Mg4Al3(HO)17 crystallizes in the triclinic P1 space group. The structure is three-dimensional. there are four inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share edges with two equivalent AlO6 octahedra and edges with four MgO6 octahedra. There are a spread of Mg?O bond distances ranging from 2.02?2.15 �. In the second Mg2+ site, Mg2+ is bonded to six O2- atoms to form edge-sharing MgO6 octahedra. There are a spread of Mg?O bond distances ranging from 2.05?2.14 �. In the third Mg2+ site, Mg2+ is bonded to six O2- atoms to form edge-sharing MgO6 octahedra. There are a spread of Mg?O bond distances ranging from 2.06?2.14 �. In the fourth Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share edges with two equivalent AlO6 octahedra and edges with four MgO6 octahedra. There are a spread of Mg?O bond distances ranging from 2.07?2.13 �. There are three inequivalent Al3+ sites. In the first Al3+ site, Al3+ is bonded to six O2- atoms to form AlO6 octahedra that share edges with two equivalent MgO6 octahedra and edges with four AlO6 octahedra. There are a spread of Al?O bond distances ranging from 1.90?2.06 �. In the second Al3+ site, Al3+ is bonded to six O2- atoms to form AlO6 octahedra that share edges with two equivalent MgO6 octahedra and edges with four AlO6 octahedra. There are a spread of Al?O bond distances ranging from 1.87?2.14 �. In the third Al3+ site, Al3+ is bonded to six O2- atoms to form edge-sharing AlO6 octahedra. There are a spread of Al?O bond distances ranging from 1.88?2.12 �. There are seventeen inequivalent H1+ sites. In the first H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H?O bond length is 0.97 �. In the second H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H?O bond length is 0.97 �. In the third H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H?O bond length is 0.96 �. In the fourth H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H?O bond length is 0.97 �. In the fifth H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H?O bond length is 0.97 �. In the sixth H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H?O bond length is 0.97 �. In the seventh H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H?O bond length is 0.97 �. In the eighth H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H?O bond length is 0.98 �. In the ninth H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H?O bond length is 0.97 �. In the tenth H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H?O bond length is 0.98 �. In the eleventh H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H?O bond length is 0.98 �. In the twelfth H1+ site, H1+ is bonded in a linear geometry to two O2- atoms. There is one shorter (1.06 �) and one longer (1.47 �) H?O bond length. In the thirteenth H1+ site, H1+ is bonded in a distorted linear geometry to two O2- atoms. There is one shorter (1.00 �) and one longer (1.64 �) H?O bond length. In the fourteenth H1+ site, H1+ is bonded in a single-bond geometry to one O2- atom. The H?O bond length is 0.98 �. In the fifteenth H1+ site, H1+ is bonded in a linear geometry to two O2- atoms. There is one shorter (1.03 �) and one longer (1.52 �) H?O bond length. In the sixteenth H1+ site, H1+ is bonded in a distorted single-bond geometry to one O2- atom. The H?O bond length is 1.01 �. In the seventeenth H1+ site, H1+ is bonded in a distorted linear geometry to two O2- atoms. There is one shorter (1.02 �) and one longer (1.62 �) H?O bond length. There are seventeen inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted single-bond geometry to one Mg2+, two equivalent Al3+, and one H1+ atom. In the second O2- site, O2- is bonded to two equivalent Mg2+, one Al3+, and one H1+ atom to form distorted corner-sharing OMg2AlH tetrahedra. In the third O2- site, O2- is bonded in a 1-coordinate geometry to three Mg2+ and one H1+ atom. In the fourth O2- site, O2- is bonded in a 1-coordinate geometry to three Mg2+ and one H1+ atom. In the fifth O2- site, O2- is bonded in a 1-coordinate geometry to three Mg2+ and one H1+ atom. In the sixth O2- site, O2- is bonded in a distorted single-bond geometry to three Mg2+ and one H1+ atom. In the seventh O2- site, O2- is bonded in a distorted single-bond geometry to three Mg2+ and one H1+ atom. In the eighth O2- site, O2- is bonded in a distorted single-bond geometry to three Mg2+ and one H1+ atom. In the ninth O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to two equivalent Mg2+, one Al3+, and one H1+ atom. In the tenth O2- site, O2- is bonded in a water-like geometry to two H1+ atoms. In the eleventh O2- site, O2- is bonded to one Mg2+, two equivalent Al3+, and one H1+ atom to form distorted OMgAl2H trigonal pyramids that share corners with two equivalent OAl3H tetrahedra and corners with two equivalent OMgAl2H trigonal pyramids. In the twelfth O2- site, O2- is bonded in a 4-coordinate geometry to three Al3+ and one H1+ atom. In the thirteenth O2- site, O2- is bonded in a distorted water-like geometry to three H1+ atoms. In the fourteenth O2- site, O2- is bonded to three Al3+ and one H1+ atom to form OAl3H tetrahedra that share corners with four OAl3H tetrahedra and corners with two equivalent OMgAl2H trigonal pyramids. In the fifteenth O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to three Al3+ and one H1+ atom. In the sixteenth O2- site, O2- is bonded in a distorted water-like geometry to two H1+ atoms. In the seventeenth O2- site, O2- is bonded to three Al3+ and one H1+ atom to form corner-sharing OAl3H tetrahedra.