# Planet Earth/6f. Mineral Identification of Hand Samples
## Nüll’s Mineral Collection

The regal elegance and luxury of Beethoven’s 7th Symphony in A major is triumph of music composed for an elite class of patrons in the city of Vienna, Austria. It was dedicated by Beethoven in 1812 to Moritz von Fries, a wealthy banker and patron of the arts. Fries’s father had established a highly successful bank, which financed its rise based on the coinage of silver. The bank minted a silver coin known as the Maria Theresa thaler. This silver coin was the official currency across German speaking counties beginning in 1741, but was adopted across the world, with the last coin minted up until 1962. It was especially used across North Africa and the Middle East, as the silver was relatively pure, and the image of the Empress Maria Theresa, ruler of Austria, Hungary and Bohemia was iconic. The ability to mint silver coinage, allow the success of Moritz von Fries to become one of the wealthiest men of his time. The music of Beethoven’s 7th Symphony embodies his importance, but Fries had to source the silver from mines, and for this he relied on a network of scientists who knew the secrets of extracting silver from rock in the mines of Saxony.
His most valuable and dedicated assistant was his chief accountant, a man named Jakob Friedrich van der Nüll. Nüll worked directly with the workers at the silver mines, and when a beautiful crystal or rock was found during ore extraction in the mine, he would ask to have it sent to him for his opulent mineral cabinet in Vienna. Over time the massive collection of minerals, crystals and rocks of Jakob Friedrich van der Nüll become well known. Nüll married Ignaz von Schwabs, the granddaughter of a famous jeweler and they lived in the Czartoryski Schlössel palace, where the mineral and crystal collection grew to over 5,000 items. Despite their beautiful diversity, the mineral and rocks in his collection were not organized in any systematic way. So he contacted Abraham Gottlob Werner, the lead professor of the Freiberg Mining Academy, asking if he knew of any students to help him organize his collection of rocks, minerals and crystals. Werner knew the man to help, a young student named Friedrich Mohs. Mohs had just joined a crew working in the mines in 1801 in Saxony, Germany, and gladly took the job in Austria to help organize and identify the rock and mineral collection of the wealthy Jakob Friedrich van der Nüll. It must had been amazing for him to come from a dirty sweaty mine, to the great opulence of Vienna in 1802. There was one major problem he faced when he arrived and that was there was no organized method of identifying rocks and minerals at this time in history.
His teacher, Abraham Gottlob Werner had classified rocks into Urgebirge (hard primitive rocks), Übergangsgebirge (transitional rocks like limestone), Flötz (bedded rocks or rocks with layers), and Aufgeschwemmte (poorly consolidated or loose rocks, like sand or gravel). Werner believed all rocks formed by water, which formed the Neptunists theory, while others believed rocks formed by fire (molten magma or lava) which was called the Plutonist theory. Today we know they form by both processes. Another prominent scientist of the time, Carl Linnaeus who devised a classification of animals and plants, also attempted to classify rocks and minerals. He had three divisions, Petrae (Lapides siplices), Minerae (Lapides compositi) and Fossilia (Lapides aggregati), in his classic book: Systema naturae. Rocks, Crystals (minerals) and Fossils he believed they grew like biological organisms in the ground. These outdated ideas and classifications were never adopted by later scientists. When Mohs arrived in Vienna he realized that the classification of rocks he learned about would not work in distinguishing the various crystals and minerals in the vast collection.
Minerals today have a very specific definition, they are naturally occurring inorganic solids with a definite chemical composition and ordered internal structure. In other words, a mineral can be described by a discrete chemical formula and a crystalline lattice structure unique to that chemical formula. Minerals are the building blocks of rocks. For Mohs this was difficult to determine without carrying out complex experiments on the minerals. For example, he would not be allowed to grind up the priceless mineral samples to see how they reacted to acids to determine what elements were in the rock. The number of known elements had only just increased from about 16 to 33 elements by 1801 when he was attempting this task. Fredrick Mohs knew that these minerals contained many of these newly discovered elements, but he would have to rely on his own keen observations and classification to apply names.

## Mohs scale of Hardness

One of those keen observations made by Mohs was that he could classify a mineral by its hardness. When you break or scratch a rock, you are breaking the chemical bonds that hold the solid together. Ionic bonds are weak, covalent bonds are strong and metallic bonds allow the solid to be malleable (ductile), like pure gold which can be easily shaped into jewelry. Hardness, and how easily a mineral is scratched can aid someone attempting to identify a mineral. By having a kit with known minerals, the level of hardness could be determined for unknown minerals. Here is Mohs hardness scale developed in his 1812 paper, and the chemical formula, with the elements that compose each mineral.
* Talc [Mg$_3$Si$_4$O$_{10}$(OH)$_2$]
* Gypsum [CaSO$_4$·2H$_2$O]
* Calcite [CaO$_3$]
* Fluorite [CaF$_2$]
* Apatite [Ca$_{10}$(PO$_4$)$_6$(OH,F,Cl)$_2$]
* Orthoclase [KAlSi$_3$O$_8$] (a type of feldspar)
* Quartz [SiO$_2$]
* Topaz [Al$_2$SiO$_4$(F,OH)$_2$]
* Corundum [Al$_2$O$_3$] (ruby gems)
* Diamond [C]
Each of these minerals could be used to scratch a mineral or crystal, and see if it leaves a scratch. Care must be done, in making sure that the test is done where it would not ruin the value of the specimen, also it was important to observe if the mineral only left a mark, like chalk and did not leave a permanent scratch. Other objects can have hardness levels that can be used to quickly test an unknown mineral. A fingernail has a hardness of 2.5, meaning that you can scratch both talc and gypsum with a fingernail. A copper penny has a hardness of 3, an iron nail has a hardness of 4, while glass has a hardness of 5.5. A knife has a hardness of around 6, while ceramic has a hardness of 6.5 to 7. Quartz is a very common mineral, which has a hardness of 7, but looks like many other transparent minerals that are softer like calcite. Modern geologists can use a pick set, with sharp needles made of material of various hardness that can be used to test unknown minerals quickly. Hardness is not the only method to identify minerals, specific density is also important.

## Specific density (also called specific gravity)

Density is the mass of an object divided by its volume. If you hold two objects of equal volume (size), the one with more density will feel heavier. Specific density is the density of an object compared to an equal volume of water, a specific density of less than 1 will mean the object will float in pure water. Nearly all minerals and rocks will not float in water (the rare exception is volcanic pumice, which has high porosity and can have a specific density as low as 0.5, and can float in water). To measure the specific density of an unknown mineral, the volume is determined by dropping the mineral sample in a graduated cylinder filled with water, and seeing how much the water level changes. The mineral sample is also weighed on a scale to determine mass. The specific density is determined by taking the mass and dividing by volume. 1 g/cm$^3$ (grams per cubic centimeter) equals the specific density of pure water. Minerals range in specific density. Gold has a specific density of 19.32 g/cm$^3$, making it very heavy relative to volume. Silver has a specific density of 10.49 g/cm$^3$. Valuable ores of these elements tend to have high specific densities. Magnetite, which is an iron oxide mineral, has a specific density of 5.17 g/cm$^3$. The most common mineral on the surface of Earth, quartz has a specific density of 2.65 g/cm$^3$. Halite (rock salt) has a specific density of only 2.16 g/cm$^3$. Hydrocarbons, that is rocks and minerals formed by carbon and hydrogen, typically have the lowest specific gravity among rocks and minerals, with coal having a specific gravity of only 1.29 g/cm$^3$. Specific density has a major influence on the abundance of minerals in the interior of the Earth. Victor Goldschmidt’s classification of lithophile, siderophile, chalcophile and atmophile elements follows from the unique density of each of these minerals, that contain these types of elements. Minerals with iron, gold, and nickel will have higher specific density, and hence be more common in the core of the Earth’s interior (siderophile elements). Specific density is a very important characteristic of minerals, and used to determine the value of gems, gold and silver jewelry, and ore deposits. It can be used to determine if you have a valuable diamond in your hand, or a piece of worthless shattered glass.

