Intrusive rock is formed when magma crystallizes and solidifies underground to form intrusions, for example plutons, dikes, sills and volcanic necks. Intrusive rock forms within Earth's crust from the crystallization of magma. Many mountain ranges, such as the Sierra Nevada in California, are formed from large granite intrusions. Intrusions are one of the two ways igneous rock. Technically an intrusion is any formation of intrusive igneous rock. In contrast, an extrusion consists of extrusive rock. Large bodies of magma that solidify underground before they reach the surface of the crust are called plutons. Plutonic rocks form 7% of the Earth's current land surface. Coarse-grained intrusive igneous rocks that form at depth within the earth are called abyssal while those that form near the surface are called subvolcanic or hypabyssal. Intrusive structures are classified according to whether or not they are parallel to the bedding planes or foliation of the country rock: if the intrusion is parallel the body is concordant, otherwise it is discordant.
An intrusive suite is a group of plutons related in time and space.. Intrusions vary from mountain-range-sized batholiths to thin veinlike fracture fillings of aplite or pegmatite. Intrusions can be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes: Batholith: a large irregular discordant intrusion Chonolith: an irregularly-shaped intrusion with a demonstrable base Cupola: a dome-shaped projection from the top of a large subterranean intrusion Dike: a narrow tabular discordant body nearly vertical Laccolith: concordant body with flat base and convex top with a feeder pipe below Lopolith: concordant body with flat top and a shallow convex base, may have a feeder dike or pipe below Phacolith: a concordant lens-shaped pluton that occupies the crest of an anticline or trough of a syncline Volcanic pipe or volcanic neck: tubular vertical body that may have been a feeder vent for a volcano Sill: a thin tabular concordant body intruded along bedding planes Stock: a smaller irregular discordant intrusive Boss: a small stock A body of intrusive igneous rock which crystallizes from magma cooling underneath the surface of the Earth is called a pluton.
If the pluton is large, it may be called a stock. Intrusive rocks are characterized by large crystal sizes, as the individual crystals are visible, the rock is called phaneritic; this is as the magma cools underground, while cooling may be fast or slow, cooling is slower than on the surface, so larger crystals grow. If it runs parallel to rock layers, it is called a sill. If an intrusion makes rocks above rise to form a dome, it is called a laccolith. How deep-seated intrusions burst through the overlying strata causes intrusive rock to be recognized: Veins spread out into branches, or branchlike parts result from filled cracks, the high temperature is evident in how they alter country rock; as heat dissipation is slow, as the rock is under pressure, crystals form, no vitreous chilled matter is present. The intrusions did not flow. Contained gases could not escape through the thick strata, thus form cavities, which can be observed; because their crystals are of the rough equal size, these rocks are said to be equigranular.
There is no distinction between a first generation of large well-shaped crystals and a fine-grained ground-mass. The minerals of each have formed in a definite order, each has had a period of crystallization that may be distinct or may have coincided with or overlapped the period of formation of some of the other ingredients. Earlier crystals originated at a time when most of the rock was still liquid and are more or less perfect. Crystals are less regular in shape because they were compelled to occupy the spaces left between the already-formed crystals; the former case is said to be idiomorphic. There are many other characteristics that serve to distinguish the members of these two groups. For example, orthoclase is feldspar from granite, while its modifications occur in lavas of similar composition; the same distinction holds for nepheline varieties. Leucite is common in lavas but rare in plutonic rocks. Muscovite is confined to intrusions; these differences show the influence of the physical conditions under which consolidation takes place.
Intrusive rocks formed at greater depths are called abyssal. Some intrusive rocks solidified in fissures as dikes and intrusive sills at shallow depth and are called subvolcanic or hypabyssal, they show structures intermediate between those of plutonic rocks. They are commonly porphyritic and sometimes vesicular. In fact, many of them are petrologically indistinguishable from lavas of similar composition. Ellicott City Granodiorite Guilford Quartz Monzonite Methods of pluton emplacement Norbeck Intrusive Suite Volcanic rock Woodstock Quartz Monzonite
Sediment is a occurring material, broken down by processes of weathering and erosion, is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation and if buried, may become sandstone and siltstone. Sediments are most transported by water, but wind and glaciers. Beach sands and river channel deposits are examples of fluvial transport and deposition, though sediment often settles out of slow-moving or standing water in lakes and oceans. Desert sand dunes and loess are examples of aeolian deposition. Glacial moraine deposits and till are ice-transported sediments. Sediment can be classified based on its grain composition. Sediment size is measured on a log base 2 scale, called the "Phi" scale, which classifies particles by size from "colloid" to "boulder". Composition of sediment can be measured in terms of: parent rock lithology mineral composition chemical make-up.
