Textural Terms in Igneous Petrology

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Textural Terms in Igneous Petrology Adcumulate - Cumulus crystals continue to grow and displace the intercumulus liquid.

the name given to a formerly open vesicle that has been filled with a secondary mineral(s) precipitated from low-t ground waters which

Andehral - Irregularly shaped compact grains not bounded by any characteristic crystal faces Annealing texture in which many small

1 Textural Terms in Igneous Petrology Adcumulate - Cumulus crystals continue to grow and displace the intercumulus liquid. Example: Opx adcumulate texture with minor interstitial chromite and plagioclase (Bushveld Complex) Amygdaloidal texture Amygdule is the name given to a formerly open vesicle that has been filled with a secondary mineral(s) precipitated from low-t ground waters which have penetrated into the rock. In this case, the amygdule is probably filled with a zeolite mineral. Andehral - Irregularly shaped compact grains not bounded by any characteristic crystal faces Annealing texture in which many small grains join to become larger grains in order to minimize surface energy. Example: Annealing of small chromite grains on the right to form large masses of chromite on the left (Bushveld Complex)

Aphanitic - Texture that consists of a mosaic of crystals too small to be seen without magnification.

intermediate members Cryptocrystalline - Texture that consists of a mosaic of minute crystals that cannot be

$Accumulation of crystals produced by crystal-melt fractionation.$

2 Aphanitic - Texture that consists of a mosaic of crystals too small to be seen without magnification. Can be either cryptocrystalline or microcrystalline Bimodal - Refers to two distinct populations without intermediate members Cryptocrystalline - Texture that consists of a mosaic of minute crystals that cannot be resolved with an optical microscope Crystallinity - Proportion of crystals in a glassy rock or in a magma Crystallites - Minute crystals that do not react visibly to polarised light under the microscope Cumulate - Accumulation of crystals produced by crystal-melt fractionation. Example: Cumulate orthopyroxene in the Bushveld Complex

Dendritic - Crystal shapes resembling tree branches.

completely bounded by its own rational crystal faces, forming a tabular,

Commonly but not necessarily formed by unrestricted growth in a liquid.

defining a compaction foliation more or less parallel to the depositional

of flow layering Glassy - A texture consisting of some proportion of glass.

3 Dendritic - Crystal shapes resembling tree branches. Example: Dentritic Olivines Embayment A term used to describe a particular crystal shape. Example: Embayment in Olivine Phenocryst Euhedral - Mineral grain completely bounded by its own rational crystal faces, forming a tabular, platy, columnar or other habit. Commonly but not necessarily formed by unrestricted growth in a liquid. Same as idiomorphic Eutaxitic texture - Flattened, welded vitroclasts defining a compaction foliation more or less parallel to the depositional surface of the pyroclastic-flow deposit in the fabric is found: NOT a type of flow layering Glassy - A texture consisting of some proportion of glass. Example: Glassy unwelded rhyolitic tuff Graphic - Magmatic texture that consists of an intergrowth of alkali feldspar and quartz, the latter in triangular and hooklike forms resembling ancient writing. Can be coarse where

quartz grains are several millimeters or microcrystalline,

or granophyric Holocrystalline - Texture made wholly of crystals.

describe rocks that contain both crystals and glass.

Example: Hypocrystalline Basalt Hypidiomorphic-granular -

4 quartz grains are several millimeters or microcrystalline, visible only with a microscope, when it is called micrographic or granophyric Holocrystalline - Texture made wholly of crystals. Example: Holocrystalline granite Hypocrytalline A term used to describe rocks that contain both crystals and glass. Example: Hypocrystalline pitchstone with perlitic cracks Example: Hypocrystalline Basalt Hypidiomorphic-granular - Magmatic phaneritic texture that consists of a mixture of euhedral, subhedral and anhedral grains

Microcrystalline - Texture that consists of a mosaic of crystals that are only visible under a microscope.

Orthocumulate - Cumulus crystals are enclosed in material that has crystallised from the interstitial melt.

crystals Example: euhedral to subhedral biotite and plagioclase crystals are surrounded by optically-continuous, gray-colored K-feldspar.

5 Microcrystalline - Texture that consists of a mosaic of crystals that are only visible under a microscope. Example: Microcrystalline Olivine basalt Myrmekite - In granitic rocks, microcrystalline texture that consists of an intergrowth of vermicular ( wormy ) quartz in a sodic plagioclase host Orthocumulate - Cumulus crystals are enclosed in material that has crystallised from the interstitial melt. Example: Opx + Chromite orthocumulate with interstitial plagioclase (Bushveld Complex) Ophitic Magmatic texture in which larger crystals called oikocrysts enclose smaller randomly oriented crystals Example: euhedral to subhedral biotite and plagioclase crystals are surrounded by optically-continuous, gray-colored K-feldspar. Parallel Growth A term used to describe a particular crystal growth orientation. Example: Parallel growth in olivine (also some skeletal material in the left) Perthite - The light gray streaks in this photomicrograph are plagioclase exsolution lamellae in gray K-feldspar. Perthite forms as an originally homogeneous feldspar exsolves two feldspars as temperature falls below the feldspar solvus during subsolidus cooling.

Phaneritic - Texture in which grains of major rock forming minerals are all large enough to be

Phanerocrystalline granite with one K-feldspar Phenocryst - Larger crystal precipitated from a melt

oikocrysts enclose smaller randomly oriented crystals Porphyritic - Inequigranular magmatic texture made

6 Phaneritic - Texture in which grains of major rock forming minerals are all large enough to be identifiable without magnification Example 2: Phanerocrystalline granite with two K-feldspars Example 3: Phanerocrystalline granite with one K-feldspar Phenocryst - Larger crystal precipitated from a melt embedded in a finer grained or glassy matrix Poikilitic - Magmatic texture in which larger crystals called oikocrysts enclose smaller randomly oriented crystals Porphyritic - Inequigranular magmatic texture made up to two grain sizes, larger crystals commonly euhedral called phenocrysts embedded in a finer-grained or glassy matrix. Example:

Porphyritic Andesite Seriate - Phaneritic inequigranular magmatic

Example 2: Seriate textured olivine basalt under crossed polarized

Example: olivines in picritic basalt Skeletal Example 2: Skeletal

development although crystal shape still apparent.

7 Porphyritic Andesite Seriate - Phaneritic inequigranular magmatic texture in which grains range more or less continuously in size. Contrast with bimodal. Example: Seriate textured olivine basalt under plane light Seriate Example 2: Seriate textured olivine basalt under crossed polarized light Skeletal Crystal shape. Example: olivines in picritic basalt Skeletal Example 2: Skeletal crystal shape developed 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 basalt containing black, ovals features that are vesicles.

Example: Vitrophyric basalt with plagioclase, augite and magnetite Vitrophyre Example 2

8 Note the acicular, white plagioclase laths throughout and the euhedral, white olivine phenocryst at the lower right. Vitrophyre - Texture in which large crystals (phenocrysts) lie in a glassy matrix. Example: Vitrophyric basalt with plagioclase, augite and magnetite Vitrophyre Example 2 phenocryst-bearing obsidian. The phenocrysts in the above photomicrograph are mostly plagioclase. The groundmass is obsidian glass

Textures of Igneous Rocks

Textures of Igneous Rocks Page 1 of 6 EENS 212 Prof. Stephen A. Nelson Petrology Tulane University This document last updated on 12-Feb-2004 Introduction to Igneous Rocks An igneous rock is any crystalline or glassy rock that forms