last modified: Sunday, 05-Jun-2016 02:03:08 CEST
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Overview of Twin Types
Quartz forms a number of different types of twins some of which are listed in the following table. Only three of them marked bold are generally accepted as twinning laws. These laws are either very common in crystals (Dauphiné Law, Brazil Law) or, although much rarer, have at least been found on a regular basis and in sufficient numbers, and are associated with certain growth forms and environmental conditions (Japan Law). There is some debate on whether some of the other twinning laws (marked italic) in fact exist in nature, others have been shown to be not of law-like nature.
| Twinning Axis |
Twinning Plane |
Composition Plane |
Type | Handedness of Subindividuals |
|
|---|---|---|---|---|---|
| Dauphiné Law | [ 0 0 0 1 ] | - | { 1 0 1 0 } | Penetration | R+R or L+L |
| Brazil Law | - | { 1 1 2 0 } | { 1 1 2 0 } | Contact / Penetration | L+R |
| Japan Law | - | { 1 1 2 2 } | { 1 1 2 2 } | Contact | all combinations |
| "Liebisch Law" or "Combined Law" |
[ 0 0 0 1 ] | { 1 1 2 0 } | - | "Mixed" | L+R |
| Esterel Law | - | { 1 0 1 1 } | { 1 0 1 1 } | Contact ? | ? |
| Sardinia Law | - | { 1 0 1 2 } | { 1 0 1 2 } | Contact ? | ? |
| Breithaupt Law | - | { 1 1 2 1 } | { 1 1 2 1 } | Contact ? | ? |
Although, for example, Japan Law twin is the correct nomenclature, I sometimes also use the term "Japanese Twin" and likewise "Brazil twin" and "Dauphiné Twin", as these terms are much more common among rockhounds.
A single crystal can be twinned in many ways, parts of it may show Dauphiné twinning, other parts Brazil twinning. In fact combinations of Brazil and Dauphiné twin portions in a single crystal aren't that uncommon, and sometimes you see references to aLiebisch or Combined Law. Although the shape of the crystals suggests the presence of a "Combined Twins", these combinations do not qualify as a twinning law by themselves, because it has been shown in electron microscopic studies that a left-handed part never borders a right-handed part that has been rotated around the c-axis.
Identifying Twins
There are complex methods to determine if and how a crystal is twinned, but a rockhound has to rely solely on crystal morphology. The characteristics of each twinning type will be described below, but beforehand I will briefly mention the four general properties of quartz crystals that help identifying twins:
- Position of x-faces
Reliable, but the x-face is not very common.
- Position of s-faces
The s-face is more common than the x-face, but one can only identify its relative position when it shows a striation. Very often the s-face is a perfect plane.
- Surface patterns caused by etching
This is very reliable, but it doesn't tell you the orientation of the crystal subindividuals. For example, you might identify a Dauphiné twin but can't tell if it's a left or right quartz.
- Oriented indentations on the surface
Reliable, but difficult to interpret, because the form of the dents also depends on the type of face.
Of course, if one relies only on these criteria one will miss many twinned crystals because of their "boring" morphology. Sometimes you can tell that a crystal is twinned, but it's impossible to tell how.
Often you read that so calledsutures coincide with twin boundaries. Sutures are more or less irregular crack-like lines visible on the crystal faces. They often are the boundaries between slightly displaced crystal subindividuals of similar orientation, which lie almost parallel to each other. These sutures are most easily observed on the m-faces of crystals with macromosaic structure.
The sutures visible on the m-faces of Japanese twins are in fact the boundary between the two twin crystals oriented differently in a law-like manner. But sutures do not indicate twin boundary between left and right-handed crystals. Sutures can only form when the neighboring crystal subindividuals show a different orientation. Left and right handed quartz crystals do not differ in their orientation with respect to the a- and c-axes, so in a twinned crystal the subindividuals lie perfectly parallel to each other (but with their crystal lattice either rotated or mirrored with respect to each other). If you see a suture, you see a slight misorientation of the two subindividuals, and their boundaries cannot coincide with a twinning plane that corresponds to a specific, law-like intergrowth.
