Hemimorphite

Hemimorphite from the 79 Mine, Eureka, Pinal County, Arizona, U.S.A., measuring 85 × 64 × 42 mm and weighing 247 grams. A good cabinet piece from what is widely regarded as one of the finest Hemimorphite localities in the world.

Hemimorphite is a zinc silicate, Zn4Si2O7(OH)2·H2O, named for its hemimorphic crystal form, meaning the two ends of its crystals develop differently rather than mirroring each other. It typically occurs as fans or crusts of bladed, often bluish to colourless crystals, forming in the oxidised zone of zinc deposits where weathering of primary Sphalerite releases zinc that later recombines as silicate and carbonate minerals. The 79 Mine's orebody, a skarn-hosted copper-lead-zinc deposit, developed an exceptionally well-mineralised oxidation zone that has made it a benchmark locality for Hemimorphite among American collectors.

The 79 Mine takes its name from a 1879 Independence Day claim and has produced fine specimens of Hemimorphite, Wulfenite, Mimetite and other secondary minerals since serious collector interest in the deposit developed last century. This piece carries a double provenance, the David Vince Collection and the Ruth Coulsell Collection, reflecting the kind of specimen pedigree that Arizona material from a classic locality like this tends to accumulate.