Forsterite is the magnesium-rich end-member of the olivine solid solution series. It is isomorphous with fayalite. Forsterite crystallizes in the orthorhombic system with cell parameters a 4.75 Å, b 10.20 Å and c 5.98 Å. Forsterite is associated with igneous and metamorphic rocks and has been found in meteorites. In 2005 it was found in cometary dust returned by the Stardust probe. In 2011 it was observed as tiny crystals in the dusty clouds of gas around a forming star. Two polymorphs of forsterite are known: ringwoodite. Both are known from meteorites. Peridot is the gemstone variety of forsterite olivine. Pure forsterite is composed of magnesium and silicon; the chemical formula is Mg2SiO4. Forsterite and tephroite are the end-members of the olivine solid solution series. Other minerals such as monticellite, an uncommon calcium-rich mineral, share the olivine structure, but solid solution between olivine and these other minerals is limited. Monticellite is found in contact metamorphosed dolomites.
Forsterite-rich olivine is the most abundant mineral in the mantle above a depth of about 400 km. Although pure forsterite does not occur in igneous rocks, dunite contains olivine with forsterite contents at least as Mg-rich as Fo92. Due to its high melting point, olivine crystals are the first minerals to precipitate from a magmatic melt in a cumulate process with orthopyroxenes. Forsterite-rich olivine is a common crystallization product of mantle-derived magma. Olivine in mafic and ultramafic rocks is rich in the forsterite end-member. Forsterite occurs in dolomitic marble which results from the metamorphism of high magnesium limestones and dolostones. Nearly pure forsterite occurs in some metamorphosed serpentinites. Fayalite-rich olivine is much less common. Nearly pure fayalite is a minor constituent in some granite-like rocks, it is a major constituent of some metamorphic banded iron formations. Forsterite is composed of the anion SiO44− and the cation Mg2+ in a molar ratio 1:2. Silicon is the central atom in the SiO44− anion.
Each oxygen atom is bonded to the silicon by a single covalent bond. The four oxygen atoms have a partial negative charge because of the covalent bond with silicon. Therefore, oxygen atoms need to stay far from each other in order to reduce the repulsive force between them; the best geometry to reduce the repulsion is a tetrahedral shape. The cations occupy two different octahedral sites which are M1 and M2 and form ionic bonds with the silicate anions. M1 and M2 are different. M2 site is larger and more regular than M1 as shown in Fig. 1. The packing in forsterite structure is dense; the space group of this structure is Pbnm and the point group is 2/m 2/m 2/m, an orthorhombic crystal structure. This structure of forsterite can form a complete solid solution by replacing the magnesium with iron. Iron can form two different cations which are Fe2+ and Fe3+; the iron ion has the same charge as magnesium ion and it has a similar ionic radius to magnesium. Fe2+ can replace the magnesium ion in the olivine structure.
One of the important factors that can increase the portion of forsterite in the olivine solid solution is the ratio of iron ions to iron ions in the magma. As the iron ions oxidize and become iron ions, iron ions cannot form olivine because of their 3+ charge; the occurrence of forsterite due to the oxidation of iron was observed in the Stromboli volcano in Italy. As the volcano fractured and volatiles escaped from the magma chamber; the crystallization temperature of the magma increased. Because iron ions were oxidized in the Stromboli magma, little iron was available to form Fe-rich olivine. Hence, the crystallizing olivine was Mg-rich, igneous rocks rich in forsterite were formed. At high pressure, forsterite undergoes a phase transition into wadsleyite. 14–15 GPa. In high-pressure experiments, the transformation may be delayed so that forsterite can remain metastable at pressures up to 50 GPa; the progressive metamorphism between dolomite and quartz react to form forsterite and carbon dioxide: 2 CaMg 2 + SiO 2 ⟶ Mg 2 SiO 4 + 2 CaCO 3 + 2 CO 2 Forsterite reacts with quartz to form the orthopyroxene mineral enstatite in the following reaction: Mg 2 SiO 4 + SiO 2 ⟶ 2 MgSiO 3 Forsterite was first described in 1824 for an occurrence at Mount Somma, Italy.
