In condensed matter physics and materials science, an amorphous or non-crystalline solid is a solid that lacks the long-range order, characteristic of a crystal. In some older books, the term has been used synonymously with glass. Nowadays, "glassy solid" or "amorphous solid" is considered to be the overarching concept, glass the more special case: Glass is an amorphous solid that exhibits a glass transition. Polymers are amorphous. Other types of amorphous solids include gels, thin films, nanostructured materials such as glass doors and windows. Amorphous materials have an internal structure made of interconnected structural blocks; these blocks can be similar to the basic structural units found in the corresponding crystalline phase of the same compound. Whether a material is liquid or solid depends on the connectivity between its elementary building blocks so that solids are characterized by a high degree of connectivity whereas structural blocks in fluids have lower connectivity. In pharmaceutical industry, the amorphous drugs were shown to have higher bioavailability than their crystalline counterparts due to the high solubility of amorphous phase.
Moreover, certain compounds can undergo precipitation in their amorphous form in vivo, they can decrease each other's bioavailability if administered together. Amorphous materials have some shortrange order at the atomic length scale due to the nature of chemical bonding. Furthermore, in small crystals a large fraction of the atoms are the crystal; the most advanced structural characterization techniques, such as x-ray diffraction and transmission electron microscopy, have difficulty in distinguishing between amorphous and crystalline structures on these length scales. Amorphous phases are important constituents of thin films, which are solid layers of a few nanometres to some tens of micrometres thickness deposited upon a substrate. So-called structure zone models were developed to describe the micro structure and ceramics of thin films as a function of the homologous temperature Th, the ratio of deposition temperature over melting temperature. According to these models, a necessary condition for the occurrence of amorphous phases is that Th has to be smaller than 0.3, the deposition temperature must be below 30% of the melting temperature.
For higher values, the surface diffusion of deposited atomic species would allow for the formation of crystallites with long range atomic order. Regarding their applications, amorphous metallic layers played an important role in the discussion of a suspected superconductivity in amorphous metals. Today, optical coatings made from TiO2, SiO2, Ta2O5 etc. and combinations of them in most cases consist of amorphous phases of these compounds. Much research is carried out into thin amorphous films as a gas separating membrane layer; the technologically most important thin amorphous film is represented by few nm thin SiO2 layers serving as isolator above the conducting channel of a metal-oxide semiconductor field-effect transistor. Hydrogenated amorphous silicon, a-Si:H in short, is of technical significance for thin-film solar cells. In case of a-Si:H the missing long-range order between silicon atoms is induced by the presence by hydrogen in the percent range; the occurrence of amorphous phases turned out as a phenomenon of particular interest for studying thin-film growth.
Remarkably, the growth of polycrystalline films is used and preceded by an initial amorphous layer, the thickness of which may amount to only a few nm. The most investigated example is represented by thin multicrystalline silicon films, where such as the unoriented molecule. An initial amorphous layer was observed in many studies. Wedge-shaped polycrystals were identified by transmission electron microscopy to grow out of the amorphous phase only after the latter has exceeded a certain thickness, the precise value of which depends on deposition temperature, background pressure and various other process parameters; the phenomenon has been interpreted in the framework of Ostwald's rule of stages that predicts the formation of phases to proceed with increasing condensation time towards increasing stability. Experimental studies of the phenomenon require a defined state of the substrate surface and its contaminant density etc. upon which the thin film is deposited. R. Zallen; the Physics of Amorphous Solids.
Wiley Interscience. S. R. Elliot; the Physics of Amorphous Materials. Longman. N. Cusack; the Physics of Structurally Disordered Matter: An Introduction. IOP Publishing. N. H. March. A. Street. P. Tosi, eds.. Amorphous Solids and the Liquid State. Springer. D. A. Adler. B. Schwartz. C. Steele, eds.. Physical Properties of Amorphous Materials. Springer. A. Inoue. Amorphous and Nanocrystalline Materials. Springer. Journal of non-crystalline solids
Failure is the state or condition of not meeting a desirable or intended objective, may be viewed as the opposite of success. Product failure ranges from failure to sell the product to fracture of the product, in the worst cases leading to personal injury, the province of forensic engineering. MIT neuroscience professor Earl K. Miller discovered that the reason why we keep repeating mistakes is because brain cells may only learn from experience when we do something right and not when we fail. Wired magazine editor Kevin Kelly explains that a great deal can be learned from things going wrong unexpectedly, that part of science's success comes from keeping blunders "small, manageable and trackable", he uses the example of engineers and programmers who push systems to their limits, breaking them to learn about them. Kelly warns against creating a culture that punishes failure harshly, because this inhibits a creative process, risks teaching people not to communicate important failures with others.