This leads to an ambiguity in which clay can be used as a composition. Sediment is transported based on the strength of the flow that carries it and its own size, volume and shape. Stronger flows will increase the lift and drag on the particle, causing it to rise, while larger or denser particles will be more to fall through the flow. Rivers and streams carry sediment in their flows; this sediment can be in a variety of locations within the flow, depending on the balance between the upwards velocity on the particle, the settling velocity of the particle. These relationships are shown in the following table for the Rouse number, a ratio of sediment fall velocity to upwards velocity. Rouse = Settling velocity Upwards velocity from lift and drag = w s κ u ∗ where w s is the fall velocity κ is the von Kármán constant u ∗ is the shear velocity If the upwards velocity is equal to the settling velocity, sediment will be transported downstream as suspended load. If the upwards velocity is much less than the settling velocity, but still high enough for the sediment to move, it will move along the bed as bed load by rolling and saltating.
If the upwards velocity is higher than the settling velocity, the sediment will be transported high in the flow as wash load. As there are a range of different particle sizes in the flow, it is common for material of different sizes to move through all areas of the flow for given stream conditions. Sediment motion can create self-organized structures such as ripples, dunes, or antidunes on the river or stream bed; these bedforms are preserved in sedimentary rocks and can be used to estimate the direction and magnitude of the flow that deposited the sediment. Overland flow can transport them downslope; the erosion associated with overland flow may occur through different methods depending on meteorological and flow conditions. If the initial impact of rain droplets dislodges soil, the phenomenon is called rainsplash erosion. If overland flow is directly responsible for sediment entrainment but does not form gullies, it is called "sheet erosion". If the flow and the substrate permit channelization, gullies may form.
The major fluvial environments for deposition of sediments include: Deltas Point bars Alluvial fans Braided rivers Oxbow lakes Levees Waterfalls Wind results in the transportation of fine sediment and the formation of sand dune fields and soils from airborne dust. Glaciers carry a wide range of sediment sizes, deposit it in moraines; the overall balance between sediment in transport and sediment being deposited on the bed is given by the Exner equation. This expression states that the rate of increase in bed elevation due to deposition is proportional to the amount of sediment that falls out of the flow; this equation is important in that changes in the power of the flow change the ability of the flow to carry sediment, this is reflected in the patterns of erosion and deposition observed throughout a stream. This can be localized, due to small obstacles. Erosion and deposition can be regional. Deposition can occur due to dam emplacement that causes the river to pool and deposit its entire load, or due to base level rise.
Seas and lakes accumulate sediment over time. The sediment can consist of terrigenous material, which originates on land, but may be deposited in either terrestrial, marine, or lacustrine environments, or of sediments originating in the body of water. Terrigenous material is supplied by nearby rivers and streams or reworked marine sediment. In the mid-ocean, the exoskeletons of dead organisms are responsible for sediment accumulation. Deposited sediments are the source of sedimentary rocks, which can contain fossils of
Limestone is a carbonate sedimentary rock, composed of the skeletal fragments of marine organisms such as coral and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. A related rock is dolostone, which contains a high percentage of the mineral dolomite, CaMg2. In fact, in old USGS publications, dolostone was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolostones or magnesium-rich limestones. About 10% of sedimentary rocks are limestones; the solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock. Limestone has numerous uses: as a building material, an essential component of concrete, as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.
Like most other sedimentary rocks, most limestone is composed of grains. Most grains in limestone are skeletal fragments of marine organisms such as foraminifera; these organisms secrete shells made of aragonite or calcite, leave these shells behind when they die. Other carbonate grains composing limestones are ooids, peloids and extraclasts. Limestone contains variable amounts of silica in the form of chert or siliceous skeletal fragment, varying amounts of clay and sand carried in by rivers; some limestones do not consist of grains, are formed by the chemical precipitation of calcite or aragonite, i.e. travertine. Secondary calcite may be deposited by supersaturated meteoric waters; this produces speleothems, such as stalactites. Another form taken by calcite is oolitic limestone, which can be recognized by its granular appearance; the primary source of the calcite in limestone is most marine organisms. Some of these organisms can construct mounds of rock building upon past generations. Below about 3,000 meters, water pressure and temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone does not form in deeper waters.
