Rocks of Virginia
Thus, of the eight common igneous minerals, only quartz, K-feldspar, and muscovite are commonly seen in sedimentary rocks. These minerals are joined in sedimentary rocks by clay minerals, calcite, dolomite, gypsum, and halite. The clay minerals form during mineral weathering. The other four minerals are salts that precipitate as water evaporates. Mar 11, · Sedimentary rocks are formed by erosion, transport in rivers, ice etc. and involve the decay and disintegration of a preexisting rock mass. Usually there are no new minerals formed, only found. When these particles eventually settle, they form alluvial gravels, sands or facetimepc.coted Reading Time: 4 mins.
Thus, of the eight common igneous minerals, only quartz, K-feldspar, and muscovite are commonly seen in sedimentary rocks.
These minerals are joined in sedimentary rocks by clay minerals, calcite, dolomite, gypsum, and halite. The clay minerals form during mineral weathering. The other four minerals are salts that precipitate as water evaporates.
The elemental constituents of these salts are ultimately also derived from mineral weathering. Common Sedimentary Minerals When igneous rocks are exposed to weathering, many of their common minerals completely dissolve or partially dissolve and xre into clay minerals.
As a result, olivine, pyroxene, plagioclase, amphibole, and biotite are uncommon in sedimentary rocks. Similarly, most distinctly metamorphic minerals either dissolve or disintegrate into particles too small to be readily noticed by a beginning geologist. Pyrite is not a rock-forming mineral, but it is whats the most popular website in the world present in sedimentary rocks that formed in roks near seawater oceans, lagoons, and coastal swamps.
Quartz, Chert, and Flint. Muscovite and Clay Minerals. Gypsum Halite. E-mail C. Jones with comments or corrections. Geology and Planetary Science Home Page.
Chemical Sedimentary Rocks
The Mineralogy of Sedimentary Rocks Because of their detrital nature, any mineral can occur in a sedimentary rock. Clay minerals, the dominant mineral produced by chemical weathering of rocks, is the most abundant mineral in mudrocks. Quartz, because it is stable. Oct 22, · These rocks are often called clastic sedimentary rocks. One of the best-known clastic sedimentary rocks is sandstone. Sandstone is formed from layers of sandy sediment that is compacted and lithified. Chemical sedimentary rocks can be found in Estimated Reading Time: 4 mins. Mar 18, · Mechanical Deposition of Sedimentary Mineral Deposits. In some cases, the ore mineral, on account of its resistance to weathering and its hardness, survived the destruction of many of the rock minerals; and, during sedimentation, on account of its weight, it was sorted out from the fragments of quartz and other resistant minerals.
Sediments are created by weathering. It is caused by reactions with air, water, salt, and acid, by freezing and thawing, and by plants and animals.
Weathering involves the decomposition of rock, and the breaking down of primary minerals in rock. The products include smaller pieces of rock, individual mineral grains, dissolved material that is carried away, and sometimes new secondary minerals. Figure 7. An original source rock igneous, metamorphic, or sedimentary is exposed to forces that cause weathering. The weathering forces may be mechanical water, wind, gravity, glaciers, waves, and frost or chemical dissolution by water, perhaps containing acids.
Often the two kinds of weathering work together. And, these processes are selective. Some minerals dissolve or react and disappear faster than other minerals. Some rocks are harder and do not break apart as easily as other rocks. Over long times — geological times — chemical weathering has a much greater effect than mechanical weathering. Even apparently dry climates have enough water to promote chemical weathering on exposed surfaces, although the weathering rate may be slow.
Mechanical weathering breaks large or solid material into smaller pieces. Clastic material , also called detritus or detrital material , may be fine grains of individual minerals or it may be lithic fragments rock fragments composed of multiple minerals.
The photo seen here Figure 7. Freezing, thawing, and the action of ice created large blocky pieces of what was originally solid bedrock. Even apparently solid granites or other rocks can be broken apart this way. Talus slopes are examples of very coarse sediment. More commonly, mechanical weathering produces smaller rock fragments, or sand, or silt composed of individual mineral grains.
