The compound described by Mg3(AsO4)2 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(1), one O(5), one O(6), and two equivalent O(2) atoms to form distorted MgO5 square pyramids that share corners with three equivalent As(1)O4 tetrahedra, corners with three equivalent Mg(2)O5 trigonal bipyramids,  an edgeedge with one Mg(1)O5 square pyramid, and  an edgeedge with one As(2)O4 tetrahedra. In the second Mg site, Mg(2) is bonded to one O(1), one O(3), one O(5), one O(6), and one O(7) atom to form distorted MgO5 trigonal bipyramids that share  a cornercorner with one Mg(3)O5 square pyramid, corners with three equivalent Mg(1)O5 square pyramids, corners with two equivalent As(1)O4 tetrahedra, and corners with three equivalent As(2)O4 tetrahedra. In the third Mg site, Mg(3) is bonded to one O(7), two equivalent O(4), and two equivalent O(8) atoms to form distorted MgO5 square pyramids that share corners with two equivalent As(1)O4 tetrahedra, corners with three equivalent As(2)O4 tetrahedra,  a cornercorner with one Mg(2)O5 trigonal bipyramid, and edges with two equivalent Mg(3)O5 square pyramids. There are two inequivalent As sites. In the first As site, As(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form AsO4 tetrahedra that share corners with two equivalent Mg(3)O5 square pyramids, corners with three equivalent Mg(1)O5 square pyramids, and corners with two equivalent Mg(2)O5 trigonal bipyramids. In the second As site, As(2) is bonded to one O(5), one O(6), one O(7), and one O(8) atom to form AsO4 tetrahedra that share corners with three equivalent Mg(3)O5 square pyramids, corners with three equivalent Mg(2)O5 trigonal bipyramids, and  an edgeedge with one Mg(1)O5 square pyramid. There are eight inequivalent O sites. In the first O site, O(7) is bonded in a trigonal planar geometry to one Mg(2), one Mg(3), and one As(2) atom. In the second O site, O(8) is bonded in a distorted trigonal planar geometry to two equivalent Mg(3) and one As(2) atom. In the third O site, O(1) is bonded in a trigonal planar geometry to one Mg(1), one Mg(2), and one As(1) atom. In the fourth O site, O(2) is bonded in a trigonal planar geometry to two equivalent Mg(1) and one As(1) atom. In the fifth O site, O(3) is bonded in a bent 120 degrees geometry to one Mg(2) and one As(1) atom. In the sixth O site, O(4) is bonded in a trigonal planar geometry to two equivalent Mg(3) and one As(1) atom. In the seventh O site, O(5) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one As(2) atom. In the eighth O site, O(6) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one As(2) atom. is represented by the CIF card [CIF]
data_Mg3(AsO4)2
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   11.569
_cell_length_b   6.386
_cell_length_c   9.165
_cell_angle_alpha   80.248
_cell_angle_beta   90.000
_cell_angle_gamma   90.000
_symmetry_Int_Tables_number   1
_cell_volume   667.318
_cell_formula_units_Z   4
loop_
 _symmetry_equiv_pos_site_id
 _symmetry_equiv_pos_as_xyz
  1  'x, y, z'
loop_
 _atom_site_type_symbol
 _atom_site_label
 _atom_site_symmetry_multiplicity
 _atom_site_fract_x
 _atom_site_fract_y
 _atom_site_fract_z
 _atom_site_occupancy
  Mg  Mg0  1  0.388  0.385  0.081  1.0
  Mg  Mg1  1  0.112  0.885  0.081  1.0
  Mg  Mg2  1  0.084  0.339  0.264  1.0
  Mg  Mg3  1  0.702  0.368  0.362  1.0
  Mg  Mg4  1  0.416  0.839  0.264  1.0
  Mg  Mg5  1  0.798  0.868  0.362  1.0
  Mg  Mg6  1  0.202  0.132  0.638  1.0
