We term such deposits laterites if unconsolidated or bauxites if lithified into rock. Bauxites and laterites are our most important source of aluminum. But, the mineralogy of a laterite depends on the composition of rocks weathered to produce it. Laterites can also be important sources of iron, manganese, cobalt, and nickel, all of which have low solubilities in water. Most laterites are aluminous. The most important aluminum ore bauxite , is a mixture of several minerals, including the polymorphs boehmite and diaspore , both AlO OH , and gibbsite, Al OH 3.
Bauxite is mined in large amounts in Australia and Indonesia, and in smaller quantities in the Americas and in Europe. In some places, relatively young laterites produce ore, but in Australia economical laterite deposits are more than 65 million years old. The term siliciclastic refers to sediments composed mostly of silicate minerals. The most common sedimentary rocks — including shale, sandstone, and conglomerate — form from siliciclastic sediments. Other, less common, kinds of sedimentary rocks consist of carbonates in limestones , iron oxides and hydroxides such as hematite or goethite , or other minerals.
Geologists classify siliciclastic sediments based on grain size. The standard classification system is the Wentworth Scale see table. Depending on size, grains may be boulders, cobbles, pebbles, gravel, sand, silt, or clay.
The word clay sometimes causes confusion. Sedimentary petrologists use the term to refer to clastic grains smaller than 0. In this text, however, we also use it to refer to minerals of the clay mineral group, no matter the grain size. Clast sizes vary from fine clay and silt to huge boulders. Small clasts are usually composed of a single mineral, generally quartz or clay.
Larger clasts are commonly lithic fragments composed of multiple minerals. The photos below show some examples. The mud comprises fine grains of silt and clay.
Quartz dominates most common sand, but the sand seen here contains mostly rosy garnet, and also epidote, zircon, magnetite, spinel, staurolite, and only minor quartz. Most of the pebbles are lithic fragments rock fragments composed of more than one mineral. These cobbles are all lithic fragments. The mineral grains in the Pfeiffer Beach sand are angular, but the clasts in the last two photos have been well rounded by abrasion caused by them being tumbled by flowing water.
Grains are about 1 mm across. Grains are cm across. The piece of wood is about 15 cm long. Wind, gravity, and other agents can move clastic material as well. Eventually, sediments are deposited when the forces of gravity overcome those trying to move them.
Large grains may not move far and are deposited first. As the energy of transportation decreases, smaller material is deposited. So, during transportation, sediments commonly become sorted , which means that sediment deposits often have relatively uniform grain size. Thus, for example, coarse material may be deposited near the headwaters of a stream, while only fine material makes it to a delta.
Sorting is not ubiquitous; streambed gravel, for example, may contain a mix of silt, sand, and larger clasts, and glacial deposits often contain a jumble of material of many different sizes.
The photo above Figure 7. After deposition, unconsolidated sediment may, over time, change into a clastic sedimentary rock by the process called lithification from lithos , the Greek word meaning stone. Lithification involves compaction and cementation of clastic material. Common cementing agents include the minerals quartz, calcite, and hematite.
Before, during, and after lithification, sedimentary rocks undergo textural or chemical changes due to heating, compaction, or reaction with groundwaters. Biological agents, including small animals or bacteria, also can be important, as can chemical agents brought in by flowing water.
We call these changes collectively diagenesis. Dissolution and removal of minerals leaching and the formation of clay or other minerals are both common during diagenesis. We call any new minerals that form, authigenic minerals. Zeolites, clays, feldspar, pyrite, and quartz can all be authigenic minerals. Although diagenesis creates many authigenic minerals, most are so fine grained that we cannot identify them without X-ray analysis.
Textural changes, including compaction and loss of pore space, are common and are part of diagenesis. Recrystallization , the changing of fine-grained rocks into coarser ones, is another form of diagenetic textural change. During recrystallization, as individual mineral grains grow together, secondary minerals may precipitate in open spaces, and more mineral cements may develop.
Consequently, rocks become harder. Diagenesis is equivalent to a low-temperature, low-pressure form of metamorphism , and the processes of sedimentation, lithification, diagenesis, and low-grade metamorphism form a continuum. Lithification changes unconsolidated sediment into a rock. Cementation by quartz, calcite, or hematite may be part of the lithification process.
