Porphyritic

Source: Wikipedia, the free encyclopedia.
This article is about a rock texture. For the type of rock, see Porphyry (geology).
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 different size class than the small needle-like crystals around it. Scale box in millimeters.

Porphyritic is an adjective used in

bimodal size ranges, meaning the rock is composed of two distinct sizes of crystal.[3]

In extrusive rocks, the phenocrysts are surrounded by a fine-grained (aphanitic) matrix or groundmass of volcanic glass or non-visible crystals, commonly seen in porphyritic basalt. Porphyritic intrusive rocks have a matrix with individual crystals easily distinguished with the eye, but one group of crystals appearing clearly much bigger than the rest, as in a porphyritic granite.

The term comes from the

texture.[4]

Formation

phenocrysts
are clearly visible, in contrast to the grey groundmass with its microscopic crystals.

Porphyritic rocks are a product of igneous differentiation, and are generally formed when a column of rising magma is cooled in two stages: In the first stage, the magma is cooled slowly deep in the crust, creating the large crystal grains, with a diameter of 2mm or more. In the final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano, creating small grains that are usually invisible to the unaided eye, typically referred to as the matrix or groundmass.[5]

The formation of large phenocrysts is due to fractional crystallization. As the melt cools, it begins crystallizing the highest melting point minerals closest to the overall composition first. This forms large, well-shaped euhedral phenocrysts. If these phenocrysts are different in density to the remaining melt, they usually settle out of solution, eventually creating cumulates. However, when this is interrupted by sudden eruption of the melt as lava, or when the density of the crystals and remaining melt remains similar, they become entrapped in the final rock.[5]

This can also occur when the

eutectic point as it cools, resulting in multiple different minerals solidifying at once and filling the remaining space simultaneously, limiting their size and shape.[5]

References

  1. ^ Dietrich, R. and Skinner, B. (1979). Rocks and Rock Minerals. See p. 108.
  2. OCLC 30508970
    .
  3. ^ "ignchrt.html". www.appstate.edu. Retrieved 2022-01-07.
  4. ISBN 0195046528
    .
  5. ^
    S2CID 219542287
    .
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