Illite
Illite is a group of related non-expanding clay minerals. Illite is a secondary mineral precipitate, an example of a phyllosilicate, or layered alumino-silicate, its structure is a 2:1 sandwich of silica tetrahedron – alumina octahedron – silica tetrahedron layers. The space between this T-O-T sequence of layers is occupied by poorly hydrated potassium cations which are responsible for the absence of swelling. Structurally, illite is quite similar to muscovite with more silicon, magnesium and water and less tetrahedral aluminium and interlayer potassium; the chemical formula is given as 24O10. It occurs as aggregates of small monoclinic grey to white crystals. Due to the small size, positive identification requires x-ray diffraction or SEM-EDS analysis. Illite occurs as an altered product of muscovite and feldspar in weathering and hydrothermal environments, it is common in sediments and argillaceous sedimentary rocks as well as in some low grade metamorphic rocks. The iron rich member of the illite group, glauconite, in sediments can be differentiated by x-ray analysis.
The cation-exchange capacity of illite is smaller than that of smectite but higher than that of kaolinite around 20 – 30 meq/100 g. Illite was first described for occurrences in the Maquoketa shale in Calhoun County, Illinois, US, in 1937; the name was derived from its type location in Illinois. Illite is called hydromica or hydromuscovite. Brammallite is a sodium rich analogue. Avalite is a chromium bearing variety, described form Mt. Avala, Serbia; the crystallinity of illite has been used as an indicator of metamorphic grade in clay-bearing rocks metamorphosed under conditions between diagenesis and low-grade metamorphism. With increasing temperature, illite is thought to undergo a transformation into muscovite. Mitchell JK. "Ch. 3: Soil Mineralogy". Fundamentals of soil behavior. New York: John Wiley and Sons, Inc. p. 32. ISBN 9780471463023
Sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particules and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes; the particles that form a sedimentary rock are called sediment, may be composed of geological detritus or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, transported to the place of deposition by water, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and piling up on the floor of water bodies. Sedimentation may occur as dissolved minerals precipitate from water solution; the sedimentary rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only 8% of the total volume of the crust.
Sedimentary rocks are only a thin veneer over a crust consisting of igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata; the study of sedimentary rocks and rock strata provides information about the subsurface, useful for civil engineering, for example in the construction of roads, tunnels, canals or other structures. Sedimentary rocks are important sources of natural resources like coal, fossil fuels, drinking water or ores; the study of the sequence of sedimentary rock strata is the main source for an understanding of the Earth's history, including palaeogeography and the history of life. The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology. Sedimentology is part of both geology and physical geography and overlaps with other disciplines in the Earth sciences, such as pedology, geomorphology and structural geology. Sedimentary rocks have been found on Mars. Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation: clastic sedimentary rocks, biochemical sedimentary rocks, chemical sedimentary rocks, a fourth category for "other" sedimentary rocks formed by impacts and other minor processes.
Clastic sedimentary rocks are composed of other rock fragments that were cemented by silicate minerals. Clastic rocks are composed of quartz, rock fragments, clay minerals, mica. Clastic sedimentary rocks, are subdivided according to the dominant particle size. Most geologists use the Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions: gravel and mud; the classification of clastic sedimentary rocks parallels this scheme. This tripartite subdivision is mirrored by the broad categories of rudites and lutites in older literature; the subdivision of these three broad categories is based on differences in clast shape, grain size or texture. Conglomerates are dominantly composed of rounded gravel, while breccias are composed of dominantly angular gravel. Sandstone classification schemes vary but most geologists have adopted the Dott scheme, which uses the relative abundance of quartz and lithic framework grains and the abundance of a muddy matrix between the larger grains.
Composition of framework grains The relative abundance of sand-sized framework grains determines the first word in a sandstone name. Naming depends on the dominance of the three most abundant components quartz, feldspar, or the lithic fragments that originated from other rocks. All other minerals are considered accessories and not used in the naming of the rock, regardless of abundance. Quartz sandstones have >90% quartz grains Feldspathic sandstones have <90% quartz grains and more feldspar grains than lithic grains Lithic sandstones have <90% quartz grains and more lithic grains than feldspar grainsAbundance of muddy matrix material between sand grains When sand-sized particles are deposited, the space between the grains either remains open or is filled with mud. "Clean" sandstones with open pore space are called arenites. Muddy sandstones with abundant muddy matrix are called wackes. Six sandstone names are possible using the descriptors for grain composition and the amount of matrix. For example, a quartz arenite would be composed of quartz grains and have little or no clayey matrix between the grains, a lithic wacke would have abundant lithic grains and abundant muddy matrix, etc.
