Robertsite, Ca3(Mn3+)42·3(H2O) (alternatively formulated Ca2(Mn3(PO4)3O2)(H2O)3), is a secondary phosphate mineral named for Willard Lincoln Roberts (1923–1987), mineralogist and professor 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 , US. Robertsite occurs at the Tip Top Mine as minute crystal aggregates and crusts found on quartz. It is a dark reddish brown to black monoclinic mineral.
It occurs as a secondary mineral in pegmatite. It is also reported from the Khoa Rang Kai phosphate deposit, Chiang Mai , Thailand It is associated with rockbridgeite, leucophosphite, collinsite in the type locality. In the guano deposit it occurs with carbonate-fluorapatite , calcite and clay minerals In the Omo Valley it occurs with mitridatite fish in Pliocene
Recently, in an exploration conducted by the Italian La Venta Geographical Association, confirmed the existence of Robertsite in the Puerto Princesa Subterranean River National Park, located about 50 kilometres (31 mi) north of the city center of Puerto Princesa * Mitridatite group: * Arseniosiderite * Ca2(Fe3+)3·3H2O * to * Ca2(Fe3+)3·3H2O * Arseniosiderite-robertsite series: * Ca2(Fe3+)3·3H2O * to * Ca3(Mn3+)42·3H2O