Gypsum originates from two fundamentally different paths. The natural kind precipitates from evaporating seawater or saline lakes and has been doing so for hundreds of millions of years, forming thick geological deposits around the world. The synthetic kind is a byproduct of modern industry, captured from power plant smokestacks, phosphate fertilizer factories, and chemical plants. Together, these sources supply the raw material for wallboard, plaster, cement, agriculture, and dozens of other uses that most people never think twice about.
How Gypsum Forms in Nature
Most natural gypsum traces back to ancient seas. When a body of saltwater becomes partially or fully cut off from the open ocean and evaporation outpaces inflow, the dissolved minerals in the water become increasingly concentrated. Calcium sulfate is one of the first salts to precipitate out, well before sodium chloride (table salt). The result is thick beds of gypsum layered in sedimentary rock, sometimes hundreds of meters deep, recording long episodes of arid climate. These deposits are called evaporites, and they are found on every continent.
The chemistry leaves fingerprints. As the brine concentrates over time, trace elements shift in predictable ways. In the Messinian-era gypsum beds of southeastern Spain, for instance, researchers found that concentrations of strontium, magnesium, and sodium increase upward through the formation while crystal size decreases, consistent with progressively saltier brine as the deposit accumulated.1Chemical Geology. Trace elements and environmental significance of Messinian gypsum deposits, the Nijar Basin, southeastern Spain Those trace-element profiles let geologists reconstruct the conditions under which a given gypsum bed formed, right down to whether the water was getting saltier or being diluted by fresh inflow.
Not all natural gypsum is marine. Some forms where sulfur-rich volcanic gases interact with calcium-bearing rocks near hot springs. Others precipitate in desert soils where groundwater wicks upward and evaporates near the surface, creating gypsum crusts and the dramatic “desert roses” that collectors prize. And in rare cases, gypsum crystallizes underground from mineral-laden groundwater circulating through fractures in rock, which is how some of the most spectacular gypsum formations on Earth came to be.
The Mineral and Its Sensitivity to Heat
Chemically, gypsum is calcium sulfate with two molecules of water locked into its crystal structure. That built-in water is what makes gypsum distinct from its close relatives. Heat it gently, and it loses some of that water to become a “hemihydrate,” the powder known as plaster of Paris. Heat it further and it loses all its water, becoming anhydrite, a related but harder and denser mineral. This family of calcium sulfate phases, gypsum, hemihydrate, and anhydrite, matters enormously for both geology and industry.
The transition from gypsum to anhydrite in nature has puzzled researchers for decades. In theory, there is a temperature above which gypsum becomes unstable in the presence of water and should convert to anhydrite. But pinning down that exact temperature has proven surprisingly difficult because anhydrite crystallizes painfully slowly at low temperatures, making direct lab measurement nearly impossible. Different research groups have arrived at different answers. A reassessment of solubility data and calorimetric measurements placed the transition at about 42 °C.2Frontiers in Nuclear Engineering. Solubility of anhydrite and gypsum at temperatures below 100°C and the gypsum-anhydrite transition temperature in aqueous solutions: a re-assessment A separate computational study using quantum-mechanical energy calculations yielded a slightly higher figure of roughly 47 °C.3PubMed Central. New Insights into the Transition Temperature of Gypsum to Anhydrite Based on DFT Calculations of Thermodynamic Properties Estimates in the broader literature range anywhere from 30 °C to 60 °C. The practical upshot is that gypsum is stable at surface temperatures and in shallow sediments, but at the elevated temperatures found a few kilometers underground, it converts to anhydrite over geological time. That is why many deep drill cores encounter anhydrite where gypsum was originally deposited.
Giant Crystals and Other Extreme Formations
The most famous gypsum formation on Earth is probably the Cave of Crystals in the Naica mine in northern Mexico, where translucent gypsum beams up to 11 meters long jut from the cave walls. These crystals grew because conditions underground were exquisitely tuned for it. The cave sat at a temperature just slightly above the gypsum-anhydrite transition point, and two chemically distinct groundwater sources mixed inside the cavity, maintaining a gentle, steady supersaturation of calcium sulfate over thousands of years. Researchers have proposed that the mixing of these fluids was controlled by the hydraulic connection between different deep caves in the system, and that it tracked cycles of warm-dry and cool-wet climate periods in the region.4Earth and Planetary Science Letters. Climatic control on the growth of gigantic gypsum crystals within hypogenic caves (Naica mine, Mexico)? The result is a geological wonder that could only form under a very narrow set of circumstances: warm enough to keep anhydrite dissolving, cool enough for gypsum to crystallize, and stable enough for the process to continue uninterrupted for an extraordinarily long time.