It also may be considered a diagenetic process. Similarly, the formation of many low-temperature minerals such as zeolites , a normal part of diagenesis, overlaps with the beginnings of metamorphism. Metamorphic petrologists often define the onset of metamorphism by the first occurrence of metamorphic minerals.
This definition can be hard to apply because many diagenetic minerals are also metamorphic minerals. Furthermore, laumontite , often considered to be formed at the lowest temperature of all metamorphic minerals, is a zeolite that is hard to distinguish from minerals that form diagenetically. Chemical weathering yields dissolved material that water transports until precipitation of chemical sediment occurs.
Several things may cause precipitation; the most common causes are evaporation, changes in temperature or acidity pH , and biological activity. Hot springs deposit a form of calcite called travertine , for example, when cooling water becomes oversaturated with CaCO 3.
This photo Figure 7. In freshwater streams or lakes, a pH change due to biological activity may cause precipitation of another form of calcite called marl. In marine settings, many reef-building organisms have shells or skeletons made of organic calcite.
Calcite and other chemical sedimentary minerals, then, precipitate in many ways. In contrast with clastic sediments, chemical sediments usually lithify at the same time they precipitate. Natural waters contain dissolved minerals, and all minerals are soluble in water to some extent.
Halides, many sulfates, and other salts have very high solubilities. Carbonate minerals, including calcite and dolomite, have moderate solubilities. Silicate minerals have relatively low solubilities. If water evaporates, it may become oversaturated in particular minerals and deposit chemical sediments, such as the salt deposits in the photo seen here Figure 7. Precipitation will continue, decreasing concentrations of dissolved material, until the solution and sediments achieve equilibrium.
Because of their high solubility, large amounts of evaporation may be necessary before salts such as halite, precipitate. In contrast, carbonate minerals calcite and dolomite and silica often precipitate early during evaporation. Silica SiO 2 , in the form of chert , is the only silicate mineral that commonly forms a chemical sedimentary rock.
So, their chemical components are common as dissolved species. As water evaporates, perhaps in a closed inland basin or an isolated sea, these minerals may precipitate to form thick beds of evaporite minerals. Besides these four minerals, many other less common minerals occur in evaporites, too. Evaporites are found in many parts of the world. Some of these pinnacles rise more than 40 m above the lakebed. These pinnacles consist of trona a hydrated sodium carbonate that precipitated from briny water.
Like all playas, this lake is dry most of the time. But, past flooding and subsequent evaporation produced thick layers of evaporite minerals. More than 25 different minerals are found in the Searles Lake bottom.
The list includes sodium and potassium carbonates, sulfates, borates, and halides. Borax hydrated sodium borate , trona, and several other minerals are profitably mined from Searles Lake sediments today. In the subsurface, massive gypsum and halite beds are common, as are the salt domes found in Texas and other Gulf Coast areas of the United States. Although generally dominated by just a few minerals, many other minerals may be present. In all, petrologists have reported nearly minerals from evaporites.
Less than a dozen are common. Evaporites may be marine deposits associated with evaporation of ocean water. They may also be non-marine , associated with freshwater lakes or other continental waters.
For water to become oversaturated, a water body must be somewhat isolated and the evaporation rate must be faster than any water flowing in.
This is most common in an arid environment. For example, at various times in the past, the Mediterranean Sea has been cut off from an ocean. Evaporation led to thick salt deposits that lie beneath the Mediterranean today.
And, today, evaporite minerals are collecting along the shores of the Dead Sea between Jordan and Israel see Figure 7. As ocean water evaporates, minerals precipitate in predictable order from those that are least soluble to those that are most soluble. Calcite is first, followed by gypsum, anhydrite, and then halite.
Many other minerals may precipitate in lesser amounts. Continental water contains different dissolved solids than marine water, so continental evaporites contain different minerals than marine evaporites. Water chemistry is also quite variable, so many different minerals are possible.
Continental evaporite deposits may contain halite, gypsum, and anhydrite but also typically have borax, trona, and many other non-marine salts. The table lists some minerals reported from evaporite deposits in North America.
