Heulandite was first named in 1822 for the English mineral collector, J. H. Heuland (see Dana 1914, p. 574–576). Although it is typically found as macroscopic (commonly 0.2–2 cm in size) crystals in cavities in mafic igneous rocks and is volumetrically minor on the Earth’s surface, heulandite also occurs in larger amounts in some sedimentary deposits, often in association with clinoptilolite. Due to the large size of typical heulandite crystals, its chemical composition and crystallographic and optical properties were easily characterized. Mineralogists recognized early that heulandite is monoclinic and that heating changes its optical properties in a predictable manner...

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