Ba10MgAs6(O12Cl)2 crystallizes in the monoclinic Pm space group. There are eight inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 9-coordinate geometry to one O(10), one O(11), one O(13), one O(14), one O(18), one O(3), one O(6), one O(7), and one O(9) atom. In the second Ba site, Ba(2) is bonded in a 7-coordinate geometry to one O(12), one O(15), one O(16), one O(2), one O(4), one O(5), and one O(8) atom. In the third Ba site, Ba(3) is bonded in a 7-coordinate geometry to one O(11), two equivalent O(10), two equivalent O(5), and two equivalent Cl(1) atoms. In the fourth Ba site, Ba(4) is bonded in a 8-coordinate geometry to one O(17), one O(9), two equivalent O(1), two equivalent O(13), and two equivalent Cl(1) atoms. In the fifth Ba site, Ba(5) is bonded in a 6-coordinate geometry to one O(15), one O(18), two equivalent O(16), two equivalent O(3), and two equivalent Cl(1) atoms. In the sixth Ba site, Ba(6) is bonded in a 8-coordinate geometry to one O(12), one O(7), two equivalent O(1), two equivalent O(10), and two equivalent Cl(1) atoms. In the seventh Ba site, Ba(7) is bonded in a 7-coordinate geometry to one O(8), two equivalent O(13), two equivalent O(16), and two equivalent Cl(1) atoms. In the eighth Ba site, Ba(8) is bonded in a 8-coordinate geometry to one O(14), one O(4), two equivalent O(3), two equivalent O(5), and two equivalent Cl(1) atoms. Mg(1) is bonded in a distorted T-shaped geometry to one O(15), one O(17), and one O(2) atom. There are six inequivalent As sites. In the first As site, As(1) is bonded in a distorted trigonal non-coplanar geometry to one O(11) and two equivalent O(13) atoms. In the second As site, As(2) is bonded in a tetrahedral geometry to one O(4), one O(6), and two equivalent O(1) atoms. In the third As site, As(3) is bonded in a tetrahedral geometry to one O(12), one O(14), and two equivalent O(16) atoms. In the fourth As site, As(4) is bonded in a tetrahedral geometry to one O(2), one O(9), and two equivalent O(3) atoms. In the fifth As site, As(5) is bonded in a tetrahedral geometry to one O(7), one O(8), and two equivalent O(5) atoms. In the sixth As site, As(6) is bonded in a tetrahedral geometry to one O(17), one O(18), and two equivalent O(10) atoms. There are eighteen inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Ba(4), one Ba(6), and one As(2) atom. In the second O site, O(2) is bonded in a distorted bent 120 degrees geometry to two equivalent Ba(2), one Mg(1), and one As(4) atom. In the third O site, O(3) is bonded in a 1-coordinate geometry to one Ba(1), one Ba(5), one Ba(8), and one As(4) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Ba(8), two equivalent Ba(2), and one As(2) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one Ba(2), one Ba(3), one Ba(8), and one As(5) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to two equivalent Ba(1) and one As(2) atom. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to one Ba(6), two equivalent Ba(1), and one As(5) atom. In the eighth O site, O(8) is bonded in a 4-coordinate geometry to one Ba(7), two equivalent Ba(2), and one As(5) atom. In the ninth O site, O(9) is bonded in a distorted single-bond geometry to one Ba(4), two equivalent Ba(1), and one As(4) atom. In the tenth O site, O(10) is bonded in a 4-coordinate geometry to one Ba(1), one Ba(3), one Ba(6), and one As(6) atom. In the eleventh O site, O(11) is bonded to one Ba(3), two equivalent Ba(1), and one As(1) atom to form distorted OBa3As tetrahedra that share corners with two equivalent Cl(1)Ba6 octahedra. The corner-sharing octahedral tilt angles are 47°. In the twelfth O site, O(12) is bonded in a 4-coordinate geometry to one Ba(6), two equivalent Ba(2), and one As(3) atom. In the thirteenth O site, O(13) is bonded in a 4-coordinate geometry to one Ba(1), one Ba(4), one Ba(7), and one As(1) atom. In the fourteenth O site, O(14) is bonded in a distorted single-bond geometry to one Ba(8), two equivalent Ba(1), and one As(3) atom. In the fifteenth O site, O(15) is bonded in a distorted see-saw-like geometry to one Ba(5), two equivalent Ba(2), and one Mg(1) atom. In the sixteenth O site, O(16) is bonded to one Ba(2), one Ba(5), one Ba(7), and one As(3) atom to form distorted OBa3As trigonal pyramids