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Rocks can be classified according to the nature, size and abundance of phenocrysts, and the presence or absence of phenocrysts is often noted when a rock name is determined. ''Aphyric'' rocks are those that have no phenocrysts, or more commonly where the rock consists of less than 1% phenocrysts (by volume); while the adjective ''phyric'' is sometimes used instead of the term ''porphyritic'' to indicate the presence of phenocrysts. Porphyritic rocks are often named using mineral name modifiers, normally in decreasing order of abundance. Thus when
{{Clear Igneous petrology
300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss
Swiss most commonly refers to:
* the adjectival form of Switzerland
* Swiss people
Swiss may also refer to: Places
* Swiss, Missouri
* Swiss, North Carolina
* Swiss, West Virginia
* Swiss, Wisconsin
Other uses
* Swiss Café, an old café located ...
side of the Mont Blanc massif, has large white phenocrysts of plagioclase
Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...
(that have trapezoid shapes when cut through). 1 euro coin (diameter 2.3 cm) for scale.
A phenocryst is an early forming, relatively large and usually conspicuous crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
distinctly larger than the grains of the rock groundmass of an igneous rock
Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava.
The magma can be derived from partial ...
. Such rocks that have a distinct difference in the size of the crystals are called porphyries, and the adjective porphyritic
Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning ...
is used to describe them. Phenocrysts often have euhedral forms, either due to early growth within a magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
, or by post-emplacement recrystallization. Normally the term ''phenocryst'' is not used unless the crystals are directly observable, which is sometimes stated as greater than in diameter. Phenocrysts below this level, but still larger than the groundmass crystals, are termed ''microphenocrysts''. Very large phenocrysts are termed '' megaphenocrysts''. Some rocks contain both microphenocrysts and megaphenocrysts. In metamorphic rock
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...
s, crystals similar to phenocrysts are called porphyroblasts.
Phenocrysts are more often found in the lighter (higher silica) igneous rocks such as felsites and andesite
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...
s, although they occur throughout the igneous spectrum including in the ultramafic
Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are usua ...
s. The largest crystals found in some pegmatite
A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic c ...
s are often phenocrysts being significantly larger than the other minerals.
Classification by phenocryst
Rocks can be classified according to the nature, size and abundance of phenocrysts, and the presence or absence of phenocrysts is often noted when a rock name is determined. ''Aphyric'' rocks are those that have no phenocrysts, or more commonly where the rock consists of less than 1% phenocrysts (by volume); while the adjective ''phyric'' is sometimes used instead of the term ''porphyritic'' to indicate the presence of phenocrysts. Porphyritic rocks are often named using mineral name modifiers, normally in decreasing order of abundance. Thus when olivine
The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
forms the primary phenocrysts in a basalt, the name may be refined from ''basalt'' to ''porphyritic olivine basalt'' or ''olivine phyric basalt''. Similarly, a basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
with olivine as the dominant phenocrysts, but with lesser amounts of plagioclase
Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...
phenocrysts, might be termed an ''olivine-plagioclase phyric basalt''.
In more complex nomenclature, a basalt with approximately 1% plagioclase phenocrysts, but 4% olivine microphenocrysts, might be termed an ''aphyric to sparsely plagioclase-olivine phyric basalt'', where plagioclase is listed before the olivine because of its larger crystals. Categorizing a rock as aphyric or as sparsely phyric is often a question of whether a significant number of crystals exceed the minimum size.
Analysis using phenocrysts
Geologists use phenocrysts to help determine rock origins and transformations because crystal formation partly depends on pressure and temperature.Other characteristics
Plagioclase
Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...
phenocrysts often exhibit zoning with a more calcic core surrounded by progressively more sodic rinds. This zoning reflects the change in magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
composition as crystallization progresses. This is described as normal zoning if the rim of the crystal shows a lower-temperature composition than the core of the crystal. Reverse zoning describes the more unusual case where the rim shows a higher-temperature composition than the core. Oscillatory zoning shows period fluctuations between low- and high-temperature compositions.
In rapakivi granite
Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...
s, phenocrysts of orthoclase
Orthoclase, or orthoclase feldspar ( endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture", because its two cleavage planes are at right angles ...
are enveloped within ''rinds'' of sodic plagioclase such as oligoclase.
In shallow intrusives or volcanic
A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often fo ...
flows phenocrysts which formed before eruption or shallow emplacement are surrounded by a fine-grained to glass
Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
y matrix
Matrix (: matrices or matrixes) or MATRIX may refer to:
Science and mathematics
* Matrix (mathematics), a rectangular array of numbers, symbols or expressions
* Matrix (logic), part of a formula in prenex normal form
* Matrix (biology), the m ...
. These volcanic phenocrysts often show flow banding, a parallel arrangement of lath
A lath or slat is a thin, narrow strip of straight-grained wood used under roof shingles or tiles, on lath and plaster walls and ceilings to hold plaster, and in lattice and trellis work.
''Lath'' has expanded to mean any type of backing m ...
-shaped crystals. These characteristics provide clues to the rocks' origins. Similarly, intragranular microfractures and any intergrowth among crystals provide additional clues.
See also
* AmygduleNotes
References
* * * The Integrated Ocean Drilling Program (IODP). (2001) Proceedings of the Ocean Drilling Program, Vol. 187 Initial Report{{Clear Igneous petrology