Calcium oxalate

Calcium oxalate (in archaic terminology, oxalate of lime) is a calcium salt of oxalate with the chemical formula CaC2O4·(H2O)x, where x varies from 0 to 3. All forms are colorless or white. The monohydrate occurs naturally as the mineral whewellite, forming envelope-shaped crystals, known in plants as raphides. The rarer dihydrate (mineral: weddellite) and trihydrate (mineral: caoxite) are also recognized. Calcium oxalates are a major constituent of human kidney stones. Calcium oxalate is also found in beerstone, a scale that forms on containers used in breweries.

Calcium oxalate
Calcium oxalate
The ball-and-stick model of calcium oxalate
IUPAC name
Calcium oxalate
3D model (JSmol)
ECHA InfoCard 100.008.419
CaC2H2O5 (monohydrate)
CaC2O4 (anhydrous)
Appearance white solid
Density 2.20 g/cm3, monohydrate[1]
Melting point 200 °C (392 °F; 473 K) decomposes (monohydrate)
0.67 mg/L (20 °C)
Main hazards Harmful, Irritant
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oilHealth code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroformReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogenSpecial hazards (white): no codeNFPA 704 four-colored diamond
Related compounds
Other anions
Calcium carbonate
Calcium acetate
Calcium formate
Other cations
Sodium oxalate
Beryllium oxalate
Magnesium oxalate
Strontium oxalate
Barium oxalate
Radium oxalate
Iron(II) oxalate
Iron(III) oxalate
Related compounds
Oxalic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Surface of a kidney stone
Scanning electron micrograph of the surface of a kidney stone showing tetragonal crystals of Weddellite (calcium oxalate dihydrate) emerging from the amorphous central part of the stone (the horizontal length of the picture represents 0.5 mm of the figured original)
Portion of CaC2O4·2H2O lattice, highlighting the connectivity of the oxalate ligand.[1](Carbon: black; Oxygen: red; Calcium: green)


Many plants accumulate calcium oxalate as it has been reported in more than 1000 different genera of plants.[2] The calcium oxalate accumulation is linked to the detoxification of calcium (Ca2+) in the plant.[3]

The poisonous plant dumb cane (Dieffenbachia) contains the substance and on ingestion can prevent speech and be suffocating. It is also found in sorrel, rhubarb (in large quantities in the leaves), cinnamon, turmeric and in species of Oxalis, Araceae, Arum Italicum, taro, kiwifruit, tea leaves, agaves, Virginia creeper (Parthenocissus quinquefolia), and Alocasia and in spinach 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.

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.[4] 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.

Chemical properties

Calcium oxalate is a combination of calcium ions and the conjugate base of oxalic acid, the oxalate anion. The aqueous solution is slightly basic, due to the basicity of the oxalate ion. The basicity of it is weaker than sodium oxalate, due to the solubility of the compound.

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 ethylene glycol poisoning.[5] This latter form is what investigators discovered in the kidneys of two victims that ultimately led to the successful prosecution and conviction of murderer Lynn Turner, who poisoned both her husband and boyfriend with ethylene glycol-based antifreeze.[6]
Calcium oxalate crystals in urine

Urine microscopy showing calcium oxalate crystals in the urine. The octahedral crystal morphology is clearly visible.

Calcium oxalate crystals (urine) - kalsiyum oksalat kristalleri (idrar) - 01

Urine microscopy showing a calcium oxalate monohydrate crystal (dumbbell shaped) and a calcium oxalate dihydrate crystal (envelope shaped) along with several erythrocytes.

Calcium oxalate crystals (urine) - kalsiyum oksalat kristalleri (idrar) - 02

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.

