Minerals and CrystalsA mineral is a naturally occurring chemical compound,usually of crystalline form and abiogenic in origin. A mineral has one specific chemical composition, whereas a rock can be an aggregate of different minerals or mineraloids. The study of minerals is called mineralogy. Minerals are distinguished by various chemical and physical properties. Differences in chemical composition and crystal structure distinguish the various species, which were determined by the mineral’s geological environment when formed. Changes in the temperature, pressure, or bulk composition of a rock mass cause changes in its minerals.Minerals can be described by their various physical properties, which are related to their chemical structure and composition. Common distinguishing characteristics include crystal structure and habit, hardness, lustre, diaphaneity, color, streak, tenacity, cleavage, fracture, parting, and specific gravity. More specific tests for describing minerals include magnetism, taste or smell, radioactivity and reaction to acid.Minerals are classified by key chemical constituents; the two dominant systems are the Dana classification and the Strunz classification. The silicate class of minerals is subdivided into six subclasses by the degree of polymerization in the chemical structure. All silicate minerals have a base unit of a [SiO4]4− silica tetrahedron—that is, a silicon cation coordinated by four oxygen anions, which gives the shape of a tetrahedron. These tetrahedra can be polymerized to give the subclasses: orthosilicates (no polymerization, thus single tetrahedra), disilicates (two tetrahedra bonded together), cyclosilicates (rings of tetrahedra), inosilicates (chains of tetrahedra), phyllosilicates (sheets of tetrahedra), and tectosilicates (three-dimensional network of tetrahedra). Other important mineral groups include the native elements, sulfides, oxides, halides, carbonates, sulfates, and phosphates.
Minerals: A mineral, by definition, is any naturally occurring, inorganic substance, often additionally characterized by an exact crystal structure. Its chemical structure can be exact, or can vary within limits. Native elements that occur naturally are also considered minerals. All minerals belong to a chemical group, which represents their affiliation with certain elements or compounds. The classified chemical groups are known as: Elements, Sulfides, Oxides, Halides, Carbonates, Nitrates, Borates, Sulfates, Chromates, Phosphates, Arsenates, Vanadates, Tungstates, Molybdates, and Silicates. Some of these chemical groups have sub-categories, which may be categorized in some mineral references as separate groups.All minerals belong to one of the six crystal groups, classified according to the way the atoms of the mineral are arranged. Minerals also have distinctive properties, such as color, hardness, crystal habit, specific gravity, luster, fracture, and tenacity. Many of these properties can vary among a single mineral, within limits. Many minerals exhibit certain properties that others do not, such as fluorescence and radioactivity. Minerals are an economic commodity; they are mined because of the need for a valuable element they contain or an intrinsic property they may have. Other minerals are mined for their beauty and rareness, thus giving many specimens an accepted worldwide value. There are well over 3,000 scientifically classified different types of minerals, and new ones are always being discovered. The vast majority are not known to professional mineral collectors, because they are rare, have no economic purpose, and for the most part do not make good specimens.There is a class of substances known as “mineraloids”. While not truly falling into the category of minerals, they are still usually classified as minerals. Two well-known examples are Mercury, which lacks a crystal structure due to its liquid state, and Opal, which also lacks a crystal structure as well as a definitive chemical formula. Despite the fact that these mineraloids lack certain essential characteristics of minerals, they are nevertheless classified as minerals in most reference guides including the acclaimed Dana’s System of Mineralogy. Another unique category of minerals is the organic minerals. While this term is technically an oxymoron, since the definition of a mineral requires it to be inorganic, there are several naturally occurring rare organic substances with a definitive chemical formula. The best known example of this is Whewellite. Most reference guides and scientific sources make an exception to these substances and still classify them as minerals.
