Texture of Igneous Rocks

Rock texture is the overall appearance of a rock based on the size and arrangement of its interlocking crystals. Crystal size is the most important aspect of igneous texture. Among the several texture varieties, three main classes can be identified phaneritic, aphanitic, and glassy textures. 

Slow All the minerals have a medium or coarse crystal size 

The crystals are so small (i.e., microlites) that individual crystals cannot be distinguished without magnification and are surrounded by a glassy matrix. Aphanitic textures form primarily when cooling rates are fast such as lava flows. 

The glassy or vitreous texture with no crystals usually indicates that the magma cooled extremely quickly and/or that it was so viscous that ions could not migrate to form crystals seed. Most glasses are related to pyroclastic igneous rocks. 

Porphyritic, Aphanitic, Phaneritic, Graphic, Ophitic Poecilitic, Intersertal, intergranular, lamproporphyric, dolerltic 

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This phenomenon is well known in fluid mechanics (Leal 1980) and was applied by Simkin (1967) to the intrasion of the mafic magma of dikes on the Isle of Skye in Scotland and by Upton and Wadsworth (1967) to the intrusion of a small sill on the volcano Piton des Neiges on Reunion. The textures of igneous rocks that result from flow differentiaton of mafic magma have been described by Marsh (1996a, b). 

Cashman K.V. (1991) Textural constraints on the kinetics of crystallization of igneous rocks. Rev. Mineral. 24, 259-314. 

Clay is an erosion product of igneous and sedimentary rocks. The weathering proceeds via mechanical and chemical processes. Its result depends on many factors, among which climate, vegetation and composition and texture of the rocks. 

Peridotite Any of a group of igneous rocks composed mainly of olivine and various pyroxenes and having a granitelike texture... 

Metamorphic rocks were formed by a secondary conversion of igneous and sedimented rocks, due to certain geological tectonic processes. The pressure, temperature, and chemical effects, particularly effects of gases and thermal solutions were the main factors in these conversions. The texture of metamorphic rocks may be characterized by the presence of layers or scales. Typical metamorphic rocks are gneisses, mica-schists, serpentines, crystalline limestones and crystalline dolomites. 

MacKenzie, W.S. Donaldson, C.H. and Guilford, C., Atlas of Igneous Rocks and Their Textures, Halstead Press, John Wiley Sons, New York, 1982,148 pp. 

Mica. Micas are platy, hydrated aluminum silicates similar to talc and are common components of igneous rock. Muscovite is commonly found in clay, shale, and slate and, therefore, is a potential health hazard. Mica is extensively used as an insulator in the electrical industry. Micas are also used in drilling muds, ornaments and decorations, floor tiles, adhesives, texture paints, rubber products, and plastics. 

Following this fxmdamental information on the fusion diagrams, we must know the densities of the igneous rocks and their magmas at all temperatures, with their changes of volume during solidification the textures and structiures produced by various conditions of solidification the latent heats concerned in fusion of the magma and in assimilation by it of other rocks and many other physico-chemical data. 

Agate that fills the voids in igneous rocks commonly exhibits the following four types of textures see also Fig. 10.18. 

The textures of kaolin (rockI include varieties similar to examples observed in igneous and melamorphic as well as sedimentary rocks. Kaolin grains and crystals may be straight or curved, sheaves, flakes, face-to-face or edge-to-edge flocculcs. interlocking crystals, tubes, scrolls, fibers, or spheres. 

PHENOCRYST. A textural term proposed by Iddings in 1892 for macroscopic crystals which are relatively much larger than the crystalline matrix of the igneous rock in which they occur. Rocks which have phenocrysts are called porphyntic, The term phenocryst is derived from the Greek, meaning show, and crystal. 

XENOMORPTTTC. The texture or fabric of an igneous rock having or characterized by crystals not bounded by their own crystal faces and which have their form impressed upon them by preexisting adjacent mineral crystals. 

Metamorphic rocks are those that have been altered in the solid state by temperature and pressure. The texture and/or composition of the original rock changes over time, but the rock never melts (otherwise it would be igneous). There are two basic types of metamorphic rocks contact and regional. 

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