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. 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. 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, 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. 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 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 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.

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