## Luster

Luster is the way that light-waves interact with a mineral’s surface. This interaction produces visible effects that enable the classification of minerals into various groups. One of the major divisions among minerals is whether they produce a metallic luster or not. Metallic minerals will appear to have the same or similar shine of polished metal or steel, and include galena, pyrite and magnetite among other minerals. This metallic luster is produced by the presence of metallic bonds within the chemical structure of the crystal. Sometimes metallic minerals will lose this metallic luster when they oxidize with oxygen in the air, making them duller in color. This oxidation is a form of tarnish, that forms over metallic bonds that easily oxidize. Pyrite disease is when pyrite tarnishes through oxidation processes, and this process worries collectors of minerals because it covers pyrite in white dull crystals. Another common luster is vitreous luster, a luster found in transparent or translucent minerals that appear like glass, and are see-through. Common minerals with vitreous luster include quartz, calcite, topaz, beryl, and fluorite. Many times, these minerals can take on a certain hue or color, but always remain fairly translucent. Very brilliant or sparkly minerals, such as diamonds and garnets are referred to as adamantine luster, as they have a high refractive index, and when cut can produce a sparkle that makes them attractive as gemstones. Both minerals with vitreous and adamantine lusters are common translucent gemstones. Non-metallic and non-translucent lusters can be described as greasy, pearly, silky, waxy, resinous, or dull. Often such classifications of luster are somewhat subjective, such that a mineral’s luster is described as simply either metallic or non-metallic.

## Color

Color maybe the most obvious way to classify minerals. Remember that as a light-wave strikes a surface of an atom, the energy can raise electrons in each atom to a higher energy state at discrete wave lengths (the release of this energy is what produces fluorescent minerals after subjected to UV-light and left in the dark, when the electron drops down its energy state). In normal light, the light that is absorbed by the surface of the mineral does not produce its color, instead it is the other visible light-waves which are reflected back or are scattered by the surface. These light-waves that are reflected back are the ones that give the surface of the mineral its color under normal light. If the surface absorbs all the visible light-waves then it would appear black. If the mineral absorbs red light (700-620 nm wavelengths), it would look green, if the mineral absorbs orange light (620-580 nm wavelengths) it would look blue, if the mineral absorbs yellow light (580-560 nm wavelengths) it would look violet, if it absorbs green light (560-490 nm wavelengths) it would look red, if it absorbs blue light (490-430 nm wavelengths), it would look orange, while if it absorbs violet light (430-380 nm wavelengths) it would look yellow. Note that this results in pairs of color which are complementary colors [red-green, orange-blue, and yellow-violet].
The elements that typically absorb visible-spectrum light are the transitional metals on the Periodic Table, such as iron, cobalt, nickel, vanadium, manganese, chromium, gold, titanium, and copper, as well as some rare earth elements. If these elements are present in a mineral they can change the mineral’s color. Some minerals are referred to as idiochromatic minerals, which means they will always have the same color, due to the presence of key elements in their chemical makeup. However, many minerals are allochromatic minerals, which means that they exhibit variable colors dependent on trace elements that are not typically part of the chemical makeup of the mineral. These trace elements can come from impurities, inclusions of other minerals, and more rarely in electron transfer between atoms or defects in the crystal lattice structure.
As a mineral, quartz is remarkable in the variety of colors it can exhibit in nature (an allochromatic mineral). These color variations frequently come from impurities or inclusions of other minerals. Amethyst (a type of quartz) is purple in color due to impurities of iron. Citrine (another type of quartz) is yellow in color due to impurities of Fe$^{3+}$ ions. Rose quartz is pink due to trace elements of titanium or manganese. Quartz can also be colored due to inclusions of other secondary minerals, such as jasper (a type of quartz, which has inclusions of hematite), or agate (a type of quartz with a wide variety of inclusions of other minerals such as calcite), these impurities and inclusions result in a wide variety of colors for a single type of mineral. Identification based on color in these allochromatic minerals is problematic.
However, idiochromatic minerals, which are minerals that only exhibit one color in nature, can be identified based on color. Examples of idiochromatic minerals include olivine (which is always green in color), garnet (which is always dark red to black in color), and orthoclase (potassium feldspar or K-spar) is typically pink in color.

## Streak

The streak of a mineral is the color the mineral displays when rubbed against an unglazed porcelain plate. This color comes from the powdered form of the mineral, which might be different than the color of the mineral in a hand sample. The streak is useful to determine different iron oxide minerals, such as magnetite, hematite, goethite, and limonite, which have different streak colors.

## Cleavage and Fracture

Minerals are formed from a lattice structure of individual atoms that are bonded together. Both cleavage and fracture describe how these bonds are broken when the mineral is subjected to stress. Some bonds will form sheet-like crystals which will split along parallel planes, while others will form a complex lattice-structure of equal bonding strength, and will break in complex fractures. Technically cleavage is when a mineral is subject to stress and splits or cleaves along a particular plane, and the mineral retains the same shape or surface as before. For example, a mineral with perfect cleavage will cleave without any rough surfaces, forming a smooth surface. Mica (both muscovite and biotite) is an example of a mineral with perfect cleavage as each sheet can be pulled apart, leaving a smooth surface. This is due to how the individual atoms are composed into individual layers. Some minerals lack cleavage and will break in jagged rough surfaces that are irregular. Some minerals can exhibit cleavage in-between the extreme of perfect cleavage and no cleavage, such as good cleavage where there is a smooth surface with residual roughness, to poor cleavage with a rough surface, but following along a specific plane or surface. Often cleavage is exhibited more in how the crystal lattice is formed than how it breaks, as perfect cleavage is found in minerals that cleave or split along these weak bonding surfaces. Fracture is how a mineral-breaks naturally, and describes the nature of this breakage, while cleavage is how the mineral cleaves or splits along planes. Fracture is what happens when you smash the mineral with a hammer, and the way it breaks apart. One characteristic type of fracture is conchoidal fracture which is found in quartz and other silica dominated minerals. Conchoidal fracture is when the mineral flakes into shards like broken glass, with a smooth bowl-shaped chip. Conchoidal fractures allow these minerals to be used as stone tools, such as arrowheads, spear tips and chisels, as these types of fractures produce sharp edges. Most archeological stone tools utilize silica minerals, such as quartz and quartz dominated rocks. Other descriptors of fracture include crumbly, splintery, jagged, uneven or smooth.

## Reaction to HCl 10% Acid

One last test, which is often used in the identification of mineral samples, is using a diluted acid (most often hydrochloric acid) to see if it reacts to a mineral. This is particularly important in determining calcite (and other carbonate minerals), a common mineral that reacts to the acid by producing CO$_2$ gas, which makes the mineral fizz or bubble when the acid is dripped on the specimen.

