals, therefore, occupy the range of densities between water and pure gold. Measuring the density of a specific mineral requires time-consuming techniques, and most geologists have developed a more intuitive sense for what is "normal" density, what is unusually heavy for its size, and what is unusually light. By "hefting" a rock, experienced geologists can usually guess if the rock is made up of minerals that contain iron or lead, for example, because it feels heavier than an average rock of the same size (see our Density module for more information).

Cleavage and fracture

Most minerals contain inherent weaknesses within their atomic structures, a plane along which the bond strength is lower than the surrounding bonds. When hit with a hammer or otherwise broken, a mineral will tend to break along that plane of pre-existing weakness. This type of breakage is called cleavage, and the quality of the cleavage varies with the strength of the bonds. Biotite, for example, has layers of extremely weak hydrogen bonds that break very easily, thus biotite breaks along flat planes and is considered to have perfect cleavage (see Figure 6). Other minerals cleave along planar surfaces of varying roughness – these are considered to have good to poor cleavage.

Figure 6: Several conchoidal fractures are visible in the mineral samples above. Note the concave surface and the curved ribs.

Cleavage and fracture (Continued)

Some minerals don't have any planes of weakness in their atomic structure. These minerals don't have any cleavage, and instead, they fracture. Quartz fractures in a distinctive fashion, called conchoidal, which produces a concave surface with a series of arcuate ribs similar to the way that glass fractures (see Figure 6). For quartz, cleavage is a distinguishing property.

Multiple Choice

Comprehension Checkpoint: A mineral with perfect cleavage

• breaks along flat lines.

• doesn't break.

Mineral classification systems

Physical properties provided the main basis for the classification of minerals from the Middle Ages through the mid-1800s. Minerals were grouped according to characteristics such as hardness, so that diamond and corundum would be in the same class of minerals. As the ability to determine the chemical composition of minerals developed, so did a new classification system. Many scientists contributed to the discovery of mineral chemical formulas, but James Dwight Dana, a mineralogist at Yale University from 1850 to 1892 (see Biography link in the Resources section), developed a classification system for minerals based on chemical composition that has survived to the present day. He grouped minerals according to their anions, such as oxides (compounds with O2-), silicates (compounds with (SiO4)4-), and sulfates (compounds with (SO4)2-). A chemical classification system meant that minerals that were grouped theoretically also tended to appear with each other in rocks since they tended to develop under similar geochemical conditions.

Mineral classification systems (Continued)

Physical properties still provide the main means for the identification of minerals, however, though they are no longer used to group minerals (from the example above, corundum is an oxide while diamond is a pure element, so by Dana's system, they are in separate groups). A composition-based grouping highlights some common mineral associations that allow geologists to make educated guesses about which minerals are present in a rock, even with only a glance. By far, the most common minerals are silicates, which make up 90% of Earth's crust. Of the many hundreds of named silicate minerals, only about eight are common, one of which is quartz. The uncommon minerals are critical, however, as they include economically important ones such as galena, which is the primary ore for lead, and apatite, a phosphate mined for phosphoric acid that is added to fertilizers. The discovery of new ore deposits depends on the ability of geologists to identify what they see in the field and recognize unusual mineral occurrences that should be explored in more detail in the laboratory. A hand lens, a pocketknife, and a lot of practice still provide the easiest and cheapest methods of identifying minerals.

Did you know?

Did you know that even though thousands of minerals have been named, only about a dozen are common in the Earth’s crust? Sophisticated laboratory equipment exists for determining the exact chemical composition of minerals, yet sometimes the most essential tools in geology are a magnifying lens and a penknife. Using just these tools, scientists can identify about 90% of what they encounter in the field.

Summary

Minerals are classified based on their chemical composition, which is expressed in their physical properties. This module, the second in a series on minerals, describes the physical properties that are commonly used to identify minerals. These include color, crystal form, hardness, density, luster, and cleavage.

Key Concepts

• Properties that help geologists identify a mineral in a rock are color, hardness, luster, crystal forms, density, and cleavage.

• Crystal form, cleavage, and hardness are determined primarily by the crystal structure at the atomic level.

• Color and density are determined primarily by the chemical composition.

• Minerals are classified based on their chemical composition.

The Silicate Minerals

BY ANNE E. EGGER, PH.D.