Kidney stones

About 80% of kidney stones are partially or entirely of the calcium oxalate type. They form when urine is persistently saturated with calcium and oxalate. 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.[7]


  1. ^ a b S. Deganello (1981). "The Structure of Whewellite, CaC2O4.H2O, at 328 K". Acta Crystallogr. B. 37: 826–829. doi:10.1107/S056774088100441X.
  2. ^ Francesci, V.R.; Nakata (2005). "Calcium oxalate in plants: formation and function". Annu Rev Plant Biol (56): 41–71.
  3. ^ Martin, G; Matteo Guggiari; Daniel Bravo; Jakob Zopfi; Guillaume Cailleau; Michel Aragno; Daniel Job; Eric Verrecchia; Pilar Junier (2012). "Fungi, bacteria and soil pH: the oxalate–carbonate pathway as a model for metabolic interaction". Environmental Microbiology. 14 (11): 2960–2970. doi:10.1111/j.1462-2920.2012.02862.x. PMID 22928486.
  4. ^ Ryan, James (27 May 2018). "What is beerstone (and how to remove it)". Retrieved 2018-05-28.
  5. ^ "Urine Crystals". Cornell University. Retrieved 12 July 2014.
  6. ^ Nash, Alanna. "The Black Widow Killer: Two men. Two murders. Too many questions". Reader's Digest. Retrieved 2009-04-26.
  7. ^ "Calcium Oxalate Data Sheet". Hummel Croton Inc. Retrieved 2017-04-23.

See also


Arum is a genus of flowering plants in the family Araceae, native to Europe, northern Africa, and western and central Asia, with the highest species diversity in the Mediterranean region. Frequently called arum lilies, they are not closely related to the true lilies Lilium. Plants in closely related Zantedeschia are also called "arum lilies".

They are rhizomatous, herbaceous perennial plants growing to 20–60 cm tall, with sagittate (arrowhead-shaped) leaves 10–55 cm long. The flowers are produced in a spadix, surrounded by a 10–40-cm-long, coloured spathe, which may be white, yellow, brown, or purple; some species are scented, others not. The fruit is a cluster of bright orange or red berries.

All parts of the plants, including the berries, are poisonous, containing significant amounts of calcium oxalate as raphides.

The genus name is the latinized form of the Greek name for these plants, aron.

Bladder stone (animal)

Bladder stones or uroliths are a common occurrence in animals, especially in domestic animals such as dogs and cats. Occurrence in other species, including tortoises, has been reported as well. The stones form in the urinary bladder in varying size and numbers secondary to infection, dietary influences, and genetics. Stones can form in any part of the urinary tract in dogs and cats, but unlike in humans, stones of the kidney are less common and do not often cause significant disease, although they can contribute to pyelonephritis and chronic renal failure. Types of stones include struvite, calcium oxalate, urate, cystine, calcium phosphate, and silicate. Struvite and calcium oxalate stones are by far the most common.


Calgranulin is an S100 calcium-binding protein that is expressed in multiple cell types, including renal epithelial cells and neutrophils.

The proteins S100A8 and S100A9 form a heterodimer called calprotectin.


Commelinoideae is a subfamily of monocotyledonous flowering plants in the dayflower family (Commelinaceae). The Commelinoideae is one of two subfamilies within the Commelinaceae and includes 39 genera (out of 41 in the family) and all but 12 of the family's several hundred known species. The subfamily is further broken down into two tribes, the Tradescantieae, which includes 26 genera and about 300 species, and the Commelineae, which contains 13 genera and about 350 species.

The Commelinoideae is separated morphologically from the other subfamily, Cartonematoideae, in having glandular microhairs, arteries containing needle-like calcium oxalate crystals called raphide canals in between the veins of the leaves, and flowers that are virtually never both yellow and actinomorphic. Molecular phylogenetics also supports the separation of the two subfamilies.


Druse can refer to:

Druze or Durzi, a Middle Eastern religious community

Druse (botany), an aggregation of calcium oxalate crystals found in certain plants

Druse (geology), an incrustation of small crystals on the surface of a rock or mineral

Drusen, pathological deposits in the eye

Druse (botany)

A druse is a group of crystals of calcium oxalate, silicates, or carbonates present in plants, and are thought to be a defense against herbivory due to their toxicity. Calcium oxalate (Ca(COO)2, CaOx) crystals are found in algae, angiosperms and gymnosperms in a total of more than 215 families. These plants accumulate oxalate in the range of 3–80% (w/w) of their dry weight through a biomineralization process in a variety of shapes. Araceae have numerous druses, multi-crystal druses and needle-shaped raphide crystals of CaOx present in the tissue. Druses are also found in leaves and bud scales of Prunus, Rosa, Allium, Vitis, Morus and Phaseolus.