## The 40 Most Common Minerals

Currently there is nearly 5,000 types of minerals that have been named, however the vast majority of these minerals are rare. Remember, a mineral is any naturally occurring inorganic solid with a definite chemical composition and ordered internal structure. So, there can be a huge variation of naturally occurring minerals that occur on planet Earth. However, the vast majority of rocks that you pick up will include only the most common 40 minerals on planet Earth. Thus, rather than spend time listing the entire list of minerals, you can learn just the most common 40 minerals (or groups of minerals) that occur on Earth’s surface. By learning how to identify these 40 minerals you will have the ability to recognize them in the various rocks that naturally occur on planet Earth, and are used in the naming of rocks.
These 40 common minerals can be divided into the following groups based on their chemistry: Halides, Carbonates, Phosphates, Sulfates, Sulfides, Oxides, Hydroxides, Native Metals, and Silicates (nearly half of these common minerals belong to the silicates).

### Silicates

The most abundant minerals on Earth’s surface are the silicates, which as their name implies, include silica (SiO$_2$). This is especially the case with minerals found in continental crust. Only about 8% of the crust is composed of non-silicate minerals. Silicates can be subdivided into orthosilicates, ring silicates, sheet silicates, chain silicates, group silicates and framework silicates, plus just various forms of silica. These subdivisions are based on the chemical arrangement of silicon and oxygen within the crystalline lattice structure of each of these minerals. Silicon bonds with oxygen to form tetrahedral molecules, which link together to form the crystalline lattice structure of these minerals. These minerals often include common elements in Earth’s crust, including calcium (Ca), sodium (Na), potassium (K), aluminum (Al), magnesium (Mg), and iron (Fe).

#### Silica

Pure silica arranged in a tetrahedra (SiO$_4$) is the mineral quartz. Quartz is one of the most abundant minerals found on the surface of Earth’s continents. Quartz is common because it is highly stable at surface temperatures and pressures on Earth, and has a relative low melting temperature. Quartz is common in sedimentary rocks, because of its hardness and stability at the surface, but also found in many igneous and metamorphic rocks. Quartz is also used in making glass and ceramics, and one of the most important building materials. As an allochromatic mineral, quartz comes in many different colors and varieties, but always exhibits a vitreous luster, or glass-like quality. Quartz also exhibits a very characteristic conchoidal fracture pattern, which can be observed in edges that have broken or are split with a rock hammer. Because it is so common, quartz is likely present in most rocks that you collect on Earth’s surface, especially from the interior of continents.
Chalcedony is a very generalized term for varieties of silica which contain submicroscopic crystals or microcrystalline impurities that add a wide variety of colors and textures to silica. These color variations and chemical variations go by numerous other names as well, including agate, jasper, opal, chert, and flint, which all fall within this category of mineral. Often the silica contains impurities of other trace elements, which give it unique colors, and a waxy luster. Opal is hydrated silica (contains H$_2$O), which is often clear or more rarely iridescent. Chert is often used as the name of the rock composed of the mineral chalcedony. Jasper or flint is used for chalcedony that contains iron oxides and is a darker red color, while agate is a multicolor variety of chalcedony. Because chalcedony does not have a definite chemical composition, it is often debated if it is a true mineral, and sometimes called a mineraloid instead. It is common in sedimentary and igneous rocks, and often forms nodules, veins and layers, likely due to very low melting temperatures, especially in the presence of water, which causes it to flow into faults and cracks in the shallow interior of the Earth.

#### Framework Silicates (the feldspars)

Orthoclase is the most common mineral in Earth’s crust, and common in igneous rocks. It is one of the end members of the three feldspars, which are recognized depending on their chemistry. Orthoclase is often called Potassium Feldspar (or K-Spar) or Alkaline Feldspar as the mineral contains potassium (abbreviated K on the Periodic Table of Elements). Orthoclase is commonly found as a pink mineral within pegmatitic granites, but also in other igneous rocks. More rarely it is found in sedimentary rocks. This is because it is not as stable on the surface of the Earth compared to quartz (orthoclase will weather into kaolinite with the dissolution of potassium). When orthoclase is found in sedimentary rocks, like sandstone, the sandstone is referred to as arkose. Orthoclase exhibits a unique two directions of cleavage and a twinning pattern in the crystal lattice structure. This gives the mineral a unique texture under thin section, and the crystal will appear to have sparkling streaks within its pearly luster surface. Microcline is closely associated with orthoclase, which has the same chemical formula, but the crystal lattice has a slightly different angle. Microcline tends to exhibit white, green and blue colors, while another crystal form is sanidine which forms at high temperatures and tends to be white to gray. Orthoclase is considered a framework silicate, as it contains potassium, aluminum and silica.
Albite is another commonly occurring end member of the feldspar group of minerals, which contains sodium (Na). It is often grouped with anorthite (the calcium feldspar) in the more general feldspar mineral plagioclase. Albite is typically found as a white mineral, and shares many properties with orthoclase and anorthite, but contains mostly sodium rather than potassium or calcium. However, it tends to grade into anorthite, as the amount of sodium is replaced by calcium, and grade into orthoclase as the amount of sodium is replaced by potassium. As one of the end members of the feldspar group, albite is typically very similar to other feldspars, in having twinning crystal structure, but is mostly white in color.
Anorthite is the third end-member of the feldspar group. Anorthite contains calcium. Because it shares many of the characteristics with albite it is often difficult to distinguish from that mineral, and often identified with albite as plagioclase, as both are commonly a white colored feldspar mineral. Just like albite, anorthite exhibits a twinning crystal structure, and is white.
Quartz and the feldspars (including orthoclase, albite, and anorthite) contain lithophile elements based on Goldschmidt’s classification; oxygen, silicon, aluminum often bonded with potassium, calcium and sodium. All these elements are common in rocks found on the surface of the Earth. In fact, between 63% and 75% of all rocks found on the surface of the Earth will contain these minerals. In continental crust, these minerals are even more common, particularly quartz. This is not true of the deeper interior of the Earth, which exhibits very different minerals. However, because you interact with these shallow crustal rocks common in continental crust these minerals are likely encountered on a daily basis, and make up a large percentage of rocks in any rock collection.

