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Feldspar goes through some twinning types
Feldspar frequently displays phase changes in what may appear externally as a single crystal but is actually two or more individuals
Individuals show differences in refraction between the crossed poles when examined in a thin section
Zone structure may show different stages of plagioclase growth
The complexity of growth can produce unusual development such as the central crystalline core of plagioclase closed by the orthoclase shell
The twins have been described by reference to the directions on the feldspar crystal
The description is given in terms of the modulators around which the twinning occurs and the structural planes in which the twins meet
Double crystals formed by simple repetition are represented by Carlsbad, Baveno, and Manebach’s twin laws
Carlsbad twins, Baveno Twin, Cupcake Manibach Carlsbad twins are common in orthoclase, and occasionally Baveno and Mannibach are seen in orthoclase
Carlsbad twins may also be present in plagioclase
Carlsbad twins often appear as two elongated singles separated by a single plane of formation: Individuals differ in extinction between polar crosses
The Baveno twins can be separated by a diagonal plane
Polymorphism or multiple twinning is abundant in plagioclase (Fig
1323a)
Twin Albit is widespread
It is recognized by the parallelism of the formation and division levels (010) (Fig
13-23b)
Parts showing white twins are useful in identifying plagioclase
The most common method of identification is the use of extinction curves
To use this method, the cut-off of the crystals is normally searched to (010)
This can be recognized by 3 factors: (1) the sharpness of the composition levels with a slight change in focus, (2) the uniformity of illumination of all slides, and (3) the equalization of the extinction angles for groups whose cufflinks were shifted to the left or right
You can also have a combination of more than one type of twinning
When Carlsbad and Albit twins are present (Fig
13-27 and 13-28), there will be four extinction modes for the crystal
Both types of cufflinks will be recognized in the forty-fifth position
Sometimes the white twin disappears and the crystal appears to be the simple Carlsbad twin
In position 0º, both the Albit and the Carlsbad twin will practically disappear
Pericline twins are polymorphic and result in ribbon-shaped individuals that can resemble white twins when viewed under a microscope
Pericline twins may be nearly normal for the direction of the level of albite formation
It is identified by its inclination with reference to the edge (001), (010) (the angle of the rhombus)
Clusters of white and Periclean twins frequently appear in alkali feldspar, especially in micro clean
The fusion results in a distinct mesh pattern between the crossed nickels
White tongues often penetrate the mesh structure at fairly regular intervals (internal growth forms the pattern known as pyrite
Alkali Feldspar Group Orthoclase: Carlsbad’s Law Twinning (C{001}=double axis) are simple twins Sanidine: usually twins according to Carlsbad’s law (C{001} axis = double axis) Sanidine: two-person twins and rarely microsynthetic microcline: Polysynthetic twining almost universal in two-way microcline twining, One according to Albite’s law ({010} = double plane), the other according to percline law (b[010] axis = double axis) gives the lattice or lattice structure, two sets of plates at right angles
Anorthoclase: (MicroClean soda) is a two-way multi-component twinning like MicroClean, but the plates are finer
It may be necessary to have an unusually thin section to expose the twin
Ferroelastic twinning in metals is a very common phenomenon
The twin laws follow simple symmetry rules and are observed in minerals like feldspar, palmerite, leucite, perovskite, etc
An important discovery in the last two decades has been that the thin regions between the twins produce different physical and chemical properties, but not the twins themselves
Mineral twinning has aroused the curiosity of mineralogists ever since Haüy established mineralogy as a discipline in 1797 [1]
In the early 20th century, many mineralogy books and research articles focused on twinning from an engineering perspective, Quartz may be the most common mineral in sandstone, but feldspar is the most abundant mineral in all other rock types; In fact, it is the most abundant mineral in the earth’s crust
Unlike quartz, feldspar is an essential component of almost all igneous rocks, through feldspar through ultrasonication
It begins to crystallize in magma at temperatures around 1000°C – 200°C warmer than quartz crystallization
Feldspar is also common in metamorphic rocks
As such, feldspar is a major (usually minor) component of most geoplastics and is reflected in its inclusion in QFL classification schemes
The two main groups of feldspars are potassium feldspar and plagioclase
They are all bas-relief in simple polarized light (similar to quartz)
Both groups have two planes that bisect each other at 90° so well that the fractured crystal fragments tend to be filled
Twinning is common
Potassium feldspar contains K and Na
The individual minerals belong to the monoclinic or triclinic crystal systems; One species, Adularia, can occur in both crystal forms
To complicate matters, K-feldspars can also occur with plagioclase
Plagioclase feldspars are also structural silicates in which mineral definitions depend on the relative proportions of Na and Ca; The most sodic mineral is albite and the most calcium anorthite
They form what is known as the solid solution series, indicating structural continuity from the sodium-dominated species to the calcium-dominated species
Thus the assignment of names such as albite or labradorite is to some extent a matter of mineral suitability, but their convenience lends itself to the classification of igneous rocks
The more plagioclase sodium (Na) tends to occur in igneous and plutonic rocks such as rhyolite and granite, the more calcium there is in basic and above-base rocks such as gabbros and basalts
The identification of minerals with each of the feldspar groups is complex
Identification generally requires detailed light microscopy, X-ray diffraction, and/or chemical analysis
From a sedimentary point of view, we tend to collect feldspar residues in either of two large groups
The main criterion used to distinguish K-feldspar from plagioclase in thin sections is the different twin laws
Plagioclase is always twinned, the most common being the following: Albate twins appear as multiple parallel plates that cross the entire section of the crystal
The width of the pair decreases and the number of plates increases in more calcareous plagioclase
Bricklin twins are similar to white twins but are oriented at different angles, usually, 90 degrees for albite twins, and Carlsbad cufflinks are a type of piercing cufflinks with a plane separating two crystalline parts
This twin diversification can also occur in K-feldspars