  Mg  Mg7  1  0.584  0.161  0.736  1.0
  Mg  Mg8  1  0.298  0.632  0.638  1.0
  Mg  Mg9  1  0.916  0.661  0.736  1.0
  Mg  Mg10  1  0.888  0.115  0.919  1.0
  Mg  Mg11  1  0.612  0.615  0.919  1.0
  As  As12  1  0.635  0.108  0.088  1.0
  As  As13  1  0.865  0.608  0.088  1.0
  As  As14  1  0.420  0.313  0.397  1.0
  As  As15  1  0.080  0.813  0.397  1.0
  As  As16  1  0.920  0.187  0.603  1.0
  As  As17  1  0.580  0.687  0.603  1.0
  As  As18  1  0.135  0.392  0.912  1.0
  As  As19  1  0.365  0.892  0.912  1.0
  O  O20  1  0.211  0.395  0.074  1.0
  O  O21  1  0.565  0.350  0.072  1.0
  O  O22  1  0.289  0.895  0.074  1.0
  O  O23  1  0.946  0.382  0.129  1.0
  O  O24  1  0.935  0.850  0.072  1.0
  O  O25  1  0.726  0.119  0.235  1.0
  O  O26  1  0.554  0.882  0.129  1.0
  O  O27  1  0.086  0.035  0.263  1.0
  O  O28  1  0.774  0.619  0.235  1.0
  O  O29  1  0.384  0.133  0.285  1.0
  O  O30  1  0.414  0.535  0.263  1.0
  O  O31  1  0.116  0.633  0.285  1.0
  O  O32  1  0.554  0.265  0.478  1.0
  O  O33  1  0.816  0.166  0.473  1.0
  O  O34  1  0.946  0.765  0.478  1.0
  O  O35  1  0.684  0.666  0.473  1.0
  O  O36  1  0.316  0.334  0.527  1.0
  O  O37  1  0.054  0.235  0.522  1.0
  O  O38  1  0.184  0.834  0.527  1.0
  O  O39  1  0.446  0.735  0.522  1.0
  O  O40  1  0.884  0.367  0.715  1.0
  O  O41  1  0.586  0.465  0.737  1.0
  O  O42  1  0.616  0.867  0.715  1.0
  O  O43  1  0.226  0.381  0.765  1.0
  O  O44  1  0.914  0.965  0.737  1.0
  O  O45  1  0.446  0.118  0.871  1.0
  O  O46  1  0.274  0.881  0.765  1.0
  O  O47  1  0.065  0.150  0.928  1.0
  O  O48  1  0.054  0.618  0.871  1.0
  O  O49  1  0.711  0.105  0.926  1.0
  O  O50  1  0.435  0.650  0.928  1.0
  O  O51  1  0.789  0.605  0.926  1.0
[/CIF]
.
The compound described by Mg3(AsO4)2 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(1), one O(5), one O(6), and two equivalent O(2) atoms to form distorted MgO5 square pyramids that share corners with three equivalent As(1)O4 tetrahedra, corners with three equivalent Mg(2)O5 trigonal bipyramids,  an edgeedge with one Mg(1)O5 square pyramid, and  an edgeedge with one As(2)O4 tetrahedra. In the second Mg site, Mg(2) is bonded to one O(1), one O(3), one O(5), one O(6), and one O(7) atom to form distorted MgO5 trigonal bipyramids that share  a cornercorner with one Mg(3)O5 square pyramid, corners with three equivalent Mg(1)O5 square pyramids, corners with two equivalent As(1)O4 tetrahedra, and corners with three equivalent As(2)O4 tetrahedra. In the third Mg site, Mg(3) is bonded to one O(7), two equivalent O(4), and two equivalent O(8) atoms to form distorted MgO5 square pyramids that share corners with two equivalent As(1)O4 tetrahedra, corners with three equivalent As(2)O4 tetrahedra,  a cornercorner with one Mg(2)O5 trigonal bipyramid, and edges with two equivalent Mg(3)O5 square pyramids. There are two inequivalent As sites. In the first As site, As(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form AsO4 tetrahedra that share corners with two equivalent Mg(3)O5 square pyramids, corners with three equivalent Mg(1)O5 square pyramids, and corners with two equivalent Mg(2)O5 trigonal bipyramids. In the second As site, As(2) is bonded to one O(5), one O(6), one O(7), and one O(8) atom to form AsO4 tetrahedra that share corners with three equivalent Mg(3)O5 square pyramids, corners with three equivalent Mg(2)O5 trigonal bipyramids, and  an edgeedge with one Mg(1)O5 square pyramid. There are