#### Sheet Silicates

Biotite is often referred to as the black mica, to distinguish it from the silver colored mica called muscovite. Biotite is often found with feldspar and quartz in common igneous rocks like granite and diorite, but also found in many metamorphic rocks like schist and phyllite. As an isochromatic mineral, biotite is always a darkish black color, but under thin sections appears a translucent dark brown-green color. The crystalline lattice structure of biotite forms sheet like crystals, which cleave perfectly between each other into lamellar sheets. Biotite is the most common sheet silicate. The crystal structure is unique, in that two layers of aluminum silicate tetrahedrons are sandwiching a magnesium and iron inner layer, and each of these triple layered crystal structures are further separated between very weak layers of potassium, which easily dissolve and weakens when weathered near the surface. This promotes the layers to split along the potassium layer of weakness. The presence of magnesium and iron give biotite a dark black color. Biotite is common, accounting for about 5% of minerals in the Earth’s crust. It is rare in sedimentary rocks because it easily weathers.
Muscovite is the silver to clear mica that is fairly common in igneous granitic rocks. It differs from biotite in color, which is due to the fact that it lacks iron and magnesium, and instead has aluminum within its crystal lattice structure. Like biotite, muscovite is a sheet silicate and easily splits or cleaves into thin sheets, which are transparent. When found in rocks, muscovite often sparkles a silver color, like fish scales or sequins. Each crystal layer is held by weak bonds with potassium, which easily weathers. As such, muscovite is rarely found in sedimentary rocks.
Kaolinite is a very soft white mineral resembling clay. It is one of the most common clay minerals, but a member of the sheet silicates. Kaolinite forms from the weathering of feldspars, with the dissolution of potassium by ground and meteoric water. Kaolinite is very stable at surface temperatures and pressures, making it an end-member in the weathering of many other types of silicate minerals. Kaolinite is common in weathered igneous and sedimentary rocks. It looks very similar to chalk in appearance, and will leave a strong white streak on a ceramic plate. Kaolinite is often used in pottery, as it provides a clay-like texture. Kaolinite is very soft and can be scratched with a fingernail. Kaolinite is the only clay mineral on this list, although there are many other clay minerals many of which are commonly found in many soils and sedimentary rock layers, as tiny clay size particles.
Talc is a common ingredient in body powers (talcum powder), as it is a very soft white mineral and easily applied as a powder to the skin without causing irritation. A common sheet mineral, that can be scratched with a fingernail, and easily broken apart, talc is a natural ingredient in many household products. Talc contains magnesium, bonded to hydrated silica, and easily cleaves into sheets. It has a remarkably similar chemistry to chrysolite (Mg3Si2O5(OH)4), a common form of asbestos. Chrysolite differs from talc in being slightly harder (2.5-3.0 on Mohs scale), due to the stronger bonds within the crystal lattice structure, and forms tiny fibers of silica. Powders made from chrysolite (asbestos) can get into the lungs and cause damage to lung tissue if breathed in. Since talc and chrysolite are so similar, talc is commonly contaminated with harder asbestos minerals, like chrysolite. One of the major producers of talcum powder in the United States payed $4.7 billion dollars in a lawsuit in 2018, because of asbestos contamination in talcum powder. Talc is common in metamorphic rocks where it forms from magnesium-rich minerals subjected to water and carbon dioxide under intense pressure and temperature. Talc has a hardness on Fredrick Mohs scale of 1, making it the softest mineral on the list.

#### Chain Silicates

The pyroxene group of minerals is a group of about 22 dark colored minerals which are formed by single chains of silica tetrahedrons (Si$_2$O$_6$), which are interspersed by metal ions between each single chain. The ions of Ca, Na, Fe, Mg, Zn, Mn, Li are spaced in one layer, while the ions of Mg, Fe, Cr, Al, Co, Mn, Sc, Ti, Vn, in another giving a large variation in chemistry of the mineral group. As a group, all these minerals are a dark black to deep green color, and exhibit many similarities despite differences in their chemical formula. Augite (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)$_2$O$_6$ is the most common member of the pyroxene group of minerals, and the type of pyroxene described below.
Pyroxene minerals are very common in many igneous rocks, including basalt and gabbro, giving these types of rocks their black color. Most pyroxene minerals are a dark black color, with a resinous or glassy shine of green or even blue. In many ways the mineral group resembles feldspars, but are much darker in color. Jadeite, a common mineral for green jade, is a type of pyroxene, noted for its beautiful dark green color. Pyroxene is a common mineral found in magma and lava composed of oceanic crust, and common in mid-ocean ridges, and newly formed crust on the ocean floor.
Like pyroxene, the amphibole group is a complex group of chain silicate minerals which differ by having double chains of silica tetrahedrons (Si$_8$O$_{22}$) with aluminum and titanium substituting in for the silicon in the crystal lattice structure. This double chain of silica tetrahedrons form one layer, which are separated by layers of ions of a huge variety of elements, resulting in complex chemical formulas. Most of the minerals within the amphibole group can only be identified with XRD, ICP-MS or other tools. The most common variety of mineral in the amphibole group is hornblende. Hornblende contains calcium and sodium, with metal ions of magnesium, iron or aluminum (Ca,Na)2–3(Mg,Fe,Al)$_5$(Al,Si)$_8$O$_{22}$(OH,F)$_2$. Hornblende is the type of mineral described below:
Amphibole is very similar to pyroxene in terms of color and characteristics. The common amphibole mineral hornblende tends to be a shiny black whereas the common pyroxene mineral augite is a duller black color. Hornblende tends to form elongate rectangular crystals whereas augite crystals tend to be blocky. Nevertheless, distinguishing pyroxene from amphibole can be tricky. In igneous rocks, tiny crystals of amphibole minerals also resemble the black shiny mineral biotite. Amphibole minerals like hornblende tend to be the only shiny opaque black mineral, without mica-like cleavage that has a relatively low density (2.9-3.4 gm/cm3). Amphibole group minerals are common in igneous rocks like basalt, gabbro, and diorite. Typically found in oceanic crust, and within regions near mid-ocean ridges, or active volcanos. The mineral is rare in sedimentary rocks, because the chain silicates easily weather to minerals such as the iron bearing goethite, aluminum bearing gibbsite, and the clay mineral kaolinite.

#### Ring silicates

Tourmaline group minerals differ in having a ring or circle arrangement of the silica tetrahedrons (Si$_6$O$_{18}$), with these rings interspersed with other elements often including iron or magnesium. This results in a wide range of colors and semi-transparent appearance for many of these minerals, making tourmaline minerals often gem quality stones in jewelry, and highly collectable in mineral collections. Tourmaline is unique from other ring silicate mineral groups in having the element boron. One of the most beautiful of the tourmaline group is the mineral, elbaite, which is a multicolored gem quality mineral. The gem term emerald is often applied to ring silicate minerals, including some varieties of the mineral elbaite, but more often emeralds are applied to the brilliant green mineral beryl. The most common mineral in the tourmaline group is schörl, as described below.
Tourmaline group minerals tend to be rare, outside of the common black variety known as schörl. However, many of these varieties of minerals are highly sought after as gemstones. Given tourmaline’s hardness of 7.5 these colorful varieties can be cut into gemstones for jewelry. Many tourmaline minerals are pleochroic, meaning that they exhibit different colors depending on the angle they are viewed. Some specimens of tourmaline sell for thousands of dollars. Tourmaline is found in igneous and metamorphic rocks, often as small prismatic semi-transparent black crystals, which can be difficult to distinguish from opaque black colored amphibole and dark green pyroxene.
Beryl is a mineral that contains the element beryllium, but is also a ring silicate mineral like tourmaline. Beryl has a simple chemical formula, with aluminum and beryllium encircling each of the rings of silica tetrahedrons. This arrangement results in crystals that are hexagonal in shape, and with a hardness above 7 on Fredrick Mohs scale. Beryl is translucent but often exhibits various colors or tints. Green colored beryl is called emerald and blue beryl aquamarine, both popular gemstones used in jewelry, but yellow, pink, white and clear colors are known to occur as well. One way to identify beryl from much more common mineral quartz is that it lacks the classic conchoidal fracture pattern of quartz. Beryl also tends to exhibit striated vertical lines in the crystal and is more columnar in shape. Beryl is a rare mineral, but considered a valuable gemstone when found. Beryl is found in plutonic granitic pegmatites (igneous rock) and some metamorphic rocks like schists.

