Ca2Mn3O8 crystallizes in the monoclinic C2/m space group. Ca(1) is bonded to one O(1), one O(2), and four equivalent O(3) atoms to form distorted CaO6 pentagonal pyramids that share corners with three equivalent Mn(1)O6 octahedra, corners with four equivalent Mn(2)O6 octahedra,  an edgeedge with one Mn(1)O6 octahedra, edges with two equivalent Mn(2)O6 octahedra, and edges with two equivalent Ca(1)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 20-56°. There are two inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form MnO6 octahedra that share corners with six equivalent Ca(1)O6 pentagonal pyramids, edges with four equivalent Mn(2)O6 octahedra, and edges with two equivalent Ca(1)O6 pentagonal pyramids. In the second Mn site, Mn(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form MnO6 octahedra that share corners with four equivalent Ca(1)O6 pentagonal pyramids, edges with two equivalent Mn(1)O6 octahedra, edges with two equivalent Mn(2)O6 octahedra, and edges with two equivalent Ca(1)O6 pentagonal pyramids. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to one Ca(1) and two equivalent Mn(2) atoms. In the second O site, O(2) is bonded to one Ca(1), one Mn(1), and two equivalent Mn(2) atoms to form distorted OCaMn3 trigonal pyramids that share  a cornercorner with one O(2)CaMn3 trigonal pyramid, corners with four equivalent O(3)Ca2Mn2 trigonal pyramids,  an edgeedge with one O(2)CaMn3 trigonal pyramid, and edges with four equivalent O(3)Ca2Mn2 trigonal pyramids. In the third O site, O(3) is bonded to two equivalent Ca(1), one Mn(1), and one Mn(2) atom to form distorted OCa2Mn2 trigonal pyramids that share corners with two equivalent O(2)CaMn3 trigonal pyramids, corners with six equivalent O(3)Ca2Mn2 trigonal pyramids, edges with two equivalent O(3)Ca2Mn2 trigonal pyramids, and edges with two equivalent O(2)CaMn3 trigonal pyramids.
Ca2Mn3O8 crystallizes in the monoclinic C2/m space group. Ca(1) is bonded to one O(1), one O(2), and four equivalent O(3) atoms to form distorted CaO6 pentagonal pyramids that share corners with three equivalent Mn(1)O6 octahedra, corners with four equivalent Mn(2)O6 octahedra,  an edgeedge with one Mn(1)O6 octahedra, edges with two equivalent Mn(2)O6 octahedra, and edges with two equivalent Ca(1)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 20-56°. The Ca(1)-O(1) bond length is 2.28 Å. The Ca(1)-O(2) bond length is 2.37 Å. There are two shorter (2.26 Å) and two longer (2.42 Å) Ca(1)-O(3) bond lengths. There are two inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form MnO6 octahedra that share corners with six equivalent Ca(1)O6 pentagonal pyramids, edges with four equivalent Mn(2)O6 octahedra, and edges with two equivalent Ca(1)O6 pentagonal pyramids. Both Mn(1)-O(2) bond lengths are 1.91 Å. All Mn(1)-O(3) bond lengths are 1.89 Å. In the second Mn site, Mn(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form MnO6 octahedra that share corners with four equivalent Ca(1)O6 pentagonal pyramids, edges with two equivalent Mn(1)O6 octahedra, edges with two equivalent Mn(2)O6 octahedra, and edges with two equivalent Ca(1)O6 pentagonal pyramids. Both Mn(2)-O(1) bond lengths are 1.86 Å. Both Mn(2)-O(2) bond lengths are 2.00 Å. Both Mn(2)-O(3) bond lengths are 1.89 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to one Ca(1) and two equivalent Mn(2) atoms. In the second O site, O(2) is bonded to one Ca(1), one Mn(1), and two equivalent Mn(2) atoms to form distorted OCaMn3 trigonal pyramids that share  a cornercorner with one O(2)CaMn3 trigonal pyramid, corners with four equivalent O(3)Ca2Mn2 trigonal pyramids,  an edgeedge with one O(2)CaMn3 trigonal pyramid, and edges with four equivalent O(3)Ca2Mn2 trigonal pyramids. In the third O site, O(3) is bonded to two equivalent Ca(1), one Mn(1), and one Mn(2) atom to form distorted OCa2Mn2 trigonal pyramids that share corners with two equivalent O(2)CaMn3 trigonal pyramids, corners with six equivalent O(3)Ca2Mn2 trigonal pyramids, edges with two equivalent O(3)Ca2Mn2 trigonal pyramids, and edges with two equivalent O(2)CaMn3 trigonal pyramids.