White Sands in New Mexico is another dramatic example. There, a shallow lake system in a closed basin has been producing gypsum crystals that weather into fine sand grains, creating an entire dune field made of gypsum rather than the quartz sand found in most deserts. The dunes cover about 700 square kilometers and are one of the largest gypsum dune fields on the planet.
Flue Gas Desulfurization Gypsum
The largest synthetic source of gypsum worldwide is a byproduct of cleaning coal-fired power plant exhaust. When coal is burned, the sulfur it contains produces sulfur dioxide, a gas that causes acid rain. To prevent that, most modern coal plants run their exhaust through a scrubber that sprays a limestone slurry into the flue gas. The sulfur dioxide reacts with the calcium in the limestone, and when the resulting slurry is oxidized, the product is calcium sulfate dihydrate: synthetic gypsum, chemically identical to the natural mineral. This process is called flue gas desulfurization, and the gypsum it produces is known as FGD gypsum.
FGD gypsum is now a major feedstock for wallboard manufacturing in many countries. From an environmental standpoint, using it instead of mining natural gypsum has measurable advantages. A life-cycle assessment comparing the two found that the total environmental load of natural gypsum board was about 6% higher than that of FGD gypsum board, with especially large differences in human toxicity (72% higher for natural) and consumption of nonrenewable resources (76% higher for natural).5Key Engineering Materials. Comparison of Life Cycle Environmental Impacts between Natural Gypsum Board and FGD Gypsum Board A separate study looking specifically at carbon footprint in the Czech Republic found that greenhouse gas emissions from producing calcined gypsum were roughly 25% lower when FGD gypsum was used instead of natural gypsum, largely because FGD gypsum requires less energy-intensive processing.6Journal of Cleaner Production. Carbon footprint analysis of calcined gypsum production in the Czech Republic
There is an irony here worth noting. As countries shift away from coal power, the supply of FGD gypsum is shrinking. Markets that grew to depend on it, particularly the wallboard industry, are now facing questions about where their gypsum will come from in the decades ahead. Some regions are increasing natural gypsum mining to fill the gap, which partly offsets the environmental gains from decarbonizing electricity.
Phosphogypsum and Its Complications
The second-largest source of synthetic gypsum is the phosphate fertilizer industry. When phosphate rock is dissolved in sulfuric acid to produce phosphoric acid (a key ingredient in most phosphate fertilizers), the calcium in the rock combines with the sulfate to form gypsum as a byproduct. This material, called phosphogypsum, is produced in enormous quantities: roughly five tons of it for every ton of phosphoric acid made.
Unlike FGD gypsum, phosphogypsum has serious contamination problems that limit its reuse. Phosphate rock naturally contains elevated levels of uranium-series radionuclides, and these concentrate in the gypsum byproduct. Phosphogypsum carries enhanced concentrations of radium-226, polonium-210, and lead-210, along with heavy metals.7PubMed. Assessment of the radiological environmental impact of using phosphogypsum as soil amendment Research on phosphogypsum stockpiles in Turkey has confirmed these elevated levels of toxic heavy metals and naturally occurring radioactive materials.8PubMed Central. Dispersion of oxides, heavy metals, and natural radionuclides in phosphogypsum stockpiles of the phosphate industries in Türkiye In the United States, the EPA has effectively banned most uses of phosphogypsum, requiring it to be stacked in large open-air piles near the plants that produce it. These stacks, some covering hundreds of acres in places like central Florida, are a long-term environmental management challenge.
Researchers continue to look for ways to purify phosphogypsum enough to make it usable, since the sheer volume of the waste is staggering. Efforts include extracting the impurities during the phosphoric acid production process itself, adjusting crystallization conditions to produce cleaner gypsum.9PubMed Central. Novel Synergistic Process of Impurities Extraction and Phophogypsum Crystallization Control in Wet-Process Phosphoric Acid If the radionuclide problem could be solved economically, phosphogypsum would go from a massive waste liability to a useful resource practically overnight.