#### Orthosilicates

Orthosilicates are defined as the group of silicate minerals where the silica tetrahedrons do not share oxygen atoms, and hence are isolated between other elements. They are sometimes called island silicates. These minerals are much more stable at high pressure and temperatures, and easily weather when near the surface since the ions that separate the individual silica tetrahedrons can dissolve over time in ground water. As such orthosilicates are much more common in the deeper mantle, within deeply buried rocks subjected to high temperatures and pressures. These minerals are found in the upper mantle derived rocks. Many of these minerals are rarely seen at the surface, typical, but likely make up a majority of the Earth’s lower crust and mantle.
Olivine contains a combination of magnesium and iron that surrounds silica tetrahedrons (SiO4), with iron more common in the deeper occurring minerals of the mantle. Olivine is actually a group of minerals, which can also contain ions of calcium and manganese, as well. Olivine is also a gemstone known as peridot, which has a beautiful green-olive color. Olivine is most common in igneous rocks, particularly basalts from oceanic crust, such as within mid-ocean ridges and common in newly formed crust in places where the mantle has risen, such as island arc volcanoes and hot spots (like the Hawaiian volcanoes). Olivine is the most common mineral group within the mantle and deeper crust (as the Fayalite-Forsterite series). Since the mantle is thick, and composes more volume than the thinner crustal rocks, olivine is the most common solid that make up the interior of the Earth. In the deep mantle the high-pressure form of olivine (called bridgmanite) is regarded as the most common mineral within the Earth, but one that is not present at Earth’s surface. Olivine is remarkably rare in continental crust, which is dominated by quartz and feldspars. However, olivine is very common in the deep mantle rocks that lay deep under the continents. It is also common within young igneous rocks of the ocean floor, as well as in lunar rocks brought back from the moon.
The garnet group of minerals are typically a dark red color, and composed of a complex crystalline lattice structure. The mineral pyrope contains mostly magnesium, while almandine contains mostly iron, other minerals in the group contain calcium or manganese. The crystal structure of garnet shows a complex framework of octahedra and tetrahedra silica. In the cubic structures, oxygen atoms are bonded to one silica tetrahedron and one silica octahedron and to two of the divalent dodecahedral sites, linking them together in a complex fashion, like a puzzle. Garnet is a fascinating mineral because it forms at great-depths in the crust and upper mantle, and these silica tetrahedrons and octahedrons are compacted tightly together in a complex crystalline structure, which results in garnet crystals having a characteristic rhombic dodecahedron shape (like 12-sided dice). Garnet is typically found in kimberlite pipes and volcanic stalks, where magma was quickly brought to the surface, as well as in many metamorphic rocks that have been subjected to intense heat and pressure, such as schist. Garnets can be fashioned into gem quality stones, and are most often a dark red in color.
Topaz is a translucent mineral which Fredrick Mohs defined as a hardness of 8. It is a silicate mineral with aluminum and fluorine. In its pure form, it is glass-like clear, but often has slight tints of color, including yellow, blue, red and green. With a hardness of 8, it is frequently fashioned into gemstones. Topaz is a fairly common gemstone in igneous rocks, particularly pegmatitic granites (slowly cooling silica-rich magma), and found in the Great Basin of Utah. In fact, topaz is Utah’s state gemstone. Topaz is basically silica surrounded by aluminum atoms, with fluorine and hydroxide (OH) anions, making it have a glass-like quality.
The mineral zircon is fairly rare, as it is composed of zirconium surrounded by silica tetrahedrals (SiO4). Zirconium is a lithophile element, but fairly rare when compared to aluminum, magnesium and calcium. Zircon crystals tend to be very small, but are fairly stable at the surface of Earth’s crust (unlike other orthosilicates). Zircon is a typical accessory mineral in many igneous rocks, but because of its stability will often be preserved in sedimentary and metamorphic rocks as well. Crystals of zircons are some of the oldest solids on Earth (besides meteorite material), since they are highly stable, both at the surface as well as deep within the Earth’s crust. Zircons are important because they can be easily dated using radioactive isotopes of uranium, which decay to lead. Zircons which has been transported and deposited in sedimentary rocks are called detrital zircons and are useful for determining how igneous rocks erode, and sediment is transported and deposited on the surface of the Earth, because each grain can be dated and traced back to its origin. Zircon has a relatively high specific density. Zircon minerals in hand size specimens are fairly rare, and found in igneous rocks, and tend to be reddish brown in color. Group Silicates
Epidote is a silicate mineral in which the some of the silica tetrahedrals are united with a shared oxygen forming a molecule of Si2O7, in which a silicon atom is surrounded by 4 oxygens, with one of the oxygen atoms shared with another silicon atom. This forms the unique double silicon molecule of Si2O7. Epidote is often dark green, with fibrous or prismatic crystals. It is found in many metamorphic rocks, such as schist and hydrothermal igneous rocks. Epidote also occurs in marble, which is metamorphized limestone, giving the rock a light green hue.
Kyanite is a fibrous blue color mineral that is composed of aluminum bonded to chains of silica, which form elongated and columnar crystals. It is a common mineral in metamorphic rocks, which is typical blue to white in color. Kyanite is important to geologists who study metamorphic rocks because it forms under high pressures and lower temperatures. The amount of kyanite, compared to andalusite and sillimanite (two other minerals composed of Al₂SiO₅) can be used to determine the history of pressure and temperature the rock was subjected to in the subsurface. Kyanite can also be found in some sedimentary rocks, but tends to weather easily.
Staurolite exhibits very characteristic twinned cross-shaped crystals which are found only in metamorphic rocks. Staurolite is a fairly rare rock found in particular pressure-temperature zones within metamorphic rocks, such as schist and gneiss. With its unique cross-like shape and brown color, staurolite is easily to identify, even when it occurs as small crystals in rocks.