The criteria for failure are dependent on context of use, may be relative to a particular observer or belief system. A situation considered to be a failure by one might be considered a success by another in cases of direct competition or a zero-sum game; the degree of success or failure in a situation may be differently viewed by distinct observers or participants, such that a situation that one considers to be a failure, another might consider to be a success, a qualified success or a neutral situation. It may be difficult or impossible to ascertain whether a situation meets criteria for failure or success due to ambiguous or ill-defined definition of those criteria. Finding useful and effective criteria, or heuristics, to judge the success or failure of a situation may itself be a significant task. Failure can be differentially perceived from the viewpoints of the evaluators. A person, only interested in the final outcome of an activity would consider it to be an Outcome Failure if the core issue has not been resolved or a core need is not met.
A failure can be a process failure whereby although the activity is completed a person may still feel dissatisfied if the underlying process is perceived to be below expected standard or benchmark. Failure to anticipate Failure to perceive Failure to carry out a taskLoser is a derogatory term for a person, unsuccessful or undesirable. A commercial failure is a company that does not reach expectations of success. Most of the items listed below had high expectations, significant financial investments, and/or widespread publicity, but fell far short of success. Due to the subjective nature of "success" and "meeting expectations," there can be disagreement about what constitutes a "major flop." For flops in computer and video gaming, see list of commercial failures in computer and video gaming For company failures related to the 1997–2001 dot-com bubble, see dot-com company See vaporware Box-office bombSometimes, "commercial failures" can receive a cult following. "Fail" is the name of a popular Internet meme where users superimpose a caption the word "fail" or "epic fail", onto photos or short videos depicting unsuccessful events or people falling short of expectations.
In July 2003, a contributor to Urban Dictionary wrote that the term, "fail," could be used as an interjection, "when one disapproves of something," citing the example: "You bought that? FAIL." This most originated as a shortened form of "You fail" or, more "You fail it," the taunting "game over" message in the 1998 Japanese video game Blazing Star, notorious for its fractured English. There is an entire Internet site dedicated to "fails" called Fail Blog; the #fail hashtag is used on the microblogging site Twitter to indicate contempt or displeasure, the image that accompanied the message that the site was overloaded is referred to as the "fail whale". Failboat or consignment of fail is a popular macro series, featuring images of cargo vessels tipping over or shedding cargo, with captions such as'the failboat has arrived', or'all aboard the failboat'; the original vessel whose image was used was the MV Cougar Ace, although the Ital Florida, the MV Napoli and the SS Normandy, sunk at her berth in New York Harbor, have appeared.
The term "miserable failure" has been popularized as a result of a known "Google bombing," which caused Google searches for the term to turn up the White House biography of George W. Bush. Perrow, Charles. Normal Accidents: Living with High-Risk Technologies. New York: Basic Books, 1983. Paperback reprint, Princeton, N. J.: Princeton University Press, 1999. ISBN 0-691-00412-9 Sandage, Scott A. Born Losers: A History of Failure in America. Cambridge, Massachusetts: Harvard University Press, 2005. ISBN 0-674-01510-X, ISBN 0-674-02107-X Designing Building Failures Zimmer, Ben, "How Fail Went From Verb to Interjection", The New York Times Magazine. Association for the Study of Failure from Japan
In archaeology, a lithic flake is a "portion of rock removed from an objective piece by percussion or pressure," and may be referred to as a chip or spall, or collectively as debitage. The objective piece, or the rock being reduced by the removal of flakes, is known as a core. Once the proper tool stone has been selected, a percussor or pressure flaker is used to direct a sharp blow, or apply sufficient force to the surface of the stone on the edge of the piece; the energy of this blow propagates through the material producing a Hertzian cone of force which causes the rock to fracture in a controllable fashion. Since cores are struck on an edge with a suitable angle for flake propagation, the result is that only a portion of the Hertzian cone is created; the process continues as the flintknapper detaches the desired number of flakes from the core, marked with the negative scars of these removals. The surface area of the core which received the blows necessary for detaching the flakes is referred to as the striking platform.
