The mineral that swells with water and can destroy entire city foundations
In short: Anhydrite is a rock-forming mineral that lacks water, making it the chemical opposite of gypsum. When exposed to moisture, it undergoes a dramatic transformation, absorbing water and expanding by up to 50 percent. This silent, subterranean swelling has caused catastrophic structural damage to buildings and city infrastructure worldwide.
Anhydrite, or anhydrous calcium sulfate, is a mineral defined by its lack of water of crystallization. While it is chemically similar to gypsum, the two exist in a constant, reversible dance dictated by environmental conditions. In humid settings or near the Earth's surface, anhydrite readily absorbs water to become gypsum. This process is not merely a chemical shift; it is a physical expansion. As anhydrite hydrates, its volume increases by 30 to 50 percent, a force capable of causing intense, tight folding in rock layers or, in human-built environments, devastating structural failure.
The consequences of this expansion were starkly illustrated in the German city of Staufen im Breisgau. Following a 2007 geothermal drilling project, subsurface water was inadvertently introduced to a deep layer of anhydrite beneath the city. As the mineral converted to gypsum, the resulting ground swelling caused uneven, extensive damage to buildings across the urban landscape. This phenomenon is a critical concern for geologists and engineers, as anhydrite is a common component of evaporite deposits and salt dome cap rocks, often lying hidden beneath the surface.
Beyond its destructive potential, anhydrite serves as a vital indicator of ancient environments. It is frequently found in salt basins and tidal flats, such as the Persian Gulf sabkhas, where it forms distinct nodular masses often described as chicken-wire anhydrite. While it is less commercially famous than its hydrated counterpart, gypsum—which is widely used in plaster, cement, and drywall—anhydrite remains a significant geological marker. Whether found in the cap rock of Texas salt domes or as rare crystals in igneous rocks, it stands as a reminder of the volatile, shifting nature of the Earth's crust.
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Sources
- Summary by Intellect · source: Anhydrite | Sulfate Mineral, Gypsum Substitute, Sedimentary Rock | Britannica
- Also: Anhydrite
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