that share corners with two equivalent Cl(1)Ba6 octahedra,  a cornercorner with one O(16)Ba3As trigonal pyramid,  an edgeedge with one Cl(1)Ba6 octahedra, and  an edgeedge with one O(16)Ba3As trigonal pyramid. The corner-sharing octahedral tilt angles range from 60-80°. In the seventeenth O site, O(17) is bonded in a 3-coordinate geometry to one Ba(4), one Mg(1), and one As(6) atom. In the eighteenth O site, O(18) is bonded in a 4-coordinate geometry to one Ba(5), two equivalent Ba(1), and one As(6) atom. Cl(1) is bonded to one Ba(3), one Ba(4), one Ba(5), one Ba(6), one Ba(7), and one Ba(8) atom to form distorted ClBa6 octahedra that share  a cornercorner with one O(11)Ba3As tetrahedra, corners with two equivalent O(16)Ba3As trigonal pyramids,  an edgeedge with one O(16)Ba3As trigonal pyramid, and faces with two equivalent Cl(1)Ba6 octahedra.
Ba10MgAs6(O12Cl)2 crystallizes in the monoclinic Pm space group. There are eight inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 9-coordinate geometry to one O(10), one O(11), one O(13), one O(14), one O(18), one O(3), one O(6), one O(7), and one O(9) atom. The Ba(1)-O(10) bond length is 2.88 Å. The Ba(1)-O(11) bond length is 2.70 Å. The Ba(1)-O(13) bond length is 3.24 Å. The Ba(1)-O(14) bond length is 3.07 Å. The Ba(1)-O(18) bond length is 2.93 Å. The Ba(1)-O(3) bond length is 3.17 Å. The Ba(1)-O(6) bond length is 2.76 Å. The Ba(1)-O(7) bond length is 2.81 Å. The Ba(1)-O(9) bond length is 2.79 Å. In the second Ba site, Ba(2) is bonded in a 7-coordinate geometry to one O(12), one O(15), one O(16), one O(2), one O(4), one O(5), and one O(8) atom. The Ba(2)-O(12) bond length is 2.89 Å. The Ba(2)-O(15) bond length is 2.49 Å. The Ba(2)-O(16) bond length is 2.93 Å. The Ba(2)-O(2) bond length is 2.96 Å. The Ba(2)-O(4) bond length is 2.83 Å. The Ba(2)-O(5) bond length is 2.89 Å. The Ba(2)-O(8) bond length is 2.70 Å. In the third Ba site, Ba(3) is bonded in a 7-coordinate geometry to one O(11), two equivalent O(10), two equivalent O(5), and two equivalent Cl(1) atoms. The Ba(3)-O(11) bond length is 2.71 Å. Both Ba(3)-O(10) bond lengths are 2.79 Å. Both Ba(3)-O(5) bond lengths are 2.74 Å. Both Ba(3)-Cl(1) bond lengths are 3.29 Å. In the fourth Ba site, Ba(4) is bonded in a 8-coordinate geometry to one O(17), one O(9), two equivalent O(1), two equivalent O(13), and two equivalent Cl(1) atoms. The Ba(4)-O(17) bond length is 2.67 Å. The Ba(4)-O(9) bond length is 2.82 Å. Both Ba(4)-O(1) bond lengths are 3.13 Å. Both Ba(4)-O(13) bond lengths are 2.62 Å. Both Ba(4)-Cl(1) bond lengths are 3.25 Å. In the fifth Ba site, Ba(5) is bonded in a 6-coordinate geometry to one O(15), one O(18), two equivalent O(16), two equivalent O(3), and two equivalent Cl(1) atoms. The Ba(5)-O(15) bond length is 2.71 Å. The Ba(5)-O(18) bond length is 2.58 Å. Both Ba(5)-O(16) bond lengths are 2.69 Å. Both Ba(5)-O(3) bond lengths are 3.18 Å. Both Ba(5)-Cl(1) bond lengths are 3.81 Å. In the sixth Ba site, Ba(6) is bonded in a 8-coordinate geometry to one O(12), one O(7), two equivalent O(1), two equivalent O(10), and two equivalent Cl(1) atoms. The Ba(6)-O(12) bond length is 2.74 Å. The Ba(6)-O(7) bond length is 3.05 Å. Both Ba(6)-O(1) bond lengths are 2.90 Å. Both Ba(6)-O(10) bond lengths are 2.80 Å. Both Ba(6)-Cl(1) bond lengths are 3.61 Å. In the seventh Ba site, Ba(7) is bonded in a 7-coordinate geometry to one O(8), two equivalent O(13), two equivalent O(16), and two equivalent Cl(1) atoms. The Ba(7)-O(8) bond length is 2.57 Å. Both Ba(7)-O(13) bond lengths are 2.74 Å. Both Ba(7)-O(16) bond lengths are 2.92 Å. Both Ba(7)-Cl(1) bond lengths are 3.25 Å. In the eighth Ba site, Ba(8) is bonded in a 8-coordinate geometry to one O(14), one O(4), two equivalent O(3), two equivalent O(5), and two equivalent Cl(1) atoms. The Ba(8)-O(14) bond length is 3.04 Å. The Ba(8)-O(4) bond length is 2.66 Å. Both Ba(8)-O(3) bond lengths are 2.75 Å. Both Ba(8)-O(5) bond lengths are 2.97 Å. Both Ba(8)-Cl(1) bond lengths are 3.53 Å. Mg(1) is bonded in a distorted T-shaped geometry to one O(15), one O(17), and one O(2) atom. The Mg(1)-O(15) bond length is 1.92 Å. The Mg(1)-O(17) bond length is 1.88 Å. The Mg(1)-O(2) bond length is 2.00 Å. There are six inequivalent As sites. In the first As