StilbiteCa

Stilbite-Ca is the calcium-rich endmember of the mineral stilbite, a member of the zeolite group within the broader family of aluminosilicates. In stilbite-Ca, calcium is the dominant extra-framework cation, with sodium and other cations present in smaller amounts. The mineral forms part of the zeolite framework, which consists of interconnected tetrahedra that create cages containing water molecules and enabling ion exchange.

Crystallography and appearance: Stilbite-Ca typically forms tabular to bladed crystals and radiating or fan-like aggregates. The color range is usually colorless to white, with pale pink, peach, or flesh tones in some varieties. It has a vitreous to pearly luster and a Mohs hardness of about 3.5 to 4. The crystal system is monoclinic, and the mineral commonly displays the characteristic zeolite habit of grouped crystals in cavities.

Occurrence and associations: Stilbite-Ca develops in low-temperature hydrothermal environments, especially in vesicles and geode-like cavities within volcanic rocks such as basalt and rhyolite. It is typically found in association with other zeolites, including natrolite, heulandite, mesolite, and other stilbite varieties. Localities with notable stilbite-Ca specimens are reported in several parts of the world, reflecting widespread but relatively delicate mineral formations.

Nomenclature and properties: Stilbite-Ca denotes the calcium-dominant endmember of the stilbite series. Its composition is variable, reflecting substitution among calcium and sodium and the presence of water in the zeolite framework. While it is valued by mineral collectors and researchers for its crystal forms and associations, stilbite-Ca is not used as an industrial ore.

See also: Stilbite, Zeolites, Mineralogy of zeolites.