Chlorite is a word that refers to two entirely unrelated things in science, and which one you encounter depends on whether you are reading about rocks or about water treatment. In mineralogy, the chlorite group is a family of green, flaky silicate minerals common in metamorphic and sedimentary rocks. In chemistry, chlorite is the ion ClO₂⁻, an oxidizing agent used in disinfection, food safety, and industrial bleaching. The two share nothing but a name, and confusing them can lead to serious misunderstandings, especially when the chemical form shows up in health misinformation.
The Chlorite Mineral Group
Chlorite minerals are sheet silicates built from alternating layers of two types: one resembling mica and the other resembling brucite (a magnesium hydroxide mineral). That layered sandwich structure gives chlorite its characteristic flaky, platy habit and a distinctive green color, though shades range from pale yellowish-green to nearly black depending on the iron and magnesium content. The name itself comes from the Greek word for green. Common members of the group include clinochlore, chamosite, and pennantite, each distinguished mainly by which metals dominate the crystal structure.
You find chlorite in an enormous range of geological settings. It forms most readily during low-grade metamorphism, when rocks are subjected to moderate heat and pressure but not enough to produce the higher-temperature minerals like garnet or staurolite. Chlorite is also a common alteration product in hydrothermal systems, where hot fluids circulating through rock convert minerals like biotite and hornblende into chlorite. And it shows up in sedimentary rocks, especially shales, where it can form during diagenesis as muds compact and warm slightly over geologic time.
A Built-In Thermometer for Geologists
One of the more useful things about chlorite is that its chemical composition shifts predictably with temperature. As the temperature of formation rises, the aluminum content in the mineral’s crystal structure increases in a systematic way. Geologists exploit this relationship as a geothermometer, using the chemistry of chlorite grains to estimate the temperatures a rock experienced during metamorphism. In regional studies of low-grade metamorphic rocks, the aluminum content of chlorite increases steadily from the lowest-temperature zeolite facies through the prehnite-pumpellyite facies and into the greenschist facies, tracking what amounts to a continuous temperature gradient.1Journal of Metamorphic Geology. Compositional variations in mafic phyllosilicates from regional low‐grade metabasites and application of the chlorite geothermometer
This temperature sensitivity also makes chlorite a practical tool for mineral exploration. In ore deposits formed by hydrothermal fluids, chlorite often records the thermal and chemical history of the system. At the Niujuan silver-gold-lead-zinc deposit in China, researchers found that the chemical variations across different types of chlorite could help identify zones of stronger alteration and guide exploration in moderate- to low-temperature ore environments.2Ore Geology Reviews. Characteristics of hydrothermal chlorite from the Niujuan Ag-Au-Pb-Zn deposit in the north margin of NCC and implications for exploration tools for ore deposits A similar approach at the Tongshankou copper-molybdenum-tungsten deposit identified four distinct types of chlorite, each tied to a different alteration zone, with formation temperatures spanning roughly 170 to 320 °C. Chlorite in the veins was enriched in metals like copper, zinc, and lead, confirming it had precipitated directly from the metal-bearing hydrothermal fluids.3Ore Geology Reviews. Chlorite chemistry of Tongshankou porphyry-related Cu–Mo–W skarn deposit, Eastern China: Implications for hydrothermal fluid evolution and exploration vectoring to concealed orebodies
What Happens When Chlorite Weathers
Chlorite is not especially resistant to weathering. In warm, wet climates it breaks down relatively quickly, losing iron and magnesium from its structure and transforming into a sequence of other clay minerals. In humid tropical soils developed on chlorite-rich bedrock, the larger sand and silt grains tend to weather into mixed-layer chlorite-vermiculite, while the finer clay fraction transforms into iron-rich smectite (nontronite) at depth and eventually into kaolinite nearer the soil surface.4Geoderma. Weathering of chlorite in a soil derived from a chloritoschist under humid tropical conditions
Under certain conditions, the breakdown goes further. In soils where iron-rich chlorite weathers intensively, the mineral can convert entirely to halloysite, kaolinite, and iron oxyhydroxides. Microscopic examination of weathered chlorite flakes reveals dramatic structural changes: large parallel sets of galleries open up as the flakes exfoliate and expand, and tiny tubes of halloysite form bridges between the gallery walls.5Clays and Clay Minerals. Evidence for Halloysite Formation from Weathering of Ferruginous Chlorite This weathering pathway matters for agriculture and engineering because the resulting clay minerals behave very differently in terms of water retention, shrink-swell behavior, and nutrient holding capacity.
