Carbonates & Sulfates
Gypsum
What it is
Hydrated calcium sulfate, one of the softest common minerals (Mohs 2). Selenite is the clear, crystalline form — named after the Greek moon goddess Selene for its pearly lunar glow. The Naica mine in Mexico contains selenite crystals over 11 meters long.
How it forms
Precipitates from evaporating seawater and salt lakes. Selenite crystals grow in water-filled caves where the temperature and chemistry remain stable for thousands of years.
CaSO₄·2H₂O
