Hard human tissues, teeth, and bone are composite materials. These composites are primarily composed of impure, calcium-deficient hydroxyapatite and collagen. The formula for stoichiometric hydroxyapatite is: Ca10(PO4)6(OH)2. However, this compound exists over a range of compositions that may be characterized in terms of a Ca/P ratio. The Ca/P ratio of stoichiometric material is 1.67. Stable compositions may have Ca/P ratios extending to 1.5:Ca9(PO4)6HOH. Structurally, hydroxyapatite contains two types of calcium sites. Four of the calcium sites per unit cell are each coordinated with 8 oxygens. Six of the calcium sites per unit cell are 7-coordinated. Six of these seven bonds are with oxygens, while the 7th is with a hydroxyl ion. Calcium deficiency results in the formation of up to one vacancy per unit cell on one of these seven coordinated calcium sites. Charge balance is maintained by the removal of one of the hydroxyl oxygens. The remaining proton becomes bound to an oxygen that is also coordinated with a phosphate. As a result, it is common to express the formula of calcium-deficient hydroxyapatite having a Ca/P ratio of 1.5 as Ca9(PO4)5(HPO4)OH.