Heart-Shaped Celestite Crystal Specimen from Madagascar
Celestine or celestite is a mineral consisting of strontium sulfate (SrSO4). The mineral is named for its occasional delicate blue color. Celestine and the carbonate mineral strontianite are the principal sources of the elementstrontium, commonly used in fireworks and in various metal alloys.
Celestine geodes are understood to form by replacement of alabaster nodules consisting of the calcium sulfates gypsum or anhydrite. Calcium sulfate is highly soluble, but strontium sulfate is mostly insoluble. Strontium-bearing solutions that come into contact with calcium sulfate nodules dissolve the calcium away, leaving a cavity. The strontium is immediately precipitated as celestine, with the crystals growing into the newly-formed cavity.