Terms you should know

• cleavage: breakage in the crystal structure of certain minerals along planes where atomic bonds are weakest

• crust: the outermost layer of Earth; the surface layer of a planet

• tetrahedron: a figure with four triangular planes; a triangular pyramid

Introduction

The mineral quartz (SiO2) is found in all rock types and all parts of the world. It occurs as sand grains in sedimentary rocks, like crystals in both igneous and metamorphic rocks, and in veins that cut through all rock types, sometimes bearing gold or other precious metals. It is so common on Earth's surface that until the late 1700s it was referred to simply as "rock crystal." Today, quartz is what most people picture when they think of the word "crystal." Quartz falls into a group of minerals called the silicates, all of which contain the elements silicon and oxygen in some proportion. Silicates are by far the most common minerals in Earth's crust and mantle, making up 95% of the crust and 97% of the mantle by most estimates. Silicates have a wide variety of physical properties, even though they often have very similar chemical formulas.

Introduction (Continued)

At 1st glance, for example, the formulas for quartz (SiO2) and olivine ((Fe, Mg)2SiO4) appear fairly similar; these seemingly minor differences, however, reflect very different underlying crystal structures and, therefore, very different physical properties. Among other differences, quartz melts at about 600° C while olivine remains solid to temperatures of nearly twice that; quartz is generally clear and colorless, whereas olivine received its name from its olive green color. The variety and abundance of the silicate minerals are a result of the nature of the silicon atom, and even more specifically, the versatility and stability of silicon when it bonds with oxygen. Pure silicon was not isolated until 1822 when the Swedish chemist Jöns Jakob Berzelius (see the Biography link in our Resources section) finally succeeded in separating silicon from its most common compound, the silicate anion (SiO4)4-. This anion takes the shape of a tetrahedron, with a Si4+ ion at the center and four O2- ions at the corners (see Figure 1); thus, the molecular anion has a net charge of -4.

Figure 1: Three ways of drawing the silica tetrahedron: a) At left, a ball & stick model, showing the silicon cation in orange surrounded by 4 oxygen anions in blue; b) At the center, a space-filling model; c) At right, a geometric shorthand.

Introduction (Continued)

The Si-O bonds within this tetrahedral structure are partially ionic and partially covalent, and they are very strong. Silica tetrahedra bond with each other and with a variety of cations in many different ways to form the silicate minerals. Although there are many hundreds of silicate minerals, only about 25 are truly common. Therefore, by understanding how these silica tetrahedra form minerals, you will be able to name and identify 95% of the rocks you encounter on Earth's surface.

Seeing the structure of the silicates

Early mineralogists grouped minerals according to physical properties, which spread the silicates across many groups because they have very different properties. By the early 1800s, however, Berzelius had begun classifying minerals based on their chemical composition rather than on their physical properties, defining groups such as the oxides and sulfides – and, of course, the silicates. At the time, Berzelius was able to determine the absolute proportions of elements within a mineral, but he could not see the internal arrangement of the atoms of those elements in their crystalline structure. A detailed view of the internal arrangement of atoms within minerals would have to wait over 100 years for the development of X-ray diffraction (XRD) by Max von Laue, and its application to determine atomic distances by the father-son team of William Henry Bragg and William Lawrence Bragg a few years later (see their biographies in our Resources section).

Seeing the structure of the silicates (Continued)

In the process of XRD, X-rays are aimed at a crystal. Electrons in the atoms within the crystal interact with the X-rays and cause them to undergo diffraction. In the same way that light can be diffracted by a grate or card (see our Light I: Particle or Wave? module for more information on this topic), X-rays are diffracted by the crystal, and a 2-dimensional pattern of constructive and destructive interference bands results. This pattern can be used to determine the distance between atoms within the crystal structure according to Bragg's Law. The Braggs' work opened up a new world of mineralogy, and they were awarded a Nobel Prize in 1915 for their work determining the crystal structures of NaCl, ZnS, and diamond. XRD revealed that even minerals with similar chemical formulas could have very different crystal structures, strongly influencing those minerals' chemical and physical properties. As scientists created XRD images of the atomic structure of minerals, they were better able to understand the nature of the bonds between atoms in the silicate and other crystals. Within a silica tetrahedron, any single Si-O bond requires half of the available bonding electrons of the O2- ion, meaning that each O2- may bond with a second ion, including another Si4+ ion. The result of this is that the silica tetrahedra can polymerize, or form chain-like compounds, by sharing an oxygen atom with a neighboring silica tetrahedron.