It is a long list. The previous chapter discussed silicate minerals common in igneous rocks. In principle, they could all be detrital grains in sediments and sedimentary rocks. In practice, most break down so quickly that they cannot be weathered or transported very much before completely decomposing. Quartz is the most resistant to weathering.
Many minerals weather to produce clays. It is no surprise, therefore, that quartz and clays are the main silicate minerals in most clastic rocks. Feldspars and sometimes muscovite may also be present but are usually subordinate to quartz.
They are absent from rocks formed from sediments transported long distances or weathered for long times. Mafic silicate minerals are exceptionally rare in sediments or sedimentary rocks. Besides quartz and clays, other silicates, including zeolites, may occasionally be present. Important nonsilicate minerals in clastic rocks include carbonates, sulfates, oxides, halide minerals and occasionally pyrite. The clay minerals include many different species; all are sheet silicates.
The sheets comprise tetrahedral layers containing mainly Si and Al, and octahedral layers containig mainly Al, Mg, and Fe. They generally contain less potassium than micas. Their layered structure and the weak bonding between layers give them a characteristic slippery feel when wet. The major differences between the different clay species are the compositions and stacking order of atomic layers.
The formulas in the box here are only approximate because clays often contain many elemental substitutions. Clay minerals account for nearly half the volume of sedimentary rocks. This makes identification of individual clay species difficult.
X-ray analysis is often necessary to tell them apart. In contrast with quartz and feldspar, clays do not form in igneous and metamorphic environments. Clays are common in shales and other sedimentary rocks. The clay species present depends on the sediment sources.
Although usually fine grained, clays can form thick beds or layers. They also develop as coatings on other minerals undergoing weathering. These generalizations are true of all clay minerals, but there is a great deal of variety. In part, the variation is due to the low temperatures at which these minerals form. At high temperatures, minerals and mineral structures tend to be simple and ordered. At low temperatures, structures are often more complex or disordered, and many different mineral varieties may form.
The three most important kinds of clays are illite, montmorillonite , and the clays of the kaolinite group. Figures 7. Kaolinites, also called kandites , vary less in composition and structure than other clays, although several kaolinite polymorphs dickite, halloysite, nacrite are known.
Kaolinite is the principal clay used to make ceramic ware because it remains white when fired in a kiln. Illite is quite similar to muscovite in some ways, but contains more Si and less K. In the process the clay expands. So, we sometimes call montmorillonite and other clays of the smectite group expandable or swelling clays. Because they absorb liquids so well, gas station operators use them to clean up spilled oil, and homeowners use them as kitty litter.
They are the major components of earthy material called bentonite , sometimes prized for its water-absorbing and cation-exchange properties.
Vermiculite , another clay of the smectite group, is often used to lighten up potting soil. Montmorillonite dominates modern clay-rich sediments and sedimentary rocks; illite dominates most sedimentary rocks that are older than about million years.
Geologists ascribe this development to ongoing diagenesis, to variations in tectonic activity resulting in changes in sediment sources, and to changes in biological activity. Many different clays have industrial uses.
Clays are widely used to make bricks, tile, paper, rubber, water pipes, and china. They are even used today by some restaurants to thicken milkshakes. Early peoples made bowls and other artifacts by shaping clay and allowing it to dry in the sun. Porcelain and china makers commonly use kaolinite. Such temperatures can be obtained over open fires, and much early pottery consists of metakaolinite.
Although metakaolinite is porous, it will not soften when wetted, in contrast to sun-baked clays. Porcelain refers to a special type of high-grade white ceramic. The Ming vase, seen in this photo Figure 7. The white color is only possible if the ceramic is made from extremely pure kaolinite. Porcelain is baked, or fired, at very high temperatures. Prior to firing, small amounts of feldspar or talc are mixed with the kaolinite.
Porcelain was first developed in China more than 1, years ago and slowly moved east through the rest of Asia and to Europe and then the Americas. Talc , a secondary mineral that forms when Mg-silicates such as olivine or pyroxene are altered, and pyrophyllite , an uncommon metamorphic mineral, are often grouped with the clays. Both are less variable in their atomic arrangements and composition and contain less H 2 O than true clays. They are transitional between clays and micas in structure and, when seen in hand specimen or thin section, are typically easier to identify than clays.