Flakes may be produced by a variety of means. Force may be introduced by pressure. Additionally, flakes may be initiated in a Hertzian, wedging fashion; when a flake is detached from its core in a Hertzian fashion, the flake propagates in a conchoidal manner from the point of impact or pressure producing a partial Hertzian cone. The cone of force leaves a distinctive bulb of applied force on the flake and a corresponding flake scar on the core. A bending initiation results when a flake initiates not at the point where the force was applied, but rather further away from the edge of the core, resulting in a flake with no Hertzian cone or bulb of applied force and few if any of the characteristics ripples or undulations seen on the ventral surface of conchoidally produced flakes. Wedging initiation is the result of a strong hammer blow. At impact, concentric radii emanate from the point of percussion, but unlike conchoidal fracture, the force travels along what would be the center of the Hertzian cone.
The bipolar reduction technique is typified by its use of wedge initiation. Like bending initiation, no bulb of applied force results from wedging initiation, although in the bipolar technique, flakes may appear to have two points of percussion, on opposite ends, because the core has been fractured by a hammer and anvil technique; the core is placed on a hard surface or "anvil" and is struck above by a hammer, thus the fracture may propagate from both ends simultaneously. The end which received the blow or pressure is referred to as the proximal end of the flake; the side displaying the bulb of force but without flake scars is called the ventral surface, while the opposite side, displaying the flake scars of previous removals, or the cortical or original rock surface, is the dorsal surface. On most natural cobbles or nodules of source material, a weathered outer rind called a cortex covers the unweathered inner material. Flakes are differentiated by the amount of cortex present on their dorsal surfaces, because the amount of cortex indicates when in the sequence of reduction the flake came from.
Primary flakes are those whose dorsal surfaces are covered with cortex. Primary flakes and secondary flakes are associated with the initial stages of lithic reduction, while tertiary flakes are more to be associated with retouching and bifacial reduction activities. Prominent bulbs of force indicate that a hard hammer percussor was used to detach the flake. Hard hammer flakes are indicative of primary reduction strategies. More moderate and diffuse bulbs may indicate the use of a soft hammer percussor—such as bone, wood, or antler—which produces the bending flakes associated with bifacial thinning and trimming; the relative abundance of each type of flake can indicate what sort of lithic work was going on at a particular spot at a particular point in time. A blade is defined as a flake with parallel or subparallel margins, at least twice as long as it is wide. There are numerous specialized types of blade flakes. Channel flakes are characteristic flakes caused by the fluting of certain Paleo-Indian projectile points.
Prismatic blades are long, narrow specialized blades with parallel margins which may be removed from polyhedral blade cores, another common lithic feature of Paleo-Indian lithic culture. Prismatic blades are triangular in cross section with several facets or flake scars on the dorsal surface. Prismatic blades begin to appear in high frequencies during the transition between the Middle and Upper Paleolithic; this lithic technology replaces the Levallois reduction technology. The striking platform is the point on the proximal portion of the flake on which the detachment blow fell or pressure was placed; this may be prepared. Termination type is a characteristic indicating the manner in which the distal end of a flake detached from
Gallium is a chemical element with symbol Ga and atomic number 31. It is in group 13 of the periodic table, thus has similarities to the other metals of the group, aluminium and thallium. Gallium does not occur as a free element in nature, but as gallium compounds in trace amounts in zinc ores and in bauxite. Elemental gallium is a soft, silvery blue metal at standard temperature and pressure, a brittle solid at low temperatures, a liquid at temperatures greater than 29.76 °C. The melting point of gallium is used as a temperature reference point. Gallium alloys are used in thermometers as a non-toxic and environmentally friendly alternative to mercury, can withstand higher temperatures than mercury; the alloy galinstan has an lower melting point of −19 °C, well below the freezing point of water. Since its discovery in 1875, gallium has been used to make alloys with low melting points, it is used in semiconductors as a dopant in semiconductor substrates. Gallium is predominantly used in electronics.
