Adularia
Potassium aluminosilicate (KAlSi3O8)
A low-temperature potassium feldspar famous for forming transparent Alpine crystals and the gem moonstone, which shows a floating blue sheen called adularescence.
- Mohs hardness
- 6-6.5
- Color
- Colorless to milky white, sometimes faintly bluish
- Type
- mineral
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Overview
Adularia is a low-temperature variety of potassium feldspar (the orthoclase-microcline group) that crystallizes in clear, glassy, often rhombohedral-looking pseudo-orthorhombic crystals. It is named for the Adula massif in the Swiss Alps, where superb transparent crystals occur in fissure veins.
When adularia contains fine alternating layers of potassium feldspar and albite, light scattering from these layers produces a soft blue-white floating glow. This optical effect, called adularescence, gives the gemstone moonstone its name and shimmer.
Chemically it is essentially KAlSi3O8, but its ordered, low-temperature structure and characteristic habit set it apart from sanidine and most common orthoclase.
Formation & geology
Adularia forms at relatively low temperatures from hydrothermal fluids, most famously in the open fissures and Alpine clefts of metamorphic terranes. As mineral-bearing water circulates through cracks in gneiss and schist, it deposits clear adularia crystals along with quartz, chlorite, and titanite.
It also forms in low-temperature hydrothermal ore veins, where it is an indicator mineral for epithermal gold-silver deposits. Classic sources include the Swiss and Austrian Alps, the Adula and St. Gotthard regions, and adularescent moonstone gravels in Sri Lanka and southern India.
How to identify it
Adularia is colorless to milky white with a vitreous luster, hardness 6-6.5, and a white streak. Crystals are often pseudo-rhombic and glassy, and gem material shows the diagnostic blue or white adularescent sheen that drifts across the stone as it is tilted.
It can be confused with quartz, but feldspar has two good cleavages meeting near 90 degrees, whereas quartz has no cleavage. The schiller distinguishes moonstone from clear quartz or chalcedony, and its lower hardness separates it from quartz (7).
Uses & significance
Adularia's chief importance is as the source of gem moonstone, one of the most popular phenomenal gemstones, cut as cabochons for rings, pendants, and beads. Fine blue-sheen moonstone from Sri Lanka is especially valued.
In economic geology, adularia is a useful pathfinder mineral for epithermal precious-metal veins. Metaphysically, moonstone is linked to intuition, the feminine, and new beginnings, making it a staple of crystal jewelry, though these properties are spiritual rather than scientific.
Frequently asked questions
Is adularia the same as moonstone?
Moonstone is gem-quality adularia (or closely related feldspar) that displays adularescence - the floating blue or white sheen. All gem moonstone of this type is adularia, but not all adularia is gem moonstone.
What causes the glow in adularia moonstone?
Microscopic alternating layers of potassium feldspar and albite scatter light, producing the soft, floating blue-white shimmer called adularescence.
How hard is adularia?
About 6 to 6.5 on the Mohs scale, so it scratches glass but is softer than quartz and can be scratched by quartz or topaz.
Where is the best moonstone found?
Sri Lanka and southern India are the classic sources of blue-sheen gem moonstone, while transparent crystal adularia is famous from the Swiss Alps.
Adularia guides
In-depth guides for identifying, valuing, and understanding Adularia.
