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"Imperial Porphyry" from the Red Sea Mountains of Egypt A waterworn cobble of porphyry Rhyolite porphyry from Colorado; scale bar in lower left is 1 cm (0.39 in). Porphyry (/ ˈ p ɔːr f ə r i / POR-fə-ree) is any of various granites or igneous rocks with coarse-grained crystals such as feldspar or quartz dispersed in a fine-grained silicate-rich, generally aphanitic matrix or groundmass.
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 ...
Examples of aphanitic igneous rock include basalt, andesite, and rhyolite. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals. Examples include obsidian.
Basanite (/ ˈ b æ s. ə ˌ n aɪ t /) is an igneous, volcanic rock with aphanitic to porphyritic texture. It is composed mostly of feldspathoids , pyroxenes , olivine , and calcic plagioclase and forms from magma low in silica and enriched in alkali metal oxides that solidifies rapidly close to the Earth's surface .
Alkali basalt is one of the rocks comprising the Pali-Aike volcanic field, in Argentina. [1]Alkali basalt or alkali olivine basalt is a dark-colored, porphyritic volcanic rock [2] usually found in oceanic and continental areas associated with volcanic activity, such as oceanic islands, continental rifts and volcanic fields. [3]
Aphanites are commonly porphyritic, having large crystals embedded in the fine groundmass, or matrix. The larger inclusions are called phenocrysts . They consist essentially of very small crystals of minerals such as plagioclase feldspar , with hornblende or augite , and may contain also biotite , quartz , and orthoclase .
This porphyritic texture is indicative of multi-stage cooling of magma. For example, porphyritic andesite will have large phenocrysts of plagioclase in a fine-grained matrix. Also in South Africa , diamonds are often mined from a matrix of weathered clay -like rock ( kimberlite ) called "yellow ground".
See: Porphyritic and Porphyry (geology). Subcategories. This category has the following 2 subcategories, out of 2 total. B. Basalt (3 C, 27 P) R. Rhyolite (1 C, 5 P)