It was named by Armand Lévy in 1824 after the English naturalist and mineral collector Adolarius Jacob Forster. Forsterite is being studied as a potential biomaterial for implants owing to its
In crystallography, the terms crystal system, crystal family, lattice system each refer to one of several classes of space groups, point groups, or crystals. Informally, two crystals are in the same crystal system if they have similar symmetries, although there are many exceptions to this. Crystal systems, crystal families and lattice systems are similar but different, there is widespread confusion between them: in particular the trigonal crystal system is confused with the rhombohedral lattice system, the term "crystal system" is sometimes used to mean "lattice system" or "crystal family". Space groups and crystals are divided into seven crystal systems according to their point groups, into seven lattice systems according to their Bravais lattices. Five of the crystal systems are the same as five of the lattice systems, but the hexagonal and trigonal crystal systems differ from the hexagonal and rhombohedral lattice systems; the six crystal families are formed by combining the hexagonal and trigonal crystal systems into one hexagonal family, in order to eliminate this confusion.
A lattice system is a class of lattices with the same set of lattice point groups, which are subgroups of the arithmetic crystal classes. The 14 Bravais lattices are grouped into seven lattice systems: triclinic, orthorhombic, rhombohedral and cubic. In a crystal system, a set of point groups and their corresponding space groups are assigned to a lattice system. Of the 32 point groups that exist in three dimensions, most are assigned to only one lattice system, in which case both the crystal and lattice systems have the same name. However, five point groups are assigned to two lattice systems and hexagonal, because both exhibit threefold rotational symmetry; these point groups are assigned to the trigonal crystal system. In total there are seven crystal systems: triclinic, orthorhombic, trigonal and cubic. A crystal family is determined by lattices and point groups, it is formed by combining crystal systems which have space groups assigned to a common lattice system. In three dimensions, the crystal families and systems are identical, except the hexagonal and trigonal crystal systems, which are combined into one hexagonal crystal family.
In total there are six crystal families: triclinic, orthorhombic, tetragonal and cubic. Spaces with less than three dimensions have the same number of crystal systems, crystal families and lattice systems. In one-dimensional space, there is one crystal system. In 2D space, there are four crystal systems: oblique, rectangular and hexagonal; the relation between three-dimensional crystal families, crystal systems and lattice systems is shown in the following table: Note: there is no "trigonal" lattice system. To avoid confusion of terminology, the term "trigonal lattice" is not used; the 7 crystal systems consist of 32 crystal classes as shown in the following table: The point symmetry of a structure can be further described as follows. Consider the points that make up the structure, reflect them all through a single point, so that becomes; this is the'inverted structure'. If the original structure and inverted structure are identical the structure is centrosymmetric. Otherwise it is non-centrosymmetric.
Still in the non-centrosymmetric case, the inverted structure can in some cases be rotated to align with the original structure. This is a non-centrosymmetric achiral structure. If the inverted structure cannot be rotated to align with the original structure the structure is chiral or enantiomorphic and its symmetry group is enantiomorphic. A direction is called polar if its two directional senses are physically different. A symmetry direction of a crystal, polar is called a polar axis. Groups containing a polar axis are called polar. A polar crystal possesses a unique polar axis; some geometrical or physical property is different at the two ends of this axis: for example, there might develop a dielectric polarization as in pyroelectric crystals. A polar axis can occur only in non-centrosymmetric structures. There cannot be a mirror plane or twofold axis perpendicular to the polar axis, because they would make the two directions of the axis equivalent; the crystal structures of chiral biological molecules can only occur in the 65 enantiomorphic space groups.
