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   Definitions of Some of the More Common Petrographic Terms Acid  - applied to igneous rocks with an abundance of silica (usually 66% or more); the term felsic is  preferred.   Alkaline rocks  - a general term applied to rocks with feldspathoids, or to rocks with unusually high concentrations of alkalies (Na  !)   Allotriomorphic granular  - a granular te ture in which most grains are anhedral. #ee aplitic.   Amydale or amygdule  - a $esicle or gas ca$ity in an igneous rock usually filled with secondary minerals such as eolites, calcite, chlorite, chalcedony, etc.   Amygduloid  - a rock (usually basaltic) containing amygdules.   Amygduloidal  - a term applied to rocks containing amygdules.   Anhedral  - a term applied to minerals not bounded by their characteristic faces. & 'llotriomorphic & enomorphic.   Antiperthite  - a perthitic intergrowth in which plagioclase encloses potash feldspar. #ee perthite.   Aphanitic  - a fine-grained te ture in which the indi$idual constituents cannot be distinguished by the unaided eye. ncludes both crystalline and glassy rocks.   Aplit e - an igneous rock (commonly occurs as $eins or dike rocks) with an aplitic te ture.   Aplitic  - a fine-grained, sugary te ture in igneous rocks inwhich the constituents are anhedral grains. #ee allotriomorphic granular.   Autolith  - an inclusion in an igneous rock to which it is genetically related (i.e., a cognate inclusion, or endogenous inclusion). *f+ enolith.   Banded structure  - due to alternating layers of different mineralogical composition or te ture.   Basic  - applied to igneous rocks with a low silica content (usually less than %); the term mafic is  preferred.   Crystallites  - tiny embryonic crystals in glassy rocks which do not polarie light   Deuteric  - essentially synonymous with late magmatic, but not hydrothermal.   Devitrification  - the process of changing from the unstable glassy state to the crystalline state.   Diabasic  - see ophitic.   Effusive  - applied to material (a$a) poured out through $olcanic $ents of fissures.    Euhedral  - a term applied to minerals bounded by their characteristic faces. & domorphic & 'utomorphic.   Eutaxiti c - a term describing the streaked appearance of certain $olcanic rocks due to alternating bands or lenses of different material, color, or te ture. ay in part be a fluidal structure, and, in part, depositional. Expansion fractures  - fractures formed in surrounding grains when a mineral e pands upon alteration. *ommonly seen around serpentinied oli$ine.   Exsolution  - the process of separation. n the solid state, the separation of two components which are capable of e isting in solid solution at higher temperature (e.g., perthite).   Fabric  - the shape and arrangement of crystals and amorphous parts in a rocks. /e ture is a function of fabric.   Felsic  - a rock with a felsitic te ture.   Felsitic  - an aphanitic te ture but applied only to acid rocks.   Flow structure  - a banded structure commonly in which platy and elongated minerals show parallel orientation. t is due to flowage during the middle or later stages of consolidation.   Fluidal structure  - similar to flow structure but commonly applied to aphanitic rocks.   Glomeroporphyriti c - a porphyritic te ture in which the phenocrysts occur in aggregates.   Granitic  - a granular te ture characteristic of a granite. & 0ypidiomorphic granular.   Granophyric  - a microscopic graphic te ture. & icrographic &icropegmatitic   Granular  - a te ture characteried by grains of appro imately e1ual sie.   Granularit y - the absolute sie of crystals in a te ture. /e ture is a function of granularity.   Graphic texture  - 'n intergrowth of two minerals (commonly 1uart and potash feldspar) gi$ing a pattern resembling cunieform or semitic characters. /he indi$idual grains of each mineral, though apparently isolated from each other, ha$e parallel optical orientation o$er small areas.   Groundmass  - commonly applied to the matri of a rock in which phenocrysts are imbedded.   olocrystalline  - composed wholly of crystalline material.   olohyaline  - composed wholly of glass.   ourglass structure  - a structure, resembling an hourglass in shape, obser$ed in certain cross-sections of minerals. yalopilitic  - a te ture where numerous microlites (commonly feldspar) are enclosed in a glassy  groundmass.   ydrothermal  - applied to heated waters from crystalliing magmas and to the effects produced by it, and to the resultant deposits formed.   