Gallium arsenide, the primary chemical compound of gallium in electronics, is used in microwave circuits, high-speed switching circuits, infrared circuits. Semiconducting gallium nitride and indium gallium nitride produce blue and violet light-emitting diodes and diode lasers. Gallium is used in the production of artificial gadolinium gallium garnet for jewelry. Gallium is considered a technology-critical element. Gallium has no known natural role in biology. Gallium behaves in a similar manner to ferric salts in biological systems and has been used in some medical applications, including pharmaceuticals and radiopharmaceuticals. Elemental gallium is not found in nature, but it is obtained by smelting. Pure gallium metal has a silvery color and its solid metal fractures conchoidally like glass. Gallium liquid expands by 3.10 %. Gallium shares the higher-density liquid state with a short list of other materials that includes water, germanium, antimony and plutonium. Gallium attacks most other metals by diffusing into the metal lattice.
For example, it diffuses into the grain boundaries of aluminium-zinc alloys and steel, making them brittle. Gallium alloys with many metals, is used in small quantities in the plutonium-gallium alloy in the plutonium cores of nuclear bombs to stabilize the plutonium crystal structure; the melting point of gallium, at 302.9146 K, is just above room temperature, is the same as the average summer daytime temperatures in Earth's mid-latitudes. This melting point is one of the formal temperature reference points in the International Temperature Scale of 1990 established by the International Bureau of Weights and Measures; the triple point of gallium, 302.9166 K, is used by the US National Institute of Standards and Technology in preference to the melting point. The melting point of gallium allows it to melt in the human hand, refreeze if removed; the liquid metal has a strong tendency to supercool below its melting point/freezing point: Ga nanoparticles can be kept in the liquid state below 90 K. Seeding with a crystal helps to initiate freezing.
Gallium is one of the four non-radioactive metals that are known to be liquid at, or near, normal room temperature. Of the four, gallium is the only one, neither reactive nor toxic and can therefore be used in metal-in-glass high-temperature thermometers, it is notable for having one of the largest liquid ranges for a metal, for having a low vapor pressure at high temperatures. Gallium's boiling point, 2673 K, is more than eight times higher than its melting point on the absolute scale, the greatest ratio between melting point and boiling point of any element. Unlike mercury, liquid gallium metal wets glass and skin, along with most other materials, making it mechanically more difficult to handle though it is less toxic and requires far fewer precautions. Gallium painted onto glass is a brilliant mirror. For this reason as well as the metal contamination and freezing-expansion problems, samples of gallium metal are supplied in polyethylene packets within other containers. Gallium does not crystallize in any of the simple crystal structures.
The stable phase under normal conditions is orthorhombic with 8 atoms in the conventional unit cell. Within a unit cell, each atom has only one nearest neighbor; the remaining six unit cell neighbors are spaced 27, 30 and 39 pm farther away, they are grouped in pairs with the same distance. Many stable and metastable phases are found as function of pressure; the bonding between the two nearest neighbors is covalent. This explains the low melting point relative to the neighbor elements and indium; this structure is strikingly similar to that of iodine and forms because of interactions between the single 4p electrons of gallium atoms, further away from the nucleus than the 4s electrons and the 3d10 core. This phenomenon recurs with mercury with its "pseudo-noble-gas" 4f145d106s2 electron configuration, liquid at room temperature; the 3d10 electrons do not shield the outer electrons well from the nucleus an
Obsidian is a occurring volcanic glass formed as an extrusive igneous rock. Obsidian is produced when felsic lava extruded from a volcano cools with minimal crystal growth, it is found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition causes a high viscosity which, upon rapid cooling, forms a natural glass from the lava. The inhibition of atomic diffusion through this viscous lava explains the lack of crystal growth. Obsidian is hard and amorphous. In the past it was used to manufacture cutting and piercing tools and it has been used experimentally as surgical scalpel blades.... among the various forms of glass we may reckon Obsidian glass, a substance similar to the stone found by Obsidius in Ethiopia. The translation into English of Natural History written by Pliny the Elder of Rome shows a few sentences on the subject of a volcanic glass called obsidian, discovered in Ethiopia by Obsidius, a Roman explorer. Obsidian is the rock formed as a result of cooled lava, the parent material.