Although the Dott classification scheme is used by sedimentologists, common names like greywacke and quartz sandstone are still used by non-specialists and in popular literature. Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles; these fine-grained particles are transported by turbulent flow in water or air, deposited as the flow calms and the particles settle out of suspension. Most authors presently
Redox
Redox is a chemical reaction in which the oxidation states of atoms are changed. Any such reaction involves both a reduction process and a complementary oxidation process, two key concepts involved with electron transfer processes. Redox reactions include all chemical reactions; the chemical species from which the electron is stripped is said to have been oxidized, while the chemical species to which the electron is added is said to have been reduced. It can be explained in simple terms: Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion. Reduction is a decrease in oxidation state by a molecule, atom, or ion; as an example, during the combustion of wood, oxygen from the air is reduced, gaining electrons from carbon, oxidized. Although oxidation reactions are associated with the formation of oxides from oxygen molecules, oxygen is not included in such reactions, as other chemical species can serve the same function; the reaction can occur slowly, as with the formation of rust, or more in the case of fire.
There are simple redox processes, such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane, more complex processes such as the oxidation of glucose in the human body. "Redox" is a portmanteau of the words "reduction" and "oxidation". The word oxidation implied reaction with oxygen to form an oxide, since dioxygen was the first recognized oxidizing agent; the term was expanded to encompass oxygen-like substances that accomplished parallel chemical reactions. The meaning was generalized to include all processes involving loss of electrons; the word reduction referred to the loss in weight upon heating a metallic ore such as a metal oxide to extract the metal. In other words, ore was "reduced" to metal. Antoine Lavoisier showed. Scientists realized that the metal atom gains electrons in this process; the meaning of reduction became generalized to include all processes involving a gain of electrons. Though "reduction" seems counter-intuitive when speaking of the gain of electrons, it might help to think of reduction as the loss of oxygen, its historical meaning.
Since electrons are negatively charged, it is helpful to think of this as reduction in electrical charge. The electrochemist John Bockris has used the words electronation and deelectronation to describe reduction and oxidation processes when they occur at electrodes; these words are analogous to protonation and deprotonation, but they have not been adopted by chemists worldwide. The term "hydrogenation" could be used instead of reduction, since hydrogen is the reducing agent in a large number of reactions in organic chemistry and biochemistry. But, unlike oxidation, generalized beyond its root element, hydrogenation has maintained its specific connection to reactions that add hydrogen to another substance; the word "redox" was first used in 1928. The processes of oxidation and reduction occur and cannot happen independently of one another, similar to the acid–base reaction; the oxidation alone and the reduction alone are each called a half-reaction, because two half-reactions always occur together to form a whole reaction.
When writing half-reactions, the gained or lost electrons are included explicitly in order that the half-reaction be balanced with respect to electric charge. Though sufficient for many purposes, these general descriptions are not correct. Although oxidation and reduction properly refer to a change in oxidation state — the actual transfer of electrons may never occur; the oxidation state of an atom is the fictitious charge that an atom would have if all bonds between atoms of different elements were 100% ionic. Thus, oxidation is best defined as an increase in oxidation state, reduction as a decrease in oxidation state. In practice, the transfer of electrons will always cause a change in oxidation state, but there are many reactions that are classed as "redox" though no electron transfer occurs. In redox processes, the reductant transfers electrons to the oxidant. Thus, in the reaction, the reductant or reducing agent loses electrons and is oxidized, the oxidant or oxidizing agent gains electrons and is reduced.
