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Definition
| Crystals with textures and structures inherited from the dominately liquid phase |
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Definition
| Rudimentary crystals too small to have distinct mineral properties |
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Definition
| Microscopic crystals large enough to have identifiable mineral properties |
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| Incompletely formed crystals in which the faces of maximum growth rate have developed leaving hollow or hopper-shaped interiors |
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| Crystals with increasingly radiating or tree-like, branching form, caused when the rate of diffusion in a magma is slower than the rate of growth |
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Definition
| Megascopically visible crystals which stand out prominetely from a finer-grained groundmass |
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Definition
| A grain completely bound by its own crystal faces. |
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Definition
| A crystal which is euhedral at one end and ragged or fractured at the other end |
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| Corroded or Embayed Crystal |
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Definition
| A phenocryst which became chemically unstable in contact with melt causing it to become partially resorbed. For example a euhedral crystal that becomes rounded or hollowed out |
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Term
| Glomeroporphyritic Texture |
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Definition
| Clustering together of phenocrysts into clumps |
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Definition
| Alignment of elements of a rock as a result of liquid flow |
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Definition
| Crisscrossing platy crystals of elongated pyroxenes or olivines. Only occurs in Komatiites |
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| The developing of cavaties as a result of freezing-in of gas bubbles escaping from a magma |
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| A highly vesicular structure similar to cinder in which gas cavaties are very numerous and roughly equidimensional, seperated by thin walls of glass or aphanitic material |
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Definition
| A rock froth, excessively cellular and usually having very thin glass walls seperating the gas bubbles. The bubbles are frequently drawn out into elongated tubes due to the flow of the froth |
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Definition
| Rocks in which gas cavities have subsequently been filled with introduced mineral material |
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Definition
| A single mineral-filled gas cavity |
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Definition
| An included fragment of wall rock in an igneous body |
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Definition
| An included single crystal from the wall rock of an intrusion or one picked up from the floor over which a lava flow moved |
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Definition
| Discontinuous spheroidal or ellipsoidal masses of mafic lavas which fit together closely like a pile of squashed marshmallows |
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Definition
| Rocks that are completely glassy and lack megascopic crystals |
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Term
| Hypohyaline/Hypocrystalline |
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Definition
| Rocks composed of crystals and glass; hypohyaline is dominantly glass, hypocrystalline is dominantly crystal |
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Definition
| Rocks composed entirely of crystals |
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Definition
| A crystal bounded in part by its own faces and in part by surfaces developed through mutual interference of adjacent crystals |
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Definition
| A crystal not bounded by its own crystal faces but whose form is impressed on it by adjacent crystals |
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Term
| Panidiomorphic Granular Texture |
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Definition
| The texture of a rock composed essentially of euhedral crystals |
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Term
| Hypidiomorphic Granular Texture (Granitic Texture) |
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Definition
| The texture of a rock composed of a mixture of anhedral and either subhedral or euhedral crystals or both |
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| Allotriomorphic Granular Texture (Aplitic Texture) |
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Definition
| The texture of a rock composed entirely of anhedral crystls. Aplitic texture usually implies fine grain size and sugary texture |
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Definition
| A uniform fine-grained texture in which individual crystals are not visible to the unaided eye |
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Definition
| A texture in which individual crystals are readily visible to the unaided eye |
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Definition
| Crystals are larger than 5mm in diameter |
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Definition
| Crystals reange from 1 to 5mm in diameter |
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Definition
| Crystals are visible but less than 1mm in diameter |
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Definition
| The texture of a rock composed of two distinct grain sizes. For example: large crystals in a phaneritic groundmass or large to small crystals in an aphanitic or glassy groundmass. The larger crystals are termed phenocrysts. All crystals of one size range belong to one generation. Thus a mineral occurring both as phencrysts and as part of the groundmass are two seperate generations of the mineral |
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Definition
| The texture of a rock consisting of grains of a wide size range but conspicuously larger than the grain size of the associated parent rock |
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Term
| Corona Structure or Reaction Rim |
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Definition
| A zone of one mineral surrounding another, completely or partially, and may occur only where two specific minerals would be in contact and not where either of those minerals were in contact with any other mineral |
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Definition
| When a mineral is surrounded by concentric shells with radial or fibrous texture |
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Definition
| Parallel alignment of tabular or elongated grains due to movement of the crystal-liquid mush |
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Definition
| Tightly pressed microlites, generally feldspar, interwoven in irregular fashion |
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Definition
| Irregular streaks apparently resulting from a drawing out of partially assimilated xenoliths during flow of a mass |
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Definition
