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Robertsite (Ca6Mn3+9(PO4)9O6(H2O)6·3H2O) or Ca2(Mn3(PO4)3O2)(H2O)3 or is a secondary phosphate species named for Willard Lincoln Roberts (1923-1987), mineralogist and professor who taught at South Dakota School of Mines in Rapid City, South Dakota. The type locality for Robertsite is the Tip Top mine, Custer County, South Dakota, USA.