Mohr's Salt

Formula: (NH₄)₂Fe(SO₄)₂·6H₂O Double sulfate of divalent iron and ammonium. Naturally occuring, it is known as mineral mohrite.

Properties

• Crystal system: monoclynic
• Crystal shape: from pure solution, flat tabular crystals are growing. If solution is conpaminated with Fe³⁺ ions, then crystals becoming more elongated.
• Color: green
• Stability on air: stable, a rare example of stable Iron (II) compound.

Preparation

Dissolve equimolar amounts of ammonium sulfate ((NH₄)₂SO₄) and iron (II) sulfate (FeSO₄·7H₂O) in water with small amount of sulfuric acid, then crystallize.

Growing

I used regular evaporation method. On air, solution is prone to slow oxidation, addition of small amount of sulfuric acid suppresses it.

Safety

The compound is generally safe, though can ittirate skin.

More photos

References

• My blog
• Wikipedia
• Mindat: mohrite
• Links in Russian:
  • Crystal growing site (much better specimens than mine...)
  • Youtube video