Class : Silicates
    Subclass : Tectosilicates
    Crystal system : Triclinic
    Chemistry : (Na,Ca)(Al,Si)₄O₈
    Rarity : Ubiquitous

Oligoclase belongs to the group of calc-sodium feldspars (plagioclase), of which it represents the most sodic term after albite (composition between An10-Ab90 and An30-Ab70). It constitutes the accessory feldspar of acidic and intermediate rocks, present in granites, in addition to orthoclase, but also in syenites, monzonites or trachytes, sometimes in diorites. Its name comes from the Greek oligos (little) and klasis (broken), because its cleavages are less good than those of albite. Oligoclase is found exceptionally in crystals, and more often forms cleavable, translucent masses, grayish white to pale greenish in color. The crystals, always lamellar, generally show the polysynthetic twins typical of plagioclase. Sunstone is a variety used in jewelry whose shimmering reflections are due to microscopic inclusions or exsolutions of hematite. It is a feldspar sometimes used in the ceramic industry.

Main photo : Oligoclase from Norway, Maine, USA © Rock Currier

Generally presents the polysynthetic twin typical of plagioclase.

Sunstone is very often imitated by a synthetic brown glass containing copper flakes. The market more readily calls this poor imitation "goldstone", it is available in many colors : blue, green, pink, etc...