Phaneritic and Aphanitic Textures: Comprehensive Comparison
Phaneritic and aphanitic are textural classifications used for igneous rocks, those formed from the cooling and solidification of molten rock (magma or lava). The key distinction between these two categories lies in the size of the mineral grains within the rock.
Phaneritic and aphanitic are two main types of igneous rock textures. They are distinguished by the size of the mineral grains in the rock.
Phaneritic Vs. Aphanitic
Phaneritic
Coarse Grained Texture (Phaneritic), Mineral Grains Easily Visible (Grains Several Mm in Size or Larger)
Phaneritic textured rocks are comprised of large crystals that are clearly visible to the eye with or without a hand lens or binocular microscope. The entire rock is made up of large crystals, which are generally 1/2 mm to several centimeters in size; no fine matrix material is present. This texture forms by slow cooling of magma deep underground in the plutonic environment.
The first-formed crystals tend to have regular shapes because they grow freely into the surrounding liquid. Later-formed crystals find themselves competing for space with their solid neighbors; they are forced to fill in the irregular gaps. Thus, you can often figure out the relative order in which the minerals crystallized from the magma.
Examples of phaneritic rocks include granite, diorite, and gabbro.
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| What Is the Difference Between Phaneritic and Aphanitic Rocks. A hand specimen of granite with phaneritic (coarse grained) texture. Principal minerals are Potassium Feldspar, Biotite Mica, and Quartz. |
Aphanitic
Fine Grained Texture (Aphanitic), Mineral Grains Smaller Than 1mm (Need Hand Lens or Microscope to See Minerals)
Aphanitic texture consists of small crystals that cannot be seen by the eye with or hand lens. The entire rock is made up of small crystals, which are generally less than 1/2 mm in size. This texture results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments. These extremely fine-grained, crystalline fabrics are formed when a magma solidifies in response to a very rapid loss of heat and dissolved gases. Emplacement in high-level dykes or eruption on to the surface can result in the development of aphanitic fabrics.
Crystalline rocks with mineral grains that cannot be distinguished from one another without magnification have an aphanitic igneous texture. Igneous rocks form by crystallization of minerals from liquid magma rising into the upper portion of Earth's crust from the lower crust and underlying mantle. Igneous rock texture indicates the rate of magmatic cooling.
Crystallization takes place either slowly in deeply buried intrusions called plutons, or rapidly at the earth's surface where magma has been extruded as lava by volcanic activity. Igneous rocks are therefore classified as either intrusive (plutonic) or extrusive (volcanic). Slow, undisturbed cooling in a well-insulated pluton is conducive to orderly arrangement of atoms and molecules into large, well-formed crystals .
Rapid cooling from a lava flow is not. Intrusive igneous rocks thus have coarse-grained, or phaneritic, textures with visible crystals, and extrusive igneous rocks have fine-grained, or aphanitic, texture. Volcanic glass , called obsidian , forms when lava is quenched and solidified so quickly that the silicate ions in the melt form no orderly atomic structure.
Examples of aphanitic rocks include basalt, rhyolite, and andesite.
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| A hand specimen of basalt with aphanitic (fine grained) texture. The dark color is due to abundant dark colored minerals (pyroxene, hornblende). |
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