heulandite

{{Infobox mineral | name = Heulandite | boxwidth = | image = Heulandite-160733.jpg | imagesize = 280px | alt = | caption = | category = Tectosilicate minerals | group = Zeolite group | formula = {{chem2|(Na,Ca,K)_{5-6}[Al_{8-9}Si_{27-28}O72]*nH2O}} | IMAsymbol = Hul | molweight = | strunz = 9.GE.05 | dana = | system = Monoclinic More than one space group | class = Prismatic (2/m) (same H-M symbol) | unit cell = | colour = colourless, yellow, green, white, pale pink | color = | habit = tabular, parallel aggregates | twinning = | cleavage = perfect basal | fracture = | tenacity = | mohs = 3.5–4 | lustre = pearly, vitreous | streak = white | diaphaneity = transparent to translucent | gravity = | density = | polish = | opticalprop = | refractive = | birefringence = | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | prop1 = | prop1text = | references = }}

Heulandite is the name of a series of tectosilicate minerals of the zeolite group. Prior to 1997, heulandite was recognized as a mineral species, but a reclassification in 1997 by the International Mineralogical Association changed it to a series name, with the mineral species being named: Heulandite-Ca Heulandite-Na Heulandite-K Heulandite-Sr Heulandite-Ba (described in 2002). Heulandite-Ca, the most common of these, is a hydrous calcium and aluminium silicate with the formula . Small amounts of sodium and potassium are usually present replacing part of the calcium. Strontium replaces calcium in the heulandite-Sr variety. The appropriate species name depends on the dominant element. The species are visually indistinguishable, and the series name heulandite is still used whenever testing has not been performed.