Talc is very soft and has a diagnostic greasy feel that generally makes identification straightforward. In contrast, pyrophyllite often looks like many other white minerals, unless it has the characteristic splay of crystals seen in the photo above Figure 7. Mineralogists have identified more than 50 different carbonate species; all contain CO 3 2- groups but some contain other anions or anionic groups. The box lists some examples.
Less common rhodochrosite, smithsonite, cerussite, strontianite, azurite, and malachite sometimes form spectacular mineral specimens. These minerals are also important ore minerals of manganese, zinc, lead, strontium, and copper. The common carbonates have relatively simple compositions and include no hydroxyl groups or H 2 O. Some relatively rare species are more complex, however, and examples are at the bottom of the list.
Calcite is the most abundant carbonate mineral. It typically forms by precipitation from oversaturated water. It also occurs in caves, where calcite forms stalactites and stalagmites. Calcite and other carbonate minerals also precipitate from hydrothermal waters warm waters , especially in ore deposits and typically in veins.
This is how most azurite and malachite, and some related minerals, are created. Many carbonate minerals can have either an inorganic or an organic origin. Inorganic marine carbonate rocks form when either calcite or aragonite precipitate from ocean water. Marl may form when carbonates precipitate on lake or stream bottoms. In contrast, organic carbonate rocks form when algae, corals, clams and other organisms create structures and shells from CaCO 3 dissolved in seawater. This may produce carbonate rocks directly, or residual shells, bones, and other material can accumulate to produce carbonate clastic rocks.
The maze coral seen in this photo Figure 7. Both calcite and dolomite are essential minerals in limestones and dolostones limestones containing dolomite instead of calcite , and may be clasts in other kinds of sedimentary rocks. Some clastic rocks are held together by fine grained carbonate cement. Both minerals are found in metamorphic rocks such as marbles and in rare igneous rocks called carbonatites. Many carbonate minerals are secondary minerals formed during weathering or diagenesis.
For example, most dolomite is secondary and forms by reaction of calcite with Mg-rich water during diagenesis. Magnesite, MgCO 3 , a related carbonate, forms as an alteration product of mafic and ultramafic rocks. Hydromagnesite a hydrated equivalent of magnesite forms by weathering of Mg-rich minerals, including olivine, serpentine, and others. The photos below are examples of coarsely crystalline carbonate minerals.
TFigure 7. The blue and green specimen in Figure 7. Both are Cu-carbonates and both are copper ore minerals. The pinkish crystals in Figure 7. Rhodochrosite is not always this color, but when it is, the color helps identify it. The azurite, malachite, and rhodochrosite come from a well-known mining district in western Colorado. The green crystals in Figure 7. Like rhodochrosite, smithsonite comes in various colors, but this green color is typical. The last photo Figure 7. Unfortunately, most mineral specimens are not as well formed or beautiful as suggested by the photos above.
So, below are three more typical examples of calcite. The calcite specimen on the left Figure 7. The blue calcite in Figure 7. Calcite cleaves easily into rhombohedral shapes like the ones seen here, and the flat surfaces are cleavage planes, not crystal faces.
Blue calcite is rare but translucent calcite is not. Rhombohedral cleavage fragments are exceptionally common. In the model of dolomite seen here Figure 7.
Carbonate CO 3 2- groups are shown as yellow triangles. Other carbonates have similar structures. We can plot the compositions of Ca-Mg-Fe carbonates on a triangular diagram similar to that used for pyroxenes in the previous chapter.
A large miscibility gap exists between the Ca-bearing carbonates and the Ca-free ones. It is similar to the gaps between wollastonite, clinopyroxene, and orthopyroxene we have already seen. And, the dolomite-ankerite series does not extend all the way across the diagram. As with the pyroxenes, the size of the miscibility gaps varies with temperature. It is simplified because melting and decarbonation reactions occur at high temperatures but are not shown.
We can see that the solvi narrow as temperature increases. At the highest temperatures on the diagram, calcite and high-magnesium calcite are stable. They are both Ca-Mg solid solutions of variable composition. Magnesite, an Mg-Ca solid solution, is stable too. At somewhat lower temperatures, high-Mg calcite changes into dolomite by an order-disorder transformation. The miscibility gaps in yellow at low temperature, mean that most carbonate compositions will unmix into calcite and dolomite, or dolomite and magnesite, solid solutions.
