What is Melanterite?
Melanterite makes mineral weathering visible. Its delicate green crystals form where sulfide minerals meet oxygen and water, and their instability preserves the changing chemistry of a mine wall in miniature.
Mineralogy and crystal structure
Reference formula: FeSO4·7H2O. Melanterite is a soft, highly hydrated monoclinic iron sulfate. It can dehydrate, oxidise and change colour in air, so a fresh blue-green crust should be treated as a time-sensitive mineralogical record rather than a stable ornament. Melanterite is classified with sulfates and has a monoclinic crystal system. Typical hardness is 2–2.5 on the Mohs scale and specific gravity is approximately 1.9–2.0.
Colour, lustre and habit
Typical colours include pale green, blue-green, white, yellow. The usual prismatic, botryoidal, crusty habit and vitreous, pearly lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Melanterite forms in or is associated with oxidation of pyrite and other iron sulfides in mine workings, coal deposits and acid-sulfate environments. Important documented localities include the United States, Spain, Germany, Italy, the United Kingdom and Japan.
Fresh green chemistry from an oxidised mine
Melanterite makes mineral weathering visible. Its delicate green crystals form where sulfide minerals meet oxygen and water, and their instability preserves the changing chemistry of a mine wall in miniature.
Collecting and care
Keep cool, sealed from fluctuating humidity and away from water; avoid touching or washing crystals.
