Gypsum is a remarkably versatile mineral that has been used by humankind for thousands of years. This soft sulfate mineral is composed of calcium sulfate dihydrate and has the chemical formula CaSO4·2H2O. Gypsum can be found in crystalline form or in massive beds called gypsum rock, and it forms in lagoons as ocean waters high in calcium and sulfate evaporate and are replenished.
Gypsum is mined and extracted from quarries and underground mines. The gypsum ore is crushed, screened, and dried if needed. It is then further crushed and ground into a fine powder called plaster of Paris. When mixed with water, this powder forms a slurry that can be poured into molds or used to bind other materials. The many uses of gypsum include plaster, drywall, chalk, and agricultural applications.
With its inherent fire-resistant properties, gypsum has become the premier building material for walls, ceilings, and partitions. It is also widely used in sculptures and architectural details. In addition, gypsum has pharmaceutical applications and is added to some foods and health products. The variety of uses for gypsum throughout history demonstrates its versatility and importance as an industrial mineral.
Where Gypsum Forms
Gypsum is formed in lagoons and other coastal areas where ocean waters high in calcium and sulfate content can slowly evaporate and be regularly replenished with new sources of water. As the water evaporates, it leaves behind the calcium and sulfate ions which combine to create calcium sulfate dihydrate – gypsum. These gypsum beds can reach thicknesses of 10 meters or more.
Gypsum is also formed in hot springs. The water from hot springs is loaded with calcium and sulfate from the rocks deep underground. When this mineral-rich water reaches the surface and cools down, gypsum precipitates out, forming large beds of gypsum sediment.
In addition to surface sources, gypsum also develops deep underground in fractures and solution cavities. Here, gypsum crystals can grow to enormous sizes uninhibited in the voids. When these gypsum deposits are later exposed at the surface through uplift and erosion, beautiful and large transparent selenite crystals are found.
Regardless of where gypsum forms, it is almost always found in sedimentary layers and beds of rock. Gypsum is the most common sulfate-bearing evaporite mineral that precipitates from brines and saltwater.
Mining Gypsum
Gypsum is mined from both quarries and underground mines using room and pillar mining techniques. Quarries are often located at the edge of gypsum beds to facilitate access and extraction. Hard overburden is removed using excavators and shovels to expose the gypsum deposits underneath.
In underground gypsum mines, shafts are driven downwards and galleries or rooms are excavated out with pillars left standing to support the mine roof. As much gypsum as possible is extracted while leaving enough pillars to prevent collapse. This is known as the room and pillar mining method.
The mined gypsum ore first goes through a jaw crusher to break down large boulders into smaller pieces. It is then fed through a series of roller mills and screens to grind the gypsum down into a fine powder. If the gypsum ore has a high moisture content, it may need to be dried in a rotary dryer first before grinding.
From Raw Gypsum to Plaster of Paris
The ground gypsum powder is called land plaster. This land plaster goes through a process called calcining in a kettle or rotary kiln to remove 75% of its water content, converting it to a fine white powder called plaster of Paris. When water is added to plaster of Paris, it hardens rapidly back into solid gypsum through the process of crystallization.
This property of plaster of Paris recombining with water to harden makes it an excellent binding material for molds, sculptures, and in the familiar gypsum plaster casts for setting broken bones. It can also be remolded and reused indefinitely. These unique properties are what make gypsum so useful to humans.
Major Uses of Gypsum
Gypsum has been used by humans for thousands of years in construction, agriculture, art, and more. Some major uses include:
Plaster and Drywall
The most common use of gypsum is in the production of plaster and drywall products for construction. When the powder of plaster of Paris is remixed with water, it hardens quickly, bonding to any shaped surface. Plaster can be directly applied to walls or poured into molds for decorative trim.
Drywall is made by sandwiching plaster of Paris between paper sheets. After the plaster hardens, drywall panels are an inexpensive and fire-resistant surfacing material. Gypsum comprises over 90% of the material used to make drywall. Its fire resistance, combined with sound dampening qualities, make drywall the leading construction material for walls and ceilings. Over 30 billion square feet of drywall is produced in North America each year.
Soil Amendment
Gypsum added to soil provides calcium and sulfate ions. This helps break up tightly bound clay soils and improves soil structure. The calcium also replaces sodium ions to reclaim sodic soils affected by high sodium. Gypsum amendments improve the physical properties of soil, promoting better plant growth and agricultural yields. It is widely used on crops such as peanuts, alfalfa, soybeans, cotton, and corn. Gypsum reduces runoff and erosion which improves water quality.
Plaster of Paris Molds and Casts
Plaster of Paris sets rapidly when remixed with water, making it ideal for casting molds and sculpting. Artisans carve original designs in clay or wax, then use plaster of Paris to create durable negative molds. Molten metals, ceramics, or new plaster can then be poured into these molds to mass produce copies. In medicine, plaster of Paris is used to create custom casts to immobilize broken bones while they heal. Dentists use plaster to make impressions of teeth for dental work. Plaster of Paris was crucial to sculpture and art before modern casting resins were developed.
Food and Pharmaceutical Additive
Gypsum has GRAS status (Generally Recognized as Safe) by the FDA. Food-grade gypsum adds calcium and texture to foods like tofu and soy milk. Brewers use gypsum to condition the water used in brewing beer. Winemakers add gypsum to control acidity and tartness. It is added to baked goods and candies as an ingredient and mold inhibitor. Gypsum also provides important minerals used in pharmaceuticals and dietary supplements.
Unique Forms and Varieties of Gypsum
In addition to its standard rock form, gypsum also develops into unique crystal varieties and habits that are highly valued:
Selenite
Selenite is the clear, transparent crystalline form of gypsum that shows glassy luster and has perfect cleavage. It commonly occurs as thin flattened crystals, but can also form “fishtail” and equant shapes. The satin-like sheen of selenite gives it a moon-like glow. Large selenite crystals are popular for metaphysical purposes and decorative displays.