Geastrum pectinatum

Geastrum pectinatum is an inedible species of mushroom belonging to the earthstar family of fungi. Although young specimens are spherical, fruit body development involves the outer layer of tissue splitting open like a star into 7 to 10 pointed rays that eventually bend back to point downward, revealing a small – 1 to 2.5 cm (0.4 to 1.0 in) broad – spore sac. The spore sac is supported by a small radially wrinkled stalk. There is a distinct conical opening (peristome) at the top of the spore sac that is up to 8 mm (0.3 in) long. It is commonly known as the beaked earthstar or the beret earthstar, in reference to the shape of the spore sac and its prominent, protruding peristome. The mass of spores and surrounding cells within the sac, the gleba, is dark-brown, and becomes powdery in mature specimens. Spores are spherical, measuring 4 to 6 micrometers in diameter, with warts on their surfaces. Although uncommon, Geastrum pectinatum has a cosmopolitan distribution, and has been collected in various locations in Europe, North and South America, Asia and Africa, where it grows on the ground in open woods. Like several other earthstars, crystals of calcium oxalate are found on G. pectinatum, and are thought to be involved in fruit body maturation.

Geastrum saccatum

Geastrum saccatum, commonly known as the rounded earthstar, is a species of mushroom belonging to the genus Geastrum. It has a worldwide distribution and is found growing on rotting wood. It is considered inedible by mushroomers because of its bitter taste. It is a common mushroom, but collections are at their peak during late summer. The opening of the outer layer of the fruiting body in the characteristic star shape is thought to be due to a buildup of calcium oxalate crystals immediately prior to dehiscence. G. saccatum is distinguished from other earthstars by the distinct circular ridge or depression surrounding the central pore. In Brazil, its common name translates to "star of the land".


Hyperoxaluria is an excessive urinary excretion of oxalate. Individuals with hyperoxaluria often have calcium oxalate kidney stones. It is sometimes called Bird's disease, after Golding Bird, who first described the condition.


An idioblast is an isolated plant cell that differs from neighboring tissues. They have various functions such as storage of reserves, excretory materials, pigments, and minerals. They could contain oil, latex, gum, resin, tannin or pigments etc. Some can contain mineral crystals such as acrid tasting and poisonous calcium oxalate or carbonate or silica.

Any of the tissue or tissue systems of plants can contain idioblasts.

Idioblasts are divided into three main categories: excretory, tracheoid, and sclerenchymatous.

Idioblasts can contain biforine cells that form crystals. The chemicals are excreted by the plant and stored in liquid or crystalline form. In bundles they are known as druse and as crystals they can be of raphide [needle] form. When the end of an idioblast is broken the crystals or other substance is ejected by internal water pressure. Idioblasts of calcium oxalate may function as a deterrent to herbivores, as a means of sequestering or storing calcium, or as a means of stiffening tissue structure.

Kidney stone disease

Kidney stone disease, also known as urolithiasis, is when a solid piece of material (kidney stone) develops in the urinary tract. Kidney stones typically form in the kidney and leave the body in the urine stream. A small stone may pass without causing symptoms. If a stone grows to more than 5 millimeters (0.2 in) it can cause blockage of the ureter resulting in severe pain in the lower back or abdomen. A stone may also result in blood in the urine, vomiting, or painful urination. About half of people will have another stone within ten years.Most stones form due to a combination of genetics and environmental factors. Risk factors include high urine calcium levels; obesity; certain foods; some medications; calcium supplements; hyperparathyroidism; gout and not drinking enough fluids. Stones form in the kidney when minerals in urine are at high concentration. The diagnosis is usually based on symptoms, urine testing, and medical imaging. Blood tests may also be useful. Stones are typically classified by their location: nephrolithiasis (in the kidney), ureterolithiasis (in the ureter), cystolithiasis (in the bladder), or by what they are made of (calcium oxalate, uric acid, struvite, cystine).In those who have had stones, prevention is by drinking fluids such that more than two liters of urine are produced per day. If this is not effective enough, thiazide diuretic, citrate, or allopurinol may be taken. It is recommended that soft drinks containing phosphoric acid (typically colas) be avoided. When a stone causes no symptoms, no treatment is needed. Otherwise pain control is usually the first measure, using medications such as nonsteroidal anti-inflammatory drugs or opioids. Larger stones may be helped to pass with the medication tamsulosin or may require procedures such as extracorporeal shock wave lithotripsy, ureteroscopy, or percutaneous nephrolithotomy.Between 1% and 15% of people globally are affected by kidney stones at some point in their lives. In 2015, 22.1 million cases occurred, resulting in about 16,100 deaths. They have become more common in the Western world since the 1970s. Generally, more men are affected than women. Kidney stones have affected humans throughout history with descriptions of surgery to remove them dating from as early as 600 BC.

