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Phaneritic (phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below Earth's surface. As magma cools slowly the minerals have time to grow and form large crystals. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye. Examples of ...
Grain size in igneous rocks results from cooling time so porphyritic rocks are created when the magma has two distinct phases of cooling. [18] Igneous rocks are classified on the basis of texture and composition. Texture refers to the size, shape, and arrangement of the mineral grains or crystals of which the rock is composed. [citation needed]
In geology, texture or rock microstructure [1] refers to the relationship between the materials of which a rock is composed. [2] The broadest textural classes are crystalline (in which the components are intergrown and interlocking crystals), fragmental (in which there is an accumulation of fragments by some physical process), aphanitic (in which crystals are not visible to the unaided eye ...
It is an igneous rock. [6] Produced from felsic lava, obsidian is rich in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. It is commonly found within the margins of rhyolitic lava flows known as obsidian flows. These flows have a high content of silica, giving them a high viscosity.
A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than 1 cm (0.4 in) and sometimes greater than 1 meter (3 ft). Most pegmatites are composed of quartz , feldspar , and mica , having a similar silicic composition to granite .
Occasionally some individual crystals (phenocrysts) are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is known as a granite porphyry. Granitoid is a general, descriptive field term for lighter-colored, coarse-grained igneous rocks.
Subsequently, the name was given to any igneous rocks with large crystals. The adjective porphyritic now refers to a certain texture of igneous rock regardless of its chemical and mineralogical composition or its color. Its chief characteristic is a large difference in size between the tiny matrix crystals and the much larger phenocrysts.
Porphyritic texture in a granite. This is an intrusive porphyritic rock. The white, square feldspar phenocrysts are much larger than crystals in the surrounding matrix; eastern Sierra Nevada, Rock Creek Canyon, California. A porphyritic volcanic sand grain, as seen under the petrographic microscope. The large grain in the middle is of a much ...