Calcium oxalate is a chemical compound that forms envelope-shaped crystals, known in plants
as raphides.
A major constituent of human kidney stones, the chemical is also found in beerstone, a
scale that forms on containers used in breweries.
Its chemical formula is CaC2O4 or Ca2.
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.
Calcium oxalate is a poisonous substance that can produce sores and numbing on ingestion
and could even be fatal.
The poisonous plant dumb cane contains the substance and on ingestion can prevent speech
and be suffocating.
It is also found in rhubarb and in species of Oxalis, Araceae, taro, kiwifruit, tea leaves,
agaves, and Alocasia and in spinach in varying amounts.
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 completely 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 flavor 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.
Hydrated forms of the compound occur naturally as three mineral species: whewellite, weddellite
and a very rare trihydrate called caoxite.
Morphology Calcium oxalate exists in monohydrate and
dihydrate form, which can be distinguished by the shape of its 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.
Effects of ingestion Even a small dose of calcium oxalate is enough
to cause intense sensations of burning in the mouth and throat, swelling, and choking
that could last for up to two weeks.
In greater doses it can cause severe digestive upset, breathing difficulties, coma or even
death.
Recovery from severe oxalate poisoning is possible, but permanent liver and kidney damage
may have occurred.
The stalks of plants in the Dieffenbachia genus produce the most severe oxalate reactions.
The needle-like oxalate crystals produce pain and swelling when they contact lips, tongue,
oral mucosa, conjunctiva, or skin.
Edema primarily is due to direct trauma from the needle-like crystals and, to a lesser
extent, by other plant toxins.
Depending on the plant ingested, mild to more severe can cause compromised airways.
One bite on the Arisaema seed pod will result in immediate swelling and burning.
It will take over 12 hours for the swelling to subside.
Treatment Medication administered at the emergency room
may include diphenhydramine, epinephrine, or famotidine, all intravenously.
Applications Calcium oxalate is used in the manufacture
of ceramic glazes.
References
See also Oxalic acid