| Small angular gas cavities in phaneritic rocks into which crystals of the rock-forming minerals project |
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Definition
| Intergrowths of quartz and alkali feldspar, quartz commonly cuneiform |
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Term
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Definition
| Worm-like or finger-like bodies of quartz enclosed in sodic plagioclase |
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Term
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Definition
| Plagioclase laths largely or entirely enclosed in pyroxene |
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Definition
| Plagioclas only partly enclosed in pyroxene |
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Definition
| Numerous grains of various minerals in random orientation completely enclosed within large, optically continuous crystals of different composition |
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Definition
| Angular interstices between feldspars occupied by some combination of olivine, pyroxene and/or Fe-Ti oxides in random orientation |
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Definition
| Interstices filled with glass, cryptocrystalline material or non-granular secondary minerals |
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Term
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Definition
| Involes albitic plagioclas overgrowths on orthoclase crystals. Occurs in some granites where the plagioclase preferentially forms on the structurally similar alkali feldspar rather than nucleating on their own |
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Term
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Definition
| Concentric shelly cracks that develop in some glasses, probably as a result of contractions. It typically develops in glasses having a notable content of water |
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Term
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Definition
| The slow process of reorganization of glass into crystals, usually of very small size. Frequently takes place first along perlitic cracks and then works out into the body of the glass |
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Definition
| Spherical bodies from microscopic size to many feet in diameter, which consist of radiating fibers and plates. This structure typically develops in silicic glasses and consists of fibers of feldspar and quartz. In many spherulites, flow structure of the surrounding glass may be traced through the bodies, indicating that they formed after flow ceased. An individual body is called a spherulite |
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Term
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Definition
| Spherulites which consist of concentric shells having hollow interspaces. The cavities are often lined with minute crystals of quartz, tridymite, cristobalite, feldspar or fayalite |
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Term
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Definition
| The rupture of rock into polygonal prisms, probably due to shrinkage on cooling. Columns are generally perpendicular to the cooling surfaces |
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Definition
| Textures developed by the mechanical crushing of rock or mineral grains |
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Term
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Definition
| One dimension is markedly greater than the other two, forming a prism with or without terminations |
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Definition
| Slender needle-like crystals with or without visible crystal faces |
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Definition
| Three distinctly different dimensions, one of which is usually much larger than the other two |
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Definition
| Two dimensions are markedly greater than the third; the crystal is usually bounded by two flat parallel faces and therefore has a uniform thickness |
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Definition
| A lens-shaped crystal, thickest in the middle and tapering to the edges, often to a very thin edge |
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Definition
| All three dimensions are equal or subequal, but the crystal has an irregular shape |
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Definition
| All three dimensions are equal or subequal, but the crystal has roughly planar sides |
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Definition
| Equidimensial grains which have rounded boundaries |
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Term
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Definition
| Rocks in which grains are of approximately the same size. The textures are particulary common in monomineralic rocks such as quartzite and marble. Preferred orientation of grains is very often present but not recognizable megascopically |
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Term
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Definition
| Crystals are equigranular and equidimensional and are generally polygonal in shape, with simple straight-line or gently curved intergranular boundaries |
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Term
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Definition
| Crystals are generally equiangular and equidimensional, or they are lenticular and have highly irregular boundaries and much interpretation of each grain into its neighbors |
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Term
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Definition
| A very fine-grained product of mechanical crushing without recrystallization of the primary minerals. Such rocks usually show directional features as thin lamallae. The crushed material is lithified by an introduced hydrothermal cement |
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Term
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Definition
| Are rocks in which there are two distinct grain sizes or noticeably different grain habits. These textures are of two fundamental origins (1) recrystallization in a polymineralic rock as a result of metamorphism without directional stresses, and (2) incomplete mechanical crushing not accompanied by development of a strong preferred orientation |
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Term
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Definition
| A crystalline texture due to metamorphic recrystallization. The term is equally applicable to those rocks with strongly preferred directions. It implies the essentially simultaneous growth of all mineral grains ina solid environment. The sizes and shapes of grains then depends on the relative ability of the different minerals to make room for themselves, that is on their crystalloblastic strength |
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Term
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Definition
| A crystalloblastic texture with two or more distinct grain sizes, equivalent to porphyritic texture in igneous rocks. The individual large crystals are called porphyroblasts, equivalent to phenocrysts in igneous rocks |
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Term
Poikiloblastic Texture
(Sieve Texture) |
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Definition