The pair of an oxidizing and reducing agent that are involved in a particular reaction is called a redox pair. A redox couple is a reducing species and its corresponding oxidizing form, e.g. Fe2+/Fe3+ Substances that have the ability to oxidize other substances are said to be oxidative or oxidizing and are known as oxidizing agents, oxidants, or oxidizers; that is, the oxidant removes electrons from another substance, is thus itself reduced. And, because it "accepts" electrons, the oxidizing agent is called an electron acceptor. Oxygen is the quintessential oxidizer. Oxidants are chemical substances with elements in high oxidation states, or else electronegative elements that can gain extra electrons by oxidizing another substance. Substances that have the ability to reduce other substances are said to be reductive or reducing and are known as
Lagoon
A lagoon is a shallow body of water separated from a larger body of water by barrier islands or reefs. Lagoons are divided into coastal lagoons and atoll lagoons, they have been identified as occurring on mixed-sand and gravel coastlines. There is an overlap between bodies of water classified as coastal lagoons and bodies of water classified as estuaries. Lagoons are common coastal features around many parts of the world. Lagoons are shallow elongated bodies of water separated from a larger body of water by a shallow or exposed shoal, coral reef, or similar feature; some authorities include fresh water bodies in the definition of "lagoon", while others explicitly restrict "lagoon" to bodies of water with some degree of salinity. The distinction between "lagoon" and "estuary" varies between authorities. Richard A. Davis Jr. restricts "lagoon" to bodies of water with little or no fresh water inflow, little or no tidal flow, calls any bay that receives a regular flow of fresh water an "estuary". Davis does state that the terms "lagoon" and "estuary" are "often loosely applied in scientific literature."
Timothy M. Kusky characterizes lagoons as being elongated parallel to the coast, while estuaries are drowned river valleys, elongated perpendicular to the coast; when used within the context of a distinctive portion of coral reef ecosystems, the term "lagoon" is synonymous with the term "back reef" or "backreef", more used by coral reef scientists to refer to the same area. Coastal lagoons are classified as inland bodies of water. Many lagoons do not include "lagoon" in their common names. Albemarle and Pamlico sounds in North Carolina, Great South Bay between Long Island and the barrier beaches of Fire Island in New York, Isle of Wight Bay, which separates Ocean City, Maryland from the rest of Worcester County, Banana River in Florida, Lake Illawarra in New South Wales, Montrose Basin in Scotland, Broad Water in Wales have all been classified as lagoons, despite their names. In England, The Fleet at Chesil Beach has been described as a lagoon. In Latin America, the term laguna in Spanish, which lagoon translates to, may be used for a small fresh water lake in a similar way a creek is considered a small river.
However, sometimes it is popularly used to describe a full-sized lake, such as Laguna Catemaco in Mexico, the third largest lake by area in the country. The brackish water lagoon may be thus explicitly identified as a "coastal lagoon". In Portuguese the same usage is found: lagoa may be a body of shallow sea water, or a small freshwater lake not linked to the sea. Lagoon is derived from the Italian laguna, which refers to the waters around Venice, the Lagoon of Venice. Laguna is attested in English by at least 1612, had been Anglicized to "lagune" by 1673. In 1697 William Dampier referred to a "Lake of Salt water" on the coast of Mexico. Captain James Cook described an island "of Oval form with a Lagoon in the middle" in 1769. Atoll lagoons form as coral reefs grow upwards while the islands that the reefs surround subside, until only the reefs remain above sea level. Unlike the lagoons that form shoreward of fringing reefs, atoll lagoons contain some deep portions. Coastal lagoons form along sloping coasts where barrier islands or reefs can develop off-shore, the sea-level is rising relative to the land along the shore.
Coastal lagoons do not form along steep or rocky coasts, or if the range of tides is more than 4 metres. Due to the gentle slope of the coast, coastal lagoons are shallow, they are sensitive to changes in sea level due to global warming. A relative drop in sea level may leave a lagoon dry, while a rise in sea level may let the sea breach or destroy barrier islands, leave reefs too deep under water to protect the lagoon. Coastal lagoons are young and dynamic, may be short-lived in geological terms. Coastal lagoons are common. In the United States, lagoons are found along more than 75 percent of the Gulf coasts. Coastal lagoons are connected to the open ocean by inlets between barrier islands; the number and size of the inlets, precipitation and inflow of fresh water all affect the nature of the lagoon. Lagoons with little or no interchange with the open ocean, little or no inflow of fresh water, high evaporation rates, such as Lake St. Lucia, in South Africa, may become saline. Lagoons with no connection to the open ocean and significant inflow of fresh water, such as the Lake Worth Lagoon in Florida in the middle of the 19th century, may be fresh.
