Calcium pyrophosphate refers to any member of a series of inorganic compound with the formula Ca2P2O7(H2O)n. They are white solids that are insoluble in water.

Deposition of dihydrate crystals in cartilage are responsible for the severe joint pain in cases of calcium pyrophosphate deposition disease (pseudo gout) whose symptoms are similar to those of gout.

[1] Ca2P2O7 is commonly used as a mild abrasive agent in toothpastes because of its insolubility and nonreactivity toward fluoride.

The stable tetrahydrate was originally reported to be rhombohedral but is now believed to be monoclinic.

[4] The anhydrous form has 3 polymorphs, α-, β-, and metastable γ[5] (Tα/β=1140ºС[6]).