Malpighian tubule system

The Malpighian tubule system is a type of excretory and osmoregulatory system found in some insects, myriapods, arachnids, and tardigrades.

The system consists of branching tubules extending from the alimentary canal that absorbs solutes, water, and wastes from the surrounding hemolymph. The wastes then are released from the organism in the form of solid nitrogenous compounds and calcium oxalate. The system is named after Marcello Malpighi, a seventeenth-century anatomist.

It is unclear as to whether the Malpighian tubules of arachnids and those of the Uniramia are homologous or the result of convergent evolution.


Nephrocalcinosis, once known as Albright's calcinosis after Fuller Albright, or Anderson-Carr kidneys, is a term originally used to describe deposition of calcium salts in the renal parenchyma due to hyperparathyroidism. The term nephrocalcinosis is used to describe the deposition of both calcium oxalate and calcium phosphate. It may cause acute kidney injury. It is now more commonly used to describe diffuse, fine, renal parenchymal calcification on radiology. It is caused by multiple different conditions and is determined progressive kidney dysfunction. These outlines eventually come together to form a dense mass. During its early stages, nephrocalcinosis is visible on x-ray, and appears as a fine granular mottling over the renal outlines. It is most commonly seen as an incidental finding with medullary sponge kidney on an abdominal x-ray.

However, it may be severe enough to cause (as well as be caused by) renal tubular acidosis or even end stage renal failure, due to disruption of the renal tissue by the deposited calcium.


Oxalate (IUPAC: ethanedioate) is the dianion with the formula C2O2−4, also written (COO)2−2. Either name is often used for derivatives, such as salts of oxalic acid, for example sodium oxalate Na2C2O4, or dimethyl oxalate ((CH3)2C2O4). Oxalate also forms coordination compounds where it is sometimes abbreviated as ox.

Many metal ions form insoluble precipitates with oxalate, a prominent example being calcium oxalate, the primary constituent of the most common kind of kidney stones.


Raphides are needle-shaped crystals of calcium oxalate as the monohydrate or calcium carbonate as aragonite, found in more than 200 families of plants.

Both ends are needle-like, but raphides tend to be blunt at one end and sharp at the other.


Ultrastructure (or ultra-structure) is the architecture of cells and biomaterials that is visible at higher magnifications than found on a standard optical light microscope. This traditionally meant the resolution and magnification range of a conventional transmission electron microscope (TEM) when viewing biological specimens such as cells, tissue, or organs. Ultrastructure can also be viewed with scanning electron microscopy and super-resolution microscopy, although TEM is a standard histology technique for viewing ultrastructure. Such cellular structures as organelles, which allow the cell to function properly within its specified environment, can be examined at the ultrastructural level.

Ultrastructure, along with molecular phylogeny, is a reliable phylogenetic way of classifying organisms. Features of ultrastructure are used industrially to control material properties and promote biocompatibility.


Weddellite (CaC2O4·2H2O) is a mineral form of calcium oxalate named for occurrences of millimeter-sized crystals found in bottom sediments of the Weddell Sea, off Antarctica. Occasionally, weddellite partially dehydrates to whewellite, forming excellent pseudomorphs of grainy whewellite after weddellite's short tetragonal dipyramids. It was first described in 1942.


Whewellite is a mineral, hydrated calcium oxalate, formula Ca C2O4·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 sedimentary nodules. However, it has also been found in hydrothermal deposits where a biological source appears improbable. For this reason, it may be classed as a true mineral.

Whewellite, or at least crystalline calcium oxalate, does also arise from biological sources. Small crystals or flakes of it are sometimes found on the surfaces of some cacti, and kidney stones frequently have the same composition.

Whewellite was named after William Whewell (1794–1866), an English polymath, naturalist and scientist, professor of moral philosophy at Cambridge and inventor of the system of crystallographic indexing.

Calcium compounds

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