Formation
(1) Hydrothermal, through precipitation from low-temperature (<200°C), weakly alkaline aqueous solutions in cavities and fissures of mafic magmatites (volcanic structures).
(2) Diagenetic, through the transformation of volcaniclastic sediments or zeolite facies metamorphism under moderate pressure and low temperatures.
(3) Autigenic, as cement in sandstones through the decomposition of unstable plagioclase or volcanic glasses.