Chlorite Minerals on Mars
Chlorite is not strictly an Earth mineral. Orbital spectroscopy from Mars missions has detected chlorite on the Martian surface, along with other phyllosilicates like nontronite and kaolinite. Geochemical modeling of water-rock interactions using the famous Martian meteorite ALH 84001 predicts that chlorite should indeed form during aqueous weathering of Martian basalts, consistent with those orbital detections.6Meteoritics & Planetary Science. Alteration minerals, fluids, and gases on early Mars: Predictions from 1‐D flow geochemical modeling of mineral assemblages in meteorite ALH 84001 The presence of chlorite on Mars is significant because it implies sustained liquid water in the planet’s past, since these minerals need water to form.
Laboratory experiments simulating meteorite impacts on phyllosilicates, including chlorite, have shown that the mineral’s crystal structure partially deforms or becomes amorphous under shock pressures. Interestingly, the near-infrared spectral signatures that orbiters use to identify minerals from space changed only modestly in shocked chlorite samples, while mid-infrared signatures shifted more dramatically. A secondary phase of quartz was also identified in the shocked chlorite.7Journal of Geophysical Research: Planets. Experimental investigation into the effects of meteoritic impacts on the spectral properties of phyllosilicates on Mars These findings help planetary scientists correctly interpret remote-sensing data from a surface that has been battered by billions of years of impacts.
The Other Chlorite: A Chemical Ion
In chemistry, chlorite (ClO₂⁻) is a completely different substance. It is an inorganic anion, a negatively charged molecule made of one chlorine atom bonded to two oxygen atoms. Its most common commercial form is sodium chlorite (NaClO₂), a white or slightly yellowish powder that dissolves easily in water and acts as a strong oxidizer. Sodium chlorite is manufactured industrially by generating chlorine dioxide gas from sodium chlorate and then reacting it with sodium hydroxide and hydrogen peroxide.8Journal of Cleaner Production. A clean production process of sodium chlorite from sodium chlorate
Sodium chlorite’s main commercial role is as a precursor to chlorine dioxide, a powerful disinfectant and bleaching agent. When sodium chlorite is acidified, it generates chlorine dioxide gas on demand. This on-site generation approach is widely used because chlorine dioxide itself is unstable and cannot be easily stored or transported. Beyond generating chlorine dioxide, sodium chlorite also finds direct use in textile and pulp bleaching and in various food safety applications.
Chlorite as a Disinfection Byproduct in Drinking Water
For most people, the practical relevance of the chlorite ion is as something showing up in their tap water. When water utilities use chlorine dioxide to disinfect drinking water, about 50 to 70 percent of the chlorine dioxide converts to chlorite as a byproduct. This matters because chlorite is regulated in many countries. In the United States, the Environmental Protection Agency sets a maximum contaminant level for chlorite in drinking water at 1.0 milligram per liter.
The tradeoff is worthwhile from a disinfection standpoint: chlorine dioxide treatment by itself does not produce significant amounts of trihalomethanes or haloacetic acids, the more familiar and well-studied disinfection byproducts associated with conventional chlorination. But it does produce chlorite.9PubMed. Formation of disinfection byproducts upon chlorine dioxide preoxidation followed by chlorination or chloramination of natural organic matter So utilities that choose chlorine dioxide avoid one category of harmful byproducts while having to manage another.
