有关斜长石环带
In plagioclase, calcium (Ca) and sodium (Na) can substitute for one another within the crystal structure. Commonly, plagioclase in igneous rocks is zoned with the center of the crystal being high in Ca; toward the edges, the crystal is progressively richer in Na (and poorer in Ca). This is interpreted as indicating that the crystal started growing when crystallization of magma began at a high temperature. (High temperatures favor Ca and low temperatures favor Na in plagioclase.) As the magma cooled, the plagioclase incorporated progressively more Na and less Ca.
Zoned Plagioclase in a Dacite from near Helena, Montana
The large plagioclase grain shows concentric compositional zoning in XP light. It is surrounded by fine grained material that is mostly quartz and feldspar; a few grains of biotite can be seen in the upper right. (Field of view is about 4.5 mm)
Oscillatory Zoning in a Plagioclase Mineral
Photomicrograph in polarized light. Banding corresponds to variations of concentration of sodium and calcium in the crystal. Scale bar: 200 microns.
Plagioclase feldspars also have good cleavage (A, left) and a dusty appearance from inclusions. They often show compositional zoning (A, right), but their most diagnostic feature is prominent lamellar twinning (B). The inclusions commonly turn out to be tiny crystals of epidote (C).
If your feldspar lacks twinning, it can be hard to tell them apart. Usually the dusty appearance, cleavage and alteration separate them from quartz. Sometimes the easiest approach is to use one of several staining methods.
Element maps showing Ca and Na zonation in plagioclase
Plagioclase, unzoned, in a hornblende diorite. Note the strong, parallel sets of albite twins, and the less visible set of pericline twins inclined almost at right angles to the albite twins. Plane polarized light, 40x
Plagioclase, zoned, in a dacite porphyry. This plagioclase has fine oscillatory zoning, in which the composition varies between more and less anorthite-rich compositions. Internal unconformities followed by euhedral overgrowths are also visible. Cross polarized light, 100x
A multiply zoned ("oscillatory") plagioclase crystal for a Chilean diorite. One possible origin for this texture is repeated injection of mafic magma -- a process well known in volcanic systems.
对于为什么会出现这种环带，有网友解释如下：
1. Minerals belonging to a solid-solution series continuously react with the surrounding liquid as it cools. A good example of a solid solution mineral is plagioclase. A cooling crystal wants to attain equilibrium with the surrounding melt. However, diffusion and ion exchanges is commonly too slow for the whole crystal to adjust to changing conditions i.e. falling temperature. This results in that only the rims of the crystals are able to reach equilibrium with the melt. This is called compositional zoning. The most common of compositional zoning is concentric zoning. This is when zones are parallel to the advancing crystal faces. This zoning reflects the growth history of the crystal. Google image: zoning in plagioclase you can see some nice examples.
2. A zoned crystal is caused by a change in the growing environment a crystal is in. Zoning may also occur in crystals due to rocks or other minerals being physically included in the growing process. In other words, zoning is a crystals way of showing impurities.
3. A texture developed in solid-solution minerals and characterized optically by changes in the colour or extinction angle of the mineral from the core to the rim. This optical zoning is a reflection of chemical zoning in the mineral. For example, a plagioclase can be zoned from a Ca-rich core to an Na-rich rim. Zoning results from the mineral's inability to maintain chemical equilibrium with magma during rapid cooling; the zonation represents a frozen picture of the continuous reaction series for that mineral. Zoning can be of three types, the first two applying mostly to plagioclase feldspars. (a) Normal zoning is where the mineral is zoned from a high-temperature core composition to a low-temperature rim composition. (b) Reverse zoning is where a mineral is zoned from a low-temperature core composition to a high-temperature rim composition. (c) Oscillatory zoning is where the mineral chemistry continuously oscillates between high- and low-temperature compositions going from the core to the rim.
补充知识：Continuous reaction series: The continuous change in composition of a solid solution mineral in order to maintain a state of equilibrium with cooling magma. The mineral changes its composition by continuously exchanging cations with the cooling magma in which it floats. The plagioclase feldspar solid-solution mineral series is a good example of a continuous reaction series. During the cooling of magma, the plagioclase continuously exchanges Ca and Al for Na and Si in order to maintain an equilibrium composition. The principle of continuous reaction was first presented by Norman Bowen in 1928.
