Micrographic texture
Encyclopedia
In petrology
, micrographic texture is a fine-grained intergrowth of quartz
and alkali feldspar
, interpreted as the last product of crystallization
in some igneous rock
s which contain high or moderately high percentages of silica. Micropegmatite is an outmoded terminology for micrographic texture.
This fine-grained texture is similar to the coarser intergrowths in certain pegmatite
s and coarse granitic veins; the quartz forms angular patches scattered through a matrix of feldspar. In polarized light the separate areas of each mineral extinguish at the same time, and this proves that, even though apparently discontinuous, they have the same crystalline orientation.
The feldspar may be considered an irregular crystal of spongy structure, the interstices being filled up by another spongy crystal of quartz. Some of the coarser-grained examples are said to be "graphic", because the intergrowths vaguely resemble ancient cuneiform lettering. Some micrographic intergrowths are similar to those characteristic of granophyre
.
Micrographic differs from graphic granite in being so much finer-grained that the texture can only be seen in a petrographic thin section with a microscope
. The feldspar
is usually orthoclase
, but can also be albite
, oligoclase
or microcline
. In at least some instances, quartz is so disposed that the two minerals have a definite relation between their crystallographic axes.
In rocks where micrographic texture is most common, it is usually interpreted as the last product of crystallization, and may represent residual melt. Commonly it has no definite form of its own, but fills up the irregular interspaces between the earlier crystallized minerals. The compositions of these residual crystallization products may represent eutectic compositions, the mixtures (quartz plus feldspar plus minor amounts of other minerals) which have the lowest fusion point. The texture may commonly form in the presence of a vapor phase as well as a silicate melt, however, and vapor-rock reactions below the solidus
may result in feldspar replacement and consequent compositional changes.
The texture should not be confused with myrmekite
or granophyre
, in which quartz forms club-shaped, curved or vermiform threads intergrown with plagioclase
feldspar
and alkali feldspar, respectively.
In some rocks the whole groundmass consists of spherulitic
growths of fibrous quartz and feldspar; in their centres there is often a quartz or feldspar crystal; the outer boundaries of the spherulites are not usually circular but irregular, owing to the interlocking of adjacent spherulites at their margins. Such textures may document solid-state devitrification of glass.
Petrology
Petrology is the branch of geology that studies rocks, and the conditions in which rocks form....
, micrographic texture is a fine-grained intergrowth of quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
and alkali feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
, interpreted as the last product of crystallization
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
in some igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s which contain high or moderately high percentages of silica. Micropegmatite is an outmoded terminology for micrographic texture.
This fine-grained texture is similar to the coarser intergrowths in certain pegmatite
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
s and coarse granitic veins; the quartz forms angular patches scattered through a matrix of feldspar. In polarized light the separate areas of each mineral extinguish at the same time, and this proves that, even though apparently discontinuous, they have the same crystalline orientation.
The feldspar may be considered an irregular crystal of spongy structure, the interstices being filled up by another spongy crystal of quartz. Some of the coarser-grained examples are said to be "graphic", because the intergrowths vaguely resemble ancient cuneiform lettering. Some micrographic intergrowths are similar to those characteristic of granophyre
Granophyre
Granophyre is a subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths such as those in the accompanying image....
.
Micrographic differs from graphic granite in being so much finer-grained that the texture can only be seen in a petrographic thin section with a microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
. The feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
is usually orthoclase
Orthoclase
Orthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...
, but can also be albite
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
, oligoclase
Oligoclase
Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite and anorthite . The albite:anorthite molar ratio ranges from 90:10 to 70:30.Oligoclase is a high sodium...
or microcline
Microcline
Microcline is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. Microcline typically contains minor amounts of sodium. It is common in granite and pegmatites. Microcline forms during slow cooling of orthoclase; it is more stable at lower temperatures...
. In at least some instances, quartz is so disposed that the two minerals have a definite relation between their crystallographic axes.
In rocks where micrographic texture is most common, it is usually interpreted as the last product of crystallization, and may represent residual melt. Commonly it has no definite form of its own, but fills up the irregular interspaces between the earlier crystallized minerals. The compositions of these residual crystallization products may represent eutectic compositions, the mixtures (quartz plus feldspar plus minor amounts of other minerals) which have the lowest fusion point. The texture may commonly form in the presence of a vapor phase as well as a silicate melt, however, and vapor-rock reactions below the solidus
Solidus (chemistry)
In chemistry, materials science, and physics, the solidus is the locus of temperatures below which a given substance is completely solid...
may result in feldspar replacement and consequent compositional changes.
The texture should not be confused with myrmekite
Myrmekite
Myrmekite describes a vermicular, or wormy, intergrowth of quartz in plagioclase. The intergrowths are microscopic in scale, typically with maximum dimensions less than 1 millimeter. The plagioclase is sodium-rich, usually albite or oligoclase. These quartz-plagioclase intergrowths are associated...
or granophyre
Granophyre
Granophyre is a subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths such as those in the accompanying image....
, in which quartz forms club-shaped, curved or vermiform threads intergrown with plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
and alkali feldspar, respectively.
In some rocks the whole groundmass consists of spherulitic
Spherulite
In petrology, spherulites are small, rounded bodies that commonly occur in vitreous igneous rocks. They are often visible in specimens of obsidian, pitchstone and rhyolite as globules about the size of millet seed or rice grain, with a duller luster than the surrounding glassy base of the rock,...
growths of fibrous quartz and feldspar; in their centres there is often a quartz or feldspar crystal; the outer boundaries of the spherulites are not usually circular but irregular, owing to the interlocking of adjacent spherulites at their margins. Such textures may document solid-state devitrification of glass.
See also
- GranophyreGranophyreGranophyre is a subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths such as those in the accompanying image....
, - Rock microstructureRock microstructureRock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature...
- Eutectic
- SolidusSolidus (chemistry)In chemistry, materials science, and physics, the solidus is the locus of temperatures below which a given substance is completely solid...
- SymplectiteSymplectiteIn mineralogy, a symplectite is a very fine intergrowth of two or more distinct mineral phases. Symplectites are largely considered to result from the breakdown of unstable minerals....