Heulandite is a series of tectosilicate minerals of the zeolite group, which includes following species: Heulandite-Ca (most common, (Ca,Na)2-3Al3(Al,Si)2Si13O36·12H2O), Heulandite-Na, Heulandite-K, Heulandite-Sr, Heulandite-Ba.
Stage 5 (>>3.0Ga)
Prominent features of Stage 5 are large-scale plate tectonics and associated crustal/hydrothermal reworking. At this time, Heulandite formed from metamorphosing of low-temperature hydrated minerals created by shallow hydrothermal alterations at near surface. With increasing depth, it crystallize in a cavity in the order: heulandite-mordenite-quartz → stilbite → resolute-scolecite → thomsonite → chabazite.
Modern Environment
Calcium-rich high-silica zeolites, heulandite occurs worldwide as major constituents of submarine volcanic sediments and as alteration products of volcanic glass in low-grade meta-volcanics and metaclastics. From rhyolitic glass heulandite formed especially in solutions of low calcium activity at temperatures between 150-250 degree Celsius. At higher temperature, heulandite is only a transitory mineral because of its high H2O content. It's also formed in the progressive sequence of diagenesis and zeolite facies metamorphism.Natural heulandite exhibits continuous solid solution between heulandite and clinoptilolite, but the Si/Al and Ca/(Na + K) ratios of different samples vary significantly.
The processes of formation of heulandite are similar between ancient times and present - metamorphism of basaltic/rhyolitic volcanic rocks at relatively low temperature is the primary mechanism.