After chemical weathering, leftover rock may have a dissolved or eroded appearance, such as the sandstone seen in Figure 7. This sandstone has weathered to obtain a honeycomb texture , typical of sandstone in which the cementation of grains is not uniform. Weathering textures are not unique to sandstone. After chemical weathering, outcrops of many sorts often become rounded or pitted. The limestone outcrop shown in Figure 7. The surfaces are a dull chalky white and the corners are all rounded. Figure 4.
Minor chemical weathering can cause minerals to alter, perhaps to oxidize rust. More intense weathering may cause some minerals to disappear. They may dissolve completely in water and be carried away in a hydrolysate water containing dissolved ions. More often, minerals react to produce secondary minerals — minerals that were not present before weathering.
Reactions that produce secondary minerals most commonly involve the reaction of water with previously existing minerals such as feldspars common in many igneous rocks , to produce clays and dissolved elements. We call such reactions hydrolysis reactions.
Secondary minerals may also form by oxidations reactions when primary minerals react with oxygen in air or water. For example, oxidation of iron-rich olivine or pyroxene commonly produces hematite Fe 2 O 3.
Mineral matter remaining after chemical weathering often includes original mineral grains that did not decompose. We sometimes call these minerals the residual minerals , or the resistate , because the minerals resisted weathering. Typical resistate minerals include quartz, clay, K-feldspar, garnet, zircon, rutile, or magnetite. After the more easily decomposed minerals break down and disappear, the resistate minerals remain to become sediment.
If we examine fresh unweathered outcrop in a road cut, rock often appears hard and shiny. Examination with a hand lens reveals that minerals have well-defined boundaries and generally sharp outlines. They may show good cleavage or crystal faces. Minerals may have their normal diagnostic colors: quartz is clear, feldspars are white or pink, muscovite is silvery and sparkly, magnetite appears metallic, and biotite and other mafic minerals appear black.
The picture is not the same if we examine outcrops exposed to weathering for a long time. After weathering, rock and most minerals have a dull or drab appearance. Grain boundaries and cleavages are obscured. Oxidation rusting and hydration may produce reddish, yellow, brown, or gray hues.
Sometimes a layer of clay or other material coats all surfaces, obscuring diagnostic minerals. The photo shown here Figure 7.
The once solid crystalline granite is now a dull earthy mass. Mafic silicates weather to create secondary clay minerals and iron oxides. Feldspars of all sorts weather to become clay minerals and dissolved material. Quartz is usually unchanged by weathering. Calcite weathers by dissolution producing dissolve ions. And aluminous minerals weather to gibbsite or other aluminum hydroxides.
The table below lists weathering products for the most common minerals. Clays and limonite a general term describing for a mix of hydrated Fe-oxides and hydroxides dominate the list. Quartz and aluminous minerals may also be produced.
While creating these secondary minerals, weathering also produces dissolved cations especially alkalis and alkali earths and anions, which may have a significant impact on water chemistry and quality. Some minerals break down more easily than others.
Geologists can compare weathering rates by looking at minerals in rock outcrops, and by studying the minerals present in sediments of different ages. The series ranked the ease with which common igneous minerals break down. Goldich found that minerals that crystallize from a magma at high temperature — minerals relatively poor in silicon and oxygen — are generally less resistant to weathering than those that crystallize at low temperature. Iron-magnesium silicates, such as olivine, pyroxene, or amphibole break down relatively easily.
Calcic feldspars, and many minerals with high solubilities in water, are also quick to decompose. Quartz, some feldspars, and some nonsilicate minerals are relatively resistant to weathering because they contain more bonds, especially Si — O bonds, that do not break easily.
It should not be surprising that minerals that characterize high-temperature igneous rocks, or those most often precipitated from water, are the first to decompose under Earth surface conditions where temperature is low and water is abundant. Sedimentologists have made comprehensive lists of the relative ease with which minerals weather.
Although there is some variation, the list shown here is typical from Birkeland 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.
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