Other Industrial Byproducts
FGD gypsum and phosphogypsum get the most attention, but several other industrial processes generate calcium sulfate as a byproduct. Fluorogypsum is produced during the manufacture of hydrofluoric acid, which is used in aluminum refining, petroleum processing, and semiconductor manufacturing. Researchers have explored using fluorogypsum as a paste filling material for underground mining voids, turning a waste product into a structural material.10PubMed Central. Study on the proportion of paste filling materials based on fluorogypsum
Borogypsum comes from the production of boric acid and boron compounds. It has been tested as a set retarder in Portland cement, a role traditionally filled by natural gypsum. Cement production relies on adding roughly 3 to 5% gypsum to clinite to control how fast the cement sets.11Cement and Concrete Research. Utilization of borogypsum as set retarder in Portland cement production Citrogypsum, a byproduct of citric acid manufacturing, and titanogypsum, from titanium dioxide pigment production, are smaller niche sources. In each case, the basic chemistry is similar: a process that generates sulfate ions in the presence of calcium ends up precipitating calcium sulfate.
From Raw Gypsum to Useful Products
Whether it comes out of a quarry or a smokestack, raw gypsum needs processing before it becomes plaster, wallboard, or a soil amendment. The most important step is calcination, which means heating the gypsum to drive off some or all of its water. At temperatures around 110 to 130 °C in open air, gypsum loses part of its water to become calcium sulfate hemihydrate, the fine white powder commonly called plaster of Paris.12PubMed Central. Setting time of construction gypsum, dental plaster, and white orthodontic gypsum When you add water back to this powder, it recrystallizes into interlocking gypsum needles and hardens, which is how plaster casts and gypsum wallboard work.
Waste gypsum, particularly from demolition of old buildings, can also be recycled back into new products. Research has shown that waste gypsum can replace natural gypsum as a set retarder in Portland cement, though the substitution is not perfectly seamless. Cement made with waste gypsum set faster than cement made with natural gypsum, by roughly 13 to 15%, because the waste material contained some hemihydrate that dissolved and reacted more quickly.13PubMed. Use of waste gypsum to replace natural gypsum as set retarders in portland cement That faster setting behavior is manageable but has to be accounted for in mix designs.
Gypsum in Agriculture
One of gypsum’s oldest uses is as a soil amendment, and the logic is straightforward. Gypsum supplies calcium and sulfur, both essential plant nutrients, without raising soil pH the way lime does. But its bigger value is in fixing “sodic” soils: soils where excess sodium has destroyed the structure, making them nearly impermeable to water and hostile to roots. The calcium in gypsum displaces the sodium on soil particles, which gets flushed away with irrigation water, gradually restoring the soil’s physical structure.
Field trials have demonstrated dramatic results. In saline-sodic soils in northeast China, applying desulfurized gypsum (FGD gypsum) at the recommended rate cut the soil’s salinity and sodicity by enormous margins and enabled rice yields where the untreated control produced nothing at all.14Geoderma. Reclamation of saline-sodic soil properties and improvement of rice (Oriza sativa L.) growth and yield using desulfurized gypsum In Ethiopia’s Rift Valley, combining gypsum with farmyard manure reduced exchangeable sodium by over 99% compared to untreated sodic soils.15PubMed Central. Elucidating Amendment Resources for Reclaiming Efficacy of Sodic Soils around Abaya and Chamo Lakes, South Ethiopia Rift Valley
For agricultural use, the source of the gypsum matters. FGD gypsum and mined natural gypsum are both widely used in farming. Phosphogypsum, as noted, carries radioactive and heavy metal contamination that can introduce those contaminants into the food chain, making it a riskier choice even where regulations allow it.7PubMed. Assessment of the radiological environmental impact of using phosphogypsum as soil amendment Farmers and agronomists need to know what they are spreading.
The Landfill Problem With Gypsum Drywall
Gypsum wallboard is the single largest use of gypsum in the developed world, and when buildings get demolished or renovated, a lot of that drywall ends up in landfills. This creates a problem that is not immediately obvious: hydrogen sulfide gas. In the oxygen-starved interior of a landfill, bacteria can convert the sulfate in gypsum into hydrogen sulfide, a toxic gas that smells like rotten eggs at low concentrations and can be deadly at high ones.
