Amphibole is an important group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain SiO4 tetrahedra, linked at the vertices and containing ions of iron and/or magnesium in their structures. Amphiboles can be green, colorless, yellow, blue, or brown; the International Mineralogical Association classifies amphiboles as a mineral supergroup, within which are two groups and several subgroups. Amphiboles crystallize into two crystal systems and orthorhombic. In chemical composition and general characteristics they are similar to the pyroxenes; the chief differences from pyroxenes are that amphiboles contain essential hydroxyl or halogen and the basic structure is a double chain of tetrahedra. Most apparent, in hand specimens, is that amphiboles form oblique cleavage planes, whereas pyroxenes have cleavage angles of 90 degrees. Amphiboles are specifically less dense than the corresponding pyroxenes. In optical characteristics, many amphiboles are distinguished by their stronger pleochroism and by the smaller angle of extinction on the plane of symmetry.
Amphiboles are the primary constituent of amphibolites. Amphiboles are minerals of either metamorphic origin. Calcium is sometimes a constituent of occurring amphiboles; those of metamorphic origin include examples such as those developed in limestones by contact metamorphism and those formed by the alteration of other ferromagnesian minerals. Pseudomorphs of amphibole after pyroxene are known as uralite; the name amphibole was used by René Just Haüy to include tremolite and hornblende. The group was so named by Haüy in allusion to the protean variety, in composition and appearance, assumed by its minerals; this term has since been applied to the whole group. Numerous sub-species and varieties are distinguished, the more important of which are tabulated below in two series; the formulae of each will be seen to be built on the general double-chain silicate formula RSi4O11. Four of the amphibole minerals are among the minerals called asbestos; these are: anthophyllite, cummingtonite/grunerite series, actinolite/tremolite series.
The cummingtonite/grunerite series is termed amosite or brown asbestos. These are called amphibole asbestos. Mining and prolonged use of these minerals can cause serious illnesses. Orthorhombic series Anthophyllite, 7Si8O222 Holmquistite, Li2Mg3Al2Si8O222 Ferrogedrite, Fe2+5Al4Si6O222Monoclinic series Tremolite, Ca2Mg5Si8O222 Actinolite, Ca25Si8O222 Cummingtonite, Fe2Mg5Si8O222 Grunerite, Fe7Si8O222 Hornblende, 0-1258O222 Glaucophane, Na23Al2Si8O222 Riebeckite, Na2Fe2+3Fe3+2Si8O222 Arfvedsonite, Na3Fe2+4Fe3+Si8O222 Richterite, Na2Ca5Si8O222 Pargasite, NaCa2Mg3Fe2+Si6Al3O222 Winchite, Mg4Si8O222 Edenite, NaCa2Mg5O222 On account of the wide variations in chemical composition, the different members vary in properties and general appearance. Anthophyllite occurs as brownish, fibrous or lamellar masses with hornblende in mica-schist at Kongsberg in Norway and some other localities. An aluminous related species is known as gedrite and a deep green Russian variety containing little iron as kupfferite.
Hornblende is an important constituent of many igneous rocks. It is an important constituent of amphibolites formed by metamorphism of basalt. Actinolite is an important and common member of the monoclinic series, forming radiating groups of acicular crystals of a bright green or greyish-green color, it occurs as a constituent of greenschists. The name is a translation of the old German word Strahlstein. Glaucophane, crocidolite and arfvedsonite form a somewhat special group of alkali-amphiboles; the first two are blue fibrous minerals, with glaucophane occurring in blueschists and crocidolite in ironstone formations, both resulting from dynamo-metamorphic processes. The latter two are dark green minerals, which occur as original constituents of igneous rocks rich in sodium, such as nepheline-syenite and phonolite. Pargasite is a rare magnesium-rich amphibole with essential sodium found in ultramafic rocks. For instance, it occurs in uncommon mantle xenoliths, carried up by kimberlite, it is hard, dense and automorphic, with a red-brown pleochroism in petrographic thin section.
List of minerals Classification of minerals - Silicates C. Michael Hogan. 2010. Calcium. Eds. A. Jorgensen, C. Cleveland. Encyclopedia of Earth. National Council for Science and the Environment. Cornelius S. Hurlbut and Cornelis Klein. 1985. Manual of Mineralogy, 20th ed. John Wiley and Sons, New York ISBN 0-471-80580-7
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
Plagioclase is a series of tectosilicate minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the plagioclase feldspar series; this was first shown by the German mineralogist Johann Friedrich Christian Hessel in 1826. The series ranges from albite to anorthite endmembers, where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. Plagioclase in hand samples is identified by its polysynthetic crystal twinning or'record-groove' effect. Plagioclase is a major constituent mineral in the Earth's crust, is an important diagnostic tool in petrology for identifying the composition and evolution of igneous rocks. Plagioclase is a major constituent of rock in the highlands of the Earth's moon. Analysis of thermal emission spectra from the surface of Mars suggests that plagioclase is the most abundant mineral in the crust of Mars.