Limestones may form in lacustrine and evaporite depositional environments. Calcite can be dissolved or precipitated by groundwater, depending on several factors, including the water temperature, pH, dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility, in which it becomes less soluble in water as the temperature increases. Impurities will cause limestones to exhibit different colors with weathered surfaces. Limestone may be crystalline, granular, or massive, depending on the method of formation. Crystals of calcite, dolomite or barite may line small cavities in the rock; when conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together, or it can fill fractures. Travertine is a banded, compact variety of limestone formed along streams where there are waterfalls and around hot or cold springs. Calcium carbonate is deposited where evaporation of the water leaves a solution supersaturated with the chemical constituents of calcite.
Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells. During regional metamorphism that occurs during the mountain building process, limestone recrystallizes into marble. Limestone is a parent material of Mollisol soil group. Two major classification schemes, the Folk and the Dunham, are used for identifying the types of carbonate rocks collectively known as limestone. Robert L. Folk developed a classification system that places primary emphasis on the detailed composition of grains and interstitial material in carbonate rocks. Based on composition, there are three main components: allochems and cement; the Folk system uses two-part names. It is helpful to have a petrographic microscope when using the Folk scheme, because it is easier to determine the components present in each sample; the Dunham scheme focuses on depositional textures. Each name is based upon the texture of the grains. Robert J. Dunham published his system for limestone in 1962.
Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles. Dunham names are for rock families, his efforts deal with the question of whether or not the grains were in mutual contact, therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk scheme, Dunham deals with the original porosity of the rock; the Dunham scheme is more useful for hand samples because it is based on texture, not the grains in the sample. A revised classification was proposed by Wright, it adds some diagenetic patterns and can be summarized as follows: See: Carbonate platform About 10% of all sedimentary rocks are limestones. Limestone is soluble in acid, therefore forms many erosional landforms; these include limestone pavements, pot holes, cenotes and gorges. Such erosion landscapes are known
A molecule is an electrically neutral group of two or more atoms held together by chemical bonds. Molecules are distinguished from ions by their lack of electrical charge. However, in quantum physics, organic chemistry, biochemistry, the term molecule is used less also being applied to polyatomic ions. In the kinetic theory of gases, the term molecule is used for any gaseous particle regardless of its composition. According to this definition, noble gas atoms are considered molecules as they are monatomic molecules. A molecule may be homonuclear, that is, it consists of atoms of one chemical element, as with oxygen. Atoms and complexes connected by non-covalent interactions, such as hydrogen bonds or ionic bonds, are not considered single molecules. Molecules as components of matter are common in organic substances, they make up most of the oceans and atmosphere. However, the majority of familiar solid substances on Earth, including most of the minerals that make up the crust and core of the Earth, contain many chemical bonds, but are not made of identifiable molecules.
No typical molecule can be defined for ionic crystals and covalent crystals, although these are composed of repeating unit cells that extend either in a plane or three-dimensionally. The theme of repeated unit-cellular-structure holds for most condensed phases with metallic bonding, which means that solid metals are not made of molecules. In glasses, atoms may be held together by chemical bonds with no presence of any definable molecule, nor any of the regularity of repeating units that characterizes crystals; the science of molecules is called molecular chemistry or molecular physics, depending on whether the focus is on chemistry or physics. Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of chemical bonds, while molecular physics deals with the laws governing their structure and properties. In practice, this distinction is vague. In molecular sciences, a molecule consists of a stable system composed of two or more atoms.
Polyatomic ions may sometimes be usefully thought of as electrically charged molecules. The term unstable molecule is used for reactive species, i.e. short-lived assemblies of electrons and nuclei, such as radicals, molecular ions, Rydberg molecules, transition states, van der Waals complexes, or systems of colliding atoms as in Bose–Einstein condensate. According to Merriam-Webster and the Online Etymology Dictionary, the word "molecule" derives from the Latin "moles" or small unit of mass. Molecule – "extremely minute particle", from French molécule, from New Latin molecula, diminutive of Latin moles "mass, barrier". A vague meaning at first; the definition of the molecule has evolved. Earlier definitions were less precise, defining molecules as the smallest particles of pure chemical substances that still retain their composition and chemical properties; this definition breaks down since many substances in ordinary experience, such as rocks and metals, are composed of large crystalline networks of chemically bonded atoms or ions, but are not made of discrete molecules.