Born of fluid, heat, and pressure, minerals dazzle us with their breathtaking colors and shapes and astonish us with their usefulness. They are forged underground, where forces that have been at work for billions of years continue to make more minerals. A mineral is a naturally occurring, inorganic (nonliving) solid having a specific chemical composition.There are over 5,300 known mineral species; over 5,070 of these have been approved by the International Mineralogical Association (IMA). The silicate minerals compose over 90% of the Earth’s crust. The diversity and abundance of mineral species is controlled by the Earth’s chemistry. Silicon and oxygen constitute approximately 75% of the Earth’s crust, which translates directly into the predominance of silicate minerals.More than 4,000 naturally occurring minerals—inorganic solids that have a characteristic chemical composition and specific crystal structure—have been found on Earth. They are formed of simple molecules or individual elements arranged in repeating chains, sheets, or three-dimensional arrays.Minerals are typically formed when molten rock, or magma, cools, or by separating out of mineral-rich water, such as that in underground caverns. In general, mineral particles are small, having formed within confined areas such as lava flows or between grains of sediments. Large crystals found in geodes and other rocks are relatively rare.Rocks themselves are made of clusters or mixtures of minerals, and minerals and rocks affect landform development and form natural resources such as gold, tin, iron, marble, and granite.To be classified as a “true” mineral, a substance must be a solid and have a crystal structure. It must also be an inorganic, naturally-occurring, homogenous substance with a defined chemical composition. The chemical composition may vary between end members of a mineral system. For example the plagioclase feldspars comprise a continuous series from sodium-rich albite (NaAlSi3O8) to calcium-rich anorthite (CaAl2Si2O8) with four recognized intermediate compositions between. Mineral-like substances that don’t strictly meet the definition are sometimes classified as mineraloids. Other natural-occurring substances are Nonminerals. Industrial minerals is a commercial term and refers to commercially valuable mined materials.
A crystal structure is the orderly geometric spatial arrangement of atoms in the internal structure of a mineral. There are 14 basic lattice arrangements of atoms in three dimensions in the six crystal systems, and all crystal structures currently recognized fit in one of these 14 arrangements. This crystal structure is based on regular internal atomic or ionic arrangement that is often visible as the mineral form. Even when the mineral grains are too small to see or are irregularly shaped the crystal structure can be determined by x-ray analysis and/or optical microscopy.
Chemistry and crystal structure define together a mineral. In fact, two or more minerals may have the same chemical composition, but differ in crystal structure (these are known as polymorphs). For example, pyrite and marcasite are both iron sulfide. Similarly, some minerals have different chemical compositions, but the same crystal structure: for example, halite (made from sodium and chlorine), galena (made from lead and sulfur) and periclase (made from magnesium and oxygen) all share the same cubic crystal structure. Crystal structure greatly influences a mineral’s physical properties. For example, though diamond and graphite have the same composition (both are pure carbon), graphite is very soft, while diamond is the hardest of all known minerals.There are currently just over 4,000 known minerals, according to the International Mineralogical Association, which is responsible for the approval of and naming of new mineral species found in nature.
Silicates—including quartz, mica, olivine, and precious minerals such as emeralds—are the most common class of minerals, as well as the major components of most rocks. Oxides, sulfides, sulfates, carbonates, and halides are other major mineral classes. Minerals are natural compounds formed through geological processes. The term “mineral” encompasses not only the material’s chemical composition but also the mineral structures. Minerals range in composition from pure elements and simple salts to very complex silicates with thousands of known forms (organic compounds are usually excluded). The study of minerals is called Mineralogy.
When they look for minerals in their own environment, students may find a single mineral specimen such as marble, which is pure calcite. However, they will probably find rocks, which are mixtures of two or more minerals. Granite, for instance, with its tiny multi-colored grains, is made of quartz, feldspar, and mica.
Minerals represent just another complex structure in this greatly varied earth. There are endless amounts of studies that can be covered when dealing with the topic of minerals. Minerals embody numerous physical properties, which make them much more interesting and complex than commonly perceived. Several of these properties are essential in mineral identification. With enough experience, a mineral can often be accurately identified by simply viewing it. However, by conducting a few simple tests, the identification procedure is exact and unmistakable.
To be classified as a “true” mineral, a substance must be a solid and have a crystal structure. It must also be an inorganic, naturally-occurring, homogenous substance with a defined chemical composition. The chemical composition may vary between end members of a mineral system. For example the plagioclase feldspars comprise a continuous series from sodium-rich albite (NaAlSi3O8) to calcium-rich anorthite (CaAl2Si2O8) with four recognized intermediate compositions between. Mineral-like substances that don’t strictly meet the definition are sometimes classified as mineraloids. Other natural-occurring substances are Nonminerals. Industrial minerals is a commercial term and refers to commercially valuable mined materials.
