Calcium oxalate (in archaic terminology, oxalate of lime) is a

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

oxalic acid Oxalic acid is an organic acid with the systematic name ethanedioic acid and chemical formula , also written as or or . It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name i ...

with the chemical formula or . It forms hydrates , where ''n'' varies from 1 to 3. Anhydrous and all hydrated forms are colorless or white. The monohydrate occurs naturally as the mineral

whewellite Whewellite is a mineral, hydrated calcium oxalate, formula calcium, Ca carbon, C2oxygen, O4·water, H2O. Because of its organic content it is thought to have an indirect biological origin; this hypothesis is supported by its presence in coal and ...

, forming envelope-shaped crystals, known in plants as raphides. The two rarer hydrates are dihydrate , which occurs naturally as the mineral weddellite, and trihydrate , which occurs naturally as the mineral caoxite, are also recognized. Some foods have high quantities of calcium oxalates and can produce sores and numbing on ingestion and may even be fatal. Cultural groups with diets that depend highly on fruits and vegetables high in calcium oxalate, such as those in

Micronesia Micronesia (, ) is a subregion of Oceania, consisting of approximately 2,000 small islands in the Northwestern Pacific Ocean. It has a close shared cultural history with three other island regions: Maritime Southeast Asia to the west, Poly ...

, reduce the level of it by boiling and cooking them. They are a constituent in 76% of human kidney stones. Calcium oxalate is also found in beerstone, a scale that forms on containers used in

breweries A brewery or brewing company is a business that makes and sells beer. The place at which beer is commercially made is either called a brewery or a beerhouse, where distinct sets of brewing equipment are called plant. The commercial brewing of be ...

Occurrence

Many plants accumulate calcium oxalate as it has been reported in more than 1000 different genera of plants. The calcium oxalate accumulation is linked to the detoxification of calcium () in the plant. Upon decomposition, the calcium oxalate is oxidised by bacteria, fungi, or wildfire to produce the soil nutrient

calcium carbonate Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...

. The

poison A poison is any chemical substance that is harmful or lethal to living organisms. The term is used in a wide range of scientific fields and industries, where it is often specifically defined. It may also be applied colloquially or figurati ...

ous plant dumb cane ('' Dieffenbachia'') contains the substance and on ingestion can prevent speech and be suffocating. It is also found in

sorrel Sorrel (''Rumex acetosa''), also called common sorrel or garden sorrel, is a perennial herbaceous plant in the family Polygonaceae. Other names for sorrel include spinach dock and narrow-leaved dock ("dock" being a common name for the genus ''Ru ...

, rhubarb (in large quantities in the leaves),

cinnamon Cinnamon is a spice obtained from the inner bark of several tree species from the genus ''Cinnamomum''. Cinnamon is used mainly as an aromatic condiment and flavouring additive in a wide variety of cuisines, sweet and savoury dishes, biscuits, b ...

turmeric Turmeric (), or ''Curcuma longa'' (), is a flowering plant in the ginger family Zingiberaceae. It is a perennial, rhizomatous, herbaceous plant native to the Indian subcontinent and Southeast Asia that requires temperatures between and high ...

and in species of '' Oxalis'',

Araceae The Araceae are a family of monocotyledonous flowering plants in which flowers are borne on a type of inflorescence called a spadix. The spadix is usually accompanied by, and sometimes partially enclosed in, a spathe (or leaf-like bract). Also ...

, '' Arum italicum'',

taro Taro (; ''Colocasia esculenta'') is a root vegetable. It is the most widely cultivated species of several plants in the family Araceae that are used as vegetables for their corms, leaves, stems and Petiole (botany), petioles. Taro corms are a ...

kiwifruit Kiwifruit (often shortened to kiwi), or Chinese gooseberry, is the edible berry (botany), berry of several species of woody vines in the genus ''Actinidia''. The most common cultivar group of kiwifruit (Actinidia chinensis var. deliciosa, ...

, tea leaves,

agave ''Agave'' (; ; ) is a genus of monocots native to the arid regions of the Americas. The genus is primarily known for its succulent and xerophytic species that typically form large Rosette (botany), rosettes of strong, fleshy leaves. Many plan ...

s, Virginia creeper ('' Parthenocissus quinquefolia''), and '' Alocasia'' and in

spinach Spinach (''Spinacia oleracea'') is a leafy green flowering plant native to Central Asia, Central and Western Asia. It is of the order Caryophyllales, family Amaranthaceae, subfamily Chenopodioideae. Its leaves are a common vegetable consumed eit ...

in varying amounts. Plants of the genus '' Philodendron'' contain enough calcium oxalate that consumption of parts of the plant can result in uncomfortable symptoms. Insoluble calcium oxalate crystals are found in plant stems, roots, and leaves and produced in idioblasts.

Vanilla Vanilla is a spice derived from orchids of the genus ''Vanilla (genus), Vanilla'', primarily obtained from pods of the flat-leaved vanilla (''Vanilla planifolia, V. planifolia''). ''Vanilla'' is not Autogamy, autogamous, so pollination ...

plants exude calcium oxalates upon harvest of the orchid seed pods and may cause

contact dermatitis Contact dermatitis is a type of acute or chronic inflammation of the skin caused by exposure to chemical or physical agents. Symptoms of contact dermatitis can include itchy or dry skin, a red rash, bumps, blisters, or swelling. These rashes ...