ypabyssal  - applied to those igneous rocks and bodies more or less intermediate between the $olcanic and plutonic types. t includes sills and dikes.   ypidiomorphic granular  - a granular te ture where the mineral constituents show boundaries in part only. (& granitic te ture.)   ypocrystalline  - composed in part of glass and in part of crystalline material.   !diomorphic granular  - a granular te ture in which the mineral constituents are mostly euhedral.   !nclusion  - a foreign body enclosed in a crystal or rock. #ee enolith.   !ntergranular texture  - a $ariety of intersertal te ture in which the interspaces are filled with granular material.   !ntersertal  - a te ture in which glass or crystalline material fills the interspaces between larger crystals (commonly feldspar laths).   euco  - when prefi ed to rock names indicates a leucocratic character.   eucocratic  - a term applied to igneous rocks unusually low in mafic minerals relati$e to the normal rock-type.   ithophysae  - concentric shells with hollow spaces between de$eloped in shrunken spherulites. /he ca$ities may be lined with minute crystals of feldspar, 1uart, or tridymite.   #afic  - a term for ferromagnesium minerals actually present in a rock. 'lso applied to rocks rich in mafic minerals.   #egascopic  - applied to obser$ations made by the unaided eye.   #ela$% #elano - - when prefi ed to rock names indicates a melacratic or melanocratic character.   #elacratic% #elanocratic  - a term applied to igneous rocks unusually rich in mafic minerals related to the normal rock-type.   #iarolitic  - possessing miarolitic ca$ities.   #iarolitic cavities  - small openings in plutonic rocks ( commonly granites) filled or lined with crystals ( 1uart, feldspar, musco$ite, fluorite, etc.). /hese represent the last phases of crystalliation.   #icrocrystalline  - a te ture in which indi$idual grains can be seen only under a microscope.    #icrogranular  - microscopically granular.   #icrographic  - microscopically graphic. & icropegmatitic, & 2ranophyric.   #icrolites  - icroscopic tabular or prismatic crystals. 3istinguished from crystallites by their capacity to  polarie light.   #icropegmatitic  - see micrographic. & icrographic, & 2ranophyric.   #yrmekite  - an intergrowth of $ermicular (worm-like) 1uart and plagioclase (generally oligoclase) usually replacing potash feldspar. 4ossibly a deuteric (late magmatic) effect, or related to deformational recrystalliation in some cases.   #yrmekitic  - the te ture found in myrmekite.   &phitic  - a te ture in which large crystals of augite enclose lath-shaped crystals of plagioclase. 's the amount of augite decreases and the relati$e amount of plagioclase increases the former may fill only the interspaces between the latter. *ommonly used synonymously with diabasicte ture.   'alagonite  - a yellow or orange, isotropic mineraloid formed by hydration and other alteration (de$itrification, o idation) of sideromelane, and constituting a characteristic part of palogonite tuffs. 'lso found as amygdule fillings in some basaltic la$as and as an alteration of the glassy skins of pillow basalts.   'egmatite  - srcinally defined to indicate a coarse, graphic te ture but now used to describe an unusually coarse-grained igneous rock with a pegmatite habit.   'erlitic  - a structure in glass showing onion-like cracks produced by contraction upon cooling.   'erthite  - an intergrowth of plagioclase (albite) and potash feldspar. n perthite potash feldspar encloses  plagioclase but in antiperthite plagioclase encloses potash feldspar. n microcline-perthite, microcline is the host. icroperthite is a microscopic perthite.   'erthitic  - describing the intergrowth of potash feldspar and plagioclase. /he te ture may be due to replacement of host by included mineral, simultaneous crystalliation of the two minerals (less probable), or the separation or unmi ing from solid solution as a result of instability induced bycooling.   'haneric $ 'haneritic , & 4hanerocrystalline.   'haneritic  - a te ture in which nearly all of the mineral constituents can be distinguished by the unaided eye.   'hanerocrystalline  & 4haneric, & 4haneritic.   'henocryst  - a large crystal in a porphyritic rock.   'ilotaxitic  - a te ture in which lath-shaped microlites (commonly feldspar) make up a felty aggregate, glass being absent or in $ery minor 1uantities. *haracteristic of some $olcanicrocks.   'leochroic halos  - colored ones in pleochroic minerals (biotite,etc.) surrounding inclusions of radioacti$e
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