Whitlockite is a mineral, an unusual form of calcium phosphate. It's actual formula is debatable depending on the authority consulted; starting from Ca3(PO4)2 it may also contain magnesium (CaMg)2(PO4)2 to the more complex Ca9(MgFe)(PO4)6PO3OH. It is a relatively rare mineral but is found in granitic pegmatite and has also been found in meteorites.
With regards to periodontal dentistry, Magnesium Whitlockite is found primarily in subgingival calculus (as opposed to supragingival calculus). It is also found more in posterior as opposed to anterior regions of the oral cavity. It comprises one component of many of the inorganic content of calculus.