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Zoned Plagioclase in a Dacite from near Helena, Montana
The large plagioclase grain shows concentric compositional zoning in XP light. It is surrounded by fine grained material that is mostly quartz and feldspar; a few grains of biotite can be seen in the upper right. (Field of view is about 4.5 mm)
 |
Oscillatory Zoning in a Plagioclase Mineral
Photomicrograph in polarized light. Banding corresponds to variations of concentration of sodium and calcium in the crystal. Scale bar: 200 microns.
 |
Plagioclase feldspars also have good cleavage (A, left) and a dusty appearance from inclusions. They often show compositional zoning (A, right), but their most diagnostic feature is prominent lamellar twinning (B). The inclusions commonly turn out to be tiny crystals of epidote (C).
If your feldspar lacks twinning, it can be hard to tell them apart. Usually the dusty appearance, cleavage and alteration separate them from quartz. Sometimes the easiest approach is to use one of several staining methods.
 |
Element maps showing Ca and Na zonation in plagioclase
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Plagioclase, unzoned, in a hornblende diorite. Note the strong, parallel sets of albite twins, and the less visible set of pericline twins inclined almost at right angles to the albite twins. Plane polarized light, 40x
 |
Plagioclase, zoned, in a dacite porphyry. This plagioclase has fine oscillatory zoning, in which the composition varies between more and less anorthite-rich compositions. Internal unconformities followed by euhedral overgrowths are also visible. Cross polarized light, 100x
 |
A multiply zoned ("oscillatory") plagioclase crystal for a Chilean diorite. One possible origin for this texture is repeated injection of mafic magma -- a process well known in volcanic systems.
对于为什么会出现这种环带，有网友解释如下：
1. Minerals belonging to a solid-solution series continuously react with the surrounding liquid as it cools. A good example of a solid solution mineral is plagioclase. A cooling crystal wants to attain equilibrium with the surrounding melt. However, diffusion and ion exchanges is commonly too slow for the whole crystal to adjust to changing conditions i.e. falling temperature. This results in that only the rims of the crystals are able to reach equilibrium with the melt. This is called compositional zoning. The most common of compositional zoning is concentric zoning. This is when zones are parallel to the advancing crystal faces. This zoning reflects the growth history of the crystal. Google image: zoning in plagioclase you can see some nice examples.
2. A zoned crystal is caused by a change in the growing environment a crystal is in. Zoning may also occur in crystals due to rocks or other minerals being physically included in the growing process. In other words, zoning is a crystals way of showing impurities.
3. A texture developed in solid-solution minerals and characterized optically by changes in the colour or extinction angle of the mineral from the core to the rim. This optical zoning is a reflection of chemical zoning in the mineral. For example, a plagioclase can be zoned from a Ca-rich core to an Na-rich rim. Zoning results from the mineral's inability to maintain chemical equilibrium with magma during rapid cooling; the zonation represents a frozen picture of the continuous reaction series for that mineral. Zoning can be of three types, the first two applying mostly to plagioclase feldspars. (a) Normal zoning is where the mineral is zoned from a high-temperature core composition to a low-temperature rim composition. (b) Reverse zoning is where a mineral is zoned from a low-temperature core composition to a high-temperature rim composition. (c) Oscillatory zoning is where the mineral chemistry continuously oscillates between high- and low-temperature compositions going from the core to the rim.
补充知识：Continuous reaction series: The continuous change in composition of a solid solution mineral in order to maintain a state of equilibrium with cooling magma. The mineral changes its composition by continuously exchanging cations with the cooling magma in which it floats. The plagioclase feldspar solid-solution mineral series is a good example of a continuous reaction series. During the cooling of magma, the plagioclase continuously exchanges Ca and Al for Na and Si in order to maintain an equilibrium composition. The principle of continuous reaction was first presented by Norman Bowen in 1928.