It is worth noting that chlorite by itself is a weak disinfectant. Testing in drinking water biofilms showed that chlorite at low concentrations achieved only modest reductions in bacteria, on the order of 0.2 to 0.3 log reductions for attached organisms. Chlorine dioxide at higher doses performed substantially better, achieving over 1 log reduction in the same biofilm system.10PubMed. Disinfectant efficacy of chlorite and chlorine dioxide in drinking water biofilms This underscores that chlorite is primarily a concern as a residual byproduct, not something utilities add intentionally for disinfection.
Removing Chlorite from Treated Water
Because chlorite is regulated, water treatment plants need strategies to keep concentrations below the legal limit. One effective approach is using ferrous iron to chemically reduce chlorite ions to harmless chloride ions. In trials, adding about 6 milligrams per liter of ferrous iron reduced chlorite concentrations from 2 milligrams per liter to below 0.3 milligrams per liter.11PubMed. The impact of ferrous ion reduction of chlorite ion on drinking water process performance The catch is that the reaction produces ferric hydroxide solids, which need to be managed in the treatment process.
Process adjustments can also reduce chlorite formation in the first place. Using potassium permanganate as a pre-oxidant before coagulation lowers the subsequent chlorine dioxide demand, which in turn reduces how much chlorite and chlorate form.12PubMed. Influence of drinking water treatments on chlorine dioxide consumption and chlorite/chlorate formation More recent research has explored adsorption-based approaches: granular activated carbon modified with quaternary ammonium surfactants can strip chlorite from water with high efficiency. One modified carbon achieved 99 percent chlorite removal in just two hours under laboratory conditions.13Journal of Environmental Chemical Engineering. Control of chlorite and chlorate in drinking water using surfactant-modified activated carbon
For smaller systems or field monitoring, portable analytical tools have also been developed. A voltammetric method using inexpensive graphite electrodes can measure chlorite concentrations on-site in water distribution networks, avoiding the need to send samples to a central laboratory.14PubMed. Determination of chlorite in drinking water by differential pulse voltammetry on graphite
Acidified Sodium Chlorite in Food Safety
Acidified sodium chlorite is approved in several countries for decontaminating meat, poultry, and fresh produce. When sodium chlorite is mixed with a food-grade acid, it generates chlorine dioxide and related oxidizing species that are highly effective at killing surface pathogens. In tests on beef carcasses inoculated with E. coli O157:H7 and Salmonella, a water wash followed by an acidified sodium chlorite spray reduced pathogen counts by roughly four log cycles, compared to about 2.3 log cycles for a water wash alone.15PubMed. Reduction of Escherichia coli O157:H7 and Salmonella typhimurium on beef carcass surfaces using acidified sodium chlorite That is the difference between removing 99 percent of bacteria and removing 99.99 percent.
At the molecular level, acidified sodium chlorite kills bacteria by overwhelming their defenses against oxidative damage. Research on Campylobacter jejuni, a common cause of foodborne illness, showed that exposure to acidified sodium chlorite shut down all vital metabolic pathways in the bacteria while activating DNA damage and repair genes, indicating severe and often lethal oxidative injury.16PubMed Central. Transcriptomic response of Campylobacter jejuni following exposure to acidified sodium chlorite The treatment leaves minimal residues on food at the concentrations used commercially, which is why regulatory agencies in the U.S. and EU have permitted its use under controlled conditions.
Toxicity of Sodium Chlorite to Humans
The same oxidizing power that makes sodium chlorite effective against bacteria makes it dangerous to human tissue. Sodium chlorite is a strong oxidizer of hemoglobin, the protein in red blood cells that carries oxygen. When it reaches the bloodstream, it converts hemoglobin to methemoglobin, a form that cannot deliver oxygen to tissues. It also generates reactive oxygen species that damage red blood cell membranes, deplete the cell’s antioxidant defenses, and lead to hemolysis, the premature destruction of red blood cells.
