Adularia — The Alpine Feldspar Behind Moonstone's Inner Light
Adularia — The Alpine Feldspar Behind Moonstone's Inner Light
A low-temperature variety of orthoclase, named for the Adula massif of the Swiss-Italian Alps
Adularia is a low-temperature variety of orthoclase feldspar, distinguished from common orthoclase by its formation conditions, its crystal habit, and the optical character of its finest specimens. The name honours the type locality on the Adula massif, the cluster of high peaks straddling the border of the Swiss canton of Graubünden and the Italian region of Lombardy, where alpine cleft mineralogy has yielded textbook specimens since the eighteenth century. Adularia matters to gemmology principally because its finely twinned, lamellar interleaving with albite produces the floating blue-white sheen known as adularescence — the optical phenomenon at the heart of fine moonstone.
Mineralogy
Adularia has the composition of orthoclase, KAlSi3O8, but crystallises at relatively low temperatures, typically below about 400 degrees Celsius, in hydrothermal alpine fissures. The low-temperature regime is the cause of the variety's distinctive habit: prismatic to pseudo-rhombohedral crystals with simple, often pristine forms, frequently colourless to milky white and transparent enough to function as facet rough or as a host for adularescence in cabochon. Mature adularia transitions structurally toward microcline as it cools further; the microstructural detail of these transitions is the subject of a substantial mineralogical literature.
The species is monoclinic, with a hardness of 6 to 6.5 on the Mohs scale, a specific gravity around 2.55 to 2.63, and refractive indices of approximately 1.518 to 1.526. Cleavage is good in two directions at near-right angles, a property that any cutter working the material must respect.
Adularescence
The optical phenomenon called adularescence — the moving, billowy, blue-to-white sheen seen in fine moonstone — arises from light scattering at the boundaries of submicroscopic lamellae of orthoclase and sodium-rich albite. As the host alkali feldspar cools through subsolvus temperatures, the originally homogeneous K-Na composition unmixes by diffusion into alternating lamellae of K-rich and Na-rich phases. Where the lamellar spacing is in the right range relative to visible-light wavelengths, the boundaries scatter shorter wavelengths preferentially, producing the cool blue floating sheen that defines fine moonstone.
Adularia from alpine clefts can show adularescence directly, but the most commercially significant moonstone — the blue sheen Sri Lankan and Indian material — is technically a fine sanidine-orthoclase intergrowth rather than pure adularia. Trade usage of adularia as a synonym for moonstone is therefore loose and often imprecise. In strict mineralogical usage, adularia denotes a low-temperature alpine orthoclase; in jewellery usage, the name is sometimes applied more broadly to any moonstone-bearing orthoclase.
Type locality and notable sources
Classic adularia comes from alpine fissures in the Adula and Gotthard massifs of Switzerland, the Penninic Alps of northern Italy, and equivalent terrains in Austria and France. These specimens, often associated with quartz, chlorite, and alpine cleft minerals such as titanite, are highly prized by mineral collectors. They are most often encountered as crystal specimens rather than cut stones.
Material with strong adularescence used in the cutting trade comes principally from Sri Lanka, southern India, and Myanmar, with significant production also from Madagascar, Tanzania, and the United States. These deposits are typically pegmatitic or metamorphic rather than alpine-cleft, and the strict mineralogical name is closer to orthoclase moonstone than to adularia in the type-locality sense.
Cutting and care
For cabochon work, the cutter orients the rough so that the lamellar planes lie roughly parallel to the base of the cabochon and the curved dome maximises the play of the sheen. Faceted adularia is rarer in commercial work but can be very beautiful in clean alpine material. Cleavage demands careful tool selection and conservative dop-blocks; thermal shock should be avoided.
Hardness at 6 to 6.5 makes adularia and orthoclase moonstone a soft stone for daily-wear ring use. We typically set fine moonstone in pendants, earrings, and protected ring designs with bezel rather than prong settings. Cleansing should be by mild soap and warm water; ultrasonic and steam cleaning are not recommended because of cleavage.
In the trade
Buyers seeking fine moonstone should think in terms of adularescence quality rather than crystallographic name. The most desirable stones show a clean, electric-blue sheen on a transparent colourless body, with the sheen visible from a wide range of viewing angles. Translucent white moonstone with weaker silvery sheen is much more common and commands a fraction of the price of fine blue-sheen material. Mineralogical adularia from alpine clefts trades primarily in the specimen market and rarely as cut stones.