Hemimorphite
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Hemimorphite from the 79 Mine, Eureka, Pinal County, Arizona, U.S.A., measuring 80 × 53 × 41 mm and weighing 147 grams. A solid cabinet piece of a species Arizona's secondary-mineral deposits are particularly well known for.
Hemimorphite is zinc silicate, Zn4Si2O7(OH)2·H2O, named for its hemimorphic crystal form, in which the two ends of each crystal develop differently rather than mirroring one another. It typically forms as fans, crusts or bladed sprays of bluish-white to colourless crystals in the oxidised zone of zinc deposits, where weathering of primary Sphalerite releases zinc that later recombines as silicate. Arizona's many skarn- and carbonate-hosted base-metal deposits have long produced oxidised zones rich in this kind of secondary zinc mineralisation, and the state remains one of the most important sources of fine Hemimorphite specimens in the world.
A well-developed fan of Hemimorphite at this size is a satisfying piece for anyone building a suite of American secondary minerals, and it carries the added interest of a named Arizona mine on its label. This piece is from the David Vince Collection.