Brushite, dicalcium phosphate dihydrate (DCPD), is a hydrated calcium phosphate mineral with the composition CaHPO4·2H2O. The structure of brushite is almost identical to that of gypsum, with their respective unit cell dimensions very similar. The phosphate of brushite is isomorphous with arsenate, which forms instead, the mineral Pharmacolite. The backbone of brushite is made up of alternating calcium and phosphorus atoms which form chains via the sharing of oxygens (the four phosphate oxygens). These chains run perpendicular to the b axis in the plane formed by the a and c axes as shown in figure 1.
Highlighting FeaturesCalcium (Ca) atoms Phosphate groups Show single calcium with coordinating oxygens Show projection down Ca-P chain All atoms
Figure 1. Calcium (green) and phosphorus (orange) chain
The phosphorus atoms have four fold coordination forming phosphate tetrahedra while the calcium atoms have eight fold coordination. Four of calcium’s bonds are with these phosphate tetrahedra, forming the backbone chains, while two more of the calcium bonds are with oxygens from neighboring chains. These last bonds which link the chains together result in a zigzag pattern or corrugated sheet. The last two calcium bonds are with the water molecules. These water molecules are what hold the sheets together.
Calcium and phosphorus make up the majority of an animal’s mineral nutrient requirements (to fulfill bone, body tissue and milk needs), therefore brushite and its anhydrous relative, monetite, are a common and widely used animal supplement.
Brushite is also of interest because it is the most soluble of the sparingly soluble calcium phosphate minerals. The fate of brushite in soils is rather transient. Typically, mineral phosphorus is added to soil in water soluble forms such as monocalcium phosphate (“triple superphosphate”) or mono- or diammonium phosphate. As the phosphorus dissolves, the high solution P concentration reacts with native soil Ca in either soluble or exchangeable form. In neutral and calcareous soils, brurhsite is one of the first reaction products and controls soluble phosphate concentrations in the soil solution for a time. But the relatively soluble brushite is unstable so after time (weeks to months) other more stable, less soluble calcium phosphate minerals will form.
Brushite forms in a slightly acidic environment, unlike many of the other calcium phosphate minerals. It is therefore a candidate for phosphorus recovery from slightly acidic wastewater containing dissolved phosphate, which can be treated with calcium hydroxide to both supply the calcium required to make brushite and raise the solution pH to restrict the solubility of brushite and cause it to precipitate; such as process is largely complete at pH 6.5.
Beevers, C. A., Acta Cryst., (1958). 11,273