The distribution of the 14 Bravais lattices into lattice systems and crystal families is given in the following table. In geometry and crystallography, a Bravais lattice is a category of translative symmetry groups in three directions; such symmetry groups consist of translations by vectors of the form R = n1a1 + n2a2 + n3a3,where n1, n2, n3 are integers and a1, a2, a3 are three non-coplanar vectors, called primitive vectors. These lattices are classified by the space group of the lattice itself, viewed as a collection of points, they represent the maximum symmetry. All crystalline materials must, by definition, fit into one of these arrangements. For convenience a Bravais lattice is depicted by a unit cell, a factor 1, 2, 3 or 4 larger than the primitive cell. Depending on the symmetry of a crystal or other pattern, the fundamental domain is again smaller, up to a factor 48; the Bravais lattices were studied by Moritz Ludwig Frankenheim in 1842, who found that there we
A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the object enters the atmosphere, various factors such as friction and chemical interactions with the atmospheric gases cause it to heat up and radiate that energy, it becomes a meteor and forms a fireball known as a shooting star or falling star. Meteorites vary in size. For geologists, a bolide is a meteorite large enough to create an impact crater. Meteorites that are recovered after being observed as they transit the atmosphere and impact the Earth are called meteorite falls. All others are known as meteorite finds; as of August 2018, there were about 1,412 witnessed falls that have specimens in the world's collections. As of 2018, there are more than 59,200 well-documented meteorite finds. Meteorites have traditionally been divided into three broad categories: stony meteorites that are rocks composed of silicate minerals.
Modern classification schemes divide meteorites into groups according to their structure and isotopic composition and mineralogy. Meteorites smaller than 2 mm are classified as micrometeorites. Extraterrestrial meteorites are such objects that have impacted other celestial bodies, whether or not they have passed through an atmosphere, they have been found on the Mars. Meteorites are always named for the places they were found a nearby town or geographic feature. In cases where many meteorites were found in one place, the name may be followed by a number or letter; the name designated by the Meteoritical Society is used by scientists and most collectors. Most meteoroids disintegrate. Five to ten a year are observed to fall and are subsequently recovered and made known to scientists. Few meteorites are large enough to create large impact craters. Instead, they arrive at the surface at their terminal velocity and, at most, create a small pit. Large meteoroids may strike the earth with a significant fraction of their escape velocity, leaving behind a hypervelocity impact crater.
The kind of crater will depend on the size, degree of fragmentation, incoming angle of the impactor. The force of such collisions has the potential to cause widespread destruction; the most frequent hypervelocity cratering events on the Earth are caused by iron meteoroids, which are most able to transit the atmosphere intact. Examples of craters caused by iron meteoroids include Barringer Meteor Crater, Odessa Meteor Crater, Wabar craters, Wolfe Creek crater. In contrast relatively large stony or icy bodies like small comets or asteroids, up to millions of tons, are disrupted in the atmosphere, do not make impact craters. Although such disruption events are uncommon, they can cause a considerable concussion to occur. Large stony objects, hundreds of meters in diameter or more, weighing tens of millions of tons or more, can reach the surface and cause large craters, but are rare; such events are so energetic that the impactor is destroyed, leaving no meteorites. Several phenomena are well documented during witnessed meteorite falls too small to produce hypervelocity craters.
The fireball that occurs as the meteoroid passes through the atmosphere can appear to be bright, rivaling the sun in intensity, although most are far dimmer and may not be noticed during daytime. Various colors have been reported, including yellow and red. Flashes and bursts of light can occur. Explosions and rumblings are heard during meteorite falls, which can be caused by sonic booms as well as shock waves resulting from major fragmentation events; these sounds can be heard with a radius of a hundred or more kilometers. Whistling and hissing sounds are sometimes heard, but are poorly understood. Following passage of the fireball, it is not unusual for a dust trail to linger in the atmosphere for several minutes; as meteoroids are heated during atmospheric entry, their surfaces experience ablation. They can be sculpted into various shapes during this process, sometimes resulting in shallow thumbprint-like indentations on their surfaces called regmaglypts. If the meteoroid maintains a fixed orientation for some time, without tumbling, it may develop a conical "nose cone" or "heat shield" shape.
As it decelerates the molten surface layer solidifies into a thin fusion crust, which on most meteorites is black. On stony meteorites, the heat-affected zone is at most a few mm deep. Reports vary. Meteorites from multiple falls, such as Bjurbole, Tagish Lake, Buzzard Coulee, have been found having fallen on lake and sea ice suggesting that they were not hot when they
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
A supernova is an event that occurs upon the death of certain types of stars. Supernovae are more energetic than novae. In Latin, nova means "new", referring astronomically to what appears to be a temporary new bright star. Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous; the word supernova was coined by Walter Baade and Fritz Zwicky in 1931. Only three Milky Way, naked-eye supernova events have been observed during the last thousand years, though many have been seen in other galaxies; the most recent directly observed supernova in the Milky Way was Kepler's Supernova in 1604, but two more recent supernova remnants have been found. Statistical observations of supernovae in other galaxies suggest they occur on average about three times every century in the Milky Way, that any galactic supernova would certainly be observable with modern astronomical telescopes. Supernovae may expel much, if not all, of the material away from a star at velocities up to 30,000 km/s or 10% of the speed of light.
