Ferrisicklerite is a phosphate of the triphylite group isotypic with olivine. The sample in this study has the composition Lil 2+ 2+ 3+ .61Na0.07Mg0.15Ca0.07Fe0.03Mnl.asFe2.45[(PO4)3.76(O4H4)0.25], space group Pmnb with a= 5.918, b= 10-037, c=4.799 A. The refinement of the structure indicates a partial occupancy of the octahedral site M(1) by Li and Na, a complete occupancy of the octahedral site M(2) by the remaining cations with isomorphous substitution between Mn 2+ and Fe 3+ in this site and the partial substitution of the group PO4 by the group (OH)4. The distortion of the coordination polyhedra can be explained in terms of the strong imbalance of the bond strengths to the anions.