Polysynthetic Twin — Stacked Lamellar Twinning in Crystal Structure
Polysynthetic Twin — Stacked Lamellar Twinning in Crystal Structure
Repeated parallel twin domains that produce diagnostic striations across many gem species
A polysynthetic twin is a crystallographic twinning configuration in which multiple parallel twin lamellae are stacked in repeated layers, forming a lamellar structure visible under magnification or between crossed polarisers. Polysynthetic twinning is distinct from the simpler contact twinning, in which two crystal domains meet along a single surface, and from penetration twinning, in which two domains interpenetrate. The polysynthetic configuration is widespread across the mineral kingdom and occurs in many gem species, including corundum, the plagioclase feldspars, calcite, and others.
Mechanism and formation
Polysynthetic twinning forms by two principal mechanisms: growth twinning, in which the alternating lamellae arise during the original crystallisation as the growing crystal repeatedly switches between two related lattice orientations; and deformation twinning, in which an originally untwinned crystal develops the lamellar structure under mechanical stress, with the lamellae forming as the crystal accommodates strain by switching lattice orientation in narrow zones rather than by gross fracturing.
The twin law — the specific symmetry relationship between adjacent lamellae — is characteristic of the species and is described by reference to a crystallographic plane or axis. The albite law in plagioclase feldspars, the Carlsbad law in alkali feldspars, the e-twin law in calcite, and the basal-plane twin law in corundum are examples of the twin laws responsible for polysynthetic twinning in major gem species.
Optical and gemmological consequences
Polysynthetic twinning has several gemmological consequences. The lamellar boundaries scatter or reflect light, sometimes producing visible striations on cleavage or polished surfaces and sometimes contributing to optical phenomena such as adularescence in moonstone or chatoyancy in some twinned gem species. Under crossed polarisers, the alternating extinction of adjacent lamellae produces a characteristic banded pattern that is diagnostic for the underlying twinning configuration.
In some cases polysynthetic twinning is a primary diagnostic feature for species or origin determination. The basal-plane polysynthetic twins in corundum, for example, are commonly observed in metamorphic ruby and sapphire and are part of the suite of inclusion features used in laboratory origin determination. In plagioclase, the presence and character of polysynthetic albite twinning is a primary criterion for the identification of plagioclase against alkali feldspar.
Detection
Polysynthetic twinning is detected by polarised-light examination, by microscopic observation of polished or cleavage surfaces, and in some cases by characteristic patterns visible to the unaided eye on weathered or etched surfaces. In faceted gems, the polariscope is the routine detection tool; the gemmologist looks for the alternating banded extinction pattern that distinguishes polysynthetic twinning from simple uniform extinction.
In the trade
Polysynthetic twinning is recorded in laboratory reports as part of the inclusion characterisation of significant stones, particularly where the twinning contributes to origin determination or to the identification of treatment status. Trade buyers of fine corundum will sometimes look for or against polysynthetic twins as visible indicators of metamorphic versus magmatic origin, with the twins more commonly observed in metamorphic stones from Mogok, Sri Lanka, and similar terrains than in magmatic stones from basaltic sources. In feldspars, the twinning is taken for granted as part of the structural identity of the species.