THE MINERAL MELANTERITE

  • Chemistry: FeSO4 - 7H2O, Hydrated Iron Sulfate.
  • Class: Sulfates
  • Group: Melanterite
  • Uses: A very minor ore of iron and as mineral specimens.
  • Specimens

Melanterite is one of only a few water soluble sulfate minerals. It forms in the near-surface secondary oxidation zone of ore deposits usually late in their development. In many mines, melanterite is an ongoing precipitate or efflorescent forming white to green encrustations, crystal aggregates and stalactites right on the sides of the mine's shafts. The primary source of the iron for melanterite is iron sulfides such as pyrite, pyrrhotite, marcasite and chalcopyrite.

A technique for removing copper from the copper sulfate mineral chalcanthite is responsible for the naming of an alternate name for melanterite. Chalcanthite, like melanterite, is soluble in water and it thus makes a solution of copper sulfate. If metallic iron is added to the solution, then metallic copper precipitates, leaving a solution of iron sulfate. This left-over solution has the same composition as a solution made from dissolving melanterite. The alternate name for melanterite is "copperas", from the Greek meaning "copper water", an allusion to the left-over solution. In a way, this could be thought of as "copper-providing water".

Attractive crystals of melanterite with a beautiful blue-green color are know to exist and are sought after. The shades toward blue come from impurities of copper which can substitute for as much as one third of the iron. The more copper, the bluer the crystals. Generally melanterite is known as having a white or green color.

Melanterite is also the name of a group of only five monoclinic sulfates of which melanterite is the only somewhat common member. Members of this group have the same basic structure as melanterite, but can have in place of iron, ions of manganese, zinc, cobalt and copper.

These are the members of the Melanterite Group:

  • Bieberite (Hydrated Cobalt Sulfate)
  • Boothite (Hydrated Copper Sulfate)
  • Mallardite (Hydrated Manganese Sulfate)
  • Melanterite (Hydrated Iron Sulfate)
  • Zinc-melanterite (Hydrated Zinc Copper Iron Sulfate)

PHYSICAL CHARACTERISTICS:

  • Color is white, green, yellowish green or blue-green.
  • Luster is vitreous to silky.
  • Transparency: Crystals are translucent to slightly transparent.
  • Crystal System is monoclinic; 2/m.
  • Crystal Habits include stubby prismatic or blocky to tabular crystals, sometimes as pseudo-octahedrons. Also acicular, fibrous and capillary and found as encrusting, stalactitic and concretionary masses.
  • Cleavage is perfect in one direct but only distinct in another.
  • Fracture is conchoidal.
  • Hardness is 2
  • Specific Gravity is approximately 1.9 (well below average).
  • Streak is white.
  • Other Characteristics: Is soluble in water and may deteriorate with absorption of water. The taste has a sweet, astringent and metallic character.
  • Associated Minerals are epsomite, chalcanthite, gypsum, pyrite, pyrrhotite, marcasite and chalcopyrite.
  • Notable Occurrences include Minas de Rio Tinto, Spain; Rammelsberg, Harz Mountains, Germany and Falun, Sweden; and in the United States at Ducktown, Tennessee; South Dakota; Colorado; Bigham Canyon, Utah; Comstock Lode, Lincoln County, Nevada; Butte, Montana; at several mines in Arizona and at The Geysers in Sonoma County and at Leona Heights, Alameda County, California.
  • Best Field Indicators are crystal habit, low density, associations, solubility in water, taste and color.
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