A crystal structure is the orderly geometric spatial arrangement of atoms in the internal structure of a mineral. There are 14 basic lattice arrangements of atoms in three dimensions in the six crystal systems, and all crystal structures currently recognized fit in one of these 14 arrangements. This crystal structure is based on regular internal atomic or ionic arrangement that is often visible as the mineral form. Even when the mineral grains are too small to see or are irregularly shaped the crystal structure can be determined by x-ray analysis and/or optical microscopy.Chemistry and crystal structure define together a mineral. In fact, two or more minerals may have the same chemical composition, but differ in crystal structure (these are known as polymorphs). For example, pyrite and marcasite are both iron sulfide. Similarly, some minerals have different chemical compositions, but the same crystal structure: for example, halite (made from sodium and chlorine), galena (made from lead and sulfur) and periclase (made from magnesium and oxygen) all share the same cubic crystal structure.
Crystal structure greatly influences a mineral’s physical properties. For example, though diamond and graphite have the same composition (both are pure carbon), graphite is very soft, while diamond is the hardest of all known minerals.There are currently just over 4,000 known minerals, according to the International Mineralogical Association, which is responsible for the approval of and naming of new mineral species found in nature.
Scientists identify elemental makeup by conducting complex test with expensive equipment, such as x-ray and high powered electron microscopes. They use these techniques to identify minerals. For the most part, though, minerals can be identified by conducting a few simple, costless tests. Once a person becomes experienced in this field, he can usually identify a mineral by observing it and taking into account its specific features, such as color and crystal formation.
Physical properties commonly used are:
- Crystal structure and habit: See the above discussion of crystal structure. A mineral may show good crystal habit or form, or it may be massive, granular or compact with only microscopically visible crystals.
- Hardness: the physical hardness of a mineral is usually measured according to the Mohs scale of mineral hardness.
- Luster indicates the way a mineral’s surface interacts with light and can range from dull to glassy (vitreous).
- Color indicates the appearance of the mineral in reflected light or transmitted light for translucent minerals (i.e. what it looks like to the naked eye).
- Streak refers to the color of the powder a mineral leaves after rubbing it on an unglazed porcelain streak plate.
- Cleavage describes the way a mineral may come apart or cleave in different ways. In thin section, cleavage is visible as thin parallel lines across a mineral.
- Fracture describes how a mineral breaks when broken contrary to its natural cleavage planes.
- Specific gravity relates the mineral mass to the mass of an equal volume of water, namely the density of the material.
- Other properties: fluorescence (response to ultraviolet light), magnetism, radioactivity, tenacity (response to mechanical induced changes of shape or form), and reactivity to dilute acids.
Chemical properties of minerals
Minerals may be classified according to chemical composition. They are here categorized by anion group. The list below is in approximate order of their abundance in the Earth’s crust. The list follows the Dana classification system.
The largest group of minerals by far are the silicates, which are composed largely of silicon and oxygen, with the addition of ions such as aluminium, magnesium, iron, and calcium. Some important rock-forming silicates include the feldspars, quartz, olivines, pyroxenes, amphiboles, garnets, and micas.
The carbonate minerals consist of those minerals containing the anion (CO3)2- and include calcite and aragonite (both calcium carbonate), dolomite (magnesium/calcium carbonate) and siderite (iron carbonate). Carbonates are commonly deposited in marine settings when the shells of dead planktonic life settle and accumulate on the sea floor. Carbonates are also found in evaporitic settings (e.g. the Great Salt Lake, Utah) and also in karst regions, where the dissolution and reprecipitation of carbonates leads to the formation of caves, stalactites and stalagmites. The carbonate class also includes the nitrate and borate minerals.
Sulfates all contain the sulfate anion, in the form SO42-. Sulfates commonly form in evaporitic settings where highly saline waters slowly evaporate, allowing the formation of both sulfates and halides at the water-sediment interface. Sulfates also occur in hydrothermal vein systems as gangue minerals along with sulfide ore minerals. Another occurrence is as secondary oxidation products of original sulfide minerals. Common sulfates include anhydrite (calcium sulfate), celestite (strontium sulfate), barite (barium sulfate), and gypsum (hydrated calcium sulfate). The sulfate class also includes the chromate, molybdate, selenate, sulfite, tellurate, and tungstate minerals.
