Mica in the context of "Optical mineralogy"

A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc located further from the subducting plate than the trench. The oceanic plate is saturated with water, mostly in the form of hydrous minerals such as micas, amphiboles, and serpentines. As the oceanic plate is subducted, it is subjected to increasing pressure and temperature with increasing depth. The heat and pressure break down the hydrous minerals in the plate, releasing water into the overlying mantle. Volatiles such as water drastically lower the melting point of the mantle, causing some of the mantle to melt and form magma at depth under the overriding plate. The magma ascends to form an arc of volcanoes parallel to the subduction zone.

Volcanic arcs are distinct from volcanic chains formed over hotspots in the middle of a tectonic plate. Volcanoes often form one after another as the plate moves over the hotspot, and so the volcanoes progress in age from one end of the chain to the other. The Hawaiian Islands form a typical hotspot chain, with the older islands to the northwest and Hawaii Island itself, which is just 400,000 years old, at the southeast end of the chain over the hotspot. Volcanic arcs do not generally exhibit such a simple age-pattern.

Granite (/ˈɡræ.nɪt/, GRAN-it) is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the continental crust of Earth, where it is found in igneous intrusions. These range in size from dikes only a few centimeters across to batholiths exposed over hundreds of square kilometers.

Granite is typical of a larger family of granitic rocks, or granitoids, that are composed mostly of coarse-grained quartz and feldspars in varying proportions. These rocks are classified by the relative percentages of quartz, alkali feldspar, and plagioclase (the QAPF classification), with true granite representing granitic rocks rich in quartz and alkali feldspar. Most granitic rocks also contain mica or amphibole minerals, though a few (known as leucogranites) contain almost no dark minerals.

Schist (/ˈʃɪst/ SHIST) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced schistosity (named for the rock). This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes or plates. This texture reflects a high content of platy minerals, such as mica, talc, chlorite, or graphite. These are often interleaved with more granular minerals, such as feldspar or quartz.

Schist typically forms during regional metamorphism accompanying the process of mountain building (orogeny) and usually reflects a medium grade of metamorphism. Schist can form from many different kinds of rocks, including sedimentary rocks such as mudstones and igneous rocks such as tuffs. Schist metamorphosed from mudstone is particularly common and is often very rich in mica (a mica schist). Where the type of the original rock (the protolith) is discernible, the schist is usually given a name reflecting its protolith, such as schistose metasandstone. Otherwise, the names of the constituent minerals will be included in the rock name, such as quartz-felspar-biotite schist.

The Aspromonte is a mountain massif in the Metropolitan City of Reggio Calabria (Calabria, southern Italy). In Italian aspro means "rough" whereas in Greek it means "white" (Άσπρος), therefore the name literally translates to either "rough mountain" or "white mountain". It overlooks the Strait of Messina, being limited by the Ionian and Tyrrhenian Seas and by the Pietrace river. The highest peak is Montalto (1,955 m (6,414 ft)). The constituting rocks are mostly gneiss, and mica schists, which form characteristic overlapping terraces. The massif is part of the Aspromonte National Park.

In the short coastal strip citrus fruits, vine and olives are grown, while at high elevations the vegetation is composed mostly by oak and holm oak under 1,000 m (3,300 ft), and by pine, Sicilian fir and beech over it. Olive trees grow in abundance. Also, the rare bergamot, the lemony-yellow fruit used in perfumes and flavouring for Earl Grey tea, only grows in the southern Aspromonte.

In geology, fissility is the ability or tendency of a rock to split along flat planes of weakness (“parting surfaces”). These planes of weakness are oriented parallel to stratification in sedimentary rocks. Fissility is differentiated from scaly fabric in hand sample by the parting surfaces’ continuously parallel orientations to each other and to stratification. Fissility is distinguished from scaly fabric in thin section by the well-developed orientation of platy minerals such as mica. Fissility is the result of sedimentary or metamorphic processes.

Planes of weakness are developed in sedimentary rocks such as shale or mudstone by clay particles aligning during compaction. Planes of weakness are developed in metamorphic rocks by the recrystallization and growth of micaceous minerals. A rock's fissility can be degraded in numerous ways during the geologic process, including clay particles flocculating into a random fabric before compaction, bioturbation during compaction, and weathering during and after uplift. The effect of bioturbation has been documented well in shale cores sampled: past variable critical depths where burrowing organisms can no longer survive, shale fissility will become more pervasive and better defined.

Peperino is an Italian word describing a brown or grey volcanic tuff, containing fragments of basalt and limestone, with disseminated crystals of augite, mica, magnetite, leucite, and other similar minerals. The name originally referred to the dark-colored inclusions, suggestive of peppercorns.

The typical peperino occurs in the Alban Hills and in Soriano nel Cimino, near Rome, and was used by the ancient Romans under the name of lapis albanus as a building stone and for the basins of fountains.

Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂. It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous end-members include siderophyllite and eastonite. Biotite was regarded as a mineral species by the International Mineralogical Association until 1998, when its status was changed to a mineral group. The term biotite is still used to describe unanalysed dark micas in the field. 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.

Members of the biotite group are sheet silicates. Iron, magnesium, aluminium, silicon, oxygen, and hydrogen form sheets that are weakly bound together by potassium ions. The term "iron mica" is sometimes used for iron-rich biotite, but the term also refers to a flaky micaceous form of haematite, and the field term Lepidomelane for unanalysed iron-rich Biotite avoids this ambiguity. Biotite is also sometimes called "black mica" as opposed to "white mica" (muscovite) – both may form in the same rocks, and in some instances side by side.

Cleavage, in mineralogy and materials science, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the microscope and to the naked eye. If bonds in certain directions are weaker than others, the crystal will tend to split along the weakly bonded planes. These flat breaks are termed "cleavage". The classic example of cleavage is mica, which cleaves in a single direction along the basal pinacoid, making the layers seem like pages in a book. In fact, mineralogists often refer to "books of mica".

Diamond and graphite provide examples of cleavage. Each is composed solely of a single element, carbon. In diamond, each carbon atom is bonded to four others in a tetrahedral pattern with short covalent bonds. The planes of weakness (cleavage planes) in a diamond are in four directions, following the faces of the octahedron. In graphite, carbon atoms are contained in layers in a hexagonal pattern where the covalent bonds are shorter (and thus even stronger) than those of diamond. However, each layer is connected to the other with a longer and much weaker van der Waals bond. This gives graphite a single direction of cleavage, parallel to the basal pinacoid. So weak is this bond that it is broken with little force, giving graphite a slippery feel as layers shear apart. As a result, graphite makes an excellent dry lubricant.

In optical mineralogy and petrography, a thin section (or petrographic thin section) is a thin slice of a rock or mineral sample, prepared in a laboratory, for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a diamond saw and ground optically flat. It is then mounted on a glass slide and then ground smooth using progressively finer abrasive grit until the sample is only 30 μm thick. The method uses the Michel-Lévy interference colour chart to determine thickness, typically using quartz as the thickness gauge because it is one of the most abundant minerals.

When placed between two polarizing filters set at right angles to each other, the optical properties of the minerals in the thin section alter the colour and intensity of the light as seen by the viewer. As different minerals have different optical properties, most rock-forming minerals can be easily identified. Plagioclase for example can be seen in the photo on the right as a clear mineral with multiple parallel twinning planes. The large blue-green minerals are clinopyroxene with some exsolution of orthopyroxene.