Seeing the structure of the silicates (Continued)

The silicates are subdivided based on the shape and bonding pattern of these polymers because the shape influences the external crystal form, the hardness, and cleavage of the mineral, the melting temperature, and the resistance to weathering. These different atomic structures produce recognizable and consistent physical properties, so it is useful to understand the structures at an atomic level to identify and classify the silicate minerals. Identifying minerals in a rock may seem like an arcane exercise, but it is only by identifying minerals that we begin to understand the history of a given rock. The most common silicate minerals fall into four types of structures, described in more detail below: isolated tetrahedra, chains of silica tetrahedra, sheets of tetrahedra, and a framework of interconnected tetrahedra. The link below opens a page in a new window, which contains 3-dimensional versions of these different structures. You can manipulate and compare the structures as you read about them.

Multiple Choice

Comprehension Checkpoint: The silica tetrahedron is made up of

• a silicon atom surrounded by four oxygen atoms.

• an oxygen atom surrounded by four silicon atoms.

Isolated tetrahedra: Olivine

The simplest atomic structure involves individual silica anions and metal cations, usually iron (Fe) and magnesium (Mg), both of which exist most commonly as ions with a charge of +2. Therefore, it takes two atoms of Fe2+ or Mg2+ (or one of each) to balance the -4 charge of the silica anion. Olivine (see Figures 2a and 2b below) is the most common silicate of this type, and it makes up most of the mantle. Because these minerals contain a relatively high proportion of iron and magnesium, they tend to be both dense and dark-colored. Because the tetrahedra are not polymerized, there are no consistent planes of internal atomic weakness, so they also have no cleavage. Garnet is another common mineral with this structure.

Figure 2a: Depiction of a single silicate tetrahedron.

Figure 2b: A picture of olivine (the green crystals), an example of a silicate structure composed of isolated tetrahedrons, with a vein of basalt (the gray material).

Chains of tetrahedra: Pyroxenes and amphiboles

When silicate anions polymerize, they share an oxygen atom with a neighboring tetrahedron. Commonly, each tetrahedron will share two of its oxygen atoms, forming long chain structures. These chains still have a net negative charge, however, and the chains bond to metal cations like Fe2+, Mg2+, and Ca2+ to balance the negative charge. These metal cations commonly bond to multiple chains, forming bridges between the chains. Single-chain silicates include a common group called the pyroxenes, which are generally dark-colored (see Figures 3a and 3b). Because the bonds within the tetrahedra are strong, planes of atomic weakness do not cross the chains; instead, pyroxenes have two cleavage planes parallel to the chains and at nearly right angles to each other.

Figure 3a: A schematic diagram of the single-chain silicate structure. Where 2 tetrahedra touch, they share an oxygen ion.

Figure 3b: Pyroxene is one of the dominant minerals in this sample of gabbro. It is a dark mineral and can be hard to recognize.

Chains of tetrahedra: Pyroxenes and amphiboles (Continued)

Double chains form when every other tetrahedron in a single chain shares a third oxygen ion with an adjoining chain (see Figure 4a). Like single chains, the double chains still maintain a net negative charge and bond to cations that can form bridges between multiple double chains.

Figure 4a: A schematic diagram of the double chain silicate structure.

Chains of tetrahedra: Pyroxenes and amphiboles (Continued)

Double chain silicates, called amphiboles, host a wider variety of cations, including Fe2+, Mg2+, Ca2+, Al3+, and Na+, and have a wide variety of colors. The most common amphibole is hornblende, a black mineral found in igneous rocks like granite and andesite (see Figures 4b and 4c). Amphiboles tend to form prismatic crystals with 2 cleavage planes at 120 degrees to each other.

Figure 4b: Individual hornblende crystals where the characteristic cleavage can be seen.

Figure 4c: Hornblende is the dark mineral in this rock.

Chains of tetrahedra: Pyroxenes and amphiboles (Continued)

Pyroxenes and amphiboles can be difficult to distinguish from one another, as they are both dark-colored, blocky minerals. A careful examination of the angle between cleavage planes, described above, is required to identify them.

Multiple Choice

Comprehension Checkpoint: The best way to tell single-chain silicate minerals from double-chain silicate minerals is by examining their

• color.

• cleavage planes.

