What is Kröhnkite?
Kröhnkite is a vivid blue hydrated copper sodium sulfate, a classic secondary mineral of arid copper deposits and a favourite for colour-focused collections.
Mineralogy and crystal structure
Reference formula: Na2Cu(SO4)2·2H2O. Kröhnkite has a monoclinic hydrated sulfate structure and commonly forms radiating sprays or crusts. Its colour comes from hydrated copper coordination, while dehydration can affect both appearance and stability. Kröhnkite is classified with sulfates and has a monoclinic crystal system. Typical hardness is 2.5–3 on the Mohs scale and specific gravity is approximately 2.9–3.0.
Colour, lustre and habit
Typical colours include blue, blue-green, green, colourless. The usual prismatic, radiating, crusty habit and vitreous, pearly lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Kröhnkite forms in or is associated with oxidised copper deposits, mine workings and sulfate-rich arid environments. Important documented localities include Chile, Australia, South Africa, Mexico and the United States.
A saturated blue that earns its place in a display
Kröhnkite brings immediate colour impact without sacrificing mineralogical depth. Blue sprays against a contrasting matrix make a memorable cabinet specimen, while the copper-sulfate chemistry explains why the colour is so intense.
Collecting and care
Keep dry and away from heat; avoid acids and protect fine sprays from touch.
