Anhydrite

Anhydrite is a calcium sulfate mineral with the chemical formula CaSO₄, closely related to gypsum but distinct in lacking water molecules in its structure. Its name is derived from the Greek anhydros, meaning “without water,” reflecting its anhydrous composition. Found in evaporite deposits, hydrothermal veins, and metamorphosed limestones, anhydrite is an important mineral both geologically and industrially. It plays a key role in sedimentary basin evolution, salt dome formation, and serves as a source of calcium and sulfur in various applications.

Chemical Composition and Crystal Structure

Anhydrite belongs to the sulfate mineral class, chemically expressed as CaSO₄, consisting of calcium, sulfur, and oxygen. It is the anhydrous form of calcium sulfate, whereas its hydrated counterpart, gypsum (CaSO₄·2H₂O), contains two water molecules per formula unit. The two minerals are often found together, as anhydrite can hydrate to gypsum under suitable conditions of temperature and humidity.
Anhydrite crystallises in the orthorhombic system, typically forming tabular, prismatic, or granular crystals. Its space group is Pnma, and the structure is composed of layers of calcium cations (Ca²⁺) and sulfate tetrahedra (SO₄²⁻). The arrangement forms a compact and tightly bonded framework, giving the mineral its notable hardness and lack of cleavage parallel to bedding in many specimens.
When exposed to water, anhydrite gradually transforms into gypsum through the reaction:
CaSO₄ + 2H₂O → CaSO₄·2H₂O
This hydration is accompanied by an increase in volume (approximately 60%), which has important implications in geology, engineering, and industry.

Physical and Optical Properties

Colour and AppearanceAnhydrite is typically colourless, white, or grey, though impurities can impart hues of blue, violet, pink, or reddish tones. Massive deposits often exhibit a banded or mottled appearance resembling marble, while transparent crystals are rare and highly prized by collectors.
Lustre and TransparencyThe mineral possesses a vitreous to pearly lustre on cleavage surfaces and may range from transparent to translucent or opaque, depending on grain size and purity.
Hardness and DensityAnhydrite has a Mohs hardness of 3 to 3.5, slightly harder than gypsum (2) but softer than calcite (3). Its specific gravity averages 2.9, higher than gypsum due to the absence of water in its structure.
Cleavage and FractureAnhydrite displays perfect cleavage in three directions at right angles, producing nearly cubic fragments. Its fracture is uneven to conchoidal, and cleavage surfaces often show pearly reflections.
Optical PropertiesOptically, anhydrite is biaxial (+), with refractive indices approximately α = 1.572, β = 1.575, γ = 1.614, and a birefringence of 0.042. It exhibits weak pleochroism and moderate relief under polarised light. Thin sections reveal characteristic interference colours, aiding in its identification under a petrographic microscope.
Other Properties

• Solubility: Slightly soluble in water, more soluble in warm acidic conditions.
• Reaction to Heat: When heated above 200 °C, gypsum loses water to form anhydrite, whereas at still higher temperatures (around 1180 °C) it decomposes to calcium oxide and sulfur trioxide.
• Taste: Anhydrite is occasionally described as having a bitter taste, though tasting minerals is not recommended.

Formation and Geological Occurrence

Anhydrite forms under evaporitic conditions where calcium and sulfate ions combine in saline waters that undergo extensive evaporation. It precipitates after the formation of more soluble minerals such as halite (NaCl), and before less soluble ones like gypsum, as part of the evaporite mineral sequence.
The typical order of precipitation in evaporitic basins is:

1. Carbonates (calcite, dolomite)
2. Gypsum (CaSO₄·2H₂O)
3. Anhydrite (CaSO₄)
4. Halite (NaCl)
5. Potassium and magnesium salts (e.g., sylvite, carnallite)

Anhydrite often occurs beneath layers of gypsum, as burial and compaction cause dehydration of gypsum at elevated temperatures and pressures. Conversely, uplift and exposure allow hydration back to gypsum.
Geological Settings:

1. Evaporite Basins: Large-scale deposits in marine or continental basins where evaporation exceeds inflow, leading to the precipitation of calcium sulfate.
2. Salt Domes and Diapirs: Deep-seated evaporites containing anhydrite may migrate plastically under pressure, forming domes. These are common in areas such as the Gulf Coast (USA) and North Sea.
3. Hydrothermal Veins: Anhydrite can precipitate from hot, sulfate-bearing hydrothermal fluids, often associated with metallic ores such as chalcopyrite, galena, and sphalerite.
4. Metamorphic Contexts: Contact metamorphism of gypsum-bearing rocks produces anhydrite, particularly in limestone or dolomite sequences.

