Textures

Texture of a magmatic rock is microscopically observed summary of features reflecting conditions of the magma crystallization, mainly size, shape and distribution of mineral grains. Textures are influenced by the degree of magma cooling, distribution and arrangement of minerals and melt composition. Clear boundary between structure and texture does not exist, because some structures are discernible by nacked eyes and others are not. More than 90 structures of magmatic rocks have been defined (Hejtman 1956). The four fundamental textures are phaneritic, aphanitic, glassy and pyroclastic.

Phaneritic (from Greek faneros – visible or obvious) texture is coarse-grained texture build up by mutually intergrown crystals visible by naked eye. The texture is characteristic of plutonic (intrusive) rocks crystallized from slowly cooling intrusions at greater depths, with sufficient time space for the crystal growth. According to grain size, very coarse-grained (> 1 cm), coarse-grained (3 mm – 1 cm), medium-grained (1-3 mm) and fine-grained (0.1-1 mm) textures are defined.

Aphanitic (from Greek aphanés – invisible) texture is characteristic of very small grains (< 0.01 mm) which can be discerned only under polarizing microscope or using another imaging technique. This textural type is typical either of rapidly cooling magma extruded on surface, or of marginal parts of shallow magmatic intrusions. Rocks with aphanitic textures are volcanic (extrusive).

Equigranular texture is phaneritic texture with equal-sized mineral grains.

Porphyric (non-equigranular) texture is that composed of larger crystals (phenocrysts, xenocrysts) emplaced in finely grained matrix. Fenocrystals originate earlier, during the period of slow cooling at greater depths within a magmatic chamber. In contrast, matrix crystallized much faster at near-surface conditions or at surface during final stage of the rock evolution. Affiliation of porphyric rocks to plutonic or volcanic class depends on grain size of the matrix which is controlled by the degree of cooling. More abundant in nature are porhyric aphanitic than porphyric phaneritic rocks, and this is why most porphyric rocks have a character of volcanic rocks. Porphyric texture, however, does not belong to fundamental texture types - it is an intermediate texture between the aphanitic and phaneritic ones. If phenocrysts create clusters, such a texture is glomeroporphyric. The term porphyric was derived from Greek porphyreos (purple, red), which described a red porphyrite (porfyro rosso antico) from Egyptian locality Djabel Dokhan.

Glassy (vitreous, hyaline) texture (from Greek hyalos and Latin vitrum – glass) is composed fully or partially of silicate glass, e.g. obsidian. The texture originates by rapid cooling, during which ions could not be organized in a crystal structure. Texture is built up by amorphous glass, microliths (germinal crystal forms) and microphenocrysts. The texture is typical of lava flows with an increased SiO2, whose increased viscosity inhibits diffusion of atoms and crystal growth. Vitrophyric texture is porphyric texture with phenocrysts isolated in glassy matrix. Glassy and vitrophyric texture occur in volcanic rocks.

Hemicrystalline texture (from Greek hemi– half) is diagnostic of rocks, in which crystallized minerals dominate over glass. The texture is most abundant in volcanic rocks, and rarely also in dykes.

Holocrystalline texture (from Greek holos – entire) is typical of fully crystallized rocks.

Pyroclastic (from Greek pyro – volcanic, magmatic, and English clastic – fragmental) or fragmental texture originates mainly during volcanic explosions. Pyroclastic rocks are composed of disintegrated fragments of various rocks called pyroclasts. Hardened rocks composed of at least 75 % porphyroclasts are called pyroclastic. Volcanic rocks with pyroclastic texture are tuffs (pyroclasts less than 2 mm in diameter), lapilli tuffs (pyroclasts 2-64 mm in size) or pyroclastic breccias or agglomerates (pyroclasts larger than 64 mm). These are composed of disintegrated fragments of glass, rocks, minerals, or pumice cemented by volcanic ash. Presence of glass among pyroclasts defines the hyaloclastic texture. Vitroclastic texture is composed of glass fragments cemented by glass.

Further textures specify magmatic rocks according to external shape (habit) of crystals:

Panallotriomorphic (panxenomorphic) texture (from Greek allotrios – other, xénos – foreign) – dominant are anhedral minerals without any crystallographic shape.