The composition of a plagioclase feldspar is denoted by its overall fraction of anorthite or albite, determined by measuring the plagioclase crystal's refractive index in crushed grain mounts, or its extinction angle in thin section under a polarizing microscope. The extinction angle varies with the albite fraction. There are several named plagioclase feldspars that fall between anorthite in the series; the following table shows their compositions in terms of constituent anorthite and albite percentages. Anorthite was named by Gustav Rose in 1823 from the Ancient Greek meaning oblique, referring to its triclinic crystallization. Anorthite is a comparatively rare mineral but occurs in the basic plutonic rocks of some orogenic calc-alkaline suites. Albite is named from the Latin albus, in reference to its unusually pure white color, it is a common and important rock-making mineral associated with the more acid rock types and in pegmatite dikes with rarer minerals like tourmaline and beryl. The intermediate members of the plagioclase group are similar to each other and cannot be distinguished except by their optical properties.
The specific gravity in each member increases 0.02 per 10% increase in anorthite. Bytownite, named after the former name for Ottawa, Canada, is a rare mineral found in more basic rocks. Labradorite is the characteristic feldspar of the more basic rock types such as diorite, andesite, or basalt and is associated with one of the pyroxenes or amphiboles. Labradorite shows an iridescent display of colors due to light refracting within the lamellae of the crystal, it is named after Labrador, where it is a constituent of the intrusive igneous rock anorthosite, composed entirely of plagioclase. A variety of labradorite known as spectrolite is found in Finland. Andesine is a characteristic mineral of rocks such as diorite which contain a moderate amount of silica and related volcanics such as andesite. Oligoclase is common in granite, syenite and gneiss, it is a frequent associate of orthoclase. The name oligoclase is derived from the Greek for little and fracture, in reference to the fact that its cleavage angle differs from 90°.
Sunstone is oligoclase with flakes of hematite. Hypersolvus List of minerals Subsolvus
Feldspars are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight. Feldspars crystallize from magma as veins in both intrusive and extrusive igneous rocks and are present in many types of metamorphic rock. Rock formed entirely of calcic plagioclase feldspar is known as anorthosite. Feldspars are found in many types of sedimentary rocks; the name feldspar derives from the German Feldspat, a compound of the words Feld, "field", Spat meaning "a rock that does not contain ore". The change from Spat to -spar was influenced by the English word spar, meaning a non-opaque mineral with good cleavage. Feldspathic refers to materials; the alternate spelling, has fallen out of use. This group of minerals consists of tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers: potassium feldspar endmember KAlSi3O8, albite endmember NaAlSi3O8, anorthite endmember CaAl2Si2O8. Solid solutions between K-feldspar and albite are called "alkali feldspar".
Solid solutions between albite and anorthite are called "plagioclase", or more properly "plagioclase feldspar". Only limited solid solution occurs between K-feldspar and anorthite, in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the Earth. Albite is considered both alkali feldspar. Alkali feldspars are grouped into two types: those containing potassium in combination with sodium, aluminum, or silicon; the first of these include: orthoclase KAlSi3O8, sanidine AlSi3O8, microcline KAlSi3O8, anorthoclase AlSi3O8. Potassium and sodium feldspars are not miscible in the melt at low temperatures, therefore intermediate compositions of the alkali feldspars occur only in higher temperature environments. Sanidine is stable at the highest temperatures, microcline at the lowest. Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition; the perthitic textures in the alkali feldspars of many granites can be seen with the naked eye.
Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope. Barium feldspars are considered alkali feldspars. Barium feldspars form as the result of the substitution of barium for potassium in the mineral structure; the barium feldspars are monoclinic and include the following: celsian BaAl2Si2O8, hyalophane 4O8. The plagioclase feldspars are triclinic; the plagioclase series follows: albite NaAlSi3O8, oligoclase AlSi2O8, andesine NaAlSi3O8—CaAl2Si2O8, labradorite AlSi2O8, bytownite AlSi2O8, anorthite CaAl2Si2O8. Intermediate compositions of plagioclase feldspar may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, the resulting two-feldspar intergrowths are too fine-grained to be visible with optical microscopes; the immiscibility gaps in the plagioclase solid solutions are complex compared to the gap in the alkali feldspars. The play of colours visible in some feldspar of labradorite composition is due to fine-grained exsolution lamellae.
