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Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning all types of igneous rocks can display some degree of porphyritic texture.
Thus porphyry was prized for monuments and building projects in Imperial Rome and thereafter. 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 ...
Porphyritic rocks are often named using mineral name modifiers, normally in decreasing order of abundance. Thus when olivine forms the primary phenocrysts in a basalt, the name may be refined from basalt to porphyritic olivine basalt or olivine phyric basalt. [5]
The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye. 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 ...
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 .
Andesite (/ ˈ æ n d ə z aɪ t /) is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende.
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".
Andesite (/ ˈ æ n d ə z aɪ t /) [1] is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende. [2]