A vug, vugh, or vugg is a small to medium-sized cavity inside rock. It may be formed through a variety of processes. Most cracks and fissures opened by tectonic activity are filled by quartz and other secondary minerals. Open spaces within ancient collapse breccias are another important source of vugs. Vugs may form when mineral crystals or fossils inside a rock matrix are removed through erosion or dissolution processes, leaving behind irregular voids; the inner surfaces of such vugs are coated with a crystal druse. Fine crystals are found in vugs where the open space allows the free development of external crystal form; the term vug is not applied to veins and fissures that have become filled, but may be applied to any small cavities within such veins. Geodes are a common vug-formed rock, although that term is reserved for more rounded crystal-lined cavities in sedimentary rocks and ancient lavas; the word vug was introduced to the English language by Cornish miners, from the days when Cornwall was a major supplier of tin.
The Cornish word was vooga, which meant "cave". Vesicular texture Rock microstructure Amygdule The dictionary definition of vug at Wiktionary
Halite known as rock salt, is a type of salt, the mineral form of sodium chloride. Halite forms isometric crystals; the mineral is colorless or white, but may be light blue, dark blue, pink, orange, yellow or gray depending on inclusion of other materials and structural or isotopic abnormalities in the crystals. It occurs with other evaporite deposit minerals such as several of the sulfates and borates; the name halite is derived from the Ancient Greek word for salt, ἅλς. Halite occurs in vast beds of sedimentary evaporite minerals that result from the drying up of enclosed lakes and seas. Salt beds may underlie broad areas. In the United States and Canada extensive underground beds extend from the Appalachian basin of western New York through parts of Ontario and under much of the Michigan Basin. Other deposits are in Ohio, New Mexico, Nova Scotia and Saskatchewan; the Khewra salt mine is a massive deposit of halite near Pakistan. Salt domes are vertical diapirs or pipe-like masses of salt that have been "squeezed up" from underlying salt beds by mobilization due to the weight of overlying rock.
Salt domes contain anhydrite and native sulfur, in addition to halite and sylvite. They are common along the Gulf coasts of Texas and Louisiana and are associated with petroleum deposits. Germany, the Netherlands and Iran have salt domes. Salt glaciers exist in arid Iran where the salt has broken through the surface at high elevation and flows downhill. In all of these cases, halite is said to be behaving in the manner of a rheid. Unusual, fibrous vein filling halite is found in France and a few other localities. Halite crystals termed hopper crystals appear to be "skeletons" of the typical cubes, with the edges present and stairstep depressions on, or rather in, each crystal face. In a crystallizing environment, the edges of the cubes grow faster than the centers. Halite crystals form quickly in some evaporating lakes resulting in modern artifacts with a coating or encrustation of halite crystals. Halite flowers are rare stalactites of curling fibers of halite that are found in certain arid caves of Australia's Nullarbor Plain.
Halite stalactites and encrustations are reported in the Quincy native copper mine of Hancock, Michigan. The worlds largest underground salt mine is the Sifto Salt Mine, it uses the Room and Pillar Mining Method. It is located half a kilometre under Lake Huron in Canada. In the United Kingdom there are three mines. Salt is used extensively in cooking as a flavor enhancer, to cure a wide variety of foods such as bacon and fish, it is used in food preservation methods across various cultures. Larger pieces dusted over food from a shaker as finishing salt. Halite is often used both residentially and municipally for managing ice; because brine has a lower freezing point than pure water, putting salt or saltwater on ice, below 0 °C will cause it to melt. It is common for homeowners in cold climates to spread salt on their sidewalks and driveways after a snow storm to melt the ice, it is not necessary to use so much salt that the ice is melted. Many cities will spread a mixture of sand and salt on roads during and after a snowstorm to improve traction.
Using Salt Brine is more effective than spreading dry salt because moisture is necessary for the freezing-point depression to work and wet salt sticks to the roads better. Otherwise the salt can be wiped away by traffic. In addition to de-icing, rock salt is used in agriculture. An example of this would be inducing salt stress to suppress the growth of annual meadow grass in turf production. Other examples involve exposing weeds to salt water to dehydrate and kill them preventing them from affecting other plants. Salt is used as a household cleaning product, its coarse nature allows for its use in various cleaning scenarios including grease/oil removal, stain removal, dries out and hardens sticky spills for an easier clean. Some cultures in Africa and Brazil, prefer a wide variety of different rock salts for different dishes. Pure salt is avoided. Many recipes call for particular kinds of rock salt, imported pure salt has impurities added to adapt to local tastes. Salt was used as a form of currency in barter systems and was exlcusively controlled by authorities and their appointees.