Calcite may contain some extra magnesium, but the other two minerals will be close to end member composition. Calcite sometimes develops exsolution textures that we can see with a thin section and a petrographic microscope.
Calcite crystals twin by several different twin laws. The drawings seen here Figure 7. The drawing on the right is called a butterfly twin , and the photo in Figure 7. Calcite also commonly contains microscopic deformation twins that we can only see with a petrographic microscope. Mineralogists have described more than sulfate minerals.
They fall into two main groups: those that contain water hydrous sulfates and those that do not anhydrous sulfates. Many others are known, but most are rare.
The drawing in Figure 7. Sulfur atoms bond to four oxygen, creating sulfate tetrahedra with composition SO 4 The tetrahedra are tightly bonded and similar, in some ways, to the SiO 4 4- groups that characterize silicates. Sulfate tetrahedra, however, do not polymerize. Besides the minerals listed in the blue box, at least a dozen other, but rare sulfates exist.
Some are listed in the table of evaporite minerals earlier in this chapter. The spectacular photo in Figure 7. The cave contains some of the largest mineral crystals in the world — up to 12 meters long. These crystals are selenite , a translucent form of gypsum. Gypsum is moderately water-soluble, so it is one of a relatively small number of common minerals that precipitate from natural water, often redissolve, and later reprecipitate somewhere else.
Most natural gypsum crystals are centimeters in size or smaller. However, extremely large crystals, such as those shown here, exist in several places around the world.
Plaster can be made from different mineral materials. Early Romans used a lime-based plaster Figure 7. At the time, the best and most productive gypsum quarries were in Montmartre, a section of Paris. Modern plasters and sheetrock both contain plaster of Paris. Complete dehydration of gypsum would produce anhydrite CaSO 4 , which is not useful as a plaster because it does not recombine easily with water. However, when plaster of Paris is mixed with water, reaction occurs quickly, giving off heat and promoting drying and hardening.
Drywall also known as sheet rock is composed of gypsum with paper front and backing. A number of different additives may be mixed with the gypsum to promote desired properties, including strength, resistance to mildew, and reduced water absorption. Drywall construction replaced the more classic lath and plaster walls in the mid s. From Winnipeg, Manitoba 9 cm across 7. It is found in thick evaporite deposits, in hydrothermal veins, and as precipitates from surface or subsurface waters, hot springs, or volcanic gases.
Anhydrite , with a composition equivalent to gypsum lacking H 2 O, is found in sedimentary rocks but alters to gypsum over time. Anhydrite is also commonly found in evaporite deposits with gypsum.
Common gypsum is white to grey, and translucent, but just about all colors are known. Normal coarse crystals of gypsum form blades or tabs. We call them selenite Figures 7. Selenite commonly twins, sometimes forming characteristic swallow tail or fishtail twins. These two types of twinning are very similar and often not distinguished from each other. Another example is in Figure 4. Satin spar is an especially fibrous variety of selenite Figure 7.
Satin spar is often opalescent and is sometimes used in jewelry, although it is soft and not very durable. Sometimes gypsum crystals form a flowery cluster called a desert rose Figure 7.
The specimen in Figure 7. In caves gypsum may form gypsum flowers , similar in many ways to desert roses. Alabaster is very fine grained gypsum that people sometimes carve or polish for building stone or for art.
Barite , barium sulfate, commonly occurs in hydrothermal deposits with copper, lead, and zinc minerals; it is often associated with anglesite and celestite.
Barite is also found in hot spring deposits, and in some evaporite deposits. The cluster of clear to light blue bladed crystals in Figure 7. And, barite sometimes occurs as concretions in sediments and sedimentary rocks, and sometimes as desert roses Figure 7. Celestite also called celestine , strontium sulfate, most often occurs in sedimentary rocks associated with gypsum, anhydrite, sulfur, or halite.
Commonly it has a diagnostic light blue color as seen in the photo here Figure 7. Celestite is the most common strontium minerals.
Strontianite strontium carbonate is the only other one of significance. Some sulfates are common as minor, and rarely major, minerals in ore deposits — typically as replacements for primary sulfides. Anglesite PbSO 4 , for example, forms during weathering or alteration as a replacement for galena PbS. The specimen seen in Figure 7. The halide group consists of minerals containing a halogen element, generally chlorine or fluorine, as an essential anion.