eight inequivalent O sites. In the first O site, O(7) is bonded in a trigonal planar geometry to one Mg(2), one Mg(3), and one As(2) atom. In the second O site, O(8) is bonded in a distorted trigonal planar geometry to two equivalent Mg(3) and one As(2) atom. In the third O site, O(1) is bonded in a trigonal planar geometry to one Mg(1), one Mg(2), and one As(1) atom. In the fourth O site, O(2) is bonded in a trigonal planar geometry to two equivalent Mg(1) and one As(1) atom. In the fifth O site, O(3) is bonded in a bent 120 degrees geometry to one Mg(2) and one As(1) atom. In the sixth O site, O(4) is bonded in a trigonal planar geometry to two equivalent Mg(3) and one As(1) atom. In the seventh O site, O(5) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one As(2) atom. In the eighth O site, O(6) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one As(2) atom. is represented by the CIF card [CIF]
data_Mg3(AsO4)2
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   11.569
_cell_length_b   6.386
_cell_length_c   9.165
_cell_angle_alpha   80.248
_cell_angle_beta   90.000
_cell_angle_gamma   90.000
_symmetry_Int_Tables_number   1
_cell_volume   667.318
_cell_formula_units_Z   4
loop_
 _symmetry_equiv_pos_site_id
 _symmetry_equiv_pos_as_xyz
  1  'x, y, z'
loop_
 _atom_site_type_symbol
 _atom_site_label
 _atom_site_symmetry_multiplicity
 _atom_site_fract_x
 _atom_site_fract_y
 _atom_site_fract_z
 _atom_site_occupancy
  Mg  Mg0  1  0.388  0.385  0.081  1.0
  Mg  Mg1  1  0.112  0.885  0.081  1.0
  Mg  Mg2  1  0.084  0.339  0.264  1.0
  Mg  Mg3  1  0.702  0.368  0.362  1.0
  Mg  Mg4  1  0.416  0.839  0.264  1.0
  Mg  Mg5  1  0.798  0.868  0.362  1.0
  Mg  Mg6  1  0.202  0.132  0.638  1.0
  Mg  Mg7  1  0.584  0.161  0.736  1.0
  Mg  Mg8  1  0.298  0.632  0.638  1.0
  Mg  Mg9  1  0.916  0.661  0.736  1.0
  Mg  Mg10  1  0.888  0.115  0.919  1.0
  Mg  Mg11  1  0.612  0.615  0.919  1.0
  As  As12  1  0.635  0.108  0.088  1.0
  As  As13  1  0.865  0.608  0.088  1.0
  As  As14  1  0.420  0.313  0.397  1.0
  As  As15  1  0.080  0.813  0.397  1.0
  As  As16  1  0.920  0.187  0.603  1.0
  As  As17  1  0.580  0.687  0.603  1.0
  As  As18  1  0.135  0.392  0.912  1.0
  As  As19  1  0.365  0.892  0.912  1.0
  O  O20  1  0.211  0.395  0.074  1.0
  O  O21  1  0.565  0.350  0.072  1.0
  O  O22  1  0.289  0.895  0.074  1.0
  O  O23  1  0.946  0.382  0.129  1.0
  O  O24  1  0.935  0.850  0.072  1.0
  O  O25  1  0.726  0.119  0.235  1.0
  O  O26  1  0.554  0.882  0.129  1.0
  O  O27  1  0.086  0.035  0.263  1.0
  O  O28  1  0.774  0.619  0.235  1.0
  O  O29  1  0.384  0.133  0.285  1.0
  O  O30  1  0.414  0.535  0.263  1.0
  O  O31  1  0.116  0.633  0.285  1.0
  O  O32  1  0.554  0.265  0.478  1.0
  O  O33  1  0.816  0.166  0.473  1.0
  O  O34  1  0.946  0.765  0.478  1.0
  O  O35  1  0.684  0.666  0.473  1.0
  O  O36  1  0.316  0.334  0.527  1.0
  O  O37  1  0.054  0.235  0.522  1.0
  O  O38  1  0.184  0.834  0.527  1.0
  O  O39  1  0.446  0.735  0.522  1.0
  O  O40  1  0.884  0.367  0.715  1.0
  O  O41  1  0.586  0.465  0.737  1.0
  O  O42  1  0.616  0.867  0.715  1.0
  O  O43  1  0.226  0.381  0.765  1.0
  O  O44  1  0.914  0.965  0.737  1.0
  O  O45  1  0.446  0.118  0.871  1.0
  O  O46  1  0.274  0.881  0.765  1.0
  O  O47  1  0.065  0.150  0.928  1.0
  O  O48  1  0.054  0.618  0.871  1.0
  O  O49  1  0.711  0.105  0.926  1.0
  O  O50  1  0.435  0.650  0.928  1.0
  O  O51  1  0.789  0.605  0.926  1.0
[/CIF]
.