Where is sedimentary rock commonly found

It is used in many different ways: as a building stone, in the production of lime an important material to improve soil for farming , glass making, industrial carbon dioxide and cement. Chalk is a form of limestone. Shale Mudstone.

Mud, silt and clay are the ingredients of shale. These are compacted to form a soft, easily broken, usually dark coloured rock. Sandstone is formed from layers of sandy sediment that is compacted and lithified. Chemical sedimentary rocks can be found in many places, from the ocean to deserts to caves. For instance, most limestone forms at the bottom of the ocean from the precipitation of calcium carbonate and the remains of marine animals with shells.

If limestone is found on land, it can be assumed that the area used to be under water. Cave formations are also sedimentary rocks, but they are produced very differently. Stalagmites and stalactites form when water passes through bedrock and picks up calcium and carbonate ions. When the chemical-rich water makes its way into a cave, the water evaporates and leaves behind calcium carbonate on the ceiling, forming a stalactite , or on the floor of the cave, creating a stalagmite.

The water drips, but the mineral remains like an icicle. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

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Like primary minerals, secondary minerals can break down and disappear, so this table compares weathering rates for both primary minerals and secondary minerals.

Weathering resistance, however, does not necessarily mean that a particular mineral is abundant in weathered materials.

Some of the minerals at the top of the list in the table are uncommon compared with others. Zircon, rutile, and tourmaline, for example, are very resistant to weathering but rarely are major components of sediments because they are only minor minerals in most parent rocks. Minerals at the bottom of the list are very unstable when exposed to the elements and, consequently, are absent from all but the youngest sediments.

After chemical weathering, dissolved material is carried away. Residual minerals and secondary minerals such as clay may remain where they form. For example, prolonged weathering of bedrock can lead to thick layers of reddish soil called laterite in tropical areas see Box , below.

Laterites vary but are always rich in oxide minerals and clays. Laterites are easily eroded. Over time, erosion by water, gravity, or wind can transport laterite debris, just like any other detrital material, away from its place of origin. Consider a tropical area with warm weather and abundant rainfall.

Weathering and leaching will be extreme, and even clay minerals may decompose. Normally soluble elements, and even relatively insoluble silica, will be dissolved and removed. The remaining material, called a residual deposit , is often composed primarily of aluminum oxides and hydroxides, the least soluble of all common minerals. 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.