Although many halides exist, only halite and sylvite are common in sedimentary rocks; they are quite rare in rocks of other sorts. Halite is typically found as rock salt in massive salt beds, often occurring with other evaporite minerals such as gypsum or anhydrite, and sometimes with sulfur.
Sylvite is much less common than halite. When found, however, it is usually associated with halite. Salt Production Methods: The pie diagram above shows the approximate amount of rock salt and rock salt equivalent produced in the United States during the calendar year Data from the United States Geological Survey.
In about 39 million tons of salt were produced in the United States. There are four important categories of rock salt production:. The United States consumes more salt than it produces.
To satisfy demand in , about 16 million tons of salt were imported. The amount of imported salt has been increasing in the past few decades. This is mostly a result of increasing demand in the United States and lower production costs in other countries. Salt Core: Photograph of a short segment of a salt core, obtained by drilling a well down to a subsurface salt layer and retrieving a cylinder of the salt.
A core of the entire salt layer is often obtained and brought to the surface for examination by a geologist, and for chemical and physical testing. The properties of numerous salt cores will be used to determine which portion of the rock layer will be mined. Companies interested in developing a salt resource located hundreds to thousands of feet below the surface usually drill numerous wells down to and through the salt layer.
They drill to learn the thickness of the salt and what types of rocks enclose it. They also obtain core samples of the salt see accompanying photo that will be used to determine its chemical and mineral composition.
The purity of the salt determines how it can be used. The depth determines the cost to build the mine. Depth also determines the electricity costs of operation, ventilation, and lifting salt, equipment, people, and water in and out of the mine.
Most of the rock salt produced in the United States is produced by traditional room and pillar mining - a mining method that is widely employed in mining for coal. This involves sending people and machines underground to remove the salt. The salt is usually hundreds of feet below Earth's surface.
To start the mine, a large-diameter shaft is drilled vertically down to the salt layer. That shaft will be equipped with lifts, much like elevators, that will be used to lower equipment, people, and supplies down to the level of the salt. Other shafts will be built down to the salt. Some of these shafts will be used to lift the mined salt up to the surface. Others will be used to bring fresh air into the mine or exhaust mine air to the surface.
Continuous Mining Machine: The machine in the photo above is a continuous miner in a salt mine. The cutting head on the right side of the photo is a rotating drum equipped with durable cutting points. As the drum rotates, the cutting head grinds the salt into small pieces which fall onto a pan directly below.
Once they have been lowered down a shaft to the level of the salt, the mining machines will begin cutting underground tunnels through the salt. A large cylindrical drum with cutting picks is mounted on the front of the mining machine. This drum rotates and cuts its way through the salt see accompanying photo. As a mining machine cuts its way through the salt, broken pieces of salt fall onto a large metal pan mounted immediately below the rotating cutting drum.
Mechanized arms rake the salt up the pan and onto a conveyor that carries the salt to a waiting wagon or to a mobile conveyor system. The conveyor will transport the salt to a mine shaft, where the salt will be lifted to the surface, or to an underground storage area. This is the basic process of underground salt mining with a continuous mining machine.
Solution mining of salt is done by injecting hot water under pressure down a well into a subsurface layer of rock salt. That same water is then withdrawn up to the surface through a nearby recovery well. While the water travels through the layer of rock salt - from the injection well to the recovery well - it dissolves a significant amount of salt.
The water is returned to the surface as a concentrated salt solution known as "brine". Most solution mining sites in the United States are operated by chemical companies who use the brine as a feedstock at a chemical manufacturing plant a short distance from the recovery well.
A few solution mining sites have ponds at the surface where they produce solar salt. Others use heat or a vacuum heating process to recrystallize the salt. These recrystallized salts have a higher purity than salt mined from the same rock unit. The recrystallized salt is a higher quality and higher purity because insoluble mineral matter and other insoluble impurities were left in the ground. Vacuum Pan Crystallizers: Six vacuum pan crystallizers connected in a series.
