Polysynthetic Albite Twins — Lamellar Twinning Diagnostic of Plagioclase
Polysynthetic Albite Twins — Lamellar Twinning Diagnostic of Plagioclase
Repeated parallel twins on the albite law that produce striations and contribute to feldspar phenomena
Polysynthetic albite twins are repeated parallel twin lamellae formed according to the albite twin law, the characteristic twinning law of plagioclase feldspars. The lamellae appear as fine, closely spaced striations visible on cleavage surfaces under magnification or as alternating extinction bands when the stone is examined between crossed polarisers. Polysynthetic albite twinning is a primary diagnostic feature in feldspar identification and is the structural basis for several of the optical phenomena that distinguish gem feldspars — the play of colour in moonstone, the iridescence in labradorite, and the schiller in sunstone.
Crystallography
The albite twin law is defined by the operation of reflection across the (010) crystallographic plane of the triclinic feldspar lattice. When growing or recrystallising plagioclase repeatedly switches lattice orientation across this plane, the result is a stack of thin lamellae, each oriented in one of two related lattice positions, alternating across the crystal. The lamellar spacing is typically in the range of a few micrometres to tens of micrometres, fine enough to require magnification but coarse enough to be readily visible under standard gemmological observation.
Polysynthetic albite twinning is most strongly developed in the more sodium-rich members of the plagioclase series — albite itself, oligoclase, and andesine — but is present to varying degrees throughout the plagioclase range. In potassium feldspar, related but distinct twinning laws — the Carlsbad and Manebach laws — predominate, and polysynthetic albite twinning is less common.
Optical effects in gem feldspars
The lamellar structure produced by polysynthetic albite twinning, often combined with exsolution lamellae of related feldspar phases, is the structural basis for the characteristic optical phenomena of gem-quality feldspar. In moonstone, fine alternating lamellae of orthoclase and albite scatter shorter wavelengths to produce the floating blue-white sheen called adularescence. In labradorite, coarser exsolution lamellae interfere with light to produce the broad iridescent flashes of red, gold, blue, and green known as labradorescence. In sunstone, oriented platelets of haematite or copper, often associated with the lamellar structure, scatter and reflect light to produce the metallic schiller for which the variety is named.
Identification
Polysynthetic albite twinning is detected directly by examination under crossed polarisers, where the alternating extinction of adjacent lamellae produces the characteristic banded extinction pattern. On polished cleavage surfaces the twinning appears as fine parallel striations under low magnification. The presence and pattern of the twinning are diagnostic for plagioclase identification and contribute to the determination of the specific composition within the plagioclase series.
In the trade
Polysynthetic albite twinning is a structural property of plagioclase feldspars and is not in itself a feature traded for or against; rather, it is the underlying cause of the optical phenomena that drive value in gem feldspars. A buyer evaluating moonstone, labradorite, or sunstone is looking at the visible expression of polysynthetic albite twinning rather than at the twinning itself, but the identification of the twinning under microscope or polariscope is a routine part of gemmological evaluation of these species.