### Oxides

Oxides are minerals that contain oxygen, the oxide anion (O$^{2−}$) bonded to another element. Since silicon, sulfur, phosphorus and carbon bond to oxygen they technically are oxides, however, in mineralogy are grouped into separate mineral groups (silicates, sulfates, phosphates and carbonates), so that mineral oxides are those minerals that contain oxygen, but lack those common elements. Instead, the mineral group of oxides typically are formed by oxygen bonded to aluminum, iron, magnesium, and other cations. In fact, ice (H$_2$O) is a mineral that would fall within this classification, as it contains oxygen bonded to hydrogen. Oxides lack silicon, which is present in all silicate minerals. Nevertheless, many oxides are very common minerals, and are found in metamorphic, igneous and sedimentary rocks. They also include important ores of iron, copper and uranium.
Fredrick Mohs defined his 9th hardness level based on corundum. Corundum is a very hard mineral that is more common than the hardest mineral— diamond, but is also known for its brightly colored gemstones. Ruby and sapphires are gemstone terms for corundum, both very hard, but transparently colored gemstones. Ruby is reddish colored gemstones of corundum, while sapphire is blue colored gemstones of corundum. Most corundum is actually a dull greenish-purple-gray color, that is fairly opaque. These color varieties come from the impurities in the aluminum oxide crystalline lattice. Rather than containing silicon, corundum contains aluminum atoms surrounded by oxygen atoms, in a densely packed crystal lattice structure. The density is greater than most transparent minerals, with a specific density of around 4 gm/cm$^3$, and would feel heavier than glass or quartz of equal volume. For corundum to form in nature the rock must contain very low amounts of silica. Often this is either in metamorphic rock that lacks silica, like marble, or in ultramafic silica-poor igneous rocks. With a hardness of 9, corundum is also found as small detrital grains in some sedimentary rocks, like sandstone. Corundum is a fairly rare mineral, but important because of its hardness, and often included in Mohs scale kits for mineral identification. Synthetic aluminum oxides, similar to corundum are being used to develop bullet-proof glass, because they are transparent, but very hard to break.
Spinel is an aluminum oxide, but contains magnesium, giving the crystal a darker color that is often black, but can be a deep purple color especially when there are impurities of iron in the crystal lattice structure. Spinel is often found in the same places as corundum, within metamorphic rocks, but also ultramafic, silica poor igneous rock. Spinel is likely much more common in the lower mantle, which is depleted in silica, with oxygen bonded to magnesium, aluminum and iron. It is a common mineral within peridotite igneous rock found in the mantle and deeply rising volcanic rocks, like kimberlite pipes. It is sometimes cut into gemstones.
Magnetite is a very heavy mineral with a density over 5 gm/cm$^3$. It is also magnetic, since it contains a large percentage of iron atoms, with a ratio of 3 iron atoms to every 4 oxygen atoms (43% iron). Because it is both very heavy and magnetic the mineral is easy to identify. Magnetite is a common mineral that can be found in igneous, metamorphic and sedimentary rocks. Gold prospectors refer to magnetite grains of loose sand, as black sand, which is heavy grains of magnetite with high density and found while gold panning. Often these black sands are removed from the gold pan by use of a magnet. Magnetite can also be found in dirt and soil, by dragging a magnet through the loose dirt, which attracts the mineral. Both iron and oxygen are fairly common in the Earth’s interior and lower mantle. Much of the occurrence of large specimens of magnetite are found in silica poor igneous and metamorphic rocks. Magnetite is an important ore of iron, and most abundant on Earth’s surface within the oldest metamorphic rocks, like those found in the rust-belt of the states of Michigan and Wisconsin. Magnetite likely was more common on Earth’s surface during its formation, but because iron is a siderophile element, it has sunk deeper into Earth’s mantle through the long process of the rock cycle.
Hematite means blood stone, because hematite gives a dark red streak when scratched across a white porcelain scratch plate, unlike the similar mineral magnetite, which leaves a black color. This red color is due to the fact that hematite has a higher ratio of oxygen. For every 2 atoms of iron, hematite has 3 atoms of oxygen (40% iron). This is still a large amount of iron, and hematite can be magnetic because of it, and still has a very high level of density, over 5 gm/cm$^3$. Hematite is more common in sedimentary rocks than magnetite, largely because it is deposited by iron-reducing microbes in the subsurface. Hematite can also form cement in sedimentary rocks, gluing grains or clasts together. Hematite is also common in iron banded formations, ancient sedimentary rocks deposited when Earth lacked significant oxygen in the oceans. It is also a common mineral in hydrothermal deposits from the oxidation (rusting) of iron minerals, at hot temperatures in the presence of water.
Goethite is a hydroxide mineral with iron bonded to oxygen and hydroxide (OH$^-$), limonite is hydrated with a molecule of water (H$_2$O). Both these minerals are actually a form of iron rust, and tend to be found in well oxygenated iron-rich sedimentary rocks, and weathering of other iron oxide minerals. Limonite is a more yellowish color, and a source for earth tones in painting (yellow ochre), while hematite is a brighter earthy red color (Indian red). This can be revealed with a streak test on white ceramic porcelain. Goethite tends to form these globular black crystals, but will produce a light color when applied to a scratch plate. Goethite is found in hydrothermal deposits as well as sedimentary rocks, as some iron-reducing bacteria produce this mineral in the subsurface. Goethite and limonite are the minerals that give many sedimentary rocks their red color, including many of the sandstones around Moab, Utah, and throughout Utah’s Red Rock Canyons. Goethite and limonite tend to form in well oxygenated soils, which cycle through wet and dry seasons, these often form in red sedimentary rocks over time.
Many of these iron oxides (Hematite, Goethite and Limonite) are more common on Mars, because without plate tectonics, iron was not drawn down into the Martian mantle, resulting in the distinct reddish color of the rocks and regolith found on the planet’s surface today.

### Sulfides

Sulfides are minerals that have sulfur, but lack oxygen. They are rare, but extremely important, as they indicate regions that were anoxic (lacking oxygen) when these minerals formed. Sulfur is a chalcophile element in Goldschimdt’s classification, as such it tends to be found in many ores associated with a group of elements, that includes gold, mercury, copper, silver, tin, zinc, and lead, among others. Hence sulfides are often associated with these types of mines. Since these minerals are often found in gold mines, they are given colorful names by prospectors, such as fool’s gold, and peacock ore.
Chalcopyrite is copper, iron and sulfur, and often referred to as peacock ore, because of the iridescent metallic colors it often exhibits. Chalcopyrite is an important ore of copper, but fairly rare on Earth. When the ore is cooked at very high temperatures with silica (sand grains), bronze can be extracted, which is a copper alloy. The Bronze Age (5,300 to 3,200 years ago) was a period of time when humans first learned how to extract copper and make bronze metal tools and jewelry from chalcopyrite. This could be done in very hot furnaces, but at lower temperatures than many iron alloys used today. Chalcopyrite was an important trade ore, for its richness of copper. Chalcopyrite is found in old igneous rocks, particularly in regions influenced by hydrothermal activity. Chalcopyrite is also found in Archean metamorphized igneous regions, called Greenstone Belts that are some of the most ancient sources of continental crust.
Pyrite is also known as fool’s gold, since it exhibits a brassy gold color, but lacks the high density of true gold, and brighter gold color. Pyrite is iron bonded to sulfur. Pyrite is abundant in hydrothermal deposits, and often found in gold mines in metamorphic and igneous rock. Pyrite is also found in marine sedimentary rocks, deposited in deep anoxic (lacking oxygen) ocean water. Pyrite tarnishes a golden white color in the presence of oxygen and moisture. This is known as pyrite disease, as it can ruin mineral specimens in collections. Pyrite is a fairly common sulfide mineral on Earth’s surface, and occurs in veins within hydrothermal igneous rocks. Pyrite weathering from mine tailings results in sulfate ions (SO$_4$$^{2-}$), which can form sulfuric acid (H$_2$SO$_4$) in aqueous solutions with water. Many old mines that are rich in pyrite result in highly polluted acidic water within the watershed that can kill fish and other organisms downstream from the mine.
Galena is a lead ore, which contains both sulfur and lead (Pb). It is also an important silver ore, since silver (Ag) can bond with sulfur to form acanthite/argentite (Ag$_2$S), which is found often with galena in hydrothermal veins or pockets in mines. Galena has historically been mined for lead, which can be easily smelted from the ore by heating it in a furnace. Galena is fairly common in hydrothermal igneous rocks, or hydrothermally altered sedimentary rocks, like limestone. In Leadville, Colorado, galena is found in pore spaces, faults, and veins where hydrothermal waters flowed through the sedimentary rock, leaving rich veins of galena and other sulfide minerals. Galena has high density of just over 7 gm/cm$^3$. This makes hand samples of the mineral very heavy when compared to other minerals. It also has a classic silver metallic luster and cubic crystal habit. After handling galena, it is good to wash your hands, since the mineral contains lead, which is toxic if ingested.

