rock textures

Igneous and Metamorphic Rock Textures

Allotriomorphic-granular (gabbroic) texture is a feature of phaneritic rocks in which all of the minerals have anhedral shapes; it is characteristic of some mafic and ultramafic rocks, such as dunite and pyroxenite.

Hypidiomorphic-granular (granitic) texture: igneous texture in which most of the mineral grains are subhedral. Typical of granite, granodiorite, quartz monzonite, etc.

Porphyritic texture: rocks in which larger grains are contained in a finer grained matrix. May occur in either intrusive or extrusive rocks, but it is most common in extrusive rocks such as basalt, andesite, dacite, and rhyolite.

Pyroclastic texture: produced by explosive volcanism, broken clasts of mineral, lithic clasts, and glass shards are common. The clasts may be either sorted or unsorted. Common in tuffs.

Intergranular texture: Plagioclase laths with interstitial pyroxene grains that are smaller than the plagioclase; commonly found in basalts.

Intersertal texture: Small feldspars with glass or altered glass interstitial to the feldspars; common texture of basalts.

Ophitic texture: Pyroxene grains partially or completely surround plagioclase laths; common texture of gabbros and basalts.

Trachytic texture: Subparallel feldspars formed during flow in volcanic rocks.

Spinifex texture: Interlacing olivine or pyroxene, a texture formed by quenching in komatiites.

Some Metamorphic Textures

Schistose texture: Strong foliation, or alignment of grains, particularly micas, in coarse-grained metamorphic rocks.

Phyllitic texture: Strong foliation in fine-grained metamorphic rocks.

Granoblastic texture: Massive, unfoliated, equigranular texture in metamorphic rocks.

Porphyroblastic texture: Metamorphic texture consisting of large grains in a finer grained matrix.