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Unit 4: How Sedimentary Processes Create Mineral Resources
- Summarize the processes that act to make sedimentary rocks.
- Explain how sedimentary processes (especially chemical weathering, erosion and deposition, and crystallization) redistribute and concentrate mineral resources.
- Explain how climate influences chemical weathering.
- Give examples and uses of mineral resources that are formed by sedimentary processes.
In this reading:
Chemical Weathering and Mineral Resources
Mineral Resources Created by Chemical Weathering (Table)
Mechanical Weathering, Erosion, Deposition, and Mineral Resources
Mineral Resources Created by Erosion and Deposition (Table)
Chemical Sedimentary Rocks and Mineral Resources
Mineral Resources that are Chemical Sedimentary Rocks (Table)
Sedimentary Rocks and Processes: A Summary
Sedimentary processes, namely weathering, erosion, crystallization, deposition, and lithification, create the sedimentary family of rocks. Most of Earth's surface is covered by sediments (like mud, sand, and gravel), and the majority of rocks you will see (the majority of rocks exposed at Earth's surface) are sedimentary rocks (like mudstone, sandstone, and limestone).
Chemical Weathering and Mineral Resources
Weathering processes break rocks into smaller pieces. There are two types of weathering: chemical and mechanical. Chemical weathering changes the chemical make-up of minerals and hence breaks them apart. For example, when sugar is stirred into water, the sugar dissolves (the sugar crystal breaks apart into the molecules that compose it; it is no longer a crystalline solid). This is a type of chemical weathering. Rust is also an example of chemical weathering; in that case, iron minerals oxidize and become smaller minerals with different compositions.
There are several things to keep in mind about chemical weathering:
- Chemical weathering requires liquid water. There will be more chemical weathering where rainfall and humidity are high.
- Different minerals chemically weather and dissolve differently (they have different solubilities). Some minerals such as halite (salt) dissolve easily (quickly). Some minerals will not dissolve. Some minerals might not dissolve quickly, but they may succumb slowly or to other types of chemical weathering. Stable minerals do not chemically weather very easily or at all.
- Chemical weathering can only act on the surface of a rock or mineral, or where a crack allows air and water into the mineral/rock. Therefore, more chemical weathering happens around these edges where the rocks have first been mechanically weathered. Mechanical weathering physically breaks a rock into smaller pieces of the same rock or minerals in that rock.
Mineral Solubility, and How Chemical Weathering Creates Mineral Reserves
As mentioned before, different minerals dissolve at different rates.
Table 1. Mineral Solubilities Mineral Time it would take a hypothetical 1 mm sphere of this mineral to dissolve in water with pH=5 (years) calcite 0.1 olivine (forsterite) 2300 plagioclase (albite) 575,000 mica (muscovite) 2,600,000 clay (kaolinite) 6,000,000 quartz 34,000,000
The above table lists how long it would take for very small pieces of minerals to dissolve. This list does not include halite (salt), which would dissolve in minutes instead of months or years. The minerals are listed from most soluble to least soluble. Halite and calcite have high solubilities, and quartz has a very low solubility and is thus more stable.
Not only do minerals have different solubilities, but elements within minerals dissolve differently. When minerals chemically weather, some elements in those minerals will readily dissolve in water and be removed by water. Other elements are not readily soluble, so will remain in a mineral. The mineral may not completely break into separate ions but will change composition (and hence become a different mineral) as one type of ion is removed.×
For example, imagine that a rock is made of minerals X and Y (purple and gray in the figure to the right, respectively). Mineral Y is made of elements C, D, and E. Mineral X is very stable, and does not react with water, similar to quartz. Water flowing through the broken rock will alter mineral Y, but X will stay intact. In mineral Y, imagine that element C (the red element in the figure) is soluble, but elements D and E are not. Water will remove C. The concentrations of elements left behind (D and E: the insoluble elements) increase in the mineral; chemical weathering thus concentrated D and E (D and E would be found in residual weathering deposits also called laterite deposits). In addition, the water containing the dissolved ions will carry element C somewhere else, where C may join with other ions and crystallize to make a mineral; and in this way, C is also concentrated. Thus chemical weathering concentrates minerals and/or elements, and turns mineral resources into mineral reserves.
Because they form right at the surface, residual weathering deposits are likely to erode away. Therefore, residual weathering deposits tend to be found in fairly young sediment layers, as older deposits will have already eroded.
