Thus felsitic textures may be described as equigranular and panidiomorphic. In this case, the olivine basalt has xenocrysts of quartz, which could not have crystallized from this magma. Embayed crystals are evidence that the mineral was unstable in the magma, and was dissolving away. Mesocumulate texture, in which liquid trapped between cumulate crystals was able to exchange material with the nearby magma, allowing more extensive overgrowths of cumulate minerals and a smaller volume of ultimately trapped liquid. There are basically seven types of igneous rock textures. Click each image to see a larger version plus information about the rock's texture. Medium-grained, 1-5 mm. The texture is sometimes further distinguished into mega-porphyritic and microporphyritic depending upon the size of the phenocrysts. Wyoming. Learn vocabulary, terms, and more with flashcards, games, and other study tools. Igneous rocks are classified based on texture and composition. lecture notes of the topic textures of igneous rocks. Halifax, Nova Scotia, Canada. Cross-polarized light, field width is 6 mm. Skaergaard intrusion, Greenland. Similarly, the shape or form of the crystals, which is best seen only in thin sections under microscope, may be described as perfect, semi perfect or totally irregular. Here the basalt originally had phenocrysts of plagioclase and numerous bubbles. When the host mineral is identified as augite and the inclusions are of plagioclase felspars, the poiklitic texture is further distinguished as ophitic. They have textures or mineralogy different than the surrounding material, and so are generally easy to spot. Notice the allanite crystal in the upper-right. Small grains of olivine, pyroxene, magnetite, and apatite hint indicate the original trapped liquid. It is a convention to broadly assess the sum of the coloured minerals in a rock and express it in percentage terms of total mineralogical composition, which gives the colour index of the mineral. The quartz grains reacted with the magma to form a garland of small augite crystals. These can be broadly divided into five categories: Following is a brief account of these textures of igneous rocks: All those textures in which majority of constituent crystals of a rock are broadly equal in size are described as equigranular textures. This is an alkaline olivine gabbro. Plane/cross-polarized light, field width is 0.6 mm. This results in a rock almost completley made of the cumulate minerals, and so very simple mineralogy. Iceland. Iceland. Texture and structure of igneous rocks. Notice the strong zoning in the alkali feldspars, with orthoclase cores, a layer of plagioclase lath inclusions, and a rim of highly twinned alkali feldspar, possibly albitic. In this case, grain textures look somewhat annealed, possibly because of deformation and annealing when it was still very hot. Iceland. Microscopic examination of thin sections of these rocks becomes essential to determine their mineralogical composition. Black Hills, South Dakota. the rock was likely formed by a violent volcanic eruption. Gilford, New Hampshire. Spherulites are spots where fibrous crystals radially grow from a nucleation point, usually during strong supercooling of the magmatic liquid. Annealed texture, in this case basalt that was held at high temperature for a long period of time. Microlites are small crystals, typically elongate, typically more or less uniform in size, and typically scattered more or less evenly through a glassy sample. Constituent mineral components in identifying the rock divisions recognised on the basis of texture essential to their! 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