Gypsum
Gypsum is a widely occurring hydrated calcium sulphate mineral with the chemical formula CaSO₄·2H₂O. It is one of the most abundant and versatile evaporite minerals on Earth, found in sedimentary, volcanic, and metamorphic environments. Gypsum is economically significant as a major industrial mineral used in cement, plaster, wallboard, fertilisers, and numerous other applications. Its softness, solubility, and crystallisation habits also make it a key indicator in sedimentary geology and environmental studies.
Known since antiquity, gypsum has been quarried for thousands of years for plaster and architectural purposes — its name originates from the Greek word gypsos, meaning “plaster.” Today, it remains an essential material in construction and agriculture, while also serving as an important mineralogical specimen due to its variety of crystal forms and transparency.
Composition and Structure
Gypsum is a hydrous calcium sulphate, chemically represented as CaSO₄·2H₂O. Its structure consists of Ca²⁺ cations, SO₄²⁻ tetrahedral anions, and water molecules. The calcium ions are coordinated by eight oxygen atoms — six from sulphate groups and two from water molecules — forming a layered crystal lattice bound by hydrogen bonds.
The structure’s layered arrangement contributes to gypsum’s:
- Perfect cleavage along one plane, producing thin flexible sheets.
- Softness, due to weak interlayer hydrogen bonds.
- Ability to lose water easily, transforming into other calcium sulphate phases.
Gypsum is polymorphous with anhydrite (CaSO₄) and bassanite (CaSO₄·0.5H₂O). Heating or dehydration converts gypsum to bassanite or anhydrite, and rehydration reverses the process. This reversible hydration–dehydration cycle underpins gypsum’s industrial use in plaster and cement production.
Varieties of Gypsum
Gypsum occurs in several distinct varieties differing in crystal habit, transparency, and fibrosity:
- Selenite – Transparent, colourless to white, well-formed crystals, often tabular or prismatic; named from the Greek selene (moon) for its soft, pearly lustre.
- Alabaster – Fine-grained, massive, and white variety used for sculpture and ornamental carvings.
- Satin spar – Fibrous, silky variety with pearly lustre and silky sheen.
- Desert rose – Rosette-shaped aggregates of tabular crystals encrusted with sand grains, typically found in arid regions.
All varieties share the same chemical composition but differ in crystal morphology and texture, resulting from varying formation environments.
Physical and Optical Properties
Gypsum’s physical characteristics are distinctive and diagnostic:
- Colour: Colourless to white, sometimes tinted grey, yellow, pink, or brown due to impurities.
- Streak: White.
- Lustre: Vitreous to silky, pearly on cleavage surfaces.
- Transparency: Transparent to translucent.
- Hardness: 1.5–2 on the Mohs scale (can be scratched by a fingernail).
- Specific gravity: 2.3–2.4.
- Cleavage: Perfect on {010}, distinct on {100}, giving thin flexible sheets.
- Fracture: Uneven to conchoidal.
- Tenacity: Flexible but not elastic.
- Crystal system: Monoclinic, typically forming tabular or prismatic crystals.
Optically, gypsum is biaxial (+) with refractive indices nα = 1.519, nβ = 1.522, and nγ = 1.530. It exhibits low birefringence (δ ≈ 0.011) and displays characteristic interference colours under polarised light microscopy.
Because of its softness, gypsum serves as the defining mineral for hardness 2 on the Mohs scale.
Formation and Geological Occurrence
Gypsum forms primarily through evaporative processes in sedimentary basins where seawater or saline lake water evaporates under arid conditions. It is commonly associated with halite (NaCl), anhydrite (CaSO₄), and dolomite (CaMg(CO₃)₂).
1. Evaporitic Environments
Gypsum precipitates when the concentration of dissolved calcium and sulphate ions in brine reaches saturation. The typical sequence of mineral precipitation in marine evaporites is:Calcite → Gypsum → Halite → Potassium and Magnesium salts.
Such deposits occur in restricted marine basins (sabkhas) and saline lakes, forming thick beds or lens-shaped masses.
2. Hydrothermal and Volcanic Environments
Gypsum can also form from hydrothermal fluids or volcanic fumaroles, where sulphur-bearing gases react with calcium-rich rocks or groundwater. In such environments, it appears as crusts, veins, or fibrous aggregates.
3. Secondary Formation
In arid regions, gypsum often precipitates as efflorescent crusts or cement in soils (gypsiferous soils). It may also form through the hydration of anhydrite in buried evaporite sequences.
4. Speleogenetic Settings
Gypsum may form in caves by oxidation of hydrogen sulphide to sulphuric acid, which then reacts with limestone:CaCO₃ + H₂SO₄ + 2H₂O → CaSO₄·2H₂O + CO₂.
This process produces elegant crystals, including large selenite crystals found in places such as the “Cave of the Crystals” (Naica, Mexico), where individual crystals can exceed 10 metres in length.