Table 2. Mineral Resources Formed by Chemical Weathering Mineral Commodity How the mineral forms Some uses gibbsite, boehmite (in the rock called bauxite) aluminum chemical weathering of aluminum-rich sediments, igneous rocks, or volcanic ash food and beverage packaging, parts in airplanes and cars garnierite and goethite nickel chemical weathering of ultramafic (low silica content) igneous rocks stainless steel, batteries goethite and hematite iron oxides, iron not currently mined for iron, but is a potential reserve steel, pigment (paint, makeup) clay minerals clay chemical weathering of feldspar minerals ceramics, paper, cat litter
Mechanical Weathering, Erosion, Deposition, and Mineral Resources
Erosion and Deposition
Mechanical weathering processes break rocks into smaller pieces without changing their compositions. The rock and mineral fragments created by mechanical weathering are called clastic sediments, and will be eroded (picked up and carried) and deposited elsewhere. Thousands or millions of years of erosion can deposit large quantities of a certain mineral in one spot, so even though the concentration of the mineral might have been low in the weathered rock, the concentration can be much higher in the sedimentary deposit. These mineral deposits are called placer deposits.
Streams are often the agents that erode (transport) these minerals. Streams can deposit sediments along the stream bank (in sand or gravel bars) and in alluvial fan or delta deposits. Once the sediments deposit in the ocean, they can be eroded further by waves and deposited in beaches. Wind can also pick up clastic sediment and concentrate placer deposits.
Sediments deposited by wind, streams, and waves are sorted by size and density. Gravity pulls sediments to the ground (or the bottom of the stream or ocean), but the wind or rushing water (or waves) pulls sediments sideways, attempting to keep them aloft. Larger and denser (higher specific gravity) sediments require more wind, stream, or wave energy to stay afloat.
If we know the size and specific gravity of the mineral that we are looking for, and we know the energy levels of a stream (or wave), then we can predict where in the stream (or shoreline) we should look for that mineral.
For example, gold tends to form small clastic sediments, but it is very dense. Therefore, small pieces of gold may deposit with large pieces of less dense clastic sediment (gravel) along streams. Therefore we should look for gold in gravel bars in modern streams, or in deposits of conglomerate that were once gravel bars.×
Before you go to the local stream to pan for gold, remember that the rock that weathered to begin with is important. If the rock that mechanically weathered to make the gravel did not contain any gold, then we will not find gold in the gravel. Gold is mostly found in igneous or metamorphic rocks. So, a gravel bar along the Meramec River in Missouri that contains only the chemical sedimentary rock chert is unlikely to contain gold. A river flowing from a granite outcrop in the Rocky Mountains, however, might have gold in its gravel bars.
Sand and gravel are important industrial mineral resources. Sand and gravel are clastic sediments, so may be eroded and deposited in environments (e.g., along streams and beaches) that can later be mined for these resources.
Table 3. Mineral Resources Formed by Mechanical Weathering, Erosion, and Deposition Mineral Commodity Specific gravity Some uses various gravel construction quartz sand, silica 2.65 (quartz) construction, glass, computer chips rutile, ilmenite titanium 4.18-4.25 (rutile), 4.7 (ilmenite) white pigment (paint), titanium metal (airplanes) chromite chromium 4.6 stainless steel rare-earth oxides rare-earth elements (REE) computer and television displays, fiber optic cables, magnets, catalytic converters in cars cassiterite tin 6.8-7.1 coating for tin cans, bronze cinnabar mercury 8.1 fluorescent lights, thermometers, production of chlorine diamond diamond 3.5 gemstone, abrasive native copper copper 8.9 wiring, antifungal and antibacterial chemicals native silver silver 10.5 mirrors, photography, electronics native gold gold 15-19.3 jewelry, electronics, currency native platinum platinum 14-19 jewelry, catalytic converters in cars, dental equipment
Chemical Sedimentary Rocks and Mineral Resources
Water contains the dissolved ions derived from chemical weathering. When water evaporates, only pure water will turn to vapor and enter the atmosphere. Any ions are left behind, and the concentration of ions in the remaining water increases. When the water becomes saturated in ions, those ions will combine to form new minerals (i.e., crystallization will occur). The new mixture of minerals is a chemical sedimentary rock. Chemical sedimentary rocks that form when water evaporates are called evaporite deposits.