. Calcium oxalate crystals are commonly found in

lichen A lichen ( , ) is a hybrid colony (biology), colony of algae or cyanobacteria living symbiotically among hypha, filaments of multiple fungus species, along with yeasts and bacteria embedded in the cortex or "skin", in a mutualism (biology), m ...

s, where they occur in two mineral forms: weddellite (CaC₂O₄·(2+x)H₂O) and whewellite (CaC₂O₄·H₂O). These crystals can form both on the surface of the lichen as a powdery coating called and within the internal structures of the lichen

thallus Thallus (: thalli), from Latinized Greek (), meaning "a green shoot" or "twig", is the vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts, lichens, and the Myxogastria. A thallus usually names the entir ...

. The type and distribution of these crystals often correlates with environmental conditions: weddellite typically forms in dry environments and can serve as a water source for the lichen, while whewellite is more common in moist habitats. In addition to water regulation, calcium oxalate crystals in lichens serve several protective functions, including shielding against excessive sunlight and potentially helping to neutralize pollutants such as

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...

. The formation of these crystals is linked to the lichen's ability to dissolve calcium from rocky substrates through the production of oxalic acid, with the amount of calcium oxalate often correlating with the calcium content of the substrate on which the lichen grows. Calcium oxalate, as ‘beerstone’, is a brownish precipitate that tends to accumulate within vats, barrels, and other containers used in the brewing of beer. If not removed in a cleaning process, beerstone will leave an unsanitary surface that can harbour microorganisms. Beerstone is composed of calcium and magnesium salts and various organic compounds left over from the brewing process; it promotes the growth of unwanted microorganisms that can adversely affect or even ruin the flavour of a batch of beer. Calcium oxalate crystals in the urine are the most common constituent of human kidney stones, and calcium oxalate crystal formation is also one of the toxic effects of

ethylene glycol poisoning Ethylene glycol poisoning is poisoning caused by drinking ethylene glycol. Early symptoms include intoxication, vomiting and abdominal pain. Later symptoms may include a decreased level of consciousness, headache, and seizures. Long term outco ...

Chemical properties

Calcium oxalate is a combination of calcium ions and the conjugate base of

, the oxalate anion. Its aqueous solutions are slightly basic because of the basicity of the oxalate ion. The basicity of calcium oxalate is weaker than that of

sodium oxalate Sodium oxalate, or disodium oxalate, is a chemical compound with the chemical formula . It is the sodium Salt (chemistry), salt of oxalic acid. It contains sodium cations and oxalate anions . It is a white, crystalline, odorless solid, that decomp ...

, due to its lower solubility in water. Solid calcium oxalate hydrate has been characterized by

X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

. It is a coordination polymer featuring planar oxalate anions linked to calcium, which also has water

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

Medical significance

Calcium oxalate can produce sores and numbing on ingestion and may even be fatal.

Morphology and diagnosis

The monohydrate and dihydrate can be distinguished by the shape of the respective crystals. * Calcium oxalate ''dihydrate'' crystals are octahedral. A large portion of the crystals in a urine sediment will have this type of morphology, as they can grow at any pH and naturally occur in normal urine. * Calcium oxalate ''monohydrate'' crystals vary in shape, and can be shaped like dumbbells, spindles, ovals, or picket fences, the last of which is most commonly seen due to

. File:Calcium oxalate crystals in urine.jpg, Urine microscopy showing calcium oxalate crystals in the urine. The octahedral crystal morphology is clearly visible. File:Calcium oxalate crystals (urine) - kalsiyum oksalat kristalleri (idrar) - 01.png, Urine microscopy showing a ''calcium oxalate monohydrate'' crystal (dumbbell shaped) and a ''calcium oxalate dihydrate'' crystal (envelope shaped) along with several erythrocytes. File:Calcium oxalate crystals (urine) - kalsiyum oksalat kristalleri (idrar) - 02.png, Urine microscopy showing several ''calcium oxalate monohydrate'' crystals (dumbbell shaped, some of them clumped) and a ''calcium oxalate dihydrate'' crystal (envelope shaped) along with several erythrocytes. File:Calcium Oxalate Detail.png, Urinary sediment showing several calcium oxalate crystals. 40X File:Urine crystals comparison.png, Comparison of different types of urinary stones. File:Histopathology of a breast cyst with calcium oxalate crystals, annotated.jpg, Histopathology of calcium oxalate crystals in a benign breast cyst, H&E stain. In the breast, they can be seen on

mammography Mammography (also called mastography; DICOM modality: MG) is the process of using low-energy X-rays (usually around 30 kVp) to examine the human breast for diagnosis and screening. The goal of mammography is the early detection of breast cance ...

and are usually benign, but can be associated with lobular carcinoma in situ.Image by Mikael Häggström, MD.
- Reference for benign/LCIS association: Last author update: 1 June 2010

Kidney stones

About 76% of kidney stones are partially or entirely of the calcium oxalate type. They form when urine is persistently saturated with calcium and oxalate. Between 1% and 15% of people globally are affected by kidney stones at some point. In 2015, they caused about 16,000 deaths worldwide. Some of the oxalate in urine is produced by the body. Calcium and oxalate in the diet play a part but are not the only factors that affect the formation of calcium oxalate stones. Dietary oxalate is an organic ion found in many vegetables, fruits, and nuts. Calcium from bone may also play a role in kidney stone formation.

Industrial applications

Calcium oxalate is used in the manufacture of

ceramic glazes Ceramic glaze, or simply glaze, is a glassy coating on ceramics. It is used for decoration, to ensure the item is impermeable to liquids and to minimize the adherence of pollutants. Glazing renders earthenware impermeable to water, sealing th ...

References

{{Authority control Oxalates Calcium compounds Kidney