This drives an expanding and fast-moving shock wave into the surrounding interstellar medium, in turn, sweeping up an expanding shell of gas and dust, observed as a supernova remnant. Supernovae create and eject the bulk of the chemical elements produced by nucleosynthesis. Supernovae play a significant role in enriching the interstellar medium with the heavier atomic mass chemical elements. Furthermore, the expanding shock waves from supernovae can trigger the formation of new stars. Supernova remnants are expected to accelerate a large fraction of galactic primary cosmic rays, but direct evidence for cosmic ray production was found only in a few of them so far, they are potentially strong galactic sources of gravitational waves. Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star or the sudden gravitational collapse of a massive star's core. In the first instance, a degenerate white dwarf may accumulate sufficient material from a binary companion, either through accretion or via a merger, to raise its core temperature enough to trigger runaway nuclear fusion disrupting the star.
In the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical collapse mechanics have been established and accepted by most astronomers for some time. Owing to the wide range of astrophysical consequences of these events, astronomers now deem supernova research, across the fields of stellar and galactic evolution, as an important area for investigation; the earliest recorded supernova HB9 was viewed by Indians 5,000-years ago and recorded in the oldest Star chart. The SN 185, was viewed by Chinese astronomers in 185 AD; the brightest recorded supernova was SN 1006, which occurred in 1006 AD and was described by observers across China, Iraq and Europe. The observed supernova SN 1054 produced the Crab Nebula. Supernovae SN 1572 and SN 1604, the latest to be observed with the naked eye in the Milky Way galaxy, had notable effects on the development of astronomy in Europe because they were used to argue against the Aristotelian idea that the universe beyond the Moon and planets was static and unchanging.
Johannes Kepler began observing SN 1604 at its peak on October 17, 1604, continued to make estimates of its brightness until it faded from naked eye view a year later. It was the second supernova to be observed in a generation. There is some evidence that the youngest galactic supernova, G1.9+0.3, occurred in the late 19th century more than Cassiopeia A from around 1680. Neither supernova was noted at the time. In the case of G1.9+0.3, high extinction along the plane of the galaxy could have dimmed the event sufficiently to go unnoticed. The situation for Cassiopeia A is less clear. Infrared light echos have been detected showing that it was a type IIb supernova and was not in a region of high extinction. Before the development of the telescope, only five supernovae were seen in the last millennium. Compared to a star's entire history, the visual appearance of a galactic supernova is brief spanning several months, so that the chances of observing one is once in a lifetime. Only a tiny fraction of the 100 billion stars in a typical galaxy have the capacity to become a supernova, restricted to either those having large mass or extraordinarily rare kinds of binary stars containing white dwarfs.
However and discovery of extragalactic supernovae are now far more common. The first such observation was of SN 1885A in the Andromeda galaxy. Today and professional astronomers are finding several hundred every year, some when near maximum brightness, others on old astronomical photographs or plates. American astronomers Rudolph Minkowski and Fritz Zwicky developed the modern supernova classification scheme beginning in 1941. During the 1960s, astronomers found that the maximum intensities of supernovae could be used as standard candles, hence indicators of astronomical distances; some of the most distant supernovae observed in 2003, appeared dimmer than expected. This supports the view. Techniques were developed for reconstructing supernovae events that have no written records of being observed; the date of the Cassiopeia A supernova event was determined from light echoes off nebulae, while the age of supernova remnant RX J0852.0-4622 was estimated from temperature
The streak of a mineral is the color of the powder produced when it is dragged across an un-weathered surface. Unlike the apparent color of a mineral, which for most minerals can vary the trail of finely ground powder has a more consistent characteristic color, is thus an important diagnostic tool in mineral identification. If no streak seems to be made, the mineral's streak is said to be colorless. Streak is important as a diagnostic for opaque and colored materials, it is less useful for silicate minerals, most of which have a white streak or are too hard to powder easily. The apparent color of a mineral can vary because of trace impurities or a disturbed macroscopic crystal structure. Small amounts of an impurity that absorbs a particular wavelength can radically change the wavelengths of light that are reflected by the specimen, thus change the apparent color. However, when the specimen is dragged to produce a streak, it is broken into randomly oriented microscopic crystals, small impurities do not affect the absorption of light.