On the other hand, lagoons with many wide inlets, such as the Wadden Sea, have strong tidal currents and mixing. Coastal lagoons tend to accumulate sediments from inflowing rivers, from runoff from the shores of the lagoon, from sediment carried into the lagoon through inlets by the tide. Large quantities of sediment may be be deposited in a lagoon when storm waves overwash barrier islands. Mangroves and marsh plants can facilitate the accumulation of sediment in a lagoon. Benthic organisms may destabilize sediments. River-mouth lagoons on mixed sand and gravel beaches form at the river-coast interface where a braided, although sometimes meandering, river interacts with a coastal environment, affected by longshore drift; the lagoons which form on the MSG coastlines are common on the east coast of the South Island of New Zealand and have long been referred to as hapua by the Māori. This classification differentiates hapua from similar lagoons located on the N
Chert
Chert is a hard, fine-grained sedimentary rock composed of crystals of quartz that are small. Quartz is the mineral form of silicon dioxide. Chert is of biological origin but may occur inorganically as a chemical precipitate or a diagenetic replacement. Geologists use chert as a generic name for any type of cryptocrystalline quartz. Chert is of biological origin, being the petrified remains of siliceous ooze, the biogenic sediment that covers large areas of the deep ocean floor, which contains the silicon skeletal remains of diatoms, silicoflagellates, radiolarians. Depending on its origin, it can contain small macrofossils, or both, it varies in color, but most manifests as gray, grayish brown and light green to rusty red. Chert occurs in carbonate rocks as oval to irregular nodules in greensand, limestone and dolostone formations as a replacement mineral, where it is formed as a result of some type of diagenesis. Where it occurs in chalk or marl, it is called flint, it occurs in thin beds, when it is a primary deposit.
Thick beds of chert occur in deep marine deposits. These thickly bedded cherts include the novaculite of the Ouachita Mountains of Arkansas and similar occurrences in Texas and South Carolina in the United States; the banded iron formations of Precambrian age are composed of alternating layers of chert and iron oxides. Chert occurs in diatomaceous deposits and is known as diatomaceous chert. Diatomaceous chert consists of beds and lenses of diatomite which were converted during diagenesis into dense, hard chert. Beds of marine diatomaceous chert comprising strata several hundred meters thick have been reported from sedimentary sequences such as the Miocene Monterey Formation of California and occur in rocks as old as the Cretaceous. In petrology the term "chert" is used to refer to all rocks composed of microcrystalline, cryptocrystalline and microfibrous quartz; the term does not include quartzite. Chalcedony is a microfibrous variety of quartz. Speaking, the term "flint" is reserved for varieties of chert which occur in chalk and marly limestone formations.
Among non-geologists, the distinction between "flint" and "chert" is one of quality – chert being lower quality than flint. This usage of the terminology is prevalent in North America and is caused by early immigrants who brought the terms from England where most true flint was indeed of better quality than "common chert". Among petrologists, chalcedony is sometimes considered separately from chert due to its fibrous structure. Since many cherts contain both microcrystalline and microfibrous quartz, it is sometimes difficult to classify a rock as chalcedony, thus its general inclusion as a variety of chert; the cryptocrystalline nature of chert, combined with its above average ability to resist weathering, recrystallization and metamorphism has made it an ideal rock for preservation of early life forms. For example: The 3.2 Ga chert of the Fig Tree Formation in the Barbeton Mountains between Swaziland and South Africa preserved non-colonial unicellular bacteria-like fossils. The Gunflint Chert of western Ontario preserves not only bacteria and cyanobacteria but organisms believed to be ammonia-consuming and some that resemble green algae and fungus-like organisms.
The Apex Chert of the Pilbara craton, Australia preserved eleven taxa of prokaryotes. The Bitter Springs Formation of the Amadeus Basin, Central Australia, preserves 850 Ma cyanobacteria and algae; the Rhynie chert of Scotland has remains of a Devonian land flora and fauna with preservation so perfect that it allows cellular studies of the fossils. In prehistoric times, chert was used as a raw material for the construction of stone tools. Like obsidian, as well as some rhyolites, felsites and other tool stones used in lithic reduction, chert fractures in a Hertzian cone when struck with sufficient force; this results in a characteristic of all minerals with no cleavage planes. In this kind of fracture, a cone of force propagates through the material from the point of impact removing a full or partial cone; the partial Hertzian cones produced during lithic reduction are called flakes, exhibit features characteristic of this sort of breakage, including striking platforms, bulbs of force, eraillures, which are small secondary flakes detached from the flake's bulb of force.