Molecules are held together by ionic bonding. Several types of non-metal elements exist only as molecules in the environment. For example, hydrogen only exists as hydrogen molecule. A molecule of a compound is made out of two or more elements. A covalent bond is a chemical bond; these electron pairs are termed shared pairs or bonding pairs, the stable balance of attractive and repulsive forces between atoms, when they share electrons, is termed covalent bonding. Ionic bonding is a type of chemical bond that involves the electrostatic attraction between oppositely charged ions, is the primary interaction occurring in ionic compounds; the ions are atoms that have lost one or more electrons and atoms that have gained one or more electrons. This transfer of electrons is termed electrovalence in contrast to covalence. In the simplest case, the cation is a metal atom and the anion is a nonmetal atom, but these ions can be of a more complicated nature, e.g. molecular ions like NH4+ or SO42−. An ionic bond is the transfer of electrons from a metal to a non-metal for both atoms to obtain a full valence shell.
Most molecules are far too small to be seen with the naked eye. DNA, a macromolecule, can reach macroscopic sizes, as can molecules of many polymers. Molecules used as building blocks for organic synthesis have a dimension of a few angstroms to several dozen Å, or around one billionth of a meter. Single molecules cannot be observed by light, but small molecules and the outlines of individual atoms may be traced in some circumstances by use of an atomic force microscope; some of the largest molecules are supermolecules. The smallest molecule is the diatomic hydrogen, with a bond length of 0.74 Å. Effective molecular radius is the size; the table of permselectivity for different substances contains examples. The chemical formula for a molecule uses one line of chemical element symbols and sometimes al
Graphite, archaically referred to as plumbago, is a crystalline form of the element carbon with its atoms arranged in a hexagonal structure. It occurs in this form and is the most stable form of carbon under standard conditions. Under high pressures and temperatures it converts to diamond. Graphite is used in lubricants, its high conductivity makes it useful in electronic products such as electrodes and solar panels. The principal types of natural graphite, each occurring in different types of ore deposits, are Crystalline small flakes of graphite occurs as isolated, plate-like particles with hexagonal edges if unbroken; when broken the edges can be angular. Ordered pyrolytic graphite refers to graphite with an angular spread between the graphite sheets of less than 1°; the name "graphite fiber" is sometimes used to refer to carbon fibers or carbon fiber-reinforced polymer. Graphite occurs in metamorphic rocks as a result of the reduction of sedimentary carbon compounds during metamorphism, it occurs in igneous rocks and in meteorites.
Minerals associated with graphite include quartz, calcite and tourmaline. The principal export sources of mined graphite are in order of tonnage: China, Canada and Madagascar. In meteorites, graphite occurs with silicate minerals. Small graphitic crystals in meteoritic iron are called cliftonite; some microscopic grains have distinctive isotopic compositions, indicating that they were formed before the Solar system. They are one of about 12 known types of mineral that predate the Solar System and have been detected in molecular clouds; these minerals were formed in the ejecta when supernovae exploded or low- to intermediate-sized stars expelled their outer envelopes late in their lives. Graphite may be the third oldest mineral in the Universe. Solid carbon comes in different forms known as allotropes depending on the type of chemical bond; the two most common are graphite. In diamond the bonds are sp3 and the atoms form tetrahedra with each bound to four nearest neighbors. In graphite they are sp2 orbital hybrids and the atoms form in planes with each bound to three nearest neighbors 120 degrees apart.
The individual layers are called graphene. In each layer, the carbon atoms are arranged in a honeycomb lattice with separation of 0.142 nm, the distance between planes is 0.335 nm. Atoms in the plane are bonded covalently, with only three of the four potential bonding sites satisfied; the fourth electron is free to migrate in the plane. However, it does not conduct in a direction at right angles to the plane. Bonding between layers is via weak van der Waals bonds, which allows layers of graphite to be separated, or to slide past each other; the two known forms of graphite and beta, have similar physical properties, except that the graphene layers stack differently. The alpha graphite may be either buckled; the alpha form can be converted to the beta form through mechanical treatment and the beta form reverts to the alpha form when it is heated above 1300 °C. The equilibrium pressure and temperature conditions for a transition between graphite and diamond is well established theoretically and experimentally.