Global Distribution
Gypsum occurs worldwide, with major deposits on every continent. Important localities include:
- Naica, Chihuahua, Mexico: Home to the world’s largest selenite crystals.
- Paris Basin, France: Historical source of plaster (“Plaster of Paris”).
- New South Wales and South Australia: Significant evaporite formations.
- Nova Scotia and Ontario, Canada: Extensive bedded gypsum deposits.
- United States: Large producers include Oklahoma, Iowa, Texas, and Michigan.
- Iran, Spain, and China: Leading global producers of gypsum for industrial use.
Natural deposits may reach thicknesses of several hundred metres, often interbedded with halite and limestone.
Chemical Behaviour and Stability
Gypsum is moderately soluble in water, with a solubility of about 2.4 g/L at 25 °C. Its solubility increases slightly with temperature up to ~40 °C, then decreases. Dissolution and reprecipitation of gypsum play crucial roles in karst processes, sediment diagenesis, and soil chemistry.
The mineral dehydrates easily upon heating:
- Below 100 °C: Partial loss of water, forming bassanite (CaSO₄·0.5H₂O).
- Above 200 °C: Complete dehydration to anhydrite (CaSO₄).
This reversible dehydration–rehydration reaction underlies the use of gypsum in plaster: heating produces plaster of Paris, which sets into solid gypsum upon adding water:CaSO₄·0.5H₂O + 1.5H₂O → CaSO₄·2H₂O.
In soils, gypsum dissolves readily, providing a source of calcium and sulphate ions, which improve soil structure and nutrient balance.
Industrial and Economic Importance
Gypsum is one of the most important industrial minerals, with extensive uses across multiple sectors.
1. Construction and Building Materials: Gypsum is used to manufacture:
- Plaster of Paris: For mouldings, casting, and sculptures.
- Gypsum plasterboard (drywall): Lightweight construction material for walls and ceilings.
- Cement additive: Enhances setting control and workability.
- Decorative plaster and stucco: Used for finishing and coating.
2. Agriculture: Gypsum serves as a soil conditioner and fertiliser, improving soil structure, reducing compaction, and supplying calcium and sulphur nutrients. It helps reclaim alkaline and sodic soils by replacing sodium ions with calcium.
3. Industrial Uses:
- Manufacture of ammonium sulphate fertiliser.
- Casting and moulding for ceramics and dental applications.
- Filler in paints, paper, and polymers.
- Flux in the glass and steel industries.
4. Energy and Environmental Applications: Synthetic gypsum is produced as a by-product of flue-gas desulphurisation (FGD) in coal-fired power plants. This FGD gypsum has identical chemical composition and is increasingly used as an environmentally sustainable alternative to natural gypsum.
Global production of gypsum exceeds 250 million tonnes per year, with the construction industry accounting for more than 75% of consumption.
Environmental and Geological Significance
Gypsum plays a vital role in sedimentary and environmental geology:
- Evaporite marker: Indicates arid climatic conditions and restricted marine basins.
- Diagenetic indicator: Records fluid migration and chemical evolution in sedimentary sequences.
- Karst processes: Its dissolution forms gypsum caves and sinkholes.
- Climate archives: Stable isotope composition (δ³⁴S, δ¹⁸O) of gypsum records ancient seawater and evaporative environments.
In soils, gypsum contributes to desalination and reclamation of sodic lands, enhancing agricultural productivity in arid and semi-arid regions.
Health, Safety, and Environmental Considerations
Gypsum is non-toxic and environmentally benign under normal conditions. It poses minimal health risks, though prolonged inhalation of fine gypsum dust may cause mild respiratory irritation. Unlike crystalline silica, gypsum dust is not carcinogenic.
Mining and quarrying can have localised environmental impacts such as landscape alteration and dust generation, but these are mitigated through reclamation practices and water management. Synthetic gypsum from FGD processes offers a sustainable alternative, reducing waste and supporting circular industrial use.
Crystallography and Mineral Associations
Gypsum crystals commonly exhibit forms {010}, {111}, {120}, and {011}. Twinning on {100} is frequent, producing swallowtail or fishtail twins, particularly in selenite crystals.
It occurs in association with:
- Halite (NaCl)
- Anhydrite (CaSO₄)
- Calcite (CaCO₃)
- Dolomite (CaMg(CO₃)₂)
- Sulphur (S)
- Aragonite and celestine (SrSO₄)
These minerals together form the typical assemblage of evaporite sequences.
Research and Technological Relevance
Modern research on gypsum spans multiple disciplines:
- Geochemistry: Stable isotope studies to trace palaeoclimatic conditions.
- Material science: Development of high-strength plasters and lightweight composites.
- Environmental engineering: Use of gypsum for carbon sequestration, acid mine drainage neutralisation, and phosphate removal from wastewater.
- Planetary geology: Detection of gypsum on Mars by the Mars Reconnaissance Orbiter confirms past water activity on the planet’s surface.
These advances demonstrate gypsum’s continued scientific significance beyond its traditional industrial applications.