Anhydrite is a mineral made of calcium sulfate (CaSO₄). It forms naturally in sedimentary environments, particularly where seawater evaporates under arid conditions, and it’s closely related to gypsum but contains no water in its crystal structure. That distinction is actually where the name comes from: “anhydrite” means “without water.” It shows up in everything from ancient salt beds to modern construction materials.
Chemical Makeup and Physical Properties
Anhydrite belongs to the orthorhombic crystal system, meaning its crystals form in three unequal axes at right angles. It has a Mohs hardness of 3 to 3.5, roughly soft enough to scratch with a copper coin. Its density sits around 2.98 g/cm³, making it noticeably heavier than gypsum. The mineral breaks along three distinct planes of cleavage, sometimes producing blocky fragments that early mineralogists called “cube spar.”
Colors range from white and gray to bluish, pinkish, or even reddish-brown depending on trace impurities. Fresh specimens can appear glassy or pearly, though prolonged exposure to moisture gradually converts surface layers to gypsum, dulling the luster over time.
How Anhydrite Forms
Anhydrite is an evaporite mineral, which means it crystallizes when mineral-rich water evaporates faster than it’s replenished. The classic setting is a shallow basin that gets cut off from the open ocean during a drop in sea level. Under hot, dry conditions, the trapped seawater becomes increasingly concentrated with dissolved salts. Once the water is oversaturated with calcium sulfate, tiny crystals begin precipitating and settling on the basin floor in thin layers.
The Permian Castile Formation in West Texas is one of the best-known examples. Roughly 250 million years ago, a barred basin in that region produced thick sequences of layered evaporites, including substantial anhydrite deposits. Anhydrite also forms deeper underground when gypsum beds get buried. Rising temperature and pressure squeeze the water molecules out of gypsum’s crystal structure, converting it to anhydrite. This process creates the contorted, folded textures geologists sometimes find in these formations.
Anhydrite vs. Gypsum
Gypsum and anhydrite are both calcium sulfate minerals, but gypsum holds two water molecules in its crystal lattice (CaSO₄·2H₂O) while anhydrite holds none (CaSO₄). This difference matters because the two minerals can convert back and forth depending on conditions. When gypsum is heated or buried deep enough, it loses its water and becomes anhydrite. When anhydrite is exposed to moisture near the surface, it slowly absorbs water and transforms into gypsum, expanding in volume by up to 60% in the process.
That expansion is geologically significant. In underground settings, the swelling can deform surrounding rock layers. In engineering contexts, unexpected anhydrite-to-gypsum conversion has caused tunnel walls and building foundations to heave. The two minerals also look different in hand specimens: gypsum is softer (Mohs 2 vs. anhydrite’s 3 to 3.5), lighter, and often forms flat, tabular crystals. Anhydrite tends to be denser, harder, and breaks into more blocky shapes.
Industrial and Construction Uses
Anhydrite’s most widespread commercial application is in construction. It serves as a set controller in Portland cement manufacturing, helping regulate how quickly the cement hardens. It’s also calcined (heated at high temperatures) to produce anhydrite cement and plaster. Research has shown that heating waste gypsum, specifically phosphogypsum from fertilizer production, to around 1000°C creates a stable anhydrite cement with strength comparable to products made from natural gypsum. This recycling pathway is appealing because it requires less energy than producing traditional building materials.
In flooring, anhydrite-based screeds have become popular for their self-leveling properties. These liquid screeds flow into place with minimal manual work, producing a smooth, even surface. They also conduct heat well, which makes them a common choice for underfloor heating systems. The screed sits directly over the heating elements and distributes warmth more efficiently than some alternatives.
Beyond construction, anhydrite has been used as a soil stabilizer and as a source of calcium and sulfur in agricultural fertilizers, though these applications account for a smaller share of total use.
Named Varieties
Several varieties of anhydrite have earned their own names based on appearance or texture. The most commercially familiar is angelite, a light blue-gray, semi-translucent variety found in Peru. Angelite is popular in the decorative stone market and is frequently carved into polished eggs, spheres, and figurines. Despite its soft, appealing color, it’s relatively fragile and sensitive to moisture, so it’s not suitable for jewelry that gets regular wear.
Other named forms are more geological curiosities. “Chicken-wire anhydrite” describes nodular masses that, in cross-section, look like a net of irregular polygons. “Tripe-stone” refers to contorted, concretionary masses with a layered, folded appearance. “Vulpinite,” from Volpino in northern Italy, is a scaly, granular variety that has historically been cut and polished for ornamental use.
Handling and Safety
Anhydrite is not classified as a hazardous substance, but its dust can irritate your skin, eyes, and respiratory system with repeated exposure. Workplace exposure limits for general dust are set at 10 mg/m³ over an eight-hour period, with a lower limit of 4 mg/m³ for the finer respirable fraction that reaches deep into the lungs. If you’re cutting, grinding, or sweeping anhydrite in any quantity, adequate ventilation and a dust mask with a particulate filter keep exposure manageable.
One practical detail worth knowing: wet calcium sulfate is slippery and, if left to dry, can harden into a concrete-like mass. Spills should be swept up mechanically while still dry rather than washed down with water.