The pressure changes linearly between 1.7 GPa at 0 K and 12 GPa at 5000 K. However, the phases have a wide region about this line where they can coexist. At normal temperature and pressure, 20 °C and 1 standard atmosphere, the stable phase of carbon is graphite, but diamond is metastable and its rate of conversion to graphite is negligible. However, at temperatures above about 4500 K, diamond converts to graphite. Rapid conversion of graphite to diamond requires pressures well above the equilibrium line: at 2000 K, a pressure of 35 GPa is needed; the acoustic and thermal properties of graphite are anisotropic, since phonons propagate along the bound planes, but are slower to travel from one plane to another. Graphite's high thermal stability and electrical and thermal conductivity facilitate its widespread use as electrodes and refractories in high temperature material processing applications. However, in oxygen-containing atmospheres graphite oxidizes to form carbon dioxide at temperatures of 700 °C and above.
Graphite is hence useful in such applications as arc lamp electrodes. It can conduct electricity due to the vast electron delocalization within the carbon layers; these valence electrons are free to move. However, the electricity is conducted within the plane of the layers; the conductive properties of powdered graphite allow its use as pressure sensor in carbon microphones. Graphite and graphite powder are valued in industrial applications for their self-lubricating and dry lubricating properties. There is a common belief that graphite's lubricating properties are due to the loose interlamellar coupling between sheets in the structure. However, it has been shown that in a vacuum environment, graphite degrades as a lubricant, due to the hypoxic conditions; this observation led to the hypothesis that the lubrication is due to the presence of fluids between the layers, such as air and water, which are adsorbed from the
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.
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K3AlSi3O102. More it refers to the dark mica series a solid-solution series between the iron-endmember annite, the magnesium-endmember phlogopite. Biotite was named by J. F. L. Hausmann in 1847 in honor of the French physicist Jean-Baptiste Biot, who performed early research into the many optical properties of mica. Biotite is a sheet silicate. Iron, aluminium, silicon and hydrogen form sheets that are weakly bound together by potassium ions, it is sometimes called "iron mica". It is sometimes called "black mica" as opposed to "white mica" – both form in the same rocks, in some instances side-by-side. Like other mica minerals, biotite has a perfect basal cleavage, consists of flexible sheets, or lamellae, which flake off, it has a monoclinic crystal system, with tabular to prismatic crystals with an obvious pinacoid termination. It has four prism faces and two pinacoid faces to form a pseudohexagonal crystal.
Although not seen because of the cleavage and sheets, fracture is uneven. It appears greenish to brown or black, yellow when weathered, it can be transparent to opaque, has a vitreous to pearly luster, a grey-white streak. When biotite is found in large chunks, they are called "books" because it resembles a book with pages of many sheets; the color of biotite is black and the mineral has a hardness of 2.5–3 on the Mohs scale of mineral hardness. Biotite dissolves in both acid and alkaline aqueous solutions, with the highest dissolution rates at low pH. However, biotite dissolution is anisotropic with crystal edge surfaces reacting 45 to 132 times faster than basal surfaces. In thin section, biotite exhibits moderate relief and a pale to deep greenish brown or brown color, with moderate to strong pleochroism. Biotite has a high birefringence which can be masked by its deep intrinsic color. Under cross-polarized light, biotite exhibits extinction parallel to cleavage lines, can have characteristic bird's eye extinction, a mottled appearance caused by the distortion of the mineral's flexible lamellae during grinding of the thin section.
Basal sections of biotite in thin section are approximately hexagonal in shape and appear isotropic under cross-polarized light. Biotite is found in a wide variety of metamorphic rocks. For instance, biotite occurs in the lava of Mount Vesuvius and in the Monzoni intrusive complex of the western Dolomites. Biotite in granite tends to be poorer in magnesium than the biotite found in its volcanic equivalent, rhyolite. Biotite is an essential phenocryst in some varieties of lamprophyre. Biotite is found in large cleavable crystals in pegmatite veins, as in New England and North Carolina USA. Other notable occurrences include Ontario Canada, it is an essential constituent of many metamorphic schists, it forms in suitable compositions over a wide range of pressure and temperature. It has been estimated. An igneous rock composed entirely of dark mica is known as a glimmerite or biotitite. Biotite may be found in association with its common alteration product chlorite; the largest documented single crystals of biotite were 7 m2 sheets found in Iveland, Norway.
Biotite is used extensively to constrain ages of rocks, by either potassium-argon dating or argon–argon dating. Because argon escapes from the biotite crystal structure at high temperatures, these methods may provide only minimum ages for many rocks. Biotite is useful in assessing temperature histories of metamorphic rocks, because the partitioning of iron and magnesium between biotite and garnet is sensitive to temperature