Major Occurrences: Extensive anhydrite deposits are found in Germany, Poland, Canada, Italy, India, the United Kingdom, Mexico, and the United States (notably Texas, Michigan, and New York). In Europe, the Zechstein Formation of the Permian period hosts some of the most significant anhydrite-bearing evaporite sequences.

Relationship with Gypsum and Other Sulfates

Anhydrite and gypsum are dimorphs—minerals of identical chemical composition but different structures due to the presence or absence of water. The transition between them is one of the most significant mineralogical hydration-dehydration reactions in geology.

• Under dry, high-temperature conditions, gypsum dehydrates to anhydrite.
• Under humid or low-temperature conditions, anhydrite rehydrates to gypsum.

This reversible reaction is not merely a laboratory curiosity but a natural geological process, shaping features such as sinkholes, breccias, and hydration fissures in sedimentary basins.
Other related calcium sulfate minerals include:

• Bassanite (CaSO₄·0.5H₂O): an intermediate phase between gypsum and anhydrite.
• Celestine (SrSO₄) and barite (BaSO₄): similar in structure but containing strontium or barium instead of calcium.

Industrial and Economic Importance

1. Source of Calcium and SulfurAnhydrite serves as a raw material for the manufacture of sulfuric acid, cement, and fertilizer. When treated with acids, it releases sulfur dioxide or sulfate compounds used in chemical industries.
2. Construction and Cement IndustryAnhydrite is used as a retarder in Portland cement to control the setting time. Finely ground anhydrite is also employed as an alternative to gypsum in self-levelling floor screeds, plasters, and wallboards. Its lower water requirement and dimensional stability make it suitable for dry indoor applications.
3. Agricultural UseGround anhydrite, like gypsum, acts as a soil conditioner. It improves calcium content and reduces soil salinity, especially in alkali soils.
4. Petroleum and Gas ExplorationAnhydrite layers serve as impermeable cap rocks above salt domes and hydrocarbon reservoirs, effectively sealing oil and gas traps. Its high density and low permeability make it a critical component of petroleum geology.
5. Decorative StoneMassive, banded, and coloured varieties of anhydrite are sometimes polished for decorative use as “alabaster,” though true alabaster refers to fine-grained gypsum. The similarity in appearance has historically caused confusion between the two.

Engineering and Environmental Considerations

Anhydrite presents challenges in engineering and construction. When buried or exposed to moisture, its transformation into gypsum causes volumetric expansion, which can crack foundations or distort underground structures. This property is of special concern in tunnel engineering, mining, and dam construction in regions with evaporite strata.
To mitigate risks, geotechnical investigations identify anhydrite-bearing layers before construction. Controlled ventilation or moisture barriers are used to prevent hydration reactions.
From an environmental perspective, anhydrite’s reaction with acidic mine waters can lead to sulfate-rich drainage, contributing to acid mine drainage problems. However, in controlled contexts, it also helps neutralise acidic effluents and is used in waste treatment and flue-gas desulfurisation.

Scientific and Geological Significance

1. Indicator of Paleoenvironmental ConditionsAnhydrite formation provides clues to palaeoclimate and basin salinity. Its presence indicates arid environments with strong evaporation, making it a key proxy in reconstructing ancient depositional settings.
2. Role in DiagenesisIn sedimentary basins, anhydrite can replace carbonate minerals during diagenesis, forming pseudomorphs or nodules. This process can influence porosity and permeability in reservoir rocks.
3. Role in Tectonics and Salt MobilityAnhydrite’s mechanical behaviour under pressure affects the flow and deformation of salt layers. In diapiric systems, anhydrite can act as a competent, brittle layer, influencing the geometry of salt domes and related structures.
4. Astrobiological and Planetary RelevanceCalcium sulfates, including anhydrite, have been detected on Mars, indicating past aqueous environments and evaporative processes similar to those on Earth. Their stability on dry planets offers insight into extraterrestrial water history and potential habitability.

Advantages, Limitations, and Substitutions

Advantages:

• Readily available and abundant in sedimentary basins.
• Useful as a calcium and sulfate source in multiple industries.
• Functions as an effective seal in hydrocarbon traps.
• Stable under dry conditions and suitable for cement formulations.

Limitations:

• Hydrates to gypsum in humid conditions, leading to expansion and instability.
• Relatively soft, limiting its structural applications.
• Easily confused with gypsum or calcite, requiring careful identification.
• Can contribute to sulfate-rich groundwater and scaling in pipelines.

Modern industries often substitute synthetic anhydrite—produced from flue-gas desulfurisation or industrial by-products—for natural sources, ensuring chemical consistency and reducing environmental extraction impacts.