The halides are the group of minerals forming the natural salts and include fluorite (calcium fluoride), halite (sodium chloride), sylvite (potassium chloride), and sal ammoniac (ammonium chloride). Halides, like sulfates, are commonly found in evaporitic settings such as playa lakes and landlocked seas such as the Dead Sea and Great Salt Lake. The halide class includes the fluoride, chloride, and iodide minerals.
Oxides are extremely important in mining as they form many of the ores from which valuable metals can be extracted. They commonly occur as precipitates close to the Earth’s surface, oxidation products of other minerals in the near surface weathering zone, and as accessory minerals in igneous rocks of the crust and mantle. Common oxides include hematite (iron oxide), magnetite (iron oxide), chromite (chromium oxide), spinel (magnesium aluminium oxide – a common component of the mantle), rutile (titanium dioxide), and ice (hydrogen oxide). The oxide class includes the oxide and the hydroxide minerals.
Many sulfides are economically important as metal ores. Common sulfides include pyrite (iron sulfide – commonly known as fools’ gold), chalcopyrite (copper iron sulfide) and galena (lead sulfide). The sulfide class also includes the selenides, the tellurides, the arsenides, the antimonides, the bismuthinides, and the sulfosalts (sulfur and a second anion such as arsenic).
The phosphate mineral group actually includes any mineral with a tetrahedral unit AO4 where A can be phosphorus, antimony, arsenic or vanadium. By far the most common phosphate is apatite which is an important biological mineral found in teeth and bones of many animals. The phosphate class includes the phosphate, arsenate, vanadate, and antimonate minerals.
The Elemental group includes metals and intermetallic elements (gold, silver, copper), semi-metals and non-metals (antimony, bismuth, graphite, sulfur). This group also includes natural alloys, such as electrum (a natural alloy of gold and silver), phosphides, silicides, nitrides and carbides (which are usually only found naturally in a few rare meteorites).
Below is a list of all of these properties. Select a property to learn about it, and how it is used as an identification procedure.
Water Solubility and Taste
Crystals: The Form
Crystals are solids that form by a regular repeated pattern of molecules connecting together. In crystals, a collection of atoms, called the Unit Cell, is repeated in exactly the same arrangement over and over throughout the entire material.Crystals have:An orderly and symmetrical atomic structure and a definite chemical composition.A definite external geometrical shape bounded by plane faces.Physical (and optical) properties which vary with direction (except for the isometric minerals).Only if a mineral is allowed to grow without interference from other minerals will it form crystals. Minerals “grow,” or crystallize, from many types of solutions. They may precipitate from evaporating sea water or crystallize from magmas when lava cools. While growing, minerals may develop a distinct crystal form with smooth, flat planes called crystal faces. The geometric pattern of the crystal faces reflects the internal atomic arrangement of the crystal structure. This is one of the most important things about a mineral.
Most minerals occur naturally as crystals. Every crystal has an orderly, internal pattern of atoms, with a distinctive way of locking new atoms into that pattern to repeat it again and again. The shape of the resulting crystaL-such as a cube (like salt) or a six-sided form (like a snowflake)-mirrors the internal arrangement of the atoms. As crystals grow, differences in temperature and chemical composition cause fascinating variations. But students will rarely find in their backyard the perfectly shaped mineral crystals that they see in a museum. This is because in order to readily show their geometric form and flat surfaces, crystals need ideal growing conditions and room to grow. When many different crystals grow near each other, they mesh together to form a conglomerated mass. This is the case with most rocks, such as granite mentioned above, which is made up of many tiny mineral crystals. The museum-quality specimens shown in the images here grew in roomy environments that allowed the geometric shapes to form uninhibited.
The internal arrangement of atoms determines all the minerals’ chemical and physical properties, including color. Light interacts with different atoms to create different colors. Many minerals are colorless in their pure state; however, impurities of the atomic structure cause color. Quartz, for example, is normally colorless, but occurs in a range of colors from pink to brown to the deep purple of amethyst, depending on the number and type of impurities in its structure. In its colorless state, quartz resembles ice. In fact, the root for crystal comes from the Greek word krystallos-ice-because the ancient Greeks believed clear quartz was ice frozen so hard it could not melt.
