Gypsum Mineral Overview

Mineral name: Gypsum
Mineral class: Sulfate

Known Facts

• Chemical formula: CaSO₄·2H₂O
• Crystal system: Monoclinic
• Mohs hardness: 2
• Color range: Colorless, white, gray, yellow, pink, brown
• Luster: Vitreous to silky (fibrous varieties)
• Streak: White
• Cleavage: Perfect in one direction, good in others
• Fracture: Uneven to splintery
• Specific gravity: Approximately 2.3
• Common locations: Worldwide (evaporite deposits)
• Uses: Plaster, drywall, cement, fertilizer, ornamental stone
• Similar minerals: Anhydrite, calcite, halite

Physical and Optical Characteristics

Gypsum is a very soft and lightweight mineral, easily scratched with a fingernail (hardness 2). It commonly forms:

• Tabular or prismatic crystals
• Massive or granular deposits
• Fibrous or silky aggregates

Several well-known varieties exist:

• Selenite: Transparent, well-formed crystals
• Satin spar: Fibrous with a silky sheen
• Alabaster: Fine-grained, massive form used for carving

Color varies widely depending on impurities, though pure gypsum is colorless or white.

Its perfect cleavage allows it to split into thin sheets, especially in selenite crystals.

Chemical Composition and Structure

Gypsum is a hydrated calcium sulfate, with the formula CaSO₄·2H₂O.

Its structure includes:

• Calcium (Ca²⁺) ions
• Sulfate (SO₄²⁻) groups
• Water molecules (H₂O) incorporated into the crystal lattice

The presence of water is essential to its structure. When heated, gypsum can lose water and transform into:

• Plaster of Paris (CaSO₄·½H₂O)
• Eventually anhydrite (CaSO₄) if fully dehydrated

This dehydration and rehydration behavior is key to many of its uses.

Formation and Geological Occurrence

Gypsum forms primarily in evaporite environments, where water evaporates and dissolved minerals crystallize.

Common formation settings include:

• Salt lakes and inland seas
• Marine evaporite basins
• Lagoonal environments in arid climates

It may also form through:

• Hydration of anhydrite
• Secondary processes in soils and sediments

Associated minerals often include:

• Halite
• Anhydrite
• Calcite

In some environments, gypsum crystals can grow to very large sizes, especially in stable, water-rich conditions.

Common Locations

Gypsum is found worldwide, with major deposits in:

• United States: Oklahoma, Texas, New Mexico, Michigan
• Spain: Notable for giant selenite crystals (e.g., Naica region)
• Mexico: Cave of the Crystals (Naica Mine)
• Canada: Nova Scotia and Ontario
• Iran and China: Extensive evaporite deposits

It is one of the most abundant sulfate minerals.

Uses and Practical Significance

Gypsum is widely used in industry and construction:

• Plaster and drywall: Primary component of wallboard
• Cement production: Controls setting time
• Plaster of Paris: Used in casting and medical applications
• Agriculture: Soil conditioner and calcium source
• Sculpture and carving: Especially alabaster

Its ability to lose and regain water makes it especially valuable in manufacturing.

Similar and Related Minerals

Gypsum can be confused with several minerals:

• Anhydrite: Similar composition but lacks water and is harder
• Calcite: Harder and reacts with acid
• Halite: Similar softness but has cubic cleavage and salty taste

Key differences include:

• Gypsum’s softness and flexibility (in thin sheets)
• Presence of water in its structure

Identification Notes for Collectors

• Easily scratched with a fingernail
• Look for tabular crystals or fibrous forms
• Check for perfect cleavage and flexible thin sheets (selenite)
• Note association with evaporite deposits
• Typically does not react with acid, unlike calcite

Gypsum is one of the most recognizable evaporite minerals, valued for its variety of forms, softness, and wide range of practical uses.