In some ancient civilizations the practice of Salting The Earth was done to make conquered land of an enemy infertile and inhospitable as an act of domination. This act is known as Salting The Earth. We see biblical reference to this practice in Judges 9:45: “he killed the people in it, pulled the wall down and sowed the site with salt.”. Salt Coarse salt Salt tectonics
Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations; the surface along which the lattice points are shared in twinned crystals is called a composition surface or twin plane. Crystallographers classify twinned crystals by a number of twin laws; these twin laws are specific to the crystal system. The type of twinning can be a diagnostic tool in mineral identification. Twinning is an important mechanism for permanent shape changes in a crystal. Twinning can be a problem in X-ray crystallography, as a twinned crystal does not produce a simple diffraction pattern. Twin laws are either defined by the direction of the twin axes. If the twin law can be defined by a simple planar composition surface, the twin plane is always parallel to a possible crystal face and never parallel to an existing plane of symmetry. If the twin law is a rotation axis, the composition surface will be irregular, the twin axis will be perpendicular to a lattice plane, but will never be an even-fold rotation axis of the existing symmetry.
For example twinning cannot occur on a new 2 fold axis, parallel to an existing 4-fold axis. In the isometric system, the most common types of twins are the Spinel Law, where the twin axis is perpendicular to an octahedral face, the Iron Cross, the interpenetration of two pyritohedrons a subtype of dodecahedron. In the hexagonal system, calcite shows the contact. Quartz shows the Brazil Law, Dauphiné Law which are penetration twins caused by transformation and Japanese Law, caused by accidents during growth. In the tetragonal system, cyclical contact twins are the most observed type of twin, such as in rutile titanium dioxide and cassiterite tin oxide. In the orthorhombic system, crystals twin on planes parallel to the prism face, where the most common is a twin which produces cyclical twins, such as in aragonite and cerussite. In the monoclinic system, twin occur most on the planes and by the Manebach Law, Carlsbad Law, Braveno Law in orthoclase, the Swallow Tail Twins in gypsum. In the triclinic system, the most twinned crystals are the feldspar minerals plagioclase and microcline.
These minerals show the Pericline Laws. Simple twinned crystals may be contact twins or penetration twins. Contact twins share a single composition surface appearing as mirror images across the boundary. Plagioclase, quartz and spinel exhibit contact twinning. Merohedral twinning occurs when the lattices of the contact twins superimpose in three dimensions, such as by relative rotation of one twin from the other. An example is metazeunerite. In penetration twins the individual crystals have the appearance of passing through each other in a symmetrical manner. Orthoclase, staurolite and fluorite show penetration twinning. If several twin crystal parts are aligned by the same twin law they are referred to as multiple or repeated twins. If these multiple twins are aligned in parallel they are called polysynthetic twins; when the multiple twins are not parallel they are cyclic twins. Albite and pyrite show polysynthetic twinning. Spaced polysynthetic twinning is observed as striations or fine parallel lines on the crystal face.
Rutile, aragonite and chrysoberyl exhibit cyclic twinning in a radiating pattern. But in general, based on the relationship between the twin axis and twin plane, there are 3 types of twinning: 1-parallel twinning, when the twin axis and compositional plane lie parallel to each other, 2-normal twining, when the twin plane and compositional plane lie and 3-complex twining, a combination of parallel twinning and normal twinning on one compositional plane. There are three modes of formation of twinned crystals. Growth twins are the result of an interruption or change in the lattice during formation or growth due to a possible deformation from a larger substituting ion. Annealing or transformation twins are the result of a change in crystal system during cooling as one form becomes unstable and the crystal structure must re-organize or transform into another more stable form. Deformation or gliding twins are the result of stress on the crystal. If a metal with face-centered cubic structure, like Al, Cu, Ag, Au, etc. is subjected to stress, it will experience twinning.
