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Heulandite crystals. Heulandite is the collective name for a series of tectosilicate minerals in the zeolite group. Zeolites are hydrous calcium aluminium silicates, in which small amounts of sodium and potassium can replace part of the calcium. Specimen from Kilpatrick, Dumbartonshire, UK. (KEYSTONE/SCIENCE PHOTO LIBRARY/NATURAL HISTORY MUSEUM, LONDON/SCIENCE PHOTO LIBRARY)