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Twinned Minerals have their own following in the mineral collecting hobby. There are collectors that are only interested in twinned minerals. Twins can add a fascinating side to otherwise boring minerals or can add yet another dimension to an already complex mineral such as calcite. There are several minerals that form classic twins such as chalcocite, fluorite, sanidine, microcline, harmotome, staurolite, gypsum, cinnabar, spinel and rutile to name a few (more are listed below). Some twins have been given colloquial names such as the "fairy cross", "iron cross" and "cog wheel" twins.
Twins form as a result of an error during crystallization. Instead of a "normal" single crystal, twins will often appear doubled where two crystals appear to be growing out of or into each other, like Siamese twins. Some twins however are not even identifiable outwardly and some minerals in fact have been found to be just a twinned variety of another mineral. Accidental relationships are not considered twins, such as when two distinct crystals grow more or less randomly side-by-side or toward each other, etc. Also epitaxial overgrowths are also not twins. These occur when one mineral of similar structure, but different chemistry, grows onto and "continues" the earlier mineral's crystal. A twin's formation is never random and follows certain defined rules called twin laws. Many types of twin laws are given their own unique names and some are well known, such as the Spinel Law or the Albite Law.
The twin laws are crystallographic in nature and are caused by flaws in the crystal structure that occur during growth or during changes in phases such as from a high temperature phase to a low temperature phase. One example of how twins form is explained by looking at how crystals grow. Most crystals grow by adding layers of atoms, one layer at a time (in a simplistic model). If the first layer is called A and the next layer which is in a different position is called B and the next C followed by another A and so forth, then a structure will be built like so: ABCABCABCABC . . . Many minerals form with such a stacking sequence. But, if an error occurs during growth a twin can be formed. If the next layer of atoms becomes misplaced and assumes the wrong position, then the following sequence will form:
Can you see the flaw? The C layer next to the middle A layer is wrong because there should be a B layer next. The rest of the sequence is then repeated as if nothing happened and the crystal grows outward in both directions until finished growing. Directly through the middle A layer, a mirror plane is produced and the right side of the crystal will be a mirror image of the left side just as a left hand is the mirror image of the right hand. The mirror is easier to see if the A is replaced by a vertical line | which represents a mirror plane as in:
Not all twins are formed this way, but it gives a good idea of how a twin is possible.
There are two general types of twin styles; contact and penetration. Contact twins have a composition plane (the twin plane) that forms at the boundary between the two twins. The composition plane is a mirror plane where the two twins can look like reflected images of each other (like a Siamese twin). The angle between the twins is critical and in some cases diagnostic. If the angle is 180 degrees then the crystals grow away from each other in opposite directions. If the angle is less than 180 degrees then the twin will have a noticeable bend. These twins form dove-tail, fish-tail and chevron shaped twins.
Many twins form penetration twins which look like two crystals that grew into and out of each other. These twins have portions of their respective twins protruding out of each other on different sides. At times it may look as if half the crystal was twisted in the wrong direction or that whoever made the crystal didn't know how it was supposed to fit together. The effect is really interesting on well formed twins. These twins can form crosses, 3-D star shapes and complex structures.
In some minerals, these two types can be repeated again and again; two, three or nearly an infinite number of times. There are two types of repeat twinning; cyclic and lamellar. Lamellar, which is also called polysynthetic twinning, forms from contact twins repeating continuously one twin after another, even on the microscopic level. Eventually a crystal composed of stacked twin layers is the result. The Feldspar Group minerals are the masters of this type of twinning which for the feldspars is know as the Albite Law.
If a composition twin has an angle of 30, 45, 60 or 90 degrees and it repeats 3, 4, 6 or so times . . . then it could form a complete circle or cyclic twin. Some classic twins form cyclic twins called "trillings"; where the mineral is composed of three twin components. Other cyclic twins can have 4, 6 or even 8 components, but trillings are the most common.
Twinning is actually rather common in the mineral kingdom, however perfectly formed twins are not. Minerals that commonly grow well formed twins are known to nearly every mineral collector. Twin collecting can be a very enjoyable hobby and most collectors own one or more. Even collectors of specific types of minerals must have their respective twins in order to have a "complete" collection. The twinning phenomena is well studied in the science of mineralogy. The study of twins is also important in crystallography, metallurgy, chemistry and biology.
Witherite (trilling)
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