The formation and migration of twin boundaries is responsible for ductility and malleability of fcc metals. Deformation twinning is a common result of regional metamorphism. Crystal twinning is used as an indicator of force direction in mountain building processes in orogeny research. Crystals that grow adjacent to each other may be aligned to resemble twinning; this parallel growth reduces system energy and is not twinning. Twinning can occur by cooperative displacement of atoms along the face of the twin boundary; this displacement of a large quantity of atoms requires significant energy to perform. Therefore, the theoretical stress required to form a twin is quite high, it is believed that twinning is associated with dislocation motion on a coordinated scale, in contrast to slip, caused by independent glide at several locations in the crystal. Twinning and slip are competitive mechanisms for crystal deformation; each mechanism is dominant under certain conditions. In fcc metals, slip is always dominant because the stres
The mineral pyrite, or iron pyrite known as fool's gold, is an iron sulfide with the chemical formula FeS2. Pyrite is considered the most common of the sulfide minerals. Pyrite's metallic luster and pale brass-yellow hue give it a superficial resemblance to gold, hence the well-known nickname of fool's gold; the color has led to the nicknames brass and Brazil used to refer to pyrite found in coal. The name pyrite is derived from the Greek πυρίτης, "of fire" or "in fire", in turn from πύρ, "fire". In ancient Roman times, this name was applied to several types of stone that would create sparks when struck against steel. By Georgius Agricola's time, c. 1550, the term had become a generic term for all of the sulfide minerals. Pyrite is found associated with other sulfides or oxides in quartz veins, sedimentary rock, metamorphic rock, as well as in coal beds and as a replacement mineral in fossils, but has been identified in the sclerites of scaly-foot gastropods. Despite being nicknamed fool's gold, pyrite is sometimes found in association with small quantities of gold.
Gold and arsenic occur as a coupled substitution in the pyrite structure. In the Carlin–type gold deposits, arsenian pyrite contains up to 0.37% gold by weight. Pyrite enjoyed brief popularity in the 16th and 17th centuries as a source of ignition in early firearms, most notably the wheellock, where a sample of pyrite was placed against a circular file to strike the sparks needed to fire the gun. Pyrite has been used since classical times to manufacture copperas. Iron pyrite was allowed to weather; the acidic runoff from the heap was boiled with iron to produce iron sulfate. In the 15th century, new methods of such leaching began to replace the burning of sulfur as a source of sulfuric acid. By the 19th century, it had become the dominant method. Pyrite remains in commercial use for the production of sulfur dioxide, for use in such applications as the paper industry, in the manufacture of sulfuric acid. Thermal decomposition of pyrite into FeS and elemental sulfur starts at 540 °C. A newer commercial use for pyrite is as the cathode material in Energizer brand non-rechargeable lithium batteries.
Pyrite is a semiconductor material with a band gap of 0.95 eV. Pure pyrite is n-type, in both crystal and thin-film forms due to sulfur vacancies in the pyrite crystal structure acting as n-dopants. During the early years of the 20th century, pyrite was used as a mineral detector in radio receivers, is still used by crystal radio hobbyists; until the vacuum tube matured, the crystal detector was the most sensitive and dependable detector available – with considerable variation between mineral types and individual samples within a particular type of mineral. Pyrite detectors occupied a midway point between galena detectors and the more mechanically complicated perikon mineral pairs. Pyrite detectors can be as sensitive as a modern 1N34A germanium diode detector. Pyrite has been proposed as an abundant, non-toxic, inexpensive material in low-cost photovoltaic solar panels. Synthetic iron sulfide was used with copper sulfide to create the photovoltaic material.. More recent efforts are working toward thin-film solar cells made of pyrite.
Pyrite is used to make marcasite jewelry. Marcasite jewelry, made from small faceted pieces of pyrite set in silver, was known since ancient times and was popular in the Victorian era. At the time when the term became common in jewelry making, "marcasite" referred to all iron sulfides including pyrite, not to the orthorhombic FeS2 mineral marcasite, lighter in color and chemically unstable, thus not suitable for jewelry making. Marcasite jewelry does not contain the mineral marcasite. China represents the main importing country with an import of around 376,000 tonnes, which resulted at 45% of total global imports. China is the fastest growing in terms of the unroasted iron pyrites imports, with a CAGR of +27.8% from 2007 to 2016. In value terms, China constitutes the largest market for imported unroasted iron pyrites worldwide, making up 65% of global imports. From the perspective of classical inorganic chemistry, which assigns formal oxidation states to each atom, pyrite is best described as Fe2+S22−.