The issue is well-documented. A survey of ten construction and demolition debris landfills in Florida found hydrogen sulfide in the gas at every single site.16PubMed. Reduced sulfur compounds in gas from construction and demolition debris landfills In laboratory simulations, drywall alone produced hydrogen sulfide concentrations as high as 63,000 parts per million, far above levels dangerous to human health. Interestingly, concrete mixed with the drywall dropped concentrations to below 1 ppm, because it raised the pH out of the range where sulfate-reducing bacteria can thrive and reacted directly with the gas.17PubMed. Hydrogen sulfide generation in simulated construction and demolition debris landfills: impact of waste composition
Chemical inhibitors can also help. Sodium molybdate interrupts the bacterial sulfate-reduction process itself, while hydrated lime makes conditions too alkaline for the bacteria to function.18PubMed. Inhibition of hydrogen sulfide generation from disposed gypsum drywall using chemical inhibitors Some jurisdictions have responded by restricting or banning gypsum drywall from landfills altogether, requiring it to be recycled instead. In parts of Europe, separate collection of drywall waste is now standard practice, and the recovered gypsum goes back into new wallboard or cement.
Gypsum on Mars
Gypsum is not exclusively an Earth mineral. NASA’s Opportunity rover found bright veins of what appeared to be gypsum running through bedrock at Endeavour Crater on Mars. The veins were about the width of a human thumb and had been deposited in fractures by flowing groundwater, according to the science team. The mission’s principal investigator called it “the single most powerful piece of evidence for liquid water on Mars that has been discovered by the Opportunity rover.”19Eos, Transactions American Geophysical Union. Mars Opportunity rover finds gypsum veins The story the veins tell is simple: water carrying dissolved calcium and sulfate flowed through cracks in the rock, and gypsum crystallized out of it.
Separately, orbital instruments have detected a large gypsum-rich area in Mars’s north polar dune field. One proposed explanation is that meltwater from polar ice infiltrated sulfide-bearing sand dunes and chemically altered them, producing gypsum both by direct mineral alteration and by evaporative crystallization in the pore spaces between sand grains.20Journal of Geophysical Research: Planets. On the origin of gypsum in the Mars north polar region The polar gypsum appears to have formed relatively recently in Martian geological terms and through a different mechanism than the ancient sulfate deposits found elsewhere on the planet. Finding gypsum on Mars matters because it is direct evidence of past water activity and tells scientists about the chemistry and temperature of that water.
Gypsum as a Habitat for Life
Back on Earth, gypsum plays an unexpected biological role. The mineral is translucent enough to let light pass through thin crusts, and that property makes it a refuge for photosynthetic microorganisms in places that are otherwise too harsh for life on the surface. In Antarctica, researchers discovered small colonies of cyanobacteria, bacteria, and fungi living inside gypsum crusts that form naturally on sandstone boulders. The organisms occupy a zone just below the crust surface where enough light penetrates for photosynthesis but the crust provides protection from the brutal cold, wind, and ultraviolet radiation of the Antarctic environment.21PubMed. A novel Antarctic microbial endolithic community within gypsum crusts
Since gypsum crusts occur on ice-free rock surfaces throughout Antarctica, they represent potential colonization sites even at the highest latitudes where almost nothing else can survive. This has made gypsum endolithic communities a point of interest for astrobiologists. If translucent mineral crusts on Earth can shelter microbial life in extreme cold and dryness, similar formations on Mars could theoretically have done the same. The Martian gypsum deposits described above make that line of reasoning slightly less hypothetical, though no evidence of Martian life has been found.
Alabaster and the Ancient Carving Tradition
Fine-grained, massive gypsum is known as alabaster, and it has been prized for decorative carving since antiquity. Egyptian and Mesopotamian artisans shaped it into vessels and ornamental objects thousands of years ago. In medieval and Renaissance Europe, gypsum alabaster rivaled marble as the material of choice for religious sculptures, tomb effigies, and altarpiece panels. English alabaster from quarries in the Midlands was exported across the continent, and workshops in Nottingham became famous for mass-produced devotional panels that ended up in churches from Scandinavia to Spain.
Modern isotope fingerprinting techniques can now trace historical alabaster artworks back to their source quarries. By analyzing sulphur, oxygen, and strontium isotope ratios, researchers have been able to distinguish between English, French, and Spanish quarry sources with high confidence, because each source area has a distinctive isotopic signature. This work has allowed art historians to reconstruct medieval trade networks for alabaster, showing which quarries supplied which workshops and how the material moved across Europe.22Chemical Geology. Tracing Medieval and Renaissance Alabaster Works of Art Back to Quarries: A Multi-Isotope (Sr, S, O) Approach Alabaster’s softness, the same quality that makes it easy to carve, also makes it vulnerable to water damage, which is why alabaster sculpture survives better indoors than out, and why so much of it has been lost from churches that suffered roof leaks over the centuries.