The specific gravity in the plagioclase series increases from albite to anorthite. Chemical weathering of feldspars results in the formation of clay minerals such as illite and kaolinite. About 20 million tonnes of feldspar were produced in 2010 by three countries: Italy and China. Feldspar is a common raw material used in glassmaking, to some extent as a filler and extender in paint and rubber. In glassmaking, alumina from feldspar improves product hardness and resistance to chemical corrosion. In ceramics, the alkalis in feldspar act as a flux. Fluxes melt at an early stage in the firing process, forming a glassy matrix that bonds the other components of the system together. In the US, about 66% of feldspar is consumed in glassmaking, including glass containers and glass fiber. Ceramics and other uses, such as fillers, accounted for the remainder. In earth sciences and archaeology, feldspars are used for K-Ar dating, argon-argon dating, luminescence dating. In October 2012, the Mars Curiosity rover analyzed a rock that turned out to have a high feldspar content.
List of minerals – A list of minerals for which there are articles on Wikipedia List of countries by feldspar production This article incorporates public domain material from the United States Geological Survey document: "Feldspar and nepheline syenite". Bonewitz, Ronald Louis. Rock and Gem. New York: DK Publishing. ISBN 978-0-7566-3342-4. Media related to Feldspar at Wikimedia Commons
A phanerite is an igneous rock whose microstructure is made up of crystals large enough to be distinguished with the unaided eye. Phaneritic texture forms when magma deep underground in the plutonic environment cools giving the crystals time to grow. Phanerites are described as coarse grained or macroscopically crystalline
Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava; the magma can be crust. The melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous rocks occur in a wide range of geological settings: shields, orogens, large igneous provinces, extended crust and oceanic crust. Igneous and metamorphic rocks make up 90–95% of the top 16 km of the Earth's crust by volume. Igneous rocks form about 15% of the Earth's current land surface. Most of the Earth's oceanic crust is made of igneous rock. Igneous rocks are geologically important because: their minerals and global chemistry give information about the composition of the mantle, from which some igneous rocks are extracted, the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted.
In terms of modes of occurrence, igneous rocks can be either extrusive. Intrusive igneous rocks make up the majority of igneous rocks and are formed from magma that cools and solidifies within the crust of a planet, surrounded by pre-existing rock; the mineral grains in such rocks can be identified with the naked eye. Intrusive rocks can be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Typical intrusive formations are batholiths, laccoliths and dikes; when the magma solidifies within the earth's crust, it cools forming coarse textured rocks, such as granite, gabbro, or diorite. The central cores of major mountain ranges consist of intrusive igneous rocks granite; when exposed by erosion, these cores may occupy huge areas of the Earth's surface. Intrusive igneous rocks that form at depth within the crust are termed plutonic rocks and are coarse-grained. Intrusive igneous rocks that form near the surface are termed subvolcanic or hypabyssal rocks and they are medium-grained.
Hypabyssal rocks are less common than plutonic or volcanic rocks and form dikes, laccoliths, lopoliths, or phacoliths. Extrusive igneous rocks known as volcanic rocks, are formed at the crust's surface as a result of the partial melting of rocks within the mantle and crust. Extrusive solidify quicker than intrusive igneous rocks, they are formed by the cooling of molten magma on the earth's surface. The magma, brought to the surface through fissures or volcanic eruptions, solidifies at a faster rate. Hence such rocks are smooth and fine-grained. Basalt is lava plateaus; some kinds of basalt solidify to form long polygonal columns. The Giant's Causeway in Antrim, Northern Ireland is an example; the molten rock, with or without suspended crystals and gas bubbles, is called magma. It rises; when magma reaches the surface from beneath water or air, it is called lava. Eruptions of volcanoes into air are termed subaerial, whereas those occurring underneath the ocean are termed submarine. Black smokers and mid-ocean ridge basalt are examples of submarine volcanic activity.
The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions: divergent boundary: 73% convergent boundary: 15% hotspot: 12%. Magma that erupts from a volcano behaves according to its viscosity, determined by temperature, crystal content and the amount of silica. High-temperature magma, most of, basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle. Long, thin basalt flows with pahoehoe surfaces are common. Intermediate composition magma, such as andesite, tends to form cinder cones of intermingled ash and lava, may have a viscosity similar to thick, cold molasses or rubber when erupted. Felsic magma, such as rhyolite, is erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes with rhyolitic magma erupt explosively, rhyolitic lava flows are of limited extent and have steep margins, because the magma is so viscous. Felsic and intermediate magmas that erupt do so violently, with explosions driven by the release of dissolved gases—typically water vapour, but carbon dioxide.
Explosively erupted pyroclastic material is called tephra and includes tuff and ignimbrite. Fine volcanic ash is erupted and forms ash tuff deposits, which ca