When a chert stone is struck against an iron-bearing surface sparks result. This makes chert an excellent tool for starting fires, both flint and common chert were used in various types of fire-starting tools, such as tinderboxes, throughout history. A primary historic use of common chert and flint was for flintlock firearms, in which the chert striking a metal plate produces a spark that ignites a small reservoir containing black powder, discharging the firearm. Cherts are subject to problems. Weathered chert develops surface pop-outs when used in concrete that undergoes freezing and thawing because of the high porosity of weathered cher
Mesozoic
The Mesozoic Era is an interval of geological time from about 252 to 66 million years ago. It is called the Age of Reptiles and the Age of Conifers; the Mesozoic is one of three geologic eras of the Phanerozoic Eon, preceded by the Paleozoic and succeeded by the Cenozoic. The era is subdivided into three major periods: the Triassic and Cretaceous, which are further subdivided into a number of epochs and stages; the era began in the wake of the Permian–Triassic extinction event, the largest well-documented mass extinction in Earth's history, ended with the Cretaceous–Paleogene extinction event, another mass extinction whose victims included the non-avian dinosaurs. The Mesozoic was a time of significant tectonic and evolutionary activity; the era witnessed the gradual rifting of the supercontinent Pangaea into separate landmasses that would move into their current positions during the next era. The climate of the Mesozoic was varied, alternating between cooling periods. Overall, the Earth was hotter than it is today.
Dinosaurs first appeared in the Mid-Triassic, became the dominant terrestrial vertebrates in the Late Triassic or Early Jurassic, occupying this position for about 150 or 135 million years until their demise at the end of the Cretaceous. Birds first appeared in the Jurassic; the first mammals appeared during the Mesozoic, but would remain small—less than 15 kg —until the Cenozoic. The flowering plants arose in the Triassic or Jurassic and came to prominence in the late Cretaceous when they replaced the conifers and other gymnosperms as the dominant trees; the phrase "Age of Reptiles" was introduced by the 19th century paleontologist Gideon Mantell who viewed it as dominated by diapsids such as Iguanodon, Megalosaurus and Pterodactylus. Mesozoic means "middle life", deriving from the Greek prefix meso-/μεσο- for "between" and zōon/ζῷον meaning "animal" or "living being"; the name "Mesozoic" was proposed in 1840 by the British geologist John Phillips. Following the Paleozoic, the Mesozoic extended 186 million years, from 251.902 to 66 million years ago when the Cenozoic Era began.
This time frame is separated into three geologic periods. From oldest to youngest: Triassic Jurassic Cretaceous The lower boundary of the Mesozoic is set by the Permian–Triassic extinction event, during which 90% to 96% of marine species and 70% of terrestrial vertebrates became extinct, it is known as the "Great Dying" because it is considered the largest mass extinction in the Earth's history. The upper boundary of the Mesozoic is set at the Cretaceous–Paleogene extinction event, which may have been caused by an asteroid impactor that created Chicxulub Crater on the Yucatán Peninsula. Towards the Late Cretaceous, large volcanic eruptions are believed to have contributed to the Cretaceous–Paleogene extinction event. 50% of all genera became extinct, including all of the non-avian dinosaurs. The Triassic ranges from 252 million to 201 million years ago, preceding the Jurassic Period; the period is bracketed between the Permian–Triassic extinction event and the Triassic–Jurassic extinction event, two of the "big five", it is divided into three major epochs: Early and Late Triassic.
The Early Triassic, about 252 to 247 million years ago, was dominated by deserts in the interior of the Pangaea supercontinent. The Earth had just witnessed a massive die-off in which 95% of all life became extinct, the most common vertebrate life on land were lystrosaurus and euparkeria along with many other creatures that managed to survive the Permian extinction. Temnospondyls would be the dominant predator for much of the Triassic; the Middle Triassic, from 247 to 237 million years ago, featured the beginnings of the breakup of Pangaea and the opening of the Tethys Sea. Ecosystems had recovered from the Permian extinction. Algae, sponge and crustaceans all had recovered, new aquatic reptiles evolved, such as ichthyosaurs and nothosaurs. On land, pine forests flourished, as did groups of insects like mosquitoes and fruit flies. Reptiles began to get bigger and bigger, the first crocodilians and dinosaurs evolved, which sparked competition with the large amphibians that had ruled the freshwater world mammal-like reptiles on land.