### Sulfates

Sulfate minerals all contain the sulfate ion SO$_4$$^{2−}$ within their crystal lattice structure. These minerals are typically found in evaporitic sedimentary rocks, where these sulfate ions bond with cations to form sulfate salts. They can also form in hydrothermal deposits, within oxidizing zones in the presence of sulfides, or from the weathering of sulfide minerals near the surface. Sulfates are more common on the surface of the Earth than sulfides, due to their abundance in sedimentary basins, particularly dry lake and ocean basins.
Gypsum is a common mineral in sedimentary rocks deposited in dry lake and ocean basins. Gypsum is an important building material for drywall, as it is fire-retardant and is nontoxic. As a soft mineral, with a hardness of 2 on Fredrick Mohs scale, gypsum can be carved and shaped into stone carvings, which is often called alabaster. Clear prismatic crystals of gypsum are often found in deserts eroding from sedimentary rocks. These crystals are called selenite “moonstones” and desert roses. Gypsum is formed from calcium cations ionically bonding to sulfate anions, with a hydrous (H$_2$O) component. As a result, gypsum easily dissolves, and is used as plaster and chalk, and grounded up for many uses. Gypsum is common in Utah, particularly in the Great Basin and Uinta Basin, where ancient lakes have dried up, leaving the mineral to be buried in the layers of sedimentary rocks.
Anhydrite is similar to gypsum, but lacks the hydrate (H$_2$O), but will weather to gypsum in the presence of water. Chemically anhydrite is called anhydrous calcium sulfate. Anhydrite is a common evaporitic mineral in the subsurface, forming thick layers in sedimentary rocks in dry ocean and lake basins, which are heated with burial resulting in dehydration of gypsum forming anhydrite. When buried these beds of anhydrite form dense barriers to the flow of water, and hydrocarbons like oil and natural gas in the subsurface. Anhydrite forms a thick pearly white mineral, and tabular crystals. Anhydrite also form salt domes, and diapirs in the subsurface, resulting in migration and flow in the presence of water. As such the mineral occurrence is important in petroleum exploration and groundwater.
Barite is a major ore of the element barium. In its pure form it is colorless and transparent, but often contains impurities of other minerals that give it a yellow brown tint. Barite occurs in evaporitic sedimentary rocks, but is also found in limestones subjected to hydrothermal activity. Because it is insoluble, non-toxic, but relatively high density, barite is often ingested to provide a radiocontrast agent when X-rays are taken of the digestive system. Oddly enough the element barium is highly poisonous (the main ingredient in rat poison), but since the barium atoms are tightly bonded to the sulfate, and are non-soluble in water and acids, they are non-toxic. Barite is most similar to the mineral gypsum, but is more dense and harder.

### Phosphates

Phosphates are characterized by having tetrahedral phosphate (PO$_4$$^{3−}$) ions and lacking silica. They are fairly rare in nature, but are important because they are mined as a source for fertilizers. Phosphate is a biological limiting element, which is needed in the growth of organic cells, and a component of organic molecules of DNA and ATP found in the cells of living organisms. In fact, one type of phosphate mineral is hydroxyapatite which is found in your bones and enamel of your teeth, Ca$_{10}$(PO$_4$)6(OH)$_2$, with a mix of fluorapatite (Ca$_{10}$(PO$_4$)6F$_2$), where fluorine can replace the OH ions. These minerals are all part of the apatite group of minerals, which also occur in rocks. Not all phosphate minerals are within the apatite group, such as the mineral turquoise. Turquoise is regarded as a valuable blue-green gemstone. Turquoise is a phosphate mineral that contains copper, giving it the distinct blue-green color valued by jewelry makers and lapidaries.
The apatite group of minerals was named by Fredrick Mohs teacher, Abraham Gottlob Werner. Apatite comes from the Greek word apatein which means to deceive in Greek, since the mineral group is sometimes difficult to identify from other minerals like feldspars. Apatite tends to have a darkish purple to green color, but many other color varieties are known. Fredrick Mohs designated apatite as the defining mineral for the hardness of 5 on his scale. The three major end-members of the apatite group are hydroxyapatite Ca$_{10}$(PO$_4$)6OH$_2$, fluorapatite Ca$_{10}$(PO$_4$)6F$_2$, and chlorapatite Ca$_{10}$(PO$_4$)6C$_{l2}$, depending on the chemical formula, with most specimens of apatite a mix of these three end members. Apatite occurs in pegmatitic igneous rocks, and hydrothermal igneous rocks. It also is the major component of bone and teeth in vertebrates, as well as some fish scales. Most fossilized bone tends to be replaced by minerals of silica or calcite, but dense apatite found in tooth enamel can be preserved over millions of years, and is highly stable in the shallow subsurface of Earth.

### Carbonates

Carbonates are a broad classification of minerals which are characterized by the presence of the carbonate ion CO$_3$$^{-2}$. Carbonates are an extremely common mineral group on the surface of the Earth, because of the high concertation of the element carbon (C) near the surface of the planet, actually since carbon dioxide (CO$_2$, a gas) and water (H$_2$O, a liquid) form carbonic acid (H$_2$CO$_3$), the same compound that gives soda pop its fizz, carbonates can be thought of as the salt of carbonic acid, however, much of the carbonate minerals found in the subsurface of Earth are also biologically produced, as carbonate minerals are used to grow shells and skeletons of many organisms that live in the oceans, lakes and rivers of the planet. In large quantities, carbonate minerals form limestone, but carbonate minerals are also found in almost any sediment deposited in water and in soils on the surface of the planet. Carbonate minerals are also important minerals in “gluing” sediments together in sedimentary rocks, as a lithifying (stone making) cement. Some carbonate minerals easily dissolve in ground water, and precipitate as crystals around grains of sand or other small clastic sediments, gluing them together to forming hard rock through a process called lithification. Carbonate minerals are also important in permineralization of organic remains to form fossils, such as the lithified remains of ancient animals and plants, such as dinosaurs.
Fredrick Mohs established calcite as his mineral hardness of 3, making it softer than the common mineral quartz (which is 7). Calcite is often distinguished from quartz by being softer on Mohs scale, however often these two minerals are difficult to tell apart when only tiny crystals are present, as both minerals are often colorless and clear. Geologists often carry a small bottle of diluted HCl acid (hydrochloric acid). By dropping some of this acid on the mineral or rock, the HCl acid will react to the CaCO3 molecules and produce CO$_2$ gas, which will bubble or fizz. This acid test will quickly distinguish calcite from quartz, even when the individual crystals are difficult to see in a rock. [CaCO$_3$ + 2HCl = CaCl$_2$ + H$_2$O (water) + CO$_2$ (gas)]. It should be noted that iron oxide minerals will also react to HCl acid, and produce a fizz, but those minerals are opaque, yellow-reddish brown to black in color. Calcite is very common on the surface of the Earth, as the mineral easily dissolves and precipitates in water depending on the pH of the water. Often rain or snow melt waters will dissolve the mineral, and as the ground water becomes more basic or evaporates it leaves behind the mineral calcite (in soils this is white remnant of calcite is called caliche). Calcite is the mineral that often forms inside faucets and pipes, and the ions of Ca$^{+2}$ and CO$_3$$^{-2}$ are common in the water that you drink. Calcite is the major mineral found in limestones, but can also make up a large percent of sandstones. Calcite also forms stalagmites and stalactites in caves. Calcite has strong birefringence (double refraction), which strongly bends light-waves which are passed through a crystal of calcite. This strong birefringence causes objects viewed through a clear piece of calcite to appear doubled, or highly displaced. Because of this optical property, calcite despite being clear or transparent, would not make a good material for glass, as you would have difficulty in seeing through the crystal, as it would distort the incoming light-waves. In crystallography, this strong birefringence makes calcite easily to identify, when polarized light is passed through the crystal under a microscope, the strong birefringence can be seen by how the crystals bends the light-waves passing through calcite. Calcite is very common in sedimentary rocks, but also can be found in hydrothermal igneous rocks that have been formed from the melt of calcite-rich sedimentary rocks or extremely hot groundwaters passing through the subsurface. Under intense pressure and heat, calcite often is replaced by dolomite, with the replacement of calcium with magnesium. Dolomite (CaMg(CO$_3$)2) is less reactive to HCl than calcite is.
Aragonite has the same chemical formula as calcite, but the atoms are arranged in a slightly different crystal lattice structure, resulting in a different style of crystal growth. Aragonite crystals are prismatic to needle shaped. These prismatic crystals are often found in modern seashells, as the mother-of-pearl lustrous colors seen inside some shells (iridescence), it is also the mineral that forms pearls. Aragonite crystals under pressure and heat will compact into calcite, so aragonite is a unique crystalline form of calcium carbonate (CaCO$_3$) found near the surface of Earth and in aquatic animals that grow shells for protection, such as corals, snails, clams, and starfish. Some animals will grow shells with calcite, while others will grow aragonite, and some grow both types of minerals to form their shells. However, with burial, these aragonite minerals will change to calcite with heat and pressure; a process geologists call diagenesis. Aragonite can be found rarely in metamorphic rocks subjected to high pressure, but low temperature such as those formed at subduction zones, with calcite-rich minerals found in marbles and blueschist rocks, becoming metastable (which means easily made unstable, but remaining stable in the ground for a long time) as aragonite crystals.
Malachite is an important ore of copper. Malachite is where copper ions have bonded with carbonate in hydrothermal low-grade metamorphic rocks within layers of limestone. As heated, groundwater passes through limestone cavities and copper will replace some of the calcium, resulting in deposits of copper, in the form of malachite. These copper deposits will always be a rich green color. Malachite is an important dye for paints as it exhibits a lush green color. It is also often carved or polished for jewelry. Historically malachite has been mined in Utah for copper in the Uinta Mountains and Brown’s Park Regions, however most of Utah’s copper production today comes from the Bingham Canyon Mine or Kennecott Copper Mine near Salt Lake City. This mine is a large deposit of chalcopyrite within igneous volcanic rocks. Malachite is often found in close association with azurite.
Azurite is a carbonate mineral of copper, which has a higher ratio of copper, which results in a characteristic blue color. As a blue mineral, azurite is an important natural dye for blue pigment, as that found in ultramarine. Azurite is unstable on the surface of the Earth, and will weather in the presence of water to malachite over time, going from a blue to green color. Azurite if often found together with malachite, in metamorphized or hydrothermally altered limestones.

