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 (Na₂O) 

Allotriomorphic granular - a granular texture in which most grains are anhedral. See aplitic. 

Amydale or amygdule - a vesicle or gas cavity in an igneous rock usually filled with secondary minerals such as zeolites, 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. = Allotriomorphic = Xenomorphic. 

Antiperthite - a perthitic intergrowth in which plagioclase encloses potash feldspar. See perthite. 

Aphanitic - a fine-grained texture in which the individual constituents cannot be distinguished by the unaided eye. Includes both crystalline and glassy rocks. 

Aplite - an igneous rock (commonly occurs as veins or dike rocks) with an aplitic texture. 

Aplitic - a fine-grained, sugary texture in igneous rocks inwhich the constituents are anhedral grains. See allotriomorphic granular. 

Autolith - an inclusion in an igneous rock to which it is genetically related (i.e., a cognate inclusion, or endogenous inclusion). Cf: xenolith. 

Banded structure - due to alternating layers of different mineralogical composition or texture. 

Basic - applied to igneous rocks with a low silica content (usually less than 52%); the term mafic is preferred. 

Crystallites - tiny embryonic crystals in glassy rocks which do not polarize 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 (Lava) poured out through volcanic vents of fissures. 

Euhedral - a term applied to minerals bounded by their characteristic faces. = Idomorphic = Automorphic. 

Eutaxitic - a term describing the streaked appearance of certain volcanic rocks due to alternating bands or lenses of different material, color, or texture. May in part be a fluidal structure, and, in part, depositional.  

Expansion fractures - fractures formed in surrounding grains when a mineral expands upon alteration. Commonly seen around serpentinized olivine. 

Exsolution - the process of separation. In the solid state, the separation of two components which are capable of existing in solid solution at higher temperature (e.g., perthite). 

Fabric - the shape and arrangement of crystals and amorphous parts in a rocks. Texture is a function of fabric. 

Felsic - a rock with a felsitic texture. 

Felsitic - an aphanitic texture but applied only to acid rocks. 

Flow structure - a banded structure commonly in which platy and elongated minerals show parallel orientation. It is due to flowage during the middle or later stages of consolidation. 

Fluidal structure - similar to flow structure but commonly applied to aphanitic rocks. 

Glomeroporphyritic - a porphyritic texture in which the phenocrysts occur in aggregates. 

Granitic - a granular texture characteristic of a granite. = Hypidiomorphic granular. 

Granophyric - a microscopic graphic texture. = Micrographic =Micropegmatitic 

Granular - a texture characterized by grains of approximately equal size. 

Granularity - the absolute size of crystals in a texture. Texture is a function of granularity. 

Graphic texture - An intergrowth of two minerals (commonly quartz and potash feldspar) giving a pattern resembling cunieform or semitic characters. The individual grains of each mineral, though apparently isolated from each other, have parallel optical orientation over small areas. 

Groundmass - commonly applied to the matrix of a rock in which phenocrysts are imbedded. 

Holocrystalline - composed wholly of crystalline material. 

Holohyaline - composed wholly of glass. 

Hourglass structure - a structure, resembling an hourglass in shape, observed in certain cross-sections of minerals.  

Hyalopilitic - a texture where numerous microlites (commonly feldspar) are enclosed in a glassy groundmass. 

Hydrothermal - applied to heated waters from crystallizing magmas and to the effects produced by it, and to the resultant deposits formed. 

Hypabyssal - applied to those igneous rocks and bodies more or less intermediate between the volcanic and plutonic types. It includes sills and dikes. 

Hypidiomorphic granular - a granular texture where the mineral constituents show boundaries in part only. (= granitic texture.) 

Hypocrystalline - composed in part of glass and in part of crystalline material. 

Idiomorphic granular - a granular texture in which the mineral constituents are mostly euhedral. 

Inclusion - a foreign body enclosed in a crystal or rock. See xenolith. 

Intergranular texture - a variety of intersertal texture in which the interspaces are filled with granular material. 

Intersertal - a texture in which glass or crystalline material fills the interspaces between larger crystals (commonly feldspar laths). 

Leuco - when prefixed to rock names indicates a leucocratic character. 

Leucocratic - a term applied to igneous rocks unusually low in mafic minerals relative to the normal rock-type. 

Lithophysae - concentric shells with hollow spaces between developed in shrunken spherulites. The cavities may be lined with minute crystals of feldspar, quartz, or tridymite. 

Mafic - a term for ferromagnesium minerals actually present in a rock. Also applied to rocks rich in mafic minerals. 

Megascopic - applied to observations made by the unaided eye. 

Mela-, Melano- - when prefixed to rock names indicates a melacratic or melanocratic character. 

Melacratic, Melanocratic - a term applied to igneous rocks unusually rich in mafic minerals related to the normal rock-type. 

Miarolitic - possessing miarolitic cavities. 

Miarolitic cavities - small openings in plutonic rocks ( commonly granites) filled or lined with crystals ( quartz, feldspar, muscovite, fluorite, etc.). These represent the last phases of crystallization. 

Microcrystalline - a texture in which individual grains can be seen only under a microscope. 

Microgranular - microscopically granular. 

Micrographic - microscopically graphic. = Micropegmatitic, = Granophyric. 

Microlites - Microscopic tabular or prismatic crystals. Distinguished from crystallites by their capacity to polarize light. 

Micropegmatitic - see micrographic. = Micrographic, = Granophyric. 