Panidiomorphic (panautomorphic) texture (from Greek ídios – own) – euhedral minerals with perfect crystal shape predominate.

Hypautomorphic texture – mineral grains are subhedral, i.e. they are partly restricted by crystallographic planes. The structure is intermediate between the two previous ones.

Further textures express their relationship to a specific rock type:

Granitic texture – mafic minerals have a higher degree of automorphism than felsic minerals, of which plagioclases have a higher degree of automorphism than K-feldspars and quartz.

Ophitic texture is composed of plagioclase laths oriented in all directions and intergranular spaces filled with mafic minerals crystallized later. If the intergranular spaces are filled with glass, the texture is hyaloophitic. Gabbroophitic texture is that of gabbros, where elongated plagioclase crystals have a higher degree of automorphism than the associated mafic minerals. If the plagioclase laths are not fully enclosed in pyroxenes, or pyroxenes are intergrown with dominant plagioclases, the texture is called as sub-ophitic.

Intersertal texture contains plagioclase and pyroxene crystals and glassy matrix with small grains of mafic minerals.

Tholeiitic texture contains pyroxenes and glass among plagioclase laths.

Poikilitic texture (from Greek poikilos – variegated, mottled) contains oikocrysts (large subhedral crystals), which enclose smaller variably oriented euhedral crystals. Due to variable orientation, the enclosed grains extinct unequally. In contrast to ophitic texture, automorphism of crystals is important in the poikilitic texture.

Poikilophitic texture contains plagioclase laths enclosed in much larger pyroxene grains.

Additional textures reflect special arrangement and habits of crystals:

Trachytic texture – small plagioclase laths are oriented in flow direction. Interstitial spaces contain cryptocrystalline material or glass. Rocks with trachytic texture often contain large plagioclase phenocrysts.

Pilotaxitic texture – small plagioclase laths are randomly oriented and grouped around larger phenocrysts. The texture is typical of aphanitic matrix of andesites and basalts.

Hyalopilitic texture – small plagioclase laths predominate over glassy matrix.

Agpaitic (nephelinic) texture – interstitial spaces between euhedral nepheline crystals are filled with mafic minerals.

Symplectitic (from Greek symplékó – to bind, to splice) is mutual integrowth of two different minerals. Symplectitic textures are subdivided to myrmekitic, graphic and granophyric.

Myrmekitic texture – rock contains fine (up to 1 mm in size) wormy intergrowths of Na-rich plagioclase and quartz, usually at contact with K-feldspar. Shapes of the myrmekitic intergrowths are rounded and originate during deformation-induced metasomatism. Myrmekitic texture occur mainly in granitic rocks.

Graphic (runic) texture (from Greek grafein – to write) is mutual intergrowth of K-feldspar and quartz originated as a final product of crystallization of magmatic rocks with medium- to high silica content. Quartz enclosed in K-feldspar acquires shapes reminiscent of those of ancient cuneiform script. Directional orientation of crystals is manifested by equant extinction. In contrast to metasomatic myrmekitic texture, the graphic texture has a primarily magmatic origin, representing a rapid crystallization close to eutectic point. The texture is often macroscopically discernible in pegmatites.

Granophyric texture is mutual integrowth of quartz and alkalic feldspar as a result of eutectic crystallization under assistance of H2O-rich fluid. The structure can evolve also during crystallization of supercooled acidic melt and is typical of intrusive rocks crystallized at shallower depths from granitic melts.

Spinifex texture is named after perenial coastal grasses that grow in sand dunes. The texture occurs only in special ultramafic volcanic rocks – komatiites. The texture originates by rapid cooling of magmas rich in Mg,Fe-silicates. The texture is composed of long acicular olivine phenocrysts and pseudomorphic secondary minerals. Typical spinifex komatiites are remarkable of large, mutually intersecting olivine crystals.

References:

Hejtman, B., 1956: Všeobecná petrografie vyvřelých hornin. Nakladatelství Československé Akademie věd, Praha, 370 pp (in Czech language).