The crystal structure described by Mg9Si5 crystallizes in the triclinic P1 space group. There are eighteen inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one Mg(14), one Si(2), one Si(6), one Si(8), and one Si(9) atom to form distorted MgMgSi4 tetrahedra that share  a cornercorner with one Si(9)Mg8 hexagonal bipyramid, corners with two equivalent Mg(5)MgSi4 tetrahedra,  an edgeedge with one Mg(5)MgSi4 tetrahedra, and  an edgeedge with one Mg(6)Si4 tetrahedra. In the second Mg site, Mg(2) is bonded in a 4-coordinate geometry to one Mg(14), one Si(10), one Si(2), one Si(5), and one Si(9) atom. In the third Mg site, Mg(3) is bonded in a 3-coordinate geometry to one Si(10), one Si(2), and one Si(9) atom. In the fourth Mg site, Mg(4) is bonded in a 4-coordinate geometry to one Si(1), one Si(10), one Si(2), and one Si(5) atom. In the fifth Mg site, Mg(5) is bonded to one Mg(14), one Si(10), one Si(2), one Si(6), and one Si(9) atom to form distorted MgMgSi4 tetrahedra that share  a cornercorner with one Si(9)Mg8 hexagonal bipyramid,  a cornercorner with one Mg(6)Si4 tetrahedra, corners with two equivalent Mg(1)MgSi4 tetrahedra,  an edgeedge with one Mg(1)MgSi4 tetrahedra, and  an edgeedge with one Mg(6)Si4 tetrahedra. In the sixth Mg site, Mg(6) is bonded to one Si(10), one Si(2), one Si(5), and one Si(9) atom to form a mixture of distorted corner and edge-sharing MgSi4 tetrahedra. In the seventh Mg site, Mg(7) is bonded in a distorted T-shaped geometry to one Si(1), one Si(4), and one Si(5) atom. In the eighth Mg site, Mg(8) is bonded in a 5-coordinate geometry to one Si(3), one Si(4), one Si(6), one Si(7), and one Si(8) atom. In the ninth Mg site, Mg(9) is bonded in a 3-coordinate geometry to one Si(1), one Si(4), and one Si(7) atom. In the tenth Mg site, Mg(10) is bonded in a 3-coordinate geometry to one Mg(14), one Si(4), one Si(6), and one Si(8) atom. In the eleventh Mg site, Mg(11) is bonded in a 3-coordinate geometry to one Si(1), one Si(5), and one Si(7) atom. In the twelfth Mg site, Mg(12) is bonded in a 3-coordinate geometry to one Si(4), one Si(7), and one Si(8) atom. In the thirteenth Mg site, Mg(13) is bonded in a 5-coordinate geometry to one Si(1), one Si(10), one Si(2), one Si(5), and one Si(7) atom. In the fourteenth Mg site, Mg(14) is bonded in a 10-coordinate geometry to one Mg(1), one Mg(10), one Mg(16), one Mg(2), one Mg(5), one Si(2), one Si(3), one Si(6), one Si(8), and one Si(9) atom. In the fifteenth Mg site, Mg(15) is bonded in a 3-coordinate geometry to one Si(1), one Si(10), and one Si(5) atom. In the sixteenth Mg site, Mg(16) is bonded in a 3-coordinate geometry to one Mg(14), one Si(3), one Si(6), one Si(8), and one Si(9) atom. In the seventeenth Mg site, Mg(17) is bonded in a distorted trigonal non-coplanar geometry to one Si(1), one Si(10), and one Si(7) atom. In the eighteenth Mg site, Mg(18) is bonded in a 5-coordinate geometry to one Si(3), one Si(4), one Si(6), one Si(8), and one Si(9) atom. There are ten inequivalent Si sites. In the first Si site, Si(1) is bonded in a 8-coordinate geometry to one Mg(11), one Mg(13), one Mg(15), one Mg(17), one Mg(4), one Mg(7), one Mg(9), and one Si(7) atom. In the second Si site, Si(2) is bonded in a 8-coordinate geometry to one Mg(1), one Mg(13), one Mg(14), one Mg(2), one Mg(3), one Mg(4), one Mg(5), and one Mg(6) atom. In the third Si site, Si(3) is bonded in a 6-coordinate geometry to one Mg(14), one Mg(16), one Mg(18), one Mg(8), one Si(4), and one Si(8) atom. In the fourth Si site, Si(4) is bonded in a 8-coordinate geometry to one Mg(10), one Mg(12), one Mg(18), one Mg(7), one Mg(8), one Mg(9), one Si(3), and one Si(6) atom. In the fifth Si site, Si(5) is bonded in a 7-coordinate geometry to one Mg(11), one Mg(13), one Mg(15), one Mg(2), one Mg(4), one Mg(6), and one Mg(7) atom. In the sixth Si site, Si(6) is bonded in a 8-coordinate geometry to one Mg(1), one Mg(10), one Mg(14), one Mg(16), one Mg(18), one Mg(5), one Mg(8), and one Si(4) atom. In the seventh Si site, Si(7) is bonded in a 7-coordinate geometry to one Mg(11), one Mg(12), one Mg(13), one Mg(17), one Mg(8), one Mg(9), and one Si(1) atom. In the eighth Si site, Si(8) is bonded in a 8-coordinate geometry to one Mg(1), one Mg(10), one Mg(12), one Mg(14), one Mg(16), one Mg(18), one Mg(8), and one Si(3) atom. In the ninth Si site, Si(9) is bonded to one Mg(1), one Mg(14), one Mg(16), one Mg(18), one Mg(2), one Mg(3), one Mg(5), and one Mg(6) atom to form distorted SiMg8 hexagonal bipyramids that share corners with two equivalent Si(10)Mg8 hexagonal bipyramids,  a cornercorner with one Mg(1)MgSi4 tetrahedra,  a cornercorner with one Mg(5)MgSi4 tetrahedra, and  an edgeedge with one Si(10)Mg8 hexagonal bipyramid. In the tenth Si site, Si(10) is bonded to one Mg(13), one Mg(15), one Mg(17), one Mg(2), one Mg(3), one Mg(4), one Mg(5), and one Mg(6) atom to form a mixture of distorted corner and edge-sharing SiMg8 hexagonal bipyramids. is represented by the CIF file [CIF]
data_Mg9Si5
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.885
_cell_length_b   7.359
_cell_length_c   13.015
_cell_angle_alpha   94.646
_cell_angle_beta   89.240
_cell_angle_gamma   116.673
_symmetry_Int_Tables_number   1
_cell_volume   587.100
_cell_formula_units_Z   2
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  0.369  0.065  0.681  1.0
  Mg  Mg1  1  0.728  0.059  0.467  1.0
  Mg  Mg2  1  0.129  0.641  0.539  1.0
  Mg  Mg3  1  0.833  0.666  0.362  1.0
  Mg  Mg4  1  0.710  0.712  0.600  1.0
  Mg  Mg5  1  0.470  0.