Alabaster
Alabaster is a fine-grained, massive form of gypsum ideal for carving. Its softness allows alabaster to be precisely sculpted into various ornamental objects and additions to buildings, statues, vases, and lamps. Light passes through the thin alabaster walls creating a glowing, translucent effect. Ornamental alabaster dates back as far as ancient Egypt.
Satin Spar
Satin spar is a fibrous form of gypsum with a silky lustre. Microscopic gypsum fibers align in parallel orientations to create the fine satin sheen that appears to glow from within. In the past, satin spar selenite was used to make decorative window panels that diffused light.
Desert Roses
Desert roses are clusters of intergrown gypsum crystals that form a rose-shaped habit. The rosette includes bladed or platy crystals fanning outward from a center. The red or brown color comes from impurities of iron oxides. Desert roses only form in arid, desert climates and are highly prized by collectors.
Unique Properties of Gypsum
Gypsum possesses unique characteristics that set it apart from other minerals and account for its widespread human use:
Soft and Easy to Cut
With a hardness of only 1.5-2 on Mohs scale, gypsum is very soft and carves easily using metal tools, wood tools, or abrasives. This allows gypsum to be precisely shaped into endless forms for construction, art, architecture, and more. Other rocks used in construction like granite are much harder and difficult to cut by comparison.
Rapidly Forms Crystal Bond
Gypsum in the form of plaster of Paris rapidly sets and recrystallizes when mixed with water. This property allows gypsum plaster to harden and bind to any shaped surface or mold quickly. Other mineral plasters require firing at high temperatures. Gypsum plaster simply needs air to facilitate the crystallization process.
Fire Resistance
Gypsum is highly fire resistant and does not burn or emit toxic fumes when exposed to fire or high heat. This makes gypsum an ideal building material in terms of fire safety. Gypsum drywall contains the spread of fires and reduces sound transmission.
Abundant and Accessible
Gypsum deposits are abundant and widespread around the globe. Major gypsum beds exist on every continent. Combined with gypsum’s lack of valuable metals or gems, this results in gypsum being extremely affordable and available. Gypsum is one of the most accessible and affordable natural minerals. Abundant gypsum helps drive global construction.
Comparison of Gypsum with Other Minerals
| Mineral | Chemical Formula | Hardness | Crystal System | Uses |
|---|---|---|---|---|
| Gypsum | CaSO4·2H2O | 1.5-2 | Monoclinic | Construction materials, plaster, agriculture, art |
| Quartz | SiO2 | 7 | Trigonal | Glass, electronics, abrasives, gemstones |
| Calcite | CaCO3 | 3 | Trigonal | Construction, limestone, marble, pigments |
| Halite | NaCl | 2.5 | Cubic | Food seasoning, industrial uses |
| Talc | Mg3Si4O10(OH)2 | 1 | Triclinic | Cosmetics, ceramics, paint |
| Fluorite | CaF2 | 4 | Cubic | Optics, hydrofluoric acid, gemstone |
Key differences:
- Gypsum is very soft at 1.5-2 hardness, useful for carving. Quartz and calcite are much harder.
- Gypsum and calcite both contain calcium, but gypsum also contains sulfate.
- Gypsum’s monoclinic crystal system differs from the trigonal systems of quartz and calcite.
- Gypsum is more used in construction while quartz is used for glass and electronics.
- Talc is the softest at 1 hardness. Fluorite is harder than gypsum.
- Gypsum’s major uses like plaster and drywall are unique compared to other minerals.
FAQs about Gypsum
What causes gypsum crystals to form in different shapes?
The crystal habit of gypsum is determined by the specific conditions as it precipitates from water such as temperature, rate of growth, and impurities present. This affects the basic molecular structure and expression of the crystals.
Why is gypsum soft enough to scratch with a fingernail?
Gypsum has a Mohs hardness of only 1.5 to 2 because of its chemical composition and crystal structure. The weaker bonds between calcium, sulfur, oxygen and water molecules result in the softness and perfect cleavage planes.
How thick can gypsum beds be?
Gypsum deposits can accumulate in beds up to 10 meters (33 feet) in thickness. Thick gypsum layers form in basins where gypsum precipitating conditions persist for long periods of time allowing large amounts to accumulate.
What gives desert roses their distinctive color?
The distinct reddish brown color of desert roses is caused by impurities of iron oxides within the gypsum crystals. The iron oxides provide the coloration to the normally white gypsum.
Why is gypsum added to soil?
Adding gypsum improves soil structure by adding calcium which replaces sodium and breaks up compacted clay. This improves fertility, drainage and root growth. The sulfate also supplies essential nutrients to plants.
How fast does plaster of Paris harden when mixed with water?
Plaster of Paris begins to harden within several minutes after mixing and will fully set within 30 minutes. The plaster recrystallizes rapidly as the water recombines with the calcium sulfate hemihydrate.
Is gypsum safe to handle?
Gypsum is non-toxic and safe for human handling. However inhalation of gypsum dust over long periods can pose a respiratory hazard so precautions should be taken to limit exposure. Always use proper ventilation and masks when handling large amounts of gypsum.
Conclusion
From agriculture to construction to art, gypsum is one of the most versatile and useful minerals on Earth. Its unique properties like softness, crystallization, and fire resistance make gypsum essential for human civilization. Gypsum has been used this way for thousands of years, and will continue to be a vital mineral into the future. Whether as crystals, in plaster and drywall, or as an everyday soil additive, gypsum has earned its reputation as a jack-of-all-trades mineral.
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