site, As(1) is bonded in a distorted trigonal non-coplanar geometry to one O(11) and two equivalent O(13) atoms. The As(1)-O(11) bond length is 1.78 Å. Both As(1)-O(13) bond lengths are 1.80 Å. In the second As site, As(2) is bonded in a tetrahedral geometry to one O(4), one O(6), and two equivalent O(1) atoms. The As(2)-O(4) bond length is 1.74 Å. The As(2)-O(6) bond length is 1.73 Å. Both As(2)-O(1) bond lengths are 1.74 Å. In the third As site, As(3) is bonded in a tetrahedral geometry to one O(12), one O(14), and two equivalent O(16) atoms. The As(3)-O(12) bond length is 1.73 Å. The As(3)-O(14) bond length is 1.72 Å. Both As(3)-O(16) bond lengths are 1.73 Å. In the fourth As site, As(4) is bonded in a tetrahedral geometry to one O(2), one O(9), and two equivalent O(3) atoms. The As(4)-O(2) bond length is 1.76 Å. The As(4)-O(9) bond length is 1.74 Å. Both As(4)-O(3) bond lengths are 1.72 Å. In the fifth As site, As(5) is bonded in a tetrahedral geometry to one O(7), one O(8), and two equivalent O(5) atoms. The As(5)-O(7) bond length is 1.71 Å. The As(5)-O(8) bond length is 1.74 Å. Both As(5)-O(5) bond lengths are 1.73 Å. In the sixth As site, As(6) is bonded in a tetrahedral geometry to one O(17), one O(18), and two equivalent O(10) atoms. The As(6)-O(17) bond length is 1.72 Å. The As(6)-O(18) bond length is 1.73 Å. Both As(6)-O(10) bond lengths are 1.72 Å. There are eighteen inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Ba(4), one Ba(6), and one As(2) atom. In the second O site, O(2) is bonded in a distorted bent 120 degrees geometry to two equivalent Ba(2), one Mg(1), and one As(4) atom. In the third O site, O(3) is bonded in a 1-coordinate geometry to one Ba(1), one Ba(5), one Ba(8), and one As(4) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Ba(8), two equivalent Ba(2), and one As(2) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one Ba(2), one Ba(3), one Ba(8), and one As(5) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to two equivalent Ba(1) and one As(2) atom. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to one Ba(6), two equivalent Ba(1), and one As(5) atom. In the eighth O site, O(8) is bonded in a 4-coordinate geometry to one Ba(7), two equivalent Ba(2), and one As(5) atom. In the ninth O site, O(9) is bonded in a distorted single-bond geometry to one Ba(4), two equivalent Ba(1), and one As(4) atom. In the tenth O site, O(10) is bonded in a 4-coordinate geometry to one Ba(1), one Ba(3), one Ba(6), and one As(6) atom. In the eleventh O site, O(11) is bonded to one Ba(3), two equivalent Ba(1), and one As(1) atom to form distorted OBa3As tetrahedra that share corners with two equivalent Cl(1)Ba6 octahedra. The corner-sharing octahedral tilt angles are 47°. In the twelfth O site, O(12) is bonded in a 4-coordinate geometry to one Ba(6), two equivalent Ba(2), and one As(3) atom. In the thirteenth O site, O(13) is bonded in a 4-coordinate geometry to one Ba(1), one Ba(4), one Ba(7), and one As(1) atom. In the fourteenth O site, O(14) is bonded in a distorted single-bond geometry to one Ba(8), two equivalent Ba(1), and one As(3) atom. In the fifteenth O site, O(15) is bonded in a distorted see-saw-like geometry to one Ba(5), two equivalent Ba(2), and one Mg(1) atom. In the sixteenth O site, O(16) is bonded to one Ba(2), one Ba(5), one Ba(7), and one As(3) atom to form distorted OBa3As trigonal pyramids that share corners with two equivalent Cl(1)Ba6 octahedra,  a cornercorner with one O(16)Ba3As trigonal pyramid,  an edgeedge with one Cl(1)Ba6 octahedra, and  an edgeedge with one O(16)Ba3As trigonal pyramid. The corner-sharing octahedral tilt angles range from 60-80°. In the seventeenth O site, O(17) is bonded in a 3-coordinate geometry to one Ba(4), one Mg(1), and one As(6) atom. In the eighteenth O site, O(18) is bonded in a 4-coordinate geometry to one Ba(5), two equivalent Ba(1), and one As(6) atom. Cl(1) is bonded to one Ba(3), one Ba(4), one Ba(5), one Ba(6), one Ba(7), and one Ba(8) atom to form distorted ClBa6 octahedra that share  a cornercorner with one O(11)Ba3As tetrahedra, corners with two equivalent O(16)Ba3As trigonal pyramids,  an edgeedge with one O(16)Ba3As trigonal pyramid, and faces with two equivalent Cl(1)Ba6 octahedra.