Gypsum

Field of view: 5 mm. Platy gypsum. The platy transparent crystals are gypsum. The brown basement is a limonite crust.

Recovered from a meteoric oxidation zone of the Besshi Massive Sulfide deposit subjected to Epidote-Amphibolite facies metamorphism. Formed in less than 50 years after the mine was closed.

Gypsum from this location was described in 1947.

The formation process of this specimen is considered by the dissolution reaction of pyrite by iron bacteria in oxygen-containing meteoric water and gypsum deposition from sulfate solution. For example, sulfate ion is formed by the following reaction:

[Inorganic reaction]

2FeS₂ (Pyrite) + 7O₂ + 2H₂O = 2Fe²⁺ + 4SO₄^2- + 4H⁺

[Iron bacteria]

4Fe²⁺ + O₂ + 4H⁺ = 4Fe³⁺ + 2H₂O

[Inorganic reaction]

FeS₂ (Pyrite) + 14Fe³⁺ + 8H₂O = 15Fe²⁺ + 2SO₄^2- + 16H⁺

and Ca ion is formed by the following reaction:

CaCO₃ (Calcite) + CO₂ + H₂O = Ca²⁺ + 2HCO^3-

Reacting Ca iron with sulfate ion, gypsum deposits.

Ca²⁺ + SO₄^2- = CaSO₄ (Gypsum)