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 (Slawson 1925). Unlike heulandite, the mineral known today as clinoptilolite typically occurs as microscopic crystals, commonly 2–20 μm in size and commonly intimately admixed with other fine-grained minerals. Although clinoptilolite is known today as the most common natural zeolite, occurring in large amounts (millions of tons) in altered volcanic tuffs and saline, alkaline-lake deposits, it is generally mentioned only in passing in beginning mineralogy texts. Clinoptilolite was not described as a distinct mineral species until 1932 (Schaller 1932). Prior to that time, the platy material described by Pirsson (1890) in amygdules in weathered basalt from the Hoodoo Mountains, Wyoming, had been classified as “mordenite,” based primarily on its chemical similarity with mordenite from Nova Scotia. In spite of the chemical similarities, Pirsson did recognize the crystallographic similarity between the Wyoming “mordenite” and heulandite. Dana’s sixth edition (1914, p. 572–573) lists both ptilolite and mordenite as members of the mordenite group. Ptilolite (which we now recognize as mordenite) is listed by Dana as commonly occurring in aggregates of needles having parallel extinction, but “mordenite” is reported by Dana to be monoclinic, having a form closely approximating that of heulandite. Schaller (1932) proposed the name clino ptilolite based on the chemical …