This formalism recognizes. These persulfide units can be viewed as derived from hydrogen disulfide, H2S2, thus pyrite would be more descriptively, not iron disulfide. In contrast, molybdenite, MoS2, features isolated sulfide centers and the oxidation state of molybdenum is Mo4+; the mineral arsenopyrite has the formula FeAsS. Whereas pyrite has S2 subunits, arsenopyrite has units, formally derived from deprotonation of H2AsSH. Analysis of classical oxidation states would recommend the description of arsenopyrite as Fe3+3−. Iron-pyrite FeS2 represents the prototype compound of the crystallographic pyrite structure; the structure is simple cubic and was among the first crystal structures solved by X-ray diffraction. It belongs to the crystallographic space group Pa3 and is denoted by the Strukturbericht notation C2. Under thermodynamic standard conditions the lattice constant a of stoichiometric iron pyrite FeS2 amounts to 541.87 pm. The unit cell is composed of a Fe face-centered cubic sublattice into.
The pyrite structure is used by other compounds MX2 of trans
A crystal or crystalline solid is a solid material whose constituents are arranged in a ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are identifiable by their geometrical shape, consisting of flat faces with specific, characteristic orientations; the scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification; the word crystal derives from the Ancient Greek word κρύσταλλος, meaning both "ice" and "rock crystal", from κρύος, "icy cold, frost". Examples of large crystals include snowflakes and table salt. Most inorganic solids are not crystals but polycrystals, i.e. many microscopic crystals fused together into a single solid. Examples of polycrystals include most metals, rocks and ice. A third category of solids is amorphous solids, where the atoms have no periodic structure whatsoever.
Examples of amorphous solids include glass and many plastics. Despite the name, lead crystal, crystal glass, related products are not crystals, but rather types of glass, i.e. amorphous solids. Crystals are used in pseudoscientific practices such as crystal therapy, along with gemstones, are sometimes associated with spellwork in Wiccan beliefs and related religious movements; the scientific definition of a "crystal" is based on the microscopic arrangement of atoms inside it, called the crystal structure. A crystal is a solid where the atoms form a periodic arrangement.. Not all solids are crystals. For example, when liquid water starts freezing, the phase change begins with small ice crystals that grow until they fuse, forming a polycrystalline structure. In the final block of ice, each of the small crystals is a true crystal with a periodic arrangement of atoms, but the whole polycrystal does not have a periodic arrangement of atoms, because the periodic pattern is broken at the grain boundaries.
Most macroscopic inorganic solids are polycrystalline, including all metals, ice, etc. Solids that are neither crystalline nor polycrystalline, such as glass, are called amorphous solids called glassy, vitreous, or noncrystalline; these have no periodic order microscopically. There are distinct differences between crystalline solids and amorphous solids: most notably, the process of forming a glass does not release the latent heat of fusion, but forming a crystal does. A crystal structure is characterized by its unit cell, a small imaginary box containing one or more atoms in a specific spatial arrangement; the unit cells are stacked in three-dimensional space to form the crystal. The symmetry of a crystal is constrained by the requirement that the unit cells stack with no gaps. There are 219 possible crystal symmetries, called crystallographic space groups; these are grouped into 7 crystal systems, such as hexagonal crystal system. Crystals are recognized by their shape, consisting of flat faces with sharp angles.
These shape characteristics are not necessary for a crystal—a crystal is scientifically defined by its microscopic atomic arrangement, not its macroscopic shape—but the characteristic macroscopic shape is present and easy to see. Euhedral crystals are those with well-formed flat faces. Anhedral crystals do not because the crystal is one grain in a polycrystalline solid; the flat faces of a euhedral crystal are oriented in a specific way relative to the underlying atomic arrangement of the crystal: they are planes of low Miller index. This occurs; as a crystal grows, new atoms attach to the rougher and less stable parts of the surface, but less to the flat, stable surfaces. Therefore, the flat surfaces tend to grow larger and smoother, until the whole crystal surface consists of these plane surfaces. One of the oldest techniques in the science of crystallography consists of measuring the three-dimensional orientations of the faces of a crystal, using them to infer the underlying crystal symmetry.
