During a mineralogical study of guano-derived sedimentary phosphate ore from the Lamphun area of northwest Thailand, robertsite [Ca₆(H₂O)₆Mn³⁺₉O₆(PO₄)₉.3H₂O] was identified and concentrated to a level suitable for X-ray diffraction and chemical analysis. Robertsite has previously been described only in association with altered phosphatic pegmatites. This new occurrence in a phosphate deposit indicates that it may be found in a broad spectrum of sedimentary rocks.