Table 4. Compositions of Brines Ion Average seawater (mg/kg) Great Salt Lake (mg/kg) Rain water from Menlo Park, CA (mg/kg) Mississippi River (mg/kg) chloride 19,350 140,000 0.8-17 24 sodium 10,760 85,600 0-9.4 20 sulfate 2,710 16,400 0.7-7.6 51 magnesium 1,290 7,200 0.7-1.2 10 calcium 411 241 0.8-1.2 38 potassium 399 4,070 0 2.9 bicarbonate 142 251 7-Apr 113 silica .5-10 48 0.3-1.2 7.9
As water evaporates, different ions reach saturation, and thus crystallize, at different times. The most insoluble minerals crystallize first, and the most soluble crystallize last. So, as salt water evaporates, calcite will crystallize first (using up the carbonate and bicarbonate ions), then gypsum (using up the sulfate), then halite (using up the sodium), and lastly potassium and magnesium salts. The minerals (and rocks they comprise) form in distinct layers, with calcite (which forms the rock limestone) on the bottom and potassium and magnesium salts on top. Because these chemical sedimentary rock contain only one mineral, beneficiation of the mined reserve (where minerals have to be separated from other minerals) may not be needed.
Because high evaporation rates are needed for crystallization, climate is important. For example, limestone (the sedimentary rock made of calcite) is presently forming in the shallow seas off the Bahamas. Also, natural highs and lows in the seafloor and/or coral reefs can create basins that favor chemical sedimentary rock formation.
Table 5. Chemical Sedimentary Rocks that are Mineral Resources Rock Mineral Commodity Some uses limestone, dolostone calcite, dolomite limestone, calcite (calcium carbonate) cement, calcium in food and supplements rock gypsum gypsum gypsum drywall rock salt halite halite (salt), chlorine (element) food additive, road deicer, source of chlorine (water treatment, bleach, etc.) rock salt sylvite potassium salt food additive, road deicer, source of potassium (fertilizer) banded iron formation magnetite, hematite iron steel
One nice thing about evaporite minerals is that they are readily soluble; that is, it is easy to remove the desired element from the mineral. For example, if soybean plants need sulfur, ground gypsum (a mineral) can be added to the soil, and the soybeans can readily remove the sulfur from the gypsum. Humans need sodium. If we eat the mineral halite, then our bodies easily remove the sodium from the salt. It would be almost impossible to remove sodium from an insoluble silicate mineral like albite (feldspar), for example.
Instead of mining the crystallized evaporite deposits, salt water or brines (even saltier water) can be mined. Salt water is, of course, found in the oceans. Brines are found in terminal lakes (lakes into which rivers flow but which have no outlets) as well as underground (the deepest groundwater is often brine).
Sedimentary Rocks and Processes: A Summary
The concept map shows how all sedimentary rocks form. The blue and tan boxes contain substances, the purple boxes show places or forces, and white boxes contain sedimentary processes (the actual events that create sedimentary rocks). These processes are further defined in the glossary. Some sedimentary rocks form with the help of living things (green boxes). (Rocks that form from or with the help of organisms are sometimes called biochemical or bioclastic sedimentary rocks.)
The different sedimentary processes can concentrate elements, making them economical to mine. In other words, sedimentary processes can turn a mineral resource into a mineral reserve.
Drever, James I. 1997. The Geochemistry of Natural Waters: Surface and Groundwater Environments, 3rd edition. Prentice Hall.
Hem, John D. 1985. Study and Interpretation of the Chemical Characteristics of Natural Water, 3rd edition. USGS Water Supply Paper 2254
Cementation: Part of lithification that involves minerals crystallizing in pore space and holding sediments together, hardening the sediment. This happens when water flows through pores between sediments. Ions in the water will combine to form minerals (crystals) that fill the pores, holding the sediments together. Some sedimentary rocks are "well cemented," making them quite hard. Some sedimentary rocks are "poorly cemented" and may crumble apart in your hand. Quartz and calcite are the two minerals that most commonly form the cement in sedimentary rocks. Return to text
Chemical Sedimentary Rock: Rocks formed by an accumulation and lithification of minerals that crystallized in water. Because the water must be saturated with ions to form these minerals, these rocks most often form in oceans (ions make seawater taste salty). These rocks include those made of minerals crystallized by organisms, which are sometimes called biochemical or bioclastic sedimentary rocks. Limestone, chert, rock salt, rock gypsum are all types of chemical sedimentary rocks. Return to text
Chemical Weathering: Processes that break a rock apart by changing the chemical composition of minerals or dissolving minerals entirely. For example, salt dropped into a glass of water will dissolve, in which case it will change from the mineral (NaCl) to ions (Na+ and Cl-) that float about in the water. Another example is rust, in which oxygen enters an iron-bearing mineral and changes it into a different (oxidized and smaller) mineral. Chemical weathering is responsible for making the oceans salty and creating all of the clay minerals on Earth. Clay minerals form when feldspar (the most common minerals in Earth's crust) chemically weather. Return to text
Clastic Sedimentary Rock: Rocks made of lithified clastic sediments. Mudstone (shale), siltstone, sandstone, and conglomerate (made of lithified gravel) are types of clastic sedimentary rock.