Myrmekite - an intergrowth of vermicular (worm-like) quartz and plagioclase (generally oligoclase) usually replacing potash feldspar. Possibly a deuteric (late magmatic) effect, or related to deformational recrystallization in some cases. 

Myrmekitic - the texture found in myrmekite. 

Ophitic - a texture in which large crystals of augite enclose lath-shaped crystals of plagioclase. As the amount of augite decreases and the relative amount of plagioclase increases the former may fill only the interspaces between the latter. Commonly used synonymously with diabasictexture. 

Palagonite - a yellow or orange, isotropic mineraloid formed by hydration and other alteration (devitrification, oxidation) of sideromelane, and constituting a characteristic part of palogonite tuffs. Also found as amygdule fillings in some basaltic lavas and as an alteration of the glassy skins of pillow basalts. 

Pegmatite - originally defined to indicate a coarse, graphic texture but now used to describe an unusually coarse-grained igneous rock with a pegmatite habit. 

Perlitic - a structure in glass showing onion-like cracks produced by contraction upon cooling. 

Perthite - an intergrowth of plagioclase (albite) and potash feldspar. In perthite potash feldspar encloses plagioclase but in antiperthite plagioclase encloses potash feldspar. In microcline-perthite, microcline is the host. Microperthite is a microscopic perthite. 

Perthitic - describing the intergrowth of potash feldspar and plagioclase. The texture may be due to replacement of host by included mineral, simultaneous crystallization of the two minerals (less probable), or the separation or unmixing from solid solution as a result of instability induced bycooling. 

Phaneric - Phaneritic, = Phanerocrystalline. 

Phaneritic - a texture in which nearly all of the mineral constituents can be distinguished by the unaided eye. 

Phanerocrystalline = Phaneric, = Phaneritic. 

Phenocryst - a large crystal in a porphyritic rock. 

Pilotaxitic - a texture in which lath-shaped microlites (commonly feldspar) make up a felty aggregate, glass being absent or in very minor quantities. Characteristic of some volcanicrocks. 

Pleochroic halos - colored zones in pleochroic minerals (biotite,etc.) surrounding inclusions of radioactive minerals (zircon, etc.) which are characterized by an intensification of the pleochroism or darkening of the host. 

Plutonic - applied to those igneous rocks and bodies which form at significant depths in the earth. 

Poikilitic - a texture in which small crystals of one mineral are enclosed by a much larger crystal unit. Ophitic and diabasic textures are varieties of poikilitic texture in which plagioclase is enclosed by pyroxene. 

Porphyritic - a texture in igneous rocks in which conspicuously large crystals (phenocrysts) are imbedded in a finer-grained or glassy groundmass. 

Pumiceous - a frothy structure in glassy rocks due to extreme vesiculation in the fluid stage by expanding and escaping gases. 

Reaction rim - a peripheral zone of minerals formed around another mineral by reaction of the latter with the magma or adjacent minerals (partial resorption). 

Resorption - the process whereby earlier formed minerals become unstable in the liquid and are partly or completely dissolved, re-fused, or changed (resorbed). 

Scoriaceous - a structure found in mafic lava with a cindery appearance due to an abundance of large vesicles. 

Segregation - a concentration into a mass or streak of certain of the early products of crystallization in a magma. 

Seriate porphyritic - a porphyritic texture in which there is nearly a complete gradation in grain size from the largest phenocrysts to the average sized grain in the groundmass. 

Sideromelane - basaltic glass; characteristic of palagonite tuff. 

Spherulite - a spheroidal mass of acicular crystals (orthoclase and quartz), radially arranged. Spherulites are frequently encountered in glassy rocks and have essentially the same composition as the glass from which they may form. 

Spherulitic - a structure in glassy rocks due to the presence of spherulites. 

Strain shadows - the wavy extinction seen in crystals due to strain. Very common in quartz. It is not to be confused with the partial extinction observed in zoned crystals. 

Subhedral - term applied to minerals bounded only in part of their characteristic faces. = Hypidiomorphic, = Hypautomorphic. 

Subporphyritic - between porphyritic and granular. 

Texture - indicates the mutual relations of crystalline and amorphous crystallinity, granularity and fabric. It should not be confused with the term structure. 

Trachytic - a texture in which tabular crystals (feldspar) have subparallel orientation as a result of flowage in a partly crystallized melt. 

Ultramafic - a term applied to igneous rocks with little or no feldspar but comprised almost entirely of mafic minerals. Chemically they may be considered as those rocks with less than 45% silica. 

Variolite - similar to a spherulite but characteristic of mafic rocks. 

Variolitic - a structure in mafic rocks due to the presence ofvariolites. 

Vesicle - an air-bubble in an aphanitic rock formed by expanding and escaping gases in the cooling lava or magma. 

Vesicular - a structure in aphanitic rocks due to an abundance ofvesicles. 

Vitrophyre - volcanic glass that contains phenocrysts or microphenocrysts. 

Xenocryst - crystals in igneous rocks that are foreign to the body of rock in which they occur. 

Xenolith - a fragment or inclusion of rock foreign to the igneous rock enclosing it. Cf. autolith. These may be partly reacted upon by the magma so as to form new minerals. 

Zoning- a generally concentric arrangement of slightly different composition material in what appears otherwise to be a homogeneous crystal. Very commonly exhibited in plagioclase. 

Content Prepared by: 

O. Don Hermes  
Department of Geology 
University of Rhode Island 
Kingston, RI 02881 
December 30, 1996 


This page prepared by Brian D. Jones 
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