Sheets: Micas and clays

When every tetrahedron shares three of its oxygen ions with neighboring tetrahedra, sheets are formed (see Figure 5a). Micas such as muscovite and biotite (see Figure 5b) are both common sheet silicates, notable for their one perfect cleavage. This perfect cleavage results from the type of bonds that occur between sheets – van der Waals bonds. Because van der Waals bonds are weak, cleavage occurs between sheets, never across sheets. Clays are another very important sheet silicate that incorporates water into their atomic structure. The presence of water lubricates the sheets and is what makes clays easy to work within forming pottery; the firing process heats the minerals to the point where the water is driven off, resulting in a rigid, durable structure such as a pot.

Figure 5b: An example of biotite.

Figure 5c: An example of muscovite. (Both biotite and muscovite are micas, which are one kind of sheet silicate.)

Framework: Quartz and feldspar

When each tetrahedron shares all of its oxygen atoms with adjacent tetrahedra, a very strong 3-dimensional framework of Si-O bonds is formed (see Figure 6a). Quartz is pure SiO2; note that the charge is now exactly balanced and no other bonding ions are needed. In the feldspars, one or two out of every four Si4+ ions is replaced by an Al3+ ion, creating a charge imbalance that must be solved through the presence of additional cations: K+, Na+, and Ca2+. There are two kinds of feldspars upon which cations are incorporated into the structure. Feldspars that contain the K+ cation are called K-feldspars, or alkali feldspar, whereas those that contain Na+ and Ca2+ are called plagioclase feldspars (see Figure 6b). This separation occurs because K+ is a much larger cation than either Ca2+ or Na+, and its presence creates a slightly expanded framework structure.

Figure 6a: An example of the 3-dimensional structure formed by a framework silicate

Figure 6b: The white, blocky minerals in the rock on the left are plagioclase feldspar; the pink minerals in the rock on the right (granite) are K-feldspar.

Framework: Quartz and feldspar (Continued)

Like olivine, quartz also has no cleavage, because there is no natural weakness within that 3-dimensional framework. The feldspars, on the other hand, have two good cleavage planes at ~90 degrees to each other, due in part to the way that the aluminum ion changes the structure slightly, opening up planes of weakness. Quartz and feldspar are generally light-colored as well, making them easily distinguishable from darker minerals like olivine and pyroxene. Quartz and feldspar together make up the bulk of the rocks we see at the surface. Plagioclase feldspar is the single most common mineral in Earth's crust, making up an estimated 39% of both continental and oceanic crust. Quartz only makes up an estimated 12% of the entire crust, but it is by far the most common mineral we see on the surface because of its resistance to weathering. Familiarity with these few minerals – olivine, garnet, pyroxene, hornblende, muscovite, biotite, K-feldspar, plagioclase, and quartz – prepares you to identify and interpret the vast majority of rocks you'll see on Earth's surface.

Multiple Choice

Comprehension Checkpoint: Quartz has no cleavage because

• its framework structure has no natural weaknesses.

• it is identical in structure to feldspar.

Silicates as a natural resource

Though we generally think of coal or oil when discussing natural resources, silicate minerals are a natural resource we can't live without on our planet, and not just because of our increasing reliance on computers. Without quartz, there would be no glass. Without the clay minerals, we would have no ceramics or pottery. We use silicate minerals in the manufacture of many building materials, including bricks and concrete. The weathering of silicate minerals on the surface of Earth produces the soils in which we grow our foods and the sand on our beaches. The properties of the minerals that are important to us are based on the versatility of the silicate anion in combination with other elements.

Did you know?

Did you know that silicates like quartz and clay are among Earth’s most important natural resources? Imagine a world without glass, bricks, pottery, or computers – all of these rely on silicate minerals. These valuable materials make up 95% of the Earth’s crust.

Summary

Understanding the structure of silicate minerals makes it possible to identify 95% of the rocks on Earth. This module covers the structure of silicates, the most common minerals in the Earth's crust. The module explains the significance of the silica tetrahedron and describes the variety of shapes it takes. X-ray diffraction is discussed in understanding the atomic structure of minerals.

Key Concepts

• Silicate minerals are the most common of Earth's minerals and include quartz, feldspar, mica, amphibole, pyroxene, and olivine.

• Silica tetrahedra made up of silicon and oxygen, forms chains, sheets, and frameworks, and bonds with other cations to form silicate minerals.

• X-ray diffraction (XRD) allows scientists to determine the crystal structure of minerals.

• The physical properties of silicate minerals are determined largely by the crystal structure.

The quiz is on the next part.