Clastic Sediments (detrital sediments): Fragments (broken pieces) of rocks and minerals. Gravel, sand, silt, and mud/clay are types of clastic sediments. Return to text
Compaction: The squeezing together of sediments at the bottom of a pile by the weight of additional sediments deposited on top. This reduces pore space and is part of lithification.
Crystallization: The combining of ions to make minerals. The minerals that form are called crystals. In sedimentary rocks, the ions are found dissolved in water. When the water becomes saturated in these ions (the concentration becomes too high), the ions combine to form minerals. Organisms (such as corals and plankton) can cause minerals to crystallize. Crystallization also happens in magma, but in that case the created minerals combine to make igneous rocks. Crystallization can also happen in very hot, squishy rocks during metamorphism. Return to text
Deposition: When sediments are dropped by the erosion agent (wind, waves, streams, gravity, or glaciers) to form a layer (bed). Return to text
Erosion: The transportation of sediments, during which erosion agents pick up and move loose sediment. The "agents" can be wind, waves, streams (the geologic word for all flowing water), glaciers, and gravity. The pieces (clastic sediments) that can be picked up by wind, streams, and waves depends on the energy level of the wind, streams, and waves. A stream with higher energy can carry large and small sediments with a range of densities. If the stream loses energy, it will drop the heaviest pieces (the largest or densest) and continue to carry smaller pieces. In this way, streams, wind, and waves sort sediment. Return to text
Evaporites: Chemical sedimentary rocks formed when salt water evaporates. Return to text
Industrial Mineral Resource: Mineral resources that are nonmetallic are often grouped together as industrial minerals. Return to text
Ions: Charged atoms or molecules. For example, Na+is a sodium atom with a +1 charge. The silicate ion is a molecule (SiO4) with a +4 charge. Ions can bond together (during crystallization) to make a mineral. When a mineral melts or chemically weathers, it is broken into ions and ceases to be a mineral. Return to text
Lithification: Processes that turn loose sediment into hard rock. This happens by compaction, when sediments are buried by newer sediment and squeezed together, and cementation, when minerals crystallize in pores and hold sediments together. Return to text
Mechanical Weathering (physical weathering): Processes break rock into smaller pieces of that same rock and/or minerals in that rock. Return to text
Mineral Reserve: A mineral or rock resource that can be extracted (mined) for a profit. This means that the resources must found in high enough concentrations and quantities, and that other factors (economic, social, governmental, technological, etc.) are favorable for its extraction. Return to text
Mineral Resource: Any mineral or rock mined from the earth and used in products. Return to text
Placer Deposits: Mineral resources collected by gravity sorting in streams, beaches, and sand dunes. Return to text
Residual Weathering Deposits (laterites): Mineral resources formed when chemical weathering removes some elements, leaving high concentrations of the mineral resource behind in the residual weathering deposit. Return to text
Solubility: A mineral property. Soluble minerals dissolve, and minerals that have high solubilities readily dissolve (break apart into their component ions). Return to text
Sorting: Sediment can be sorted by size and or composition during deposition. If sediment is sorted, then most of the deposited sediment all has the same size or composition. For example, streams commonly sort sediment, so that in one part of the stream you might find a gravel bar (the size of most of the sediment is gravel), and along another place there might be a sand bar (the size of most of the sediment is sand). Return to text
Stability: A mineral property. Stable minerals do not chemically weather very readily, whereas unstable minerals will chemically weather. Return to text
Weathering: Processes that break rocks and minerals apart. There are two types of weathering processes: mechanical and chemical. Return to text