Perthite: Gemstone Information

Named after the town of Perth, Ontario, Canada, where it was first found, Perthite is the general variety name for alkali feldspar consisting of parallel or subparallel intergrowths in which the potassium-rich phase (commonly microcline but may be orthoclase) appears to be the host from which the sodium-rich phase (commonly albite but may be oligoclase) exsolved; such exsolved areas may be visible to the naked eye, typically forming strings, lamellae, blebs, films or irregular veinlets (the term ‘flame perthite’ has been used in this context). Perthite classification is based on the size of the exsolved phase. Thus, where the texture is invisible to the naked eye but can be resolved with a microscope, it is called microperthite. Amazonite (macro-perthite) is an example of perthitic structure that can be seen by the naked eye and moonstone (crypto-perthite) is an example where the perthitic structure cannot be resolved with the microscope. If there is more plagioclase than potassium feldspar, it is called ‘antiperthite’. Moonstone from Moonstone Hill, Queensland, has been classified as an anorthoclase-crypto-perthite antiperthite. Perthite is often pink, red or honey coloured and materials exhibiting a rich golden
labradorescence against a flesh-red to reddish brown or white background have been used as ornamental stones. Perthites often show silvery reflections. A glassy tan microcline Perthite from a pegmatite in Custer, South Dakota, USA, shows blue-white fluorescence. Fine perthites occur at Dungannon, Ontario, and other localities in Canada. Perthite is used for carving and also cabochoned. Parting may be seen in some perthites.
Perthite Refractive Indices
α = 1.522–1.527 Indices between those of microcline and albite.
β = 1.526–1.532 The orthoclase (K-feldspar) will have the lower
γ = 1.530–1.538 RI and the albite will have a higher RI.
DR = 0.005–0.013