Melanterite and Gypsum

About Melanterite

• Melanterite is a hydrated iron sulphate.
• It is formed through the oxidative weathering and decomposition of primary iron sulfide minerals, most commonly pyrite (FeS₂) or its unstable polymorph marcasite.
• It is often found in mines as a post-mining formation on mine walls.
• Properties:
  • Colour: It is colorless to white or green, also greenish-blue to blue with increased copper content and colourless to pale green in transmitted light.
    • Melanterite has conchoidal fracture and displays cleavage in two directions.
  • It is highly soluble in water.
  • Formula: Fe²⁺(H₂O)₆(SO₄) · H₂O
• Recent findings: Melanterite has been detected in Neyveli, Tamil Nadu.
  • It forms under temporary wet, acidic and oxidising conditions and provides evidence of short-lived water activity relevant to understanding Mars’ geological history.

About Gypsum

• Gypsum is a common sulfate mineral.
• It is found in large beds along with other evaporite minerals, especially in Permian and Triassic sedimentary formations.
  • It is also found in salt pans and saline lakes and is a key part of cap rock on salt domes.
• Properties:
  • Forms: It appears in various forms, including selenite (well-developed crystals), satin spar (fibrous, silky, and opalescent), alabaster (fine-grained and massive), and gypsite (earthy and pulverulent).
  • Composition: It  is composed of hydrated calcium sulfate.
  • Colour: It is colourless to white, often tinged other hues due to impurities and colourless in transmitted light.
  • Formulae: CaSO₄·2H₂O
• Applications: It is used as a fluxing agent, fertilizer, filler, and retarder in portland cement.
  • When calcined, it becomes plaster of Paris, used in building materials.
• Leading Producers: Its deposits occur in many countries, but Spain, Thailand, the United States, Turkey, and Russia are among the leading producers.
• Recent findings: Gypsum is one of the two hydrous sulphate minerals identified in the Atacama Desert in Chile and the Tiruchirappalli Badlands, a semi-arid geological and palaeoclimatic region in Tamil Nadu.
  • It records past water activity and can preserve potential biosignatures within its mineral structure, making it significant for astrobiological investigations.