Scientists typically describe crystals as “growing,” even though they are not alive. In subterranean gardens, they branch and bristle as trillions of atoms connect in regular three-dimensional patterns. Each crystal starts small and grows as more atoms are added. Many grow from water rich in dissolved minerals, but they also grow from melted rock and even vapor. Under the influence of different temperatures and pressures, atoms combine in an amazing array of crystal shapes. It is this variety and perfection of form and symmetry that has long drawn scientists to the study of minerals. Symmetry is a regular, repeated pattern of component parts. Symmetry is everywhere in nature-the paired wings of a butterfly, the whorls and petals in a sunflower, the pattern of a snowflake, the legs of a spider-and minerals are no exception. In crystals, these repeated patterns occur within the basic atomic structure and reflect the pattern of faces of the crystal. You often can see the characteristic symmetry of a mineral crystal with the naked eye, but if the crystal is tiny, then you may need to look at it with a magnifying glass or microscope (as will be demonstrated in Lesson Plan 2). Recognizing symmetrical patterns in crystals may be difficult at first, but experience helps: the more specimens you look at, the more symmetry-and crystals-you will recognize. However, some specimens do not have well-formed crystals and are difficult even for experts to classify.
Gems: The Chosen Few
Many minerals form beautiful crystals, but the most prized of all are gemstones. Uncut gems are often fairly ordinary looking. It’s only when they are cut and polished that they obtain the brilliance and luster that makes them so valued.Gems are mineral crystals that have been cut and polished. Jewelers, for example, may select certain crystals to cut into gems because of their extraordinary color, but select others such as diamonds for their hardness and flashes of color (known as “fire”). In general, any attractive, relatively flawless crystal can be cut into a gem. Although almost any of the four thousand kinds of minerals could be cut into a gemstone, in practice most gems used in jewelry today come from only about fifteen different minerals.Size, beauty, rarity, and durability are the basic criteria that determine a gem’s value, though a rich and interesting history can add to that worth. Because of their beauty and the skill and artistry necessary to cut gemstones from crystals, gems have always been expensive. For centuries, gems belonged almost exclusively to royalty. One factor in the value of a gem is its weight. Gem weight is measured in carats; one metric carat equals 0.2 grams (.007 ounces). The word carat comes from carob, a Mediterranean tree whose seed was for centuries the standard for weighing precious stones.Historically gems have been divided into precious and semiprecious classes. There are a number of semiprecious gems, many quite beautiful, but diamonds, rubies, sapphires, and emeralds continue to qualify as “precious.” (At one time, amethyst was also considered a precious gem, but large reserves were later found in Brazil, reducing its value.)
Diamonds, made of carbon atoms, are the hardest natural substance found on Earth. Formed under extremely high pressure hundreds of miles underground, they are found in very few locations around the world. Graphite is also made of carbon atoms, but with a different arrangement—explaining why diamond is the hardest mineral and graphite (used in pencil lead) is one of the softest.
Rubies are formed of a mineral called corundum, comprised of aluminum oxide. The red color is caused by traces of chromium. Corundum also forms sapphire in many colors, which generally come from trace mixtures of iron, titanium, and chromium.
Emeralds are formed of a mineral called beryl whose chemical formula is a complex mix of beryllium, aluminum, silicon, and oxygen. The color comes from additional traces of chromium and vanadium. Different trace elements can produce other colors, allowing beryl to form semiprecious stones such as aquamarine.
Minerals and gems are classified by their physical properties, including hardness, luster, color, density, and magnetism. They’re also identified by the ways in which they break, or the type of mark, or streak, that they leave when rubbed on a laboratory tool called a streak plate.
In the Smithsonian collection, the largest cut diamond is the Portuguese Diamond, weighing an astonishing 127.01 carats (just under one ounce)! The Oppenheimer Diamond at 253.7 carats is one of the largest uncut diamonds known to exist (since the Oppenheimer Diamond is not cut, it is not a gem). The most famous diamond of them all, the deep blue Hope Diamond (45.52 carats), is surrounded by sixteen white diamonds and suspended from a platinum chain bearing forty-six more diamonds. Although visitors may appreciate only its size and craftsmanship when they look through the display case, in fact they are looking back in history at a formation process that occurred more than a billion years ago.