Andesine: Gemstone Information
Andesine is only a minor constituent in most granites and syenites but is the dominant feldspar in a wide range of igneous rocks of basic to intermediate and certain igneous rocks called appropriately andesites. It is also found in some metamorphic rocks as a minor constituent. Localities include Greenland, Andes Mountains (hence the name
Andesine) and Norway. Large crystals are found in the San Gabriel Mountains of California and pale yellow to colourless andesine (5 cm+) is found at deposits in north-eastern Idaho, China (red and green stones – occasionally referred to as ‘sunstone’) and Democratic Republic of Congo (red stones up to 30 cts), Japan, India, South Africa, Argentina, France, Italy and Germany.
The properties of weakly pleochroic (orange-red to red), andesine from the Democratic Republic of Congo lie close to the andesine–labradorite boundary. Red feldspars with labradorite composition and properties have been described from this locality (see labradorite and sunstone). The inference is that this material has a wide range of properties and chemical composition that span the andesine–labradorite boundary.
Crystals are translucent to transparent, polysynthetic twinning is ubiquitous.
• Colour: Mostly white or grey; can be red, orange-red, greenish or yellowish
• Lustre: Vitreous to dull
• Varieties: Lavanite (an andesine feldspar similar to sunstone, but without the schiller inclusions).
• Crystal System: Triclinic
• Refractive index: α 1.543–1.554; β 1.547–1.559; γ 1.552–1.562; α 1.551– γ 1.560 Democratic Republic of Congo Biaxial (+ve/ve).
Density: 2.66–2.68: 2.67 (Congo)
• Hardness: 6–6.5
• Dispersion: Weak
• Cleavage/fracture: Is perfect in one and good in another direction forming nearly right-angled prisms/conchoidal to uneven fracture – brittle.
• Colour varieties: Red andesine, up to 33.42 cts (very rarely green or yellow) from Democratic Republic of Congo. Irregular colour distribution has been noted.
• Inclusions: Lath-like groups of minute reflective inclusions (presumed to be copper by analogy to Oregon sunstone) producing schiller observed in red andesine from the Democratic Republic of Congo.
Twin planes are also reported
• Fluorescence: SWUV – blue, pink, yellow, yellowish brown. Weak red emission with an even weaker blue surface related luminescence (SWUV) has been reported for red andesine from the Democratic Republic of Congo which under LWUV fluoresced a weak to medium orange
• Absorption spectra: Red andesine from the Democratic Republic of Congo showed increasing absorption across the visible spectrum from red to violet on which was superimposed a moderately broad band with a max at =565 nm attributed to minute, non light scattering, copper particles. Optical effects: Chatoyancy, Asterism, Colour Change, ADR Red and green andesine from the Democratic Republic of Congo has been reported as showing an ‘Alexandrite effect’, showing alternating colours depending on the light source. Green material from China (=An44Ab54Or2 on the basis of analyses on a single specimen) shows a similar effect, turning red and losing transparency under incandescent incident light. The loss of transparency appears to be associated with light scattering and the scattering may also be responsible for the colour change. No copper was detected in the specimens analysed by SEM-EDAX. There has been speculation as to where the red/green material actually comes from; one or other or both localities.