Why Do Some Crystals Look Like Two Grown Together? Sharing a Plane
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Two crystals of the same substance joined in a specific orientation, sharing part of their structure, look like an accident and follow strict rules. The patterns identify minerals and record what happened to them.
What twinning is
A twin is two or more portions of the same crystalline substance joined in a definite symmetrical relationship rather than at random. The two parts share a plane, an axis or a point, and the orientation of one relative to the other is fixed by the structure rather than arbitrary, which is what distinguishes a twin from two crystals that simply grew against each other. The relationship can be described as a reflection across a plane, a rotation about an axis or an inversion through a point, and the specific relationship that occurs for a given substance is characteristic of it, following from how the atoms are arranged and which alternative arrangements are nearly as favourable energetically.
How twins form
Three mechanisms produce them and each says something different about the specimen:
- •Growth twinning, where the error occurs as the crystal grows and an atom attaches in the alternative orientation
- •Transformation twinning, where a crystal changes structure on cooling and adopts both orientations in different regions
- •Deformation twinning, where stress forces part of a crystal to shear into the twin orientation
- •Contact twins, where the two parts meet along a plane
- •Penetration twins, where the two appear to pass through each other
- •Repeated twinning, producing stacks of thin alternating layers
What it tells a geologist
Twinning is diagnostic in several ways. Particular twin forms are characteristic of particular minerals and are used in identification, with several named after the localities where they were first described, and a distinctive twin visible in hand specimen frequently settles what a mineral is. Deformation twins record that the rock was stressed after the crystal formed, and their abundance and orientation indicate how much and in which direction, which contributes to reconstructing structural history. Transformation twins record that the mineral passed through a temperature at which its structure changed, which constrains the conditions. Under a microscope with polarised light, repeated twinning produces a striped appearance that identifies an entire mineral group immediately, which is the single most used diagnostic feature in rock thin sections.
The famous twins
Several twin forms are distinctive enough to have names and to be recognised on sight. Staurolite forms crosses, with two prisms intersecting at sixty or ninety degrees, which produced a folk name referring to a cross and a long history as an amulet. Gypsum forms swallowtail twins, two blades meeting at an angle. Calcite, quartz and fluorite each have several named twin laws. Feldspar twinning produces the fine parallel striping visible on cleavage surfaces that distinguishes one group from another in hand specimen, which is among the most useful field tests in igneous rocks. Diamonds twin into flattened triangular forms that are harder to cut. In each case the form follows from the structure and is reproducible rather than accidental.
Where it matters beyond rocks
The phenomenon has consequences in materials and in industry. Twinning is a mechanism by which metals deform, alongside the sliding of planes past each other, and it dominates in some metals and at low temperatures, which affects how they behave under load and how they are worked. Twins in semiconductor crystals are defects that disrupt electrical behaviour, so growing single crystals free of them is a substantial part of what semiconductor manufacture involves. Twinned crystals complicate the determination of structures by X-ray diffraction, since the pattern produced is a superposition of two orientations and must be deconvolved, which is a recurring problem in crystallography and has produced its own body of technique. And in gemstones, twinning affects both appearance and how a stone can be cut.
The takeaway
Two parts of one substance joined in a fixed symmetrical relationship, related by reflection, rotation or inversion, is a twin rather than two crystals grown together. Growth, transformation on cooling and deformation under stress each produce them and each records something different. Repeated twinning under polarised light identifies a whole mineral group immediately in thin section.