Extrusive formation of obsidian may occur when felsic lava cools at the edges of a felsic lava flow or volcanic dome or when lava cools during sudden contact with water or air. Intrusive formation of obsidian may occur. Tektites were once thought by many to be obsidian produced by lunar volcanic eruptions, though few scientists now adhere to this hypothesis. Obsidian is mineral-like, but not a true mineral, it is sometimes classified as a mineraloid. Though obsidian is dark in color, similar to mafic rocks such as basalt, obsidian's composition is felsic. Obsidian consists of SiO2 70% or more. Crystalline rocks with obsidian's composition include rhyolite; because obsidian is metastable at the Earth's surface, no obsidian has been found, older than Cretaceous age. This breakdown of obsidian is accelerated by the presence of water. Having a low water content when newly formed less than 1% water by weight, obsidian becomes progressively hydrated when exposed to groundwater, forming perlite. Pure obsidian is dark in appearance, though the color varies depending on the presence of impurities.
Iron and other transition elements may give the obsidian a dark brown to black color. Few samples are nearly colorless. In some stones, the inclusion of small, radially clustered crystals spherulites of the mineral cristobalite in the black glass produce a blotchy or snowflake pattern. Obsidian may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled; these bubbles can produce interesting effects such as a golden sheen. An iridescent, rainbow-like sheen is caused by inclusions of magnetite nanoparticles. Obsidian can be found in locations, it can be found in Argentina, Azerbaijan, Canada, Georgia, Greece, El Salvador, Iceland, Japan, Mexico, New Zealand, Papua New Guinea, Scotland and the United States. Obsidian flows which may be hiked on are found within the calderas of Newberry Volcano and Medicine Lake Volcano in the Cascade Range of western North America, at Inyo Craters east of the Sierra Nevada in California.
Yellowstone National Park has a mountainside containing obsidian located between Mammoth Hot Springs and the Norris Geyser Basin, deposits can be found in many other western U. S. states including Arizona, New Mexico, Utah, Washington and Idaho. Obsidian can be found in the eastern U. S. states of Virginia, as well as North Carolina. There are only four major deposit areas in the central Mediterranean: Lipari, Pantelleria and Monte Arci. Ancient sources in the Aegean were Gyali. Acıgöl town and the Göllü Dağ volcano were the most important sources in central Anatolia, one of the more important source areas in the prehistoric Near East; the first known archaeological evidence of usage was in Kariandusi and other sites of the Acheulian age dated 700,000 BC, although the number of objects found at these sites were low relative to the Neolithic. Use of obsidian in pottery of the Neolithic in the area around Lipari was found to be less at a distance representing two weeks journeying. Anatolian sources of obsidian are known to have been the material used in the Levant and modern-day Iraqi Kurdistan from a time beginning sometime about 12,500 BC.
