« Back to Glossary Index. General term from the fine-grained, not discernible part of a rock. In igneous rocks, this is the part of the rock that is not phenocrysts, and can help in determining the composition of extrusive rocks.

What is the definition of Groundmass?

: the fine-grained or glassy base of a porphyry in which the larger distinct crystals are embedded.

What is the Groundmass made of?

The groundmass contains microlites of plagioclase, amphibole, abundant magnetite crystals, and felsic glass.

How is Groundmass formed?

groundmass The finer-grained material of igneous rocks in which larger crystals (phenocrysts) and xenoliths are enclosed. In lavas, the fine ground-mass is formed when the magma is erupted on to the Earth's surface and is cooled rapidly by air convection over the flow. ...

What is a phenocrysts in geology?

A relatively large crystal embedded in a finer-grained or glassy igneous rock. The presence of phenocrysts gives the rock a porphyritic texture (see illustration). Phenocrysts are represented most commonly by feldspar, quartz, biotite, hornblende, pyroxene and olivine.

3.4.3 Igneous Textures: Porphyritic Rocks have Phenocrysts embedded in a Groundmass matrix

15 related questions found

What is the relationship between phenocrysts and groundmass?

Large igneous crystals are designated phenocrysts if they contrast sharply in size with the enclosing groundmass; this implies a size ratio of at least about 5:1. Porphyritic texture consists of phenocrysts in a groundmass and is typical of the rock porphyry.

Is quartz an Aphanitic?

Aphanites are commonly porphyritic, having large crystals embedded in the fine groundmass, or matrix. ... They consist essentially of very small crystals of minerals such as plagioclase feldspar, with hornblende or augite, and may contain also biotite, quartz, and orthoclase.

What is granitic composition?

The chemical composition of granite is typically 70-77% silica, 11-13% alumina, 3-5% potassium oxide, 3-5% soda, 1% lime, 2-3% total iron, and less than 1% magnesia and titania. Volcanic rock of equivalent chemical composition and mineralogy is called rhyolite.

Where are porphyritic rocks found?

When the ground mass is composed of small crystals of feldspar and quartz, the rock is called granite-porphyry. Both are of the composition of granite, and both are found in the elvan dikes of the tin district in Cornwall. Quartz- porphyry is common in the Porcupine gold camp, Ontario.

How do porphyritic igneous rocks form?

A porphyritic texture is developed when magma that has been slowly cooling and crystallising within the Earth's crust is suddenly erupted at the surface, causing the remaining uncrystallised magma to cool rapidly. This texture is characteristic of most volcanic rocks. ... This texture is exhibited by some volcanic rocks.

What is the meaning of Phaneritic?

[ făn′ə-rĭt′ĭk ] Of or relating to an igneous rock in which the crystals are so coarse that individual minerals can be distinguished with the naked eye. Phaneritic rocks are intrusive rocks that cooled slowly enough to allow significant crystal growth. Compare aphanitic.

What type of igneous rock is andesite?

Andesite is a fine-grained, extrusive igneous or volcanic rock. It is dark grey and made up of equal amounts of light and dark minerals, although the crystals are too small to be seen without a magnifier. Occasionally andesite may contain some larger crystals.

Is glassy a texture?

If a rock looks like a block of (colored) glass, with no visible mineral crystals, it has a glassy texture. Superficially, a glassy texture suggests cooling that was so extremely fast that no crystals could form. However, composition is also vitally important.

What is a matrix in rocks?

matrix, in geology, the material in which something is embedded, either the natural rock that holds crystals, fossils, pebbles, mineral veins, and the like, or the fine-grained materials that surround larger grains in a rock—e.g., silt and clay particles in a sandstone or tiny crystals in a crystalline rock, sometimes ...

What does Phenocryst mean?

: a crystal foreign to the rock in which it occurs.

Is Grandma a noun or verb?

GRANDMA (noun) definition and synonyms | Macmillan Dictionary.

What kind of rock is porphyritic?

Porphyritic texture is an igneous rock texture in which large crystals are set in a finer-grained or glassy groundmass. Porphyritic textures occur in coarse, medium and fine-grained igneous rocks. Usually the larger crystals, known as phenocrysts, formed earlier in the crystallisation sequence of the magma.

What is porphyritic rock used for?

Thus, "imperial"-grade porphyry was prized for monuments and building projects in Imperial Rome and thereafter. Subsequently, the name was given to any igneous rocks with large crystals. The adjective porphyritic now refers to a certain texture of igneous rock regardless of its chemical and mineralogical composition.

What is the differences between phenocrysts and porphyritic?

Phaneritic: any coarse-grained igneous rock, often intrusive, usually formed as a result of a longer cooling history (ex. granite, gabbro). Porphyritic: an igneous rock with one mineral (called the phenocryst) exhibiting a grain size larger than the remainder of the minerals (called the groundmass).

What is granite used for?

It is widely used for architectural facades, construction materials, ornamental stone and monuments. Over 40% of dimension stone quarried is granite. Crushed granite is used as a durable construction material in asphalt and concrete used in highway and infrastructure projects.

What are conglomerate rocks?

Conglomerate is a sedimentary rock made of rounded pebbles and sand that is usually held together (cemented) by silica, calcite or iron oxide. It is a stone similar to sandstone but the rock particles are rounded or angular gravel rather than sand.

What type of rock is pumice?

Pumice is pyroclastic igneous rock that was almost completely liquid at the moment of effusion and was so rapidly cooled that there was no time for it to crystallize. When it solidified, the vapours dissolved in it were suddenly released, the whole mass swelling up into a froth that immediately consolidated.

Is obsidian a Phaneritic?

The result is a natural amorphous glass with few or no crystals. Examples include obsidian. ... The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye. Examples of phaneritic igneous rocks are gabbro, diorite and granite.

Does granite have groundmass?

The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a completely crystalline rock. ... Occasionally some individual crystals (phenocrysts) are larger than the groundmass, in which case the texture is known as porphyritic.

Is obsidian a felsic rock?

Though obsidian is usually dark in color, similar to mafic rocks such as basalt, the composition of obsidian is extremely felsic. Obsidian consists mainly of SiO₂ (silicon dioxide), usually 70% by weight or more. Crystalline rocks with a similar composition include granite and rhyolite.