Clinoptilolite is a natural zeolite comprising a microporous arrangement of silica and alumina tetrahedra. These zeolite crystals form when volcanic molten lava meets the sea. This chemical reaction creates a crystal with a cage-like, porous structure and negative charge, making it one of the rare, negatively-charged minerals in nature. Engineers have been exploiting this feature, using zeolites as filters for heavy metals which typically have a positive charge. Clinoptilolite, in particular, is a great zeolite for filtration applications. It was even used to cleanup radioactive spills at Chernobyl.
The photo above shows stunning orange crystals of clinoptilotite-Ca on a crust of calcite and clay minerals. The zeolite was mined in Las Palmas Province, Canary Islands, Spain.