H.264-movie, 256x256 px:
Dauphiné Law twins areelectrical twins: their optical properties are similar to untwinned crystals, but mechanical pressure along the a axis does not cause an electrostatic polarization of the crystals. Dauphiné Law twin domains cannot be visualized in polarized light, because adjacent domains have the same handedness and rotate the light's polarization planes in the same direction.
In many crystals the pattern of domains gets more complex and irregular towards the tips of the crystal, while the base is made of a few twin domains that may be arranged as roughly triangular sectors (Friedlaender, 1951).
To my knowledge, this etching is natural, and I have not enhanced the contrast of the image in any way. To see such a strong pattern, one normally needs to etch the crystal with hydrofluoric acid, but the hot fluids in a pegmatite or miarole pocket like the one the crystal probably came from are also very aggressive agents.
Since the x-faces are on the right side of the prism faces, this is a right/right twin.
In addition to the x-faces, there are also two types of steep rhombohedral faces that sit in between the prism and the (normal) rhombohedral faces.
H.264-movie, 256x256px:
The tiny interlocked quartz grains that make up cryptocrystalline quartz varieties are mostly heavily twinned polysynthetically according to the Brazil Law on a microscopic scale.
Brazil Law twins areoptical twins: Light waves that pass through a right or left quartz along its c axis change the orientation of their oscillation plane in either left or right direction. The effects of the right and left parts of the twinned crystal will at least partially cancel out each other, and if the left and right quartz portions are equal, the light will pass apparently unaltered. In a slice of a twinned crystal the left- and right-handed portions can still be made visible in polarized light, of course.
Japan Law twin quartzes have initially been labeled as La Gardette twins, after the first locality in the French Alps where this type of twinning has been described before the large number of localities in Japan were discovered. Japanese twins are quite rare and a newly discovered locality will always be mentioned in collector's magazines.
LikeBrazil twins, Japan Law twins are contact twins. The twinning plane is { 1 1 2 2 }, corresponding to the form ξ (the Greek letter xi), a negative trigonal bipyramid, similar to the positive trigonal bipyramid and its s-face, but flatter. The form ξ is a very rare crystal face that lies between the r and z face. Interestingly, Japan Law twins can additionally be Brazil as well as Dauphiné Law twins.
Copyright © 2005-2016, A.C. A k h a v a n
Impressum
- Source: http://www.quartzpage.de/crs_twins.html
Dauphiné Law Twins
dauphine_twin_1.mp4 372kb
960x960 122kb
Brazil Law Twins
brazil_twin_1.mp4 376kb
1.) www.faden.it presents images of Brazil Law twins
2.) on the website of Uniun Cristallina a very good image of a Brazil Law twin can be found.
Japan Law Twins
The Japan Law is the only common twinning law of quartz where the two twin crystals are visible as individuals. In a Japan Law twin the c-axis of two crystals meet at an angle of 84°33', with two of the m prism faces of both crystals being parallel. The result is usually a V-shaped twin, rarely a slightly oblique cross. Very often the Japan Law twin looks platy, because the crystal grows faster between the two arms.
Footnotes
1
Some say that there is no "pseudosymmetry" and accordingly call the habit of such twins hexagonal. I disagree: natural crystals - twinned or untwinned - never show symmetry as a whole. Their shape obeys certain mathematical relations, so they all share certain geometrical properties, but only idealized crystals are symmetric. When one calls a mineral "isometric", for example, she does not mean to say that the mineral in all its occurrences is isometric, but that the structural elements that determine the external shape of the crystals show isometric symmetry. In that sense, a crystal is said to have pseudohexagonal habit when its external shape suggests the presence of an internal hexagonal structure, while, in fact, the crystal lattice possesses a different symmetry.
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