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Igneous Petrology and the Nature of Magmas
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Many volcanic rocks contain large crystals surrounded by very fine-grained or glassy material. Such rocks have a porphyritic texture and are called porphyries. Figure 5.30 shows such a porphyritic rock, from the Swedish island Alnö, that contains large crystals of grayish rounded quartz and blockier crystals of whitish feldspar surrounded by a darker fine-grained matrix. The two grain sizes suggest a two-stage cooling history. The large crystals, termed phenocrysts, grew slowly over a long time before eruption (as evidenced by their size). Subsequently, magma containing the crystals moved upward and, during rapid cooling at Earth’s surface, the finer-grained material solidified. The fine-grained matrix material, termed the groundmass, generally contains microscopic or submicroscopic crystals (because they formed quickly and did not have time to grow large) and often glass (because cooling was so fast that no crystals could form).
Igneous Rocks
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
Orthoclase or other alkali feldspar usually forms over half the rock, with smaller amount of plagioclase (oligoclase); the dark minerals may be biotite, hornblende, or a pyroxene; and apatite, sphene, zircon and opaque iron oxides are accessories. A little quartz may be present, filling interstices between the other minerals. The texture is coarse-grained, sometimes porphyritic. Hand specimens are usually pale coloured.
Formation of Cu–Au porphyry deposits: hydraulic quartz veins, magmatic processes and constraints from chlorine
Published in Australian Journal of Earth Sciences, 2023
G. N. Phillips, J. R. Vearncombe, J. D. Clemens, A. Day, A. F. M. Kisters, B. P. Von der Heyden
An igneous body hosting part of the mineralisation is referred to here as the stock, and a large igneous body inferred at depth is referred to as the precursor pluton or batholith (Figures 2 and 3). Both terms are used in the recent literature, including Sillitoe (2010). The term porphyritic describes an igneous rock that contains crystals with a bimodal size distribution—larger crystals in a finer-grained groundmass.