| A texture in which large porphyroblasts include numerous small crystal grains. During growth the larger crystal apparently grew around mineral grains which could not be accomodated in the composition of the growing large crystal |
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Term
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Definition
| The crystalloblastic texture of polymineralic rocks in which there is no preffered orientation of grains |
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Term
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Definition
| Textures produced by mechanical crushing without essential recrystallization. These rocks may or may not show directional features |
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Term
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Definition
| The general term used to include all planar textures and structures of metamorphic rocks that were developed during metamorphism. The foliation may be defined by layering of contrasting mineralogies (gneissosity) by planar preferred orientations of individual grains (schistosity), by planar fracture surfaces (cleavage) or by any combination of these three |
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Term
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Definition
| Structures which show preferred direction in metamorphic rocks and are defined by more or less closely spaced fractures. Several types of cleavage are recognized. The orientation of cleavage to regional structure varies with the type of cleavage |
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Term
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Definition
| Physically distinct fractures developed by mechanical rupture. In this type of cleavage the fractures are distinct breaks separated by unfractured rock which will break readily in all directions. Apparently fracture cleavage does not involve parallel orientation of mineral grains or aggregates |
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Definition
| Closely spaced distinct fractures developed by mechanical deformation. In this type of cleavage all portions of the rock tend to split more readily in one direction than in any other. Rocks with slaty cleavage are microcrystalline but show a characteristic sheen on the fracture surface. Reorientation of flaky grains or development of lenticular grains parallel to the cleavage is characteristic |
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Term
| Flow Cleavage (Schistosity) |
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Definition
| The ability of metamorphic rocks to split preferentially in one direction as a result of parallelism of megascopically visible micaceous mineral flakes |
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Term
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Definition
| Fracture cleavage involving visible displacement along the fracture. Micaceous minerals may be dragged by slippage to positions parallel to the fractures |
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Term
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Definition
| The ability to split into long pencil-like fragments, usually due to intersection of two cleavages or one cleavage and bedding |
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Definition
| Slaty, or flow cleavage parallel to the bedding. This may be due to load or to isoclinal folding and should not be confused with bedding plane fissility of unmetamorphosed rocks |
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Term
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Definition
| A planar texture defined by parallelism of megascopically visible platy, prismatic, or lenticular crystals. A schistose texture is one which shows schistosity. Parallelism of flakes of micas, chlorites, talc graphite, hematite etc., are characteristic. The flakes may or may not show a linear parallelism superimposed on the planar. There may be associated mineral segregation into bands or streaks, but only those portions dominated by parallel platy crystals can be said to be schistose |
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Term
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Definition
| Structures in which the planar element is represented by mineral segregation into bands, lenses, and irregular planar masses. The segregation of minerals may be of several origins. Metamorphic differentiation results in segregation by concentrating the more mobile mineral phases in shear zones, leaving the unsheared portion of the rock deficient in the mobile constituents and enriched in the relatively immobile phases. The growths of porphroblasts is often accompanied by secretionary growth of more mobile constituents |
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Term
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Definition
| A banded structure with alternating bands of an equigranular mosaic texture, usually of quartzo-feldspathic material, and a schistose texture, usually dominated by micas and hornblende. The bands are not usually continuous over long distances but lens out. They are, however, thin compared with their lateral extent |
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Term
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Definition
| A lenticular structure composed of an aggregate of grains which in cross-section has the shape of an eye. Most commonly augen structures are developed by the growth of a large porphyroblast forcing aside the surrounding rock and allowing the secretionary growth of mobile constituents in the corners of the eyes |
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Definition
| A structure developed by dynamic metamorphism in which lenticular masses of coarse-grained or uncrushed material are seperated by wavy zones of finger-grained foliated material |
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Definition
| A lenticular structure developed when competent units embedded in a yielding matrix are pulled apart by deformation. Boudinage structures are often developed from small dikes or veins in scistose rocks or from quartzite or limestone units in deformed argillaceous rocks. Individual pillow-shaped unit is called a boudin |
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Term
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Definition
| Contorted veins or dikelets which show irregular patterns. The veins are usually quartz or pegmatite embedded in foliated rocks. They may be concordant with the host-rock structure but most are commonly discordant. The folds are usually thickened on their axes and thinned on their limbs |
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Term
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Definition
| The general term for the parallel orientation of textural or structural features that are linear. Prismatic, bladed, or rod-shaped elements, or lines of intersection of planes, have their longest dimension parallel. Linear textures are linear elements which show as a result of parallelism of elongated single crystals. Prismatic crystals, such as hornblende, and bladed crystals, such as kyanite, and sometimes biotite, are the more common examples. The linear texture is usually superimposed on a planar element. Linear structures are polygranular units that show parallelism of an elongated dimension. The linear element may be a deformed relict structure or the result of new crystal growth and deformation |
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