They are approximately 30 feet in height. These crystallizers are in a sugar plant, but similar units are used in salt crystallization. Vacuum pan salt is produced in large enclosed tanks known as vacuum pans or vacuum salt crystallizers. The tanks are filled with brine, which is heated by injecting steam into the tank.
The steam heats the brine and causes it to boil. As the brine boils it produces additional steam, which is fed into a second vacuum pan and causes its water to boil. Three or more vacuum pans are connected in a series to make efficient use of the steam. As the brine in the tanks becomes saturated with salt, crystals of salt begin to form in the tanks.
Salt produced in a vacuum pan has a much higher purity than salt produced from an underground mine. Vacuum pan salt is usually made from brine produced by solution mining. When the salt was dissolved underground, solid particle impurities in the salt were not dissolved and remained underground. Vacuum pan salt also has a different texture - its crystals are very small.
Some vacuum pan salt also has a different crystal shape - flake-shaped crystals instead of cubic-shaped crystals. Flake salt is a specialty product sold for table use. It is also preferred by many producers of baked goods, butter, cheese, and other products. Mechanized Solar Salt Production: Heavy equipment is used to construct salt pans and to harvest their thick layers of salt. This bulldozer is used on a solar salt plantation on Bonaire, an island of the Leeward Antilles , in the Caribbean Sea.
People have been producing solar salt from ocean water for thousands of years. Solar salt can be produced in parts of the world where evaporation rates significantly exceed precipitation rates. The name "solar" means that the sun acts as a heat source to evaporate ocean water or brine brought up from the subsurface , leaving behind its dissolved salt. Historically, the salt was produced in shallow ponds, built along the seashore with inlets just below the level of high tide.
Then, twice each day, at the instant that high tide reaches its peak, water flows into the ponds to fill them with salt-laden water. Then for the next 12 hours, the water evaporates to produce salt. This process has been operated as an industry for thousands of years. How much salt can be made? That depends upon the climate, the weather, the surface area of the ponds, how they are built - and the knowledge of the people who tend them.
For each liter of water they evaporate, about 35 grams of salt about six teaspoons can potentially be recovered. In the United States a few million tons of solar salt is produced each year. Brazil , Mexico , and India produce more solar salt than the United States. However, most other countries have domestic salt production.
In India, production of salt by artisanal methods is an important source of employment even though the work pays little, is extremely demanding, and exposure to sun and salt water takes a heavy toll on the workers. Estimated salt production by country from the United States Geological Survey. The United States Geological Survey reports that "almost every country in the world has salt deposits or solar evaporation operations of various sizes".
Eight countries China , the United States , India , Germany , Australia , Canada , Chile , and Mexico each produced at least 9 million tons of salt in calendar year The accompanying pie chart illustrates their relative importance.
Much of the salt produced in India is solar salt made using artisanal methods. Unlike most other mineral commodities, the natural salt resource is enormous. Ocean water can provide an essentially unlimited amount of salt. The amount of salt held in subsurface rock units is enough to easily supply the world for centuries. Salt Consumption in the United States: The chart above shows the general categories of salt consumption in the United States.
Highway deicing and chemical manufacturing are the most important uses of salt. Highway Deicing: A truck spreading rock salt on an asphalt highway in the United Kingdom. Rock salt and brine are applied to highways in many countries where ice and snow can create a hazard. The leading use of rock salt in the United States is highway deicing. The amount of highway salt consumed varies significantly from year to year, depending upon weather conditions.
Variations in the need for highway salt are responsible for most of the year-to-year variation in national consumption. The rock salt is mined, crushed to appropriate size, and usually applied to roads and highways without further processing. When applied dry, much of the salt can bounce off of the road or be scattered by the wind of passing traffic. If the salt is applied wet, more of it sticks to the road, and the loss of salt is reduced.
Use of rock salt as a road and highway deicer has some environmental problems. Two are related to salt dissolved in water entering the ground on the roadside.
The salty water can kill roadside vegetation. It can also contaminate shallow groundwater that might be withdrawn from personal or private water supply wells. It might also discharge into local streams where it can kill or sicken plants, fish, and animals. Soda Ash: Salt is one of the raw materials used to make soda ash at this chemical plant in Sowa, Botswana.
The second most important use of salt in the United States is as a feedstock for the chemical industry.
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