The first attested civilized use is dated to the late fifth millennium BC, known from excavations at Tell Brak. Obsidian was valued in Stone Age cultures because, like flint, it could be fractured to produce sharp blades or arrowheads. Like all glass and some other types of occurring rocks, obsidian breaks with a characteristic conchoidal fracture, it was polished to create early mirrors. Modern archaeologists have developed a relative dating system, obsidian hydration dating, to calculate the age of obsidian artifacts. In the Ubaid in the 5th millennium BC, blades were manufactured from obsidian extracted from outcrops located in modern-day Turkey. Ancient Egyptians used obsidian imported from the eastern Mediterranean and southern Red Sea regions. Obsidian was used in ritual circumcisions because of its deftness and sharpness. In the eastern Mediterranean
A crystallite is a small or microscopic crystal which forms, for example, during the cooling of many materials. The orientation of crystallites can be random with no preferred direction, called random texture, or directed due to growth and processing conditions. Fiber texture is an example of the latter. Crystallites are referred to as grains; the areas where crystallites meet are known as grain boundaries. Polycrystalline or multicrystalline materials, or polycrystals are solids that are composed of many crystallites of varying size and orientation. Most inorganic solids are polycrystalline, including all common metals, many ceramics and ice; the extent to which a solid is crystalline has important effects on its physical properties. Sulfur, while polycrystalline, may occur in other allotropic forms with different properties. Although crystallites are referred to as grains, powder grains are different, as they can be composed of smaller polycrystalline grains themselves. While the structure of a crystal is ordered and its lattice is continuous and unbroken, amorphous materials, such as glass and many polymers, are non-crystalline and do not display any structures as their constituents are not arranged in an ordered manner.
Polycrystalline structures and paracrystalline phases are in between these two extremes. Crystallite size is measured from X-ray diffraction patterns and grain size by other experimental techniques like transmission electron microscopy. Solid objects large enough to see and handle are composed of a single crystal, except for a few cases. Most materials are polycrystalline, made of a large number of single crystals – crystallites – held together by thin layers of amorphous solid; the crystallite size can vary from a few nanometers to several millimeters. If the individual crystallites are oriented at random, a large enough volume of polycrystalline material will be isotropic; this property helps the simplifying assumptions of continuum mechanics to apply to real-world solids. However, most manufactured materials have some alignment to their crystallites, resulting in texture that must be taken into account for accurate predictions of their behavior and characteristics; when the crystallites are ordered with just some random spread of orientations, one has a mosaic crystal.
Material fractures can be a transgranular fracture. There is an ambiguity with powder grains: a powder grain can be made of several crystallites. Thus, the "grain size" found by laser granulometry can be different from the "grain size" found by X-ray diffraction, by optical microscopy under polarised light, or by scanning electron microscopy. Coarse grained rocks are formed slowly, while fine grained rocks are formed on geological time scales. If a rock forms quickly, such as the solidification of lava ejected from a volcano, there may be no crystals at all; this is. Grain boundaries are interfaces. A grain boundary is a single-phase interface, with crystals on each side of the boundary being identical except in orientation; the term "crystallite boundary" is sometimes, though used. Grain boundary areas contain those atoms that have been perturbed from their original lattice sites and impurities that have migrated to the lower energy grain boundary. Treating a grain boundary geometrically as an interface of a single crystal cut into two parts, one of, rotated, we see that there are five variables required to define a grain boundary.
The first two numbers come from the unit vector. The third number designates the angle of rotation of the grain; the final two numbers specify the plane of the grain boundary. Grain boundaries disrupt the motion of dislocations through a material. Dislocation propagation is impeded because of the stress field of the grain boundary defect region and the lack of slip planes and slip directions and overall alignment across the boundaries. Reducing grain size is therefore a common way to improve strength without any sacrifice in toughness because the smaller grains create more obstacles per unit area of slip plane; this crystallite size-strength relationship is given by the Hall–Petch relationship. The high interfacial energy and weak bonding in grain boundaries makes them preferred sites for the onset of corrosion and for the precipitation of new phases from the solid. Grain boundary migration plays an important role in many of the mechanisms of creep. Grain boundary migration occurs when a shear stress acts on the grain boundary plane and causes the grains to slide.
This means that fine-grained materials have a poor resistance to creep relative to coarser grains at high temperatures, because smaller grains contain more atoms in grain boundary sites. Grain boundaries cause deformation in that they are sources and sinks of point defects. Voids in a material tend to gather in a grain boundary, if this happens to a critical extent, the material could fracture. During grain boundary migration, the rate determining step depends on the angle between two adjacent grains. In a small angle dislocation boundary, the migration rate depends on vacancy diffusion between dislocations. In a high angle dislocation boundary, this depends on the atom transport by single atom jumps from the shrinki
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