Following the bloom of the Middle Triassic, the Late Triassic, from 237 to 201 million years ago, featured frequent heat spells and moderate precipitation. The recent warming led to a boom of dinosaurian evolution on land as those one began to separate from each other, as well as first pterosaurs. During the Late Triassic, some advanced cynodonts gave rise to the first Mammaliaformes. All this climatic change, resulted in a large die-out known as the Triassic-Jurassic extinction event, in which many archosaurs, most synapsids, all large amphibians became extinct, as well as 34% of marine life, in the Earth's fourth mass extinction event; the cause is debatable. The Jurassic ranges from 200 million years to 145 million years ago and features three major epochs: The Early Jurassic, the Middle Jurassic, the L
Limonite
Limonite is an iron ore consisting of a mixture of hydrated iron oxide-hydroxides in varying composition. The generic formula is written as FeO·nH2O, although this is not accurate as the ratio of oxide to hydroxide can vary quite widely. Limonite is one of the three principal iron ores, the others being hematite and magnetite, has been mined for the production of iron since at least 2500 BCE. Limonite is named for the Greek word λειμών, meaning "wet meadow", or λίμνη, meaning “marshy lake” as an allusion to its occurrence as bog iron ore in meadows and marshes. In its brown form it is sometimes called brown iron ore. In its bright yellow form it is sometimes called yellow iron ore. Limonite is dense with a specific gravity varying from 2.7 to 4.3. It varies in colour from a bright lemony yellow to a drab greyish brown; the streak of limonite on an unglazed porcelain plate is always brownish, a character which distinguishes it from hematite with a red streak, or from magnetite with a black streak.
The hardness is variable, but in the 4 - 5.5 range. Although defined as a single mineral, limonite is now recognized as a mixture of related hydrated iron oxide minerals, among them goethite, akaganeite and jarosite. Individual minerals in limonite may form crystals, but limonite does not, although specimens may show a fibrous or microcrystalline structure, limonite occurs in concretionary forms or in compact and earthy masses; because of its amorphous nature, occurrence in hydrated areas limonite presents as a clay or mudstone. However, there are limonite pseudomorphs after other minerals such as pyrite; this means that chemical weathering transforms the crystals of pyrite into limonite by hydrating the molecules, but the external shape of the pyrite crystal remains. Limonite pseudomorphs have been formed from other iron oxides and magnetite. Limonite forms from the hydration of hematite and magnetite, from the oxidation and hydration of iron rich sulfide minerals, chemical weathering of other iron rich minerals such as olivine, pyroxene and biotite.
It is the major iron component in lateritic soils. It is deposited in run-off streams from mining operations. One of the first uses was as a pigment; the yellow form produced yellow ochre for which Cyprus was famous, while the darker forms produced more earthy tones. Roasting the limonite changed it to hematite, producing red ochres, burnt umbers and siennas. Bog iron ore and limonite mudstones are mined as a source of iron, although commercial mining of them has ceased in the United States. Iron caps or gossans of siliceous iron oxide form as the result of intensive oxidation of sulfide ore deposits; these gossans were used by prospectors as guides to buried ore. In addition the oxidation of those sulfide deposits which contained gold resulted in the concentration of gold in the iron oxide and quartz of the gossans. Goldbearing limonite gossans were productively mined in the Shasta County, California mining district. Similar deposits were mined near Rio Tinto in Mount Morgan in Australia. In the Dahlonega gold belt in Lumpkin County, Georgia gold was mined from limonite-rich lateritic or saprolite soil.
The gold of the primary veins was concentrated into the limonites of the weathered rocks. In another example the weathered iron formations of Brazil served to concentrate gold with the limonite of the resulting soils. While the first iron ore was meteoric iron, hematite was far easier to smelt, in Africa, where the first evidence of iron metallurgy occurs, limonite is the most prevalent iron ore. Before smelting, as the ore was heated and the water driven off and more of the limonite was converted to hematite; the ore was pounded as it was heated above 1250 °C, at which temperature the metallic iron begins sticking together and non-metallic impurities are thrown off as sparks. Complex systems developed, notably in Tanzania, to process limonite. Nonetheless and magnetite remained the ores of choice when smelting was by bloomeries, it was only with the development of blast furnaces in 1st century BCE in China and about 1150 CE in Europe, that the brown iron ore of limonite could be used to best advantage.
As regards to the use of limonite for pigments, it was one of the earliest man-used materials and can be seen in Neolithic cave paintings and pictographs. Bog iron Iron ore Ore genesis Mineral galleries Mindat Gold and limonite
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