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A porphyroblast is a large 
Some garnet porphyroblasts contain curving trails of quartz and other mineral inclusions that record rotation of the crystals relative to their surroundings. However, the question of how much porphyroblasts actually rotate in an external reference frame fixed to the Earth's surface during metamorphism and deformation has long been the subject of debate. The question focused on so-called ''"spiral garnets"'', also known as ''"snowball garnets"'', whose inclusion trails define spiral patterns. These microstructures are interpreted classically as having formed by shearing induced rotation of a growing garnet crystal. Later research, however, led to an alternative formation model in which a porphyroblast grows over a developing microfold while maintaining a stable position in the external reference frame. Repetition of this process can then produce complex spiral-shaped patterns. Although many researchers continue to adopt the classic rotational model, most researchers who have published research testing both models by measuring the orientations of porphyroblasts have come to support the modern interpretation.
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ...
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, macr ...
in a 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, cau ...
which has grown within the finer grained matrix
Matrix most commonly refers to:
* ''The Matrix'' (franchise), an American media franchise
** '' The Matrix'', a 1999 science-fiction action film
** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...
. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape.
The most common porphyroblasts in metapelites (metamorphosed mudstones and siltstone
Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, ...
s) are garnet
Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.
All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different ...
s and staurolites, which stand out in well- foliated metapelites (such as schist
Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes ...
s) against the platy mica
Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
matrix.
A similar type of crystal is a ''phenocryst
300px, Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter 2.3 cm) for scale.
A phenocryst is an early forming, relati ...
'', a large crystal in an igneous rock
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or l ...
. Porphyroblasts are often confused with '' porphyroclasts'', which can also be large outstanding crystals, but which are ''older'' than the matrix of the rock.
If a porphyroblastic mineral has small inclusions of minerals within it, the mineral is described as poikiloblast
A poikiloblast is a porphyroblast mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs na ...
ic. This observation can help interpret deformation history.
A rock which has many porphyroblasts is described as having a ''porphyroblastic texture''.
As porphyroblasts grow, the foliation may be preserved as oriented inclusions trapped by the porphyroblast as it overgrows them, and this is helpful for tracking changing deformation planes.
In metamorphic rocks that experience deformation during metamorphism, porphyroblasts may grow before, during, or after the phase of deformation recorded by the matrix minerals. The relationship of porphyroblast growth to deformation is typically evaluated by comparing the shape orientation of trails of mineral inclusions in the porphyroblast to the matrix fabric.
Some garnet porphyroblasts contain curving trails of quartz and other mineral inclusions that record rotation of the crystals relative to their surroundings. However, the question of how much porphyroblasts actually rotate in an external reference frame fixed to the Earth's surface during metamorphism and deformation has long been the subject of debate. The question focused on so-called ''"spiral garnets"'', also known as ''"snowball garnets"'', whose inclusion trails define spiral patterns. These microstructures are interpreted classically as having formed by shearing induced rotation of a growing garnet crystal. Later research, however, led to an alternative formation model in which a porphyroblast grows over a developing microfold while maintaining a stable position in the external reference frame. Repetition of this process can then produce complex spiral-shaped patterns. Although many researchers continue to adopt the classic rotational model, most researchers who have published research testing both models by measuring the orientations of porphyroblasts have come to support the modern interpretation.
See also
*List of rock textures
This page is intended to be a list of rock textural and morphological terms.
A
* Adcumulate
* Agglomeritic
* Adamantine a type of lustre
* Amygdaloidal
* Anhedral
* Antitaxial veins
* Aphanitic
* Aplitic; aplite
* Augen textured g ...
* Rock microstructure
Rock microstructure includes the texture and small-scale structures of a rock. The words ''texture'' and ''microstructure'' are interchangeable, with the latter preferred in modern geological literature. However, ''texture'' is still acceptable ...
References
{{Reflist Metamorphic petrology