Porphyritic granite (Precambrian; St. Cloud area, Minnesota, USA) 2 : 無料・フリー素材/写真
Porphyritic granite (Precambrian; St. Cloud area, Minnesota, USA) 2 / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | Porphyritic granite from the Precambrian of Minnesota, USA.Gray, glassy crystals = quartzPinkish crystals = potassium feldsparWhitish gray crystals = plagioclase feldsparBlack crystals = hornblende amphiboleIgneous rocks form by the cooling & crystallization of hot, molten rock (magma & lava). If this happens at or near the land surface, or on the seafloor, they are extrusive igneous rocks. If this happens deep underground, they are intrusive igneous rocks. Most igneous rocks have a crystalline texture, but some are clastic, vesicular, frothy, or glassy.Granite is a common intrusive igneous rock. Garden-variety granites are composed of quartz, potassium feldspar (K-feldspar), sodic plagioclase feldspar, hornblende amphibole, and mica. Granites have a felsic chemistry. Felsic igneous rocks are generally light-colored, have greater than 65% silica (“silica” = SiO2 chemistry) (felsic has also been defined as greater than 70% silica), are rich in potassium (K) & sodium (Na), and are dominated by the minerals quartz and K-feldspar. Many granites have a phaneritic texture (coarsely crystalline; all crystals between 1 mm and 1 cm in size). Porphyritic granite has the same chemistry & mineralogy as ordinary granite, but differs in the size range of its crystals. Porphyritic granites have a mix of large & small crystals, the result of two cooling rates, but all crystals are still fairly decent-sized. Large crystals in a porphyritic rock are called phenocrysts. The smaller crystals make up the groundmass. Porphyritic granites typically have K-feldspar phenocrysts (pinkish). The groundmass is typically quartz (grayish), sodic plagioclase feldspar (whitish-gray), amphibole (black), plus sometimes biotite mica and/or muscovite mica.Locality: unrecorded site at or near St. Cloud, central Minnesota, USA |
| 撮影日 | 2021-10-22 10:06:05 |
| 撮影者 | James St. John |
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