The surface across which the mineral is dragged is called a "streak plate", is made of unglazed porcelain tile. In the absence of a streak plate, the unglazed underside of a porcelain bowl or vase or the back of a glazed tile will work. Sometimes a streak is more or described by comparing it with the "streak" made by another streak plate; because the trail left behind results from the mineral being crushed into powder, a streak can only be made of minerals softer than the streak plate, around 7 on the Mohs scale of mineral hardness. For harder minerals, the color of the powder can be determined by filing or crushing with a hammer a small sample, usually rubbed on a streak plate. Most minerals that are harder have an unhelpful white streak; some minerals leave a streak similar to their natural color, such as lazurite. Other minerals leave surprising colors, such as fluorite, which always has a white streak, although it can appear in purple, yellow, or green crystals. Hematite, black in appearance, leaves a red streak which accounts for its name, which comes from the Greek word "haima", meaning "blood."
Galena, which can be similar in appearance to hematite, is distinguished by its gray streak. Bishop, A. C.. R.. R.. Cambridge Guide to Minerals and Fossils. Cambridge: Cambridge University Press. Pp. 12–13. Holden, Martin; the Encyclopedia of Gemstones and Minerals. New York: Facts on File. p. 251. ISBN 1-56799-949-2. Schumann, Walter. Minerals of the World. New York: Sterling Publishing. Pp. 18–16. ISBN 0-00-219909-2. Physical Characteristics of Minerals, at Introduction to Mineralogy by Andrea Bangert What is Streak? from the Mineral Gallery
Mohs scale of mineral hardness
The Mohs scale of mineral hardness is a qualitative ordinal scale characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. Created in 1812 by German geologist and mineralogist Friedrich Mohs, it is one of several definitions of hardness in materials science, some of which are more quantitative; the method of comparing hardness by observing which minerals can scratch others is of great antiquity, having been mentioned by Theophrastus in his treatise On Stones, c. 300 BC, followed by Pliny the Elder in his Naturalis Historia, c. 77 AD. While facilitating the identification of minerals in the field, the Mohs scale does not show how well hard materials perform in an industrial setting. Despite its lack of precision, the Mohs scale is relevant for field geologists, who use the scale to identify minerals using scratch kits; the Mohs scale hardness of minerals can be found in reference sheets. Mohs hardness is useful in milling, it allows assessment of.
The scale is used at electronic manufacturers for testing the resilience of flat panel display components. The Mohs scale of mineral hardness is based on the ability of one natural sample of mineral to scratch another mineral visibly; the samples of matter used by Mohs are all different minerals. Minerals are chemically pure solids found in nature. Rocks are made up of one or more minerals; as the hardest known occurring substance when the scale was designed, diamonds are at the top of the scale. The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, or the softest material that can scratch the given material. For example, if some material is scratched by apatite but not by fluorite, its hardness on the Mohs scale would fall between 4 and 5. "Scratching" a material for the purposes of the Mohs scale means creating non-elastic dislocations visible to the naked eye. Materials that are lower on the Mohs scale can create microscopic, non-elastic dislocations on materials that have a higher Mohs number.
While these microscopic dislocations are permanent and sometimes detrimental to the harder material's structural integrity, they are not considered "scratches" for the determination of a Mohs scale number. The Mohs scale is a purely ordinal scale. For example, corundum is twice as hard as topaz; the table below shows the comparison with the absolute hardness measured by a sclerometer, with pictorial examples. On the Mohs scale, a streak plate has a hardness of 7.0. Using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale; the table below incorporates additional substances that may fall between levels: Comparison between hardness and hardness: Mohs hardness of elements is taken from G. V. Samsonov in Handbook of the physicochemical properties of the elements, IFI-Plenum, New York, USA, 1968. Cordua, William S. "The Hardness of Minerals and Rocks". Lapidary Digest, c. 1990