scapolite

Scapolite is a mineral group of aluminum silicates that occurs in a solid-solution series between two endmembers: marialite, which is sodium-rich, and meionite, which is calcium-rich. The minerals share a similar framework structure built from silicon and aluminum tetrahedra, with sodium and/or calcium occupying interstitial sites and chlorine or carbonate varieties appearing in some compositions. As a result, scapolite minerals can exhibit a range of chemistries and colors.

Scapolite forms in metamorphic and hydrothermal settings, most commonly in contact metamorphosed carbonate rocks (skarns) and calc-silicate rocks. It also appears in alpine-type metamorphic terrains and in some igneous environments. Crystals are typically elongated and prismatic, and the group can occur as coarse, well-formed crystals in favorable conditions or as granular aggregates in rock. Associated minerals often include calcite, diopside, wollastonite, grossular, and garnet.

In hand specimens, scapolite is usually white to colorless, but impurities can impart pale yellows, pinks, or purples. It has a vitreous luster, and its hardness is commonly around 5.5 to 6 on the Mohs scale. Cleavage is imperfect in two directions. While not among the most widely used gemstones, transparent or translucent scapolite crystals have been cut as collector’s stones and, in some localities, as decorative gems. Scapolite is valued by mineral collectors for its crystal forms and its occurrence in distinctive skarn associations.