Anhydrite
Anhydrite is a calcium sulfate mineral with the chemical formula CaSO4. It is closely related to gypsum (CaSO4·2H2O), but contains no water molecules in its crystal structure. The name "anhydrite" is derived from the Greek word "anhydros," meaning "waterless," referring to its lack of hydration compared to gypsum.
Properties and Identification:
- Chemical Formula: CaSO4
- Crystal System: Orthorhombic
- Color: Typically white, gray, or colorless; can also be bluish, reddish, or brownish due to impurities.
- Luster: Pearly to vitreous.
- Hardness: 3 to 3.5 on the Mohs scale.
- Cleavage: Perfect on {010} and {100}, good on {110}.
- Fracture: Uneven.
- Specific Gravity: 2.9 to 3.0.
- Streak: White.
- Distinguishing Features: Its hardness, cleavage, and lack of water content (compared to gypsum) are key identifiers. Anhydrite feels slightly heavier than gypsum due to its higher specific gravity.
Occurrence and Formation:
Anhydrite typically forms in evaporite deposits, in environments where saline water evaporates, concentrating dissolved salts. It is often found in sedimentary basins associated with gypsum, halite (rock salt), and other evaporite minerals. It can also occur as a hydrothermal vein mineral.
Anhydrite is unstable in the presence of water at surface temperatures and pressures. Over geological time, it can slowly hydrate and transform into gypsum, a process that involves an increase in volume. This expansion can cause significant stress in surrounding rock, potentially leading to fracturing and ground deformation.
Uses:
Anhydrite has several industrial uses, including:
- Cement production: As a source of sulfate for regulating the setting time of cement.
- Soil conditioning: As a source of calcium and sulfur for improving soil fertility.
- Desiccant: Due to its ability to absorb moisture (though much less effectively than dedicated desiccants).
- Source of sulfur: Can be used as a source of sulfur in chemical processes.
- Ornamental stone: Sometimes used in carving and lapidary work, although its softness and cleavage make it less desirable than other materials.
Associated Minerals:
Commonly found in association with gypsum, halite, dolomite, calcite, celestite, and sulfur.
Varieties:
While distinct varieties are not formally recognized, variations in color and texture are common due to the presence of impurities.