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Geology is the Way

Porphyritic texture

The porphyritic texture is a type of texture occurring in volcanic and intrusive igneous rocks defined by the presence of larger crystals, called phenocrysts, surrounded by a matrix of considerably smaller crystals or igneous glass, called groundmass. The word derives from the Ancient Greek porphyra, ‘purple’, in reference to “imperial porphyry”, an ancient porphyritic ornamental stone from Egypt extensively used by the Romans. The term ‘porphyritic’ actually denotes the relative difference in grain size between phenocrysts and the surrounding groundmass and it is not linked to absolute grain sizes. A rock can be considered to be ‘porphyritic’ if a remarkable difference in grain size between the phenocrysts and the groundmass can be detected. For example, volcanic rocks are said to be porphyritic already if they contain crystals of a few millimeters that are set in a very fine-grained or glassy (aphanitic) matrix with no crystals or barely visible crystals. Porphyritic granites, on the other hand, are characterized by megacrystals that are several centimeters long and are set in a matrix which actually consist of medium- to coarse-grained crystals (1-5  mm or more) but that are significantly smaller than the phenocrysts. Phenocrysts that are smaller than 0.5 mm (i.e. detectable under the microscope) are referred to as microphenocrysts. Since porphyritic textures may occur in a wide range of different igneous rocks, it is good practice to complete the description of porphyritic rocks with a textural term describing the texture of the groundmass (e.g. porphyritic rock with aphanitic groundmass). The term porphyritic can also be used as an adjective in the name of a rock (e.g. porphyritic andesite). The related term ‘porphyry‘ is a general term denoting any igneous rock with a porphyritic texture.

Porphyritic texture sketch

Meaning of porphyritic textures
The presence of large crystals in a fine-grained groundmass is commonly regarded to indicate that the phenocrystals crystallized slowly during magma ascent or in a magma chamber, before the crystallization of the surrounding matrix. In the case of volcanic rocks, phenocrystals are considered to have formed at depth before the emission of lava on the surface and the rapid crystallization of the groundmass. In plutonic rocks, phenocrysts are generally considered the earliest products of crystallization and are thought to form during magma ascent or during incipient crystallization of the pluton.

Seriate and hiatal porphyritic textures
In porphyritic rocks, phenocrysts may show a continuous decrease in grain size down to the grain size of the groundmass (seriate texture) or show sudden breaks, ‘hiatuses’, in the grain size range (hiatal texture). There are several interpretations of these textures. A seriate texture might indicate continuous nucleation and crystallization of new crystals during cooling, until the final crystallization of the groundmass. On the other hand, hiatal textures might indicate that the first crystals that nucleated continued to grow and no new crystals nucleated before the final crystallization of the magma or that the groundmass surrounding phenocrysts started to crystallize rapidly at some point. Many igneous rocks may show a combination of seriate and hiatal textures, i.e. a continuous grain size-range interrupted by one or more hiatuses, depending on how complicated their crystallization sequence was.

Glomerophyritic texture
A glomerophyritic texture is a type of porphyritic texture in which phenocrysts are grouped into aggregates termed ‘glomerocrysts’. Glomerocrysts form because new crystals exploit the surface tension of other crystals to grow becoming interpenetrated with other crystals or because crystals stick together while growing and colliding in a melt. Glomerocrysts can be confused with xenoliths and, in general, their recognition from cognate xenoliths is very difficult (i.e. fragments of rocks produced by the same or a closely-related magma). An in-depth explanation of the origin of glomerophyritic textures is available at Alexstrekeisen.it.

Porphyritic volcanic rocks

porphyritic basalt

Porphyritic basalt with phenocrysts of plagioclase (white). The sample is 8 cm wide. Isle of Mull, Scotland. Photo © Siim Sepp.

porphyritic trachyte

Porphyritic trachyte with large phenocrystals of alkali feldspar (white-pink) surrounded by a very fine-grained groundmass of plagioclase and biotite. 5.1 cm across. Bannockburn, Ontario, Canada. Photo © James St. John.

porphyritic rhyodacite

Porphyritic rhyodacite with phenocrysts of light colored and partially transparent sanidine surrounded by a fine-grained groundmass. O’Leary Porphyry, Sunset Crater National Monument, Arizona, USA. Photo © James St. John.

porphyritic anorthoclase phonolite

Porphyritic phonolite with large phenocrysts of anorthoclase (pale colors) in a very fine-grained groundmass of sanidine and nepheline. Sample is 6.6 cm across. Mt. Kenya volcano, Kenya. Photo © James St. John.

plagioclase glomerocrysts in andesite

In a glomerophyritic rock phenocrysts occur as aggregates of two or more crystals surrounded by the groundmass. Pic: plagioclase glomerocrysts in andesite. Field of view: 5 cm. Photo © Richard Droker.

Porphyritic plutonic rocks

alkali feldspar phenocrysts in monzogranite

Porphyritic monzogranite with megacrystals of alkali feldspar. Monte Capanne monzogranite. Sant’Andrea, island of Elba, Italy.

porphyritic monzogranite

Porphyritic monzogranite with megacrystals of alkali feldspar. Monte Capanne monzogranite. Sant’Andrea, island of Elba, Italy.

alkali feldspar phenocrysts in monzogranite

Close-up of the images above highlighting the relatively large size (5-15 cm) of the alkali feldspar (orthoclase) phenocrysts relatively to the surrounding monzogranite (0.5 – 1.0 cm). Sant’Andrea, island of Elba, Italy. [see post]

porphyritic granite

Porphyritic granite with prismatic phenocrysts of alkali feldspar in a matrix of plagioclase (white), quartz (grey, transparent) and biotite (black). Land’s end granite, Cornwall, U.K. Photo © Margaret W. Carruthers.

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Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies

 

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