The crystal structures within a porphyritic rock depend on the minerals that make up the phenocrysts and the groundmass. Here's a breakdown:
Phenocrysts:
* Structure: They typically exhibit euhedral to subhedral crystal forms, meaning they have well-developed faces and sharp angles. This is because they had more time to grow in a slowly cooling magma chamber.
* Common Minerals: Depending on the rock type, phenocrysts can be made of various minerals like:
* Feldspars: (plagioclase, orthoclase)
* Quartz: (SiO2)
* Olivine: (Mg,Fe)2SiO4
* Pyroxene: (Ca,Mg,Fe)Si2O6
* Amphibole: Ca2(Mg,Fe,Al)5Si8O22(OH)2
* Biotite: K(Mg,Fe)3AlSi3O10(OH,F)2
Groundmass:
* Structure: The groundmass can have different crystal structures, depending on how the magma cooled:
* Crystalline: This means the minerals in the groundmass are well-defined crystals, but they are much smaller than the phenocrysts. Common crystal structures include:
* Equigranular: All crystals are roughly the same size
* Inequigranular: Crystals are of different sizes
* Poikilitic: Large crystals enclose many smaller crystals
* Glassy: The groundmass may be partially or fully glassy, indicating rapid cooling.
Examples of Porphyritic Rocks:
* Porphyritic basalt: Phenocrysts of olivine, pyroxene, or plagioclase in a glassy or finely crystalline groundmass.
* Porphyritic granite: Phenocrysts of quartz, feldspar, and mica in a granular groundmass.
* Porphyritic andesite: Phenocrysts of plagioclase, hornblende, and pyroxene in a finely crystalline groundmass.
In summary:
Porphyritic rocks are not defined by a single crystal structure but by their texture. The specific crystal structures present depend on the mineral composition of the phenocrysts and the groundmass, which varies depending on the type of porphyritic rock.