Laboratory studies on human red blood cells exposed to sodium chlorite showed methemoglobin levels increasing by five- to fifty-twofold compared to untreated controls, while the enzyme that normally reverses methemoglobin formation lost up to 93 percent of its activity. Reactive oxygen species increased by three- to twenty-onefold, and the cells’ ability to quench free radicals collapsed.17PubMed. Sodium chlorite increases production of reactive oxygen species that impair the antioxidant system and cause morphological changes in human erythrocytes Earlier animal and in vitro work confirmed that chlorine compounds, including chlorite, deplete glutathione (a key antioxidant molecule) in red blood cells in a dose-dependent fashion, and that this oxidative stress likely drives the cascade of blood-related damage.18PubMed Central. Toxicological effects of chlorine dioxide, chlorite and chlorate
Beyond blood effects, animal studies have identified reproductive and endocrine impacts. Male rats exposed to chlorite at 100 parts per million or higher in their drinking water showed a significant increase in abnormal sperm shape and decreased sperm motility. Pups exposed in utero and through nursing had reduced thyroid hormone levels at several weeks of age. These reproductive endpoints appeared to be among the most sensitive indicators of chlorite toxicity, triggered at lower doses than other clinical signs.19PubMed. Sodium chlorite administration in Long-Evans rats: reproductive and endocrine effects
What Happens When Someone Drinks Concentrated Sodium Chlorite
Acute poisoning cases illustrate just how dangerous concentrated sodium chlorite is. A published case report describes a 55-year-old man who ingested less than 100 milliliters of a 28 percent sodium chlorite solution in a suicide attempt. He arrived at the hospital cyanotic, with lowered consciousness, no urine output, and chocolate-brown colored serum, a hallmark of severe methemoglobinemia. Lab tests confirmed 40 percent methemoglobin, a level that causes profound tissue oxygen deprivation. Treatment required methylene blue to reverse the methemoglobin, dialysis to remove the toxin and manage kidney failure, and red blood cell transfusions to replace the cells being destroyed by hemolysis. Even with aggressive treatment, the patient developed disseminated intravascular coagulation before eventually recovering.20PubMed Central. A case of severe chlorite poisoning successfully treated with early administration of methylene blue, renal replacement therapy, and red blood cell transfusion: case report
Sodium chlorite is also documented as a cause of methemoglobinemia and severe hemolysis in accidental or lower-dose ingestion scenarios, even from concentrations found in some household and industrial cleaning products.21PubMed Central. A Case of Methemoglobinemia and Hemolysis from Sodium Chlorite Ingestion
The “Miracle Mineral Supplement” Problem
Perhaps the most alarming dimension of chlorite toxicity is its deliberate promotion as a health product. Since the mid-2000s, a product variously called Miracle Mineral Supplement (MMS) or Miracle Mineral Solution has been marketed online as a cure for conditions ranging from malaria to autism to COVID-19. MMS is typically a 28 percent sodium chlorite solution that users are instructed to mix with an acid, generating chlorine dioxide, and drink. Some protocols direct users to administer it to children or use it as an enema.
The U.S. Food and Drug Administration has repeatedly warned that ingesting these products is equivalent to drinking bleach and has described reports of severe vomiting, severe diarrhea, life-threatening drops in blood pressure from dehydration, and acute liver failure associated with their use.22International Journal of Medicine and Medical Sciences. The chlorine dioxide controversy: A deadly poison or a cure for COVID-19? No credible clinical trial has shown benefit from drinking chlorine dioxide or sodium chlorite solutions for any medical condition. The chemistry described throughout this article, where sodium chlorite destroys red blood cells, depletes antioxidants, and causes kidney failure at high doses, explains precisely why these products are harmful. The fact that chlorine dioxide kills bacteria on beef carcasses does not mean it is safe or therapeutic inside a human body. The mechanism that makes it lethal to microbes on a surface is the same mechanism that damages human cells.
Regulatory agencies in many countries have issued warnings, seized shipments, and prosecuted sellers, but MMS continues to circulate through social media and alternative health networks. If you encounter products described as “water purification drops” with instructions to activate them with acid before drinking, that is almost certainly a sodium chlorite product, and it should not be consumed.