Calcium arsenate

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| Name = Calcium arsenate
| ImageFile = Calciumarsenaat. png
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|IUPACName=Calcium arsorate
| ImageName = Calcium arsenate
| ImageFile2 = Arsenian Wapnia. jpg
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| ImageName2 = Calcium arsenate
| OtherNames = Calcium orthoarsenate Cucumber dust Tricalcium arsenate Tricalcium ortho arsenate
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|Section3=Chembox Hazards
| NFPA H = 4
| NFPA R = 0
| NFPA F = 0
| NFPA S =
| LD50 = 20 mg/kg (rat, oral)82 mg/kg (rat, oral)74 mg/kg (mouse, oral)50 mg/kg (rabbit, oral)38 mg/kg (dog, oral)
| PEL = TWA 0.010 mg/m3
| IDLH = 5 mg/m3 (as As) A related mineral is ferrarisite (|Ca5H2(AsO4)4*9H2O.

Calcium arsenate use is now banned in the UK, and its use is strictly regulated in the United States. It is currently the active ingredient in TURF Cal manufactured by Mallinckrodt, it is one of the few herbicides – used mainly for the control of Poa annua and crabgrass that hinders earthworm activity. Its label states that it will "reduce and inhibit earthworm activity and survival" and is only recommended against serious earthworm infestations in places such as golf course greens.

Calcium arsenate is highly toxic, having both carcinogenic and systemic health effects. The Occupational Safety and Health Administration has set a permissible exposure limit at 0.01 mg/m3 over an eight hour time weighted average, while the National Institute for Occupational Safety and Health recommends a limit five times less (0.002 mg/m3). It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U. S. Emergency Planning and Community Right to Know Act (42 U. S. C. 11002), and is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.

Weilite is the monohydrogenated counterpart, Ca(HAsO4), while švenekite – the dihydrogenated one, Ca(H2AsO4)2. Hydrated analogues of weilite are haidingerite (monohydrate) and pharmacolite (dihydrate), with the latter name reflecting arsenic related toxicity. Examples of more complex, hydrated Ca arsenates with some anions hydrogenated, are ferrarisite, guérinite, sainfeldite, vladimirite, and jeankempite.