Gypsum is a common sulfate mineral (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)). This soft, non-metallic mineral forms primarily in sedimentary settings when saline water evaporates. Gypsum is one of the most widely used industrial minerals in the world. Although it is a single chemical compound, its appearance is variable, ranging from transparent to opaque and from colorless to various hues.
The Defining Color of Pure Gypsum
The intrinsic color of chemically pure gypsum is colorless, appearing clear and transparent to translucent. This transparent form often occurs as large, well-developed crystals. The lack of color means light passes through the mineral without being significantly absorbed or scattered.
However, massive deposits of gypsum are often perceived as white or gray. This white appearance results from physical factors, not intrinsic mineral color. Microscopic internal fractures, air pockets, or water inclusions within the crystal structure scatter light, which the human eye perceives as white.
How Impurities Create Color Variations
The broad spectrum of colors found in natural gypsum results from trace elements and foreign materials incorporated during its formation. These impurities are often trapped within the crystal lattice or the surrounding matrix as the mineral precipitates. The geological environment dictates which elements are included, leading to a wide range of shades.
Iron oxides are responsible for warm tones, staining the gypsum yellow, orange, pink, or reddish-brown. The presence of clay or fine-grained silicates can cause the gypsum to appear gray or brown. Black or dark gray coloration is usually caused by inclusions of organic matter or sulfides.
Common Forms and Their Appearance
Gypsum occurs in several distinct varieties defined by their crystal habit. Selenite refers to the transparent, often colorless, crystalline form. These crystals are exceptionally clear, allowing light to pass through them unimpeded.
Alabaster is a fine-grained, massive form historically valued for ornamental carving. It is often white or pale-hued due to dispersed impurities, which give it a soft, translucent quality. Satin Spar is a fibrous variety recognized by its silky or pearly luster, typically appearing white or pale yellow. This unique sheen is caused by light reflecting off the parallel crystal fibers.
Practical Applications of Gypsum
The majority of gypsum extracted globally is utilized in the construction industry. Its primary use is in the manufacture of wallboard (drywall) and plaster of Paris. Gypsum is also incorporated into Portland cement, where it acts as a retarder to control the setting time and prevent the cement from hardening too quickly.
For most industrial applications, the mineral’s chemical purity and physical state are more important than its color. In agriculture, gypsum is applied as a soil amendment to improve soil structure and water penetration, and to supply plants with calcium and sulfur. While colored varieties are sometimes used for decorative items, the white or gray massive forms are most relevant for large-scale construction.