The large crystals, because of their prominence in the rock are called phenocrysts. Many rocks with an overall fine-grained texture display scattered minerals that are more than 1 mm across. A porphyritic texture forms when a rock has had two different stages of cooling resulting in two distictly different crystal sizes. Phenocrysts may have sharp edges and well-formed crystal faces or they may be corroded and somewhat irregular. Porphyritic and Poiklitic textures are important examples of such textures. This shows that the large crystals had a chance to form in a magma while it was migrating and cooling. iv. Porphyritic textures develop when conditions during cooling of a magma change relatively quickly. Examples of phaneritic igneous rocks are gabbro, diorite and granite. A seamless stone texture with porphyritic granite blocks arranged in a herringbone pattern. Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group. Examples of phaneritic igneous rocks are gabbro, diorite and granite. This texture showing large sized crystals within a matrix of small crystals is the porphyritic texture. Porphyritic texture Porphyritic texture is an igneous rock texture in which large crystals are set in a finer-grained or glassy groundmass. Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals. Description. The image represents a physical area of 3224 x 2543 mm (126.9 x 100.1 inches) in total, with each individual block measuring approximately 750 x 50 mm (29.5 x 2 inches). The Porphyritic Texture is characterised by the presence of a few conspicuously large sized crystals (the phenocrysts) which are embedded in a fine-grained ground mass or matrix. GLASSY TEXTURE (caused by rapid cooling); fresh fractures display a conchoidal fracture like broken glass. Large crystals are called Phenocrysts and form first by cooling slowly, they are surrounded by a finer grained groundmass which cooled faster. PORPHYRITIC TEXTURE - an igneous rock in which PHENOCRYSTS (large crystals) are surrounded by a fine groundmass (very small crystals). Examples of porphyritic rocks are: andesite porphyry and rhyolite porphyry. Porphyritic textures develop when conditions during cooling of a magma change relatively quickly. Most people chose this as the best definition of porphyritic: Of or containing porphyry... See the dictionary meaning, pronunciation, and sentence examples. C - has fragmental texture; B - has a porphyritic texture; A - has coarse-grained texture Activity 2.3 Using Color Index to Determine Composition Use the color index shown at the bottom of Figure 2.10 as well as the photos of Samples A-F in Figure 2.11 to complete the following statements. Usually the larger crystals, known as phenocrysts, formed earlier in the crystallisation sequence of the magma. This porphyritic texture indicates that the magma sat and cooled a bit below the Earth's surface, thus giving time for the large crystals to grow, before erupting onto the surface and cooling very quickly. Porphyritic textures occur in coarse, medium and fine-grained igneous rocks. In the rock are called phenocrysts and form first by cooling slowly, they are surrounded a... ( caused by rapid cooling ) ; fresh fractures display a conchoidal fracture like broken glass the magma rock! That the large crystals are called phenocrysts and form first by cooling slowly, are... 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