What is Aphanitic texture in geology?

Aphanitic – This texture describes very fine grained rock where individual crystals can be seen only with the aid of a microscope, i.e. the rock is mostly groundmass. An aphanitic texture is developed when magma is erupted at the Earth’s surface and cools too quickly for large crystals to grow.

What is aphanitic form?

aphanitic An igneous rock texture characterized by mineral grains which are too small to be identified without a petrological microscope. These extremely fine-grained, crystalline fabrics are formed when a magma solidifies in response to a very rapid loss of heat and dissolved gases.

What is aphanitic and pyroclastic texture?

There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic. Aphanitic (a = not, phaner = visible) rocks, in contrast to phaneritic rocks, typically form from lava which crystallize rapidly on or near Earth’s surface.

What does it mean if an igneous rock has an aphanitic texture?

Aphanitic rock is igneous rock in which the grain or crystalline structure is too fine to be seen by the unaided eye. Such rock is formed when the material solidifies at or near the surface so that the cooling is rather rapid. Such rocks are termed “extrusive” rocks.

What rocks have an aphanitic texture?

Examples of aphanitic igneous rock include basalt, andesite and rhyolite. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals. Examples include obsidian and pumice.

How do you identify aphanitic rocks?

Aphanitic igneous rocks have a fine-grained texture and most of their crystals are too small to see with the naked eye. You will need to use a magnifying glass to observe the crystals in aphanitic rocks. Igneous rocks that form too quickly for crystals to form have what is called a glassy texture.

What is an aphanitic igneous rock?

APHANITIC TEXTURE – Igneous rocks that form on the earth’s surface have very fine-grained texture because the crystals are too small to see without magnification. PHANERITIC TEXTURE – Igneous rocks with large, visible crystals because the rock formed slowly in an underground magma chamber.

What does aphanitic look like?

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.

What is the aphanitic equivalent of gabbro?

Basalt

Basalt is the aphanitic (fine-textured) extrusive equivalent of gabbro. From the Latin word “pumex”, meaning “pumice”. A vesicular, glassy volcanic rock, generally of rhyolite composition (felsic). Often light in color.

Is gabbro an aphanitic?

Gabbro is distinct from anorthosite, which contains less than 10% mafic minerals. Coarse-grained gabbroids are produced by slow crystallization of magma having the same composition as the lava that solidifies rapidly to form fine-grained (aphanitic) basalt.

Is andesite an aphanitic?

Andesite is an aphanitic (fine-grained) igneous rock that is intermediate in its content of silica and low in alkali metals.

Is obsidian aphanitic or phaneritic?

Classification of Igneous Rocks

What is pegmatite texture?

A pegmatitic texture is one in which the mineral grains are exceptionally large. The largest ones are, by convention, more than about 3 cm long. This texture is found in intrusive rocks. The extra large size does not mean that they cooled extra slowly.

What is the difference between aphanitic and phaneritic rocks?

Rocks that are aphanitic have a grain size below 1 millimetre. On the other hand, phaneritic rocks have grains that are coarse and the grain size is usually between 1 millimetre and 10 millimetres.

What is the texture of gabbro rock?

Gabbro

Is gabbro hard or soft?

Gabbro is a coarse-grained, dark-colored, intrusive igneous rock. It is usually black or dark green in color and composed mainly of the minerals plagioclase and augite. It is the most abundant rock in the deep oceanic crust.

Is dacite mafic or felsic?

felsic

Dacite is a felsic extrusive rock, intermediate in composition between andesite and rhyolite. It is often found associated with andesite, and forms lava flows, dikes, and, in some cases, massive intrusions in the centres of old volcanoes.

Does gabbro have biotite?

Gabbro may also contain small amounts of olivine (olivine gabbro) amphibole (hornblende gabbro) and biotite. The quartz content in gabbro is less than 5% of total volume, but quartz gabbros or monzogabbros are also known to occur, and are probably derived from magma that was oversaturated with silica.

What is basalt texture?

Basalts show, almost always, aphanitic or fine-grained mineral texture resulting from rapid cooling of volcanic magma on or close to the surface of Earth. The component minerals are so fine that cannot be identifiable by the unaided eyes.

What is the texture of peridotite?

Peridotite

How do you identify peridotite?

Classic peridotite is bright green with some specks of black, although most hand samples tend to be darker green. Peridotitic outcrops typically range from earthy bright yellow to dark green; this is because olivine is easily weathered to iddingsite.

What is the difference between peridotite and olivine?

Peridotite contains at least 40 percent olivine and some pyroxene. Unlike the olivine grains, the pyroxene grains in peridotite have a visible cleavage when viewed under a hand lens. Peridotite forms much of Earth’s mantle and can occur as nodules that are brought up from the mantle by kimberlite or basalt magmas.

What is the grain size of peridotite?

∼60–70 µm across

F-type peridotites are fine-grained (olivine, ∼60–70 µm across).

What type of rock is phyllite?

Phyllite

Is phyllite a soft rock?

Phyllite is a foliated metamorphic rock rich in tiny sheets of sericite mica. Phyllite is a durable and soft rock and used as decorative aggregates, floor tiles, and as exterior building, or facing stone. The other uses include cemetery markers, commemorative tablets, creative artwork, and writing slates.

What type of texture does phyllite have?

phyllitic sheen

Phyllites are said to have a texture called “phyllitic sheen,” and are usually classified as having formed through low-grade metamorphic conditions through regional metamorphism metamorphic facies. Phyllite has good fissility (a tendency to split into sheets).