Sanidine: Gemstone Information

Sanidine is an end member of a series of the alkali feldspars whose series ranges from pure NaAlSi3O8 to pure KAlSi3O8. This series only exists at high temperatures. Optical properties and X-ray techniques are the only good ways to distinguish sanidine from orthoclase, microcline and anorthoclase. Sanidine has a flattened crystal habit and simple twinning is not infrequent. Occurrence Sanidine is a common constituent in acid and intermediate volcanic rocks such as rhyolites, dacites and trachytes; where the rock cooled quickly at above approximately 900 °C, sanidine is the stable structure. Also in lamproites, a leucite-nepheline dolerite and in a basaltic tuff. Notable occurrences include Germany, USA – Idaho, Colorado and New Mexico – Russia, Italy and Madagascar (transparent colourless to yellow gemmy crystals have come from pegmatite derived feldspar veins cutting coarse marbles at Ampandrandrava, Toliara (Tuléar) Province and Itrongahy Fianarantsoa Province. The yellow gemmy crystals, originally described as orthoclase and now identified as high to low sanidine, are actually found over a rather large area, from the classic Itrongahy region to the south-east for at least 40 km. Rich yellow facetable pieces up to 2,500 cts have been reported. Appearance Transparent to translucent • Lustre: Vitreous • Colour: Colourless, yellow, light brown, light grey, smoky; the tone being defined by the size of the stone. Properties • Crystal system: Monoclinic – biaxial ve • Refractive index: α 1.518–1.527; β 1.523–1.532; γ 1.524–1.534. Yellow sanidine (formally identified as orthoclase) from Madagascar with iron up to 3% will show increases in α and β of =0.003/wt% of Fe2O3. Iron rich material having values in the upper part of the range. The RI range for brown Rhineland, Germany, material have been reported as α 1.516–1.520; β 1.521–1.525; γ 1.522–1.526 and DR –0.005 to 0.007. Colourless crystals up to 1 cm; RI α 1.516–1.519; β 1.520–1.522; γ 1.521–1.523; DR 0.003–0.005 have been reported from volcanic tuffs at Ashton, Idaho, USA • Birefringence: 0.006–0.008 • Pleochroism: Weak • Density: 2.56–2.62 (SG Rhineland material 2.57–2.58) • Hardness: 6 • Dispersion: Weak • Cleavage/fracture is prismatic, excellent to good in two directions forming nearly right angled prisms. Fracture is conchoidal or uneven – may be somewhat brittle • Inclusions: Not diagnostic though some melt inclusions may be visible • Optical effects: chatoyancy, asterism, colour change, ADR See moonstone • Absorption Spectra (400–700 nm): Ferric iron (Fe3) may substitute in the tetrahedral Si/Al site producing a yellow hue. Absorption spectra of yellow sanidine (previously identified as orthoclase) from Madagascar show a weak line or band at 448 nm and a slightly stronger one at 420 nm in the blue and violet respectively. A stronger band at 375 nm can be observed using a spectrophotometer. The smoky colour in some sanidine is the result of ionizing radiation interacting with Al which produces strong absorption increasing from 500 to 400 nm in smoky coloured sanidine from Eifel, Rhineland, Germany. Not diagnostic • Fluorescence: Weak reddish orange under both LWUV and SWUV (Madagascar) • Treatment: Some sanidine subjected to iridation turns amber or smoky colored.