• Chemistry: KFe4(AsO4)(OH)4 - 7H2O, Hydrated Potassium Iron Arsenate Hydroxide.
  • Class: Phosphates
  • Subclass: Arsenates
  • Group: Pharmacosiderite.
  • Uses: Only as mineral specimens.
  • Specimens

Pharmacosiderite is a rare secondary arsenate mineral. It typically forms in the oxidation zones of ore deposits. Pharmacosiderite forms from the alteration of arsenopyrite, tennantite and other primary arsenic minerals, and to a much rarer extent, it forms directly from precipitation of hydrothermal solutions.

It typically forms cubes as it is isometric in symmetry. This is rather unusual for an arsenate or even a phosphate class mineral. It can be confused with the popular and common mineral fluorite. But fluorite has perfect cleavage and is harder.

Pharmacosiderite in respect to its name can be confused with several other similarly named minerals. Pharmacolite is also an arsenate mineral and the two are sometimes associated, but is otherwise unrelated to pharmacosiderite. Picropharmacolite is likewise an arsenate but it also is not related, but by name. And finally the carbonate mineral siderite only shares iron as a common element in its formula to pharmacosiderite. Siderite is derived from the Greek word for iron. Pharmaco- is derived from the Greek word for poison in allusion to the arsenic content.

Pharmacosiderite lends its good name to a small mineral group called The Pharmacosiderite Group. The Pharmacosiderite Group is a set of closely related arsenates of which pharmacosiderite is the most common member. These are the most recognized members of the Pharmacosiderite Group and their respective formulas:

  • Alumopharmacosiderite KAl4(AsO4)(OH)4 - 6.5H2O
  • Pharmacosiderite KFe4(AsO4)(OH)4 - 7H2O
  • Sodium pharmacosiderite (Na, K)Fe4(AsO4)(OH)5 - 7H2O


  • Color is dark olive-green, yellow, brown or red.
  • Luster is adamantine or vitreous to greasy on fractured surfaces.
  • Transparency: Specimens are transparent to translucent.
  • Crystal System is isometric.
  • Crystal Habits include cubes and more rarely octahedrons, also granular and massive.
  • Cleavage is indistinct.
  • Fracture is conchoidal.
  • Hardness is 2.5
  • Specific Gravity is approximately 2.7 - 2.9 (average for translucent minerals)
  • Streak is white.
  • Other Characteristics: Crystals are often diagonally striated and are sectile.
  • Associated Minerals are barite, azurite, arthurite, olivenite, mixite, clinoclase, pharmacolite and limonite.
  • Notable Occurrences include the Carharrack Mine and a few other mines of Cornwall, England; St Leonard, Haute Vienne, France; Clara Mine, Black Forest, Germany and from the USA at Santa Cruz County, Arizona; Utah and South Dakota.
  • Best Field Indicators are crystal habits, color, associations, lack of cleavage and softness.
Some Colorful Members of the Colorful Phosphates Class


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