textures manual with photos

8/3/2019 Textures Manual With Photos

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Textural Terms in Igneous Petrology

Adcumulate - Cumulus crystals continue to growand displace the intercumulus liquid. Example:Opx adcumulate texture with minor interstitialchromite and plagioclase (Bushveld Complex)

Amygdaloidal texture – Amygdule is the namegiven to a formerly open vesicle that has beenfilled with a secondary mineral(s) precipitatedfrom low-T ground waters which have penetratedinto the rock. In this case, the amygdule isprobably filled with a zeolite mineral.

Andehral - Irregularly shaped compact grainsnot bounded by any characteristic crystal faces

Annealing – texture in which many small grains join to become larger grains in order to minimizesurface energy. Example: Annealing of smallchromite grains on the right to form largemasses of chromite on the left (Bushveld

Complex)



Aphanitic - Texture that consists of a mosaic ofcrystals too small to be seen withoutmagnification. Can be either cryptocrystalline ormicrocrystalline

Bimodal - Refers to two distinct populationswithout intermediate members

Cryptocrystalline - Texture that consists of amosaic of minute crystals that cannot beresolved with an optical microscope

Crystallinity - Proportion of crystals in a glassyrock or in a magma

Crystallites - Minute crystals that do not reactvisibly to polarised light under the microscope

Cumulate - Accumulation of crystals produced bycrystal-melt fractionation. Example: Cumulate

orthopyroxene in the Bushveld Complex



Dendritic - Crystal shapes resembling treebranches. Example: Dentritic Olivines

Embayment – A term used to describe aparticular crystal shape. Example: Embayment inOlivine Phenocryst

Euhedral - Mineral grain completely bounded byits own rational crystal faces, forming a tabular,platy, columnar or other habit. Commonly but notnecessarily formed by unrestricted growth in aliquid. Same as idiomorphic

Eutaxitic texture - Flattened, weldedvitroclasts defining a compaction foliation moreor less parallel to the depositional surface of thepyroclastic-flow deposit in the fabric is found:NOT a type of flow layering

Glassy - A texture consisting of some proportionof glass. Example: Glassy unwelded rhyolitic tuff

Graphic - Magmatic texture that consists of anintergrowth of alkali feldspar and quartz, the

latter in triangular and hooklike formsresembling ancient writing. Can be coarse where



quartz grains are several millimeters ormicrocrystalline, visible only with a microscope,when it is called micrographic or granophyric

Holocrystalline - Texture made wholly ofcrystals. Example: Holocrystalline granite

Hypocrytalline – A term used to describe rocksthat contain both crystals and glass. Example:Hypocrystalline pitchstone with perlitic cracks

Example: Hypocrystalline Basalt Hypidiomorphic-granular - Magmatic phaneritictexture that consists of a mixture of euhedral,subhedral and anhedral grains



Microcrystalline - Texture that consists of amosaic of crystals that are only visible under amicroscope. Example: Microcrystalline Olivinebasalt

Myrmekite - In granitic rocks, microcrystallinetexture that consists of an intergrowth ofvermicular (“wormy”) quartz in a sodic plagioclasehost

Orthocumulate - Cumulus crystals are enclosedin material that has crystallised from theinterstitial melt. Example: Opx + Chromiteorthocumulate with interstitial plagioclase(Bushveld Complex)

Ophitic – Magmatic texture in which largercrystals called oikocrysts enclose smallerrandomly oriented crystals Example: euhedral tosubhedral biotite and plagioclase crystals aresurrounded by optically-continuous, gray-coloredK-feldspar.

Parallel Growth – A term used to describe aparticular crystal growth orientation. Example:Parallel growth in olivine (also some skeletalmaterial in the left)

Perthite - The light gray streaks in thisphotomicrograph are plagioclase exsolutionlamellae in gray K-feldspar. Perthite forms as anoriginally homogeneous feldspar exsolves twofeldspars as temperature falls below thefeldspar solvus during subsolidus cooling.

http://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/plagtwins.X.html

http://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/plagtwins.X.html



Phaneritic - Texture in which grains of majorrock forming minerals are all large enough to beidentifiable without magnification

Example 2: Phanerocrystalline granite with twoK-feldspars

Example 3: Phanerocrystalline granite with oneK-feldspar

Phenocryst - Larger crystal precipitated from amelt embedded in a finer grained or glassymatrix

Poikilitic - Magmatic texture in which largercrystals called oikocrysts enclose smaller

randomly oriented crystals

Porphyritic - Inequigranular magmatic texturemade up to two grain sizes, larger crystals

commonly euhedral called phenocrysts embeddedin a finer-grained or glassy matrix. Example:



Porphyritic Andesite

Seriate - Phaneritic inequigranular magmatictexture in which grains range more or less

continuously in size. Contrast with bimodal.Example: Seriate textured olivine basalt underplane light

Seriate Example 2: Seriate textured olivinebasalt under crossed polarized light

Skeletal – Crystal shape. Example: olivines inpicritic basalt

Skeletal Example 2: Skeletal crystal shapedeveloped in olivine

Subhedral – Poor crystal shape development

although crystal shape still apparent. Example:Subhedral olivine in picritic basalt

Vesicular – A term given to a rock that contains

lots of vesicles. Example: Scoriaceous basaltcontaining black, ovals features that are vesicles.



Note the acicular, white plagioclase lathsthroughout and the euhedral, white olivinephenocryst at the lower right.

Vitrophyre - Texture in which large crystals(phenocrysts) lie in a glassy matrix. Example:Vitrophyric basalt with plagioclase, augite andmagnetite

Vitrophyre – Example 2 phenocryst-bearingobsidian. The phenocrysts in the abovephotomicrograph are mostly plagioclase. Thegroundmass is obsidian glass

textures manual with photos

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