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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 ...
Photomicrograph of a porphyritic-aphanitic felsic rock, from the Middle Eocene in the Blue Ridge Mountains of Virginia. Plagioclase phenocrysts (white) and hornblende phenocryst (dark; intergrown with plagioclase) are set in a fine matrix of plagioclase laths that show flow structure.
The significance of porphyritic texture as an indication that magma forms through different stages of cooling was first recognized by the Canadian geologist, Norman L. Bowen, in 1928. [8] Porphyritic texture is particularly common in andesite, with the most prominent phenocrysts typically composed of plagioclase feldspar.
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 ...
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".
Porphyritic textures develop when conditions during the cooling of magma change relatively quickly. The earlier formed minerals will have formed slowly and remain as large crystals, whereas, sudden cooling causes the rapid crystallization of the remainder of the melt into a fine-grained (aphanitic) matrix .
Quartz-porphyry, in layman's terms, is a type of volcanic rock containing large porphyritic crystals of quartz. [ 1 ] [ 2 ] These rocks are classified as hemi-crystalline acid rocks . Structure
Overall, the granite is a porphyritic rock that is composed of plagioclase feldspar, quartz, and biotite, and contains large pink microcline chunks throughout the matrix. The microcline megacrysts appear as blocky, subhedral grains that are generally around 2 cm in length, but can range from 1 to 4 cm.