### Halides

The halides are a group of important minerals that are composed of anions of halide elements such as fluoride (F$^−$), chloride (Cl$^−$), bromide (Br$^−$), and iodide (I$^−$). These minerals include many commonly occurring salts, and are frequently found in evaporitic deposits, where ancient seas have evaporated, leaving behind thick layers of these salts. They are often found in association with sulfate minerals.
Halite is the mineralogical term for table salt. It is mined around the world for use in curing food, and as a flavor enhancer in everyday cooking. Halite easily dissolves in water, into ions of sodium (Na+) and chloride (Cl-). As these ions are ionically bonded together, they are easily broken in the presence of water H2O, which is a polarized molecule. Halite is almost exclusively found in evaporitic deposits, left behind from ancient remains of seas, oceans and saline lakes. Halite is one of the principal minerals mined from the Great Basin, particularly from the Great Salt Lake basin near Salt Lake City, and across northwestern Utah.
Fredrick Mohs established fluorite for his hardness of 4. Fluorite is a fairly hard mineral, which belongs to the halide group of minerals, and exhibits a clear appearance. Often fluorite is tinted various colors in nature, most often a light purple color. Because of its relative softness, fluorite is rarely cut into gemstones, although mineralogical specimens are often collected, because they form beautiful octahedral crystals. Fluorite will exhibit fluorescence under ultraviolet light, although many carbonate minerals also fluoresce with UV light, caused by the falling of electron energy states. Fluorite is often found in hydrothermal metamorphic rocks and igneous rocks, often in regions enriched in galena. Fluorite is mined as an ore of fluoride, which is used in many applications.

### Native Metals

Native metals are a group of minerals composed of elements like gold, silver, and copper that occur naturally as metallic bonded atoms. A metal is any material that conducts electricity, exhibits a metallic luster and is malleable or ductile. These traits are a result of the individual bonds between the atoms in native metals sharing electrons through metallic bonding, hence these materials are excellent conductors of electricity. Native metals are rare, since most native metals will easily oxidize in the presence of oxygen (this is called tarnish). Iron oxides are much more common in the shallow subsurface than native metals, iron is most often found as an iron oxide. (Native forms of iron and nickel are very rare, and mostly restricted to meteorites). Gold, silver and copper are common native metals found in nature, and these were an early natural source for these valuable metals. Despite being present in nature, native metals are very rare, but economically important source for these precious metals. Naturally occurring gold nuggets and flakes are examples of native metal minerals. Naturally occurring sulfur and carbon (as graphite) are sometimes grouped within native metal group of minerals, since they are naturally occurring pure forms of a single element, although both sulfur and carbon are technically non-metals since they are not composed of metallic bonds. It is very rare that elements, such as gold, silver, platinum and copper occur naturally in pure form, and one of the reasons these minerals are sought after. Native copper is more common than the other native metals, but still rare and collectable, and most sources of copper are other naturally occurring minerals that contain copper.
As a siderophile element, gold (Au) is very rare on the surface of the Earth, but can be enriched in hydrothermal active volcanic regions. These deposits of gold are formed when ions dissolved in underground water are heated and passed through veins or faults in the subsurface, allowing the gradual accumulations of these atoms to form over many years. Gold is nearly always associated with igneous and metamorphic rocks, although these veins can erode forming what are called placer deposits in sedimentary rocks. A placer deposit is a deposit of gold formed by erosion of these enriched veins of gold, however because of gold’s high density the gold will often accumulate, while other minerals will be washed and transported downstream. These accumulations can be panned, dredged and sluiced by gold prospectors looking for gold. They are using this characteristic high density to find gold in the river or stream deposits. Gold’s density is 19.3 gm/cm$^3$, much greater than any other mineral listed! Gold is also mined underground or by stripe mining the surface, this is called lode mining, and a mother lode is the original source of eroded gold. Gold is fairly soft, with a hardness just above a finger nail (2.5-3), making it easy to scratch or to leave a dent in pure gold, as it is ductile as well, easy to bend without breaking it. This property makes gold a good material for jewelry.
Fredrick Mohs work in organizing minerals had a dramatic effect on the understanding of the occurrence and distributions of minerals in the surface of the Earth, as it became much easier to identify minerals by geologists. These 40 minerals listed are a small sample of the true diversity of minerals that occur in nature, and can be found in the Earth. However, knowing these 40 minerals will enable you to identify nearly 99% of the minerals you will likely encounter as you pick up rocks and examine them closely. The most common minerals that you are likely to find on the surface of the interior of continents is quartz, particularly in sedimentary rocks. The amount of quartz decreases with depth into Earth, as well as within oceanic crust (in subduction and mid-ocean zones). The percentage of each type of mineral in a rock will be important in the naming of different types of rocks. Rock names are based on the type of material that compose the rock, in particular the mineralogy and texture (grain or crystal sizes) found in the rock.
Book Page Navigation :
| Previous | Current | Next |
| e. The Rock Cycle and Rock Types (Igneous, Metamorphic and Sedimentary). | f. Mineral Identification of Hand Samples. | g. Common Rock Identification. |