A crystal's habit is its visible external shape. This is determined by the crystal structure, the specific crystal chemistry and bonding, the conditions under which the crystal formed. By volume and weight, the largest concentrations of crystals in the Earth are part of its solid bedrock. Crystals found in rocks range in size from a fraction of a millimetre to several centimetres across, although exceptionally large crystals are found; as of 1999, the world's largest known occurring crystal is a crystal of beryl from Malakialina, Madagascar, 18 m long and 3.5 m in diameter, weighing 380,000 kg. Some crystals have formed by magmatic and metamorphic processes, giving origin to large masses of crystalline rock; the vast majority of igneous rocks are formed from molten magma and the degree of crystallization depends on the conditions under which they solidified. Such rocks as granite, which have cooled slowly and under great pressures, have crystallized.
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns that repeat along the principal directions of three-dimensional space in matter; the smallest group of particles in the material that constitutes this repeating pattern is the unit cell of the structure. The unit cell reflects the symmetry and structure of the entire crystal, built up by repetitive translation of the unit cell along its principal axes; the translation vectors define the nodes of the Bravais lattice. The lengths of the principal axes, or edges, of the unit cell and the angles between them are the lattice constants called lattice parameters or cell parameters; the symmetry properties of the crystal are described by the concept of space groups. All possible symmetric arrangements of particles in three-dimensional space may be described by the 230 space groups.
The crystal structure and symmetry play a critical role in determining many physical properties, such as cleavage, electronic band structure, optical transparency. Crystal structure is described in terms of the geometry of arrangement of particles in the unit cell; the unit cell is defined as the smallest repeating unit having the full symmetry of the crystal structure. The geometry of the unit cell is defined as a parallelepiped, providing six lattice parameters taken as the lengths of the cell edges and the angles between them; the positions of particles inside the unit cell are described by the fractional coordinates along the cell edges, measured from a reference point. It is only necessary to report the coordinates of a smallest asymmetric subset of particles; this group of particles may be chosen so that it occupies the smallest physical space, which means that not all particles need to be physically located inside the boundaries given by the lattice parameters. All other particles of the unit cell are generated by the symmetry operations that characterize the symmetry of the unit cell.
The collection of symmetry operations of the unit cell is expressed formally as the space group of the crystal structure. Vectors and planes in a crystal lattice are described by the three-value Miller index notation; this syntax uses the indices ℓ, m, n as directional orthogonal parameters, which are separated by 90°. By definition, the syntax denotes a plane that intercepts the three points a1/ℓ, a2/m, a3/n, or some multiple thereof; that is, the Miller indices are proportional to the inverses of the intercepts of the plane with the unit cell. If one or more of the indices is zero, it means. A plane containing a coordinate axis is translated so that it no longer contains that axis before its Miller indices are determined; the Miller indices for a plane are integers with no common factors. Negative indices are indicated with horizontal bars, as in. In an orthogonal coordinate system for a cubic cell, the Miller indices of a plane are the Cartesian components of a vector normal to the plane. Considering only planes intersecting one or more lattice points, the distance d between adjacent lattice planes is related to the reciprocal lattice vector orthogonal to the planes by the formula d = 2 π | g ℓ m n | The crystallographic directions are geometric lines linking nodes of a crystal.
The crystallographic planes are geometric planes linking nodes. Some directions and planes have a higher density of nodes; these high density planes have an influence on the behavior of the crystal as follows: Optical properties: Refractive index is directly related to density. Adsorption and reactivity: Physical adsorption and chemical reactions occur at or near surface atoms or molecules; these phenomena are thus sensitive to the density of nodes. Surface tension: The condensation of a material means that the atoms, ions or molecules are more stable if they are surrounded by other similar species; the surface tension of an interface thus varies according to the density on the surface. Microstructural defects: Pores and crystallites tend to have straight grain boundaries following higher density planes. Cleavage: This occurs preferentially parallel to higher density planes. Plastic deformation: Dislocation glide occurs preferentially parallel to higher density planes; the perturbation carried by the dislocation is along a dense direction.
The shift of one node in a more dense direction requires a lesser distortion of the crystal lattice. Some directions and planes are defined by symmetry of the crystal system. In monoclinic, rhombohedral and trigonal/hexagonal systems there is one unique axis which has higher rotational symmetry than the other two axes; the basal plane is the plane perpendicular to the principal axis in these crystal systems. For triclinic and cubic crystal systems the axis designation is arbitrary and there is no principal axis. For the special case of simple cubic crystals, the lattice vectors are orthogonal and of equal length. So, in this common case, the Miller indices and both denote normals/directions in Cartesian coordinates. For cubic crystals with lattice constant a, the spacing d between adjacent l
Magma is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, evidence of magmatism has been discovered on other terrestrial planets and some natural satellites. Besides molten rock, magma may contain suspended crystals and gas bubbles. Magma is produced by melting of the mantle and/or the crust at various tectonic settings, including subduction zones, continental rift zones, mid-ocean ridges and hotspots. Mantle and crustal melts migrate upwards through the crust where they are thought to be stored in magma chambers or trans-crustal crystal-rich mush zones. During their storage in the crust, magma compositions may be modified by fractional crystallization, contamination with crustal melts, magma mixing, degassing. Following their ascent through the crust, magmas may feed a volcano or solidify underground to form an intrusion. While the study of magma has relied on observing magma in the form of lava flows, magma has been encountered in situ three times during geothermal drilling projects—twice in Iceland, once in Hawaii.
Most magmatic liquids are rich in silica. Silicate melts are composed of silicon, aluminium, magnesium, calcium and potassium; the physical behaviours of melts depend upon their atomic structures as well as upon temperature and pressure and composition. Viscosity is a key melt property in understanding the behaviour of magmas. More silica-rich melts are more polymerized, with more linkage of silica tetrahedra, so are more viscous. Dissolution of water drastically reduces melt viscosity. Higher-temperature melts are less viscous. Speaking, more mafic magmas, such as those that form basalt, are hotter and less viscous than more silica-rich magmas, such as those that form rhyolite. Low viscosity leads to less explosive eruptions. Characteristics of several different magma types are as follows: Ultramafic SiO2 < 45% Fe–Mg > 8% up to 32%MgO Temperature: up to 1500°C Viscosity: Very Low Eruptive behavior: gentle or explosive Distribution: divergent plate boundaries, hot spots, convergent plate boundaries.
At any given pressure and for any given composition of rock, a rise in temperature past the solidus will cause melting. Within the solid earth, the temperature of a rock is controlled by the geothermal gradient and the radioactive decay within the rock; the geothermal gradient averages about 25 °C/km with a wide range from a low of 5–10 °C/km within oceanic trenches and subduction zones to 30–80 °C/km under mid-ocean ridges and volcanic arc environments. It is very difficult to change the bulk composition of a large mass of rock, so composition is the basic control on whether a rock will melt at any given temperature and pressure; the composition of a rock may be considered to include volatile phases such as water and carbon dioxide. The presence of volatile phases in a rock under pressure can stabilize a melt fraction; the presence of 0.8% water may reduce the temperature of melting by as much as 100 °C. Conversely, the loss of water and volatiles from a magma may cause it to freeze or solidify.
A major portion of all magma is silica, a compound of silicon and oxygen. Magma contains gases, which expand as the magma rises. Magma, high in silica resists flowing, so expanding gases are trapped in it. Pressure builds up until the gases blast out in a dangerous explosion. Magma, poor in silica flows so gas bubbles move up through it and escape gently. Melting of solid rocks to form magma is controlled by three physical parameters: temperature and composition; the most common mechanisms of magma generation in the mantle are decompression melting and lowering of the solidus. Mechanisms are discussed further in the entry for igneous rock; when rocks melt, they do so and because most rocks are made of several minerals, which all have different melting points. As a rock melts, for example, its volume changes; when enough rock is melted, the small globules of melt soften the rock. Under pressure within the earth, as little as a fraction of a percent of partial melting may be sufficient to cause melt to be squeezed from its source.
Melts can stay in place long enough to melt to 20% or 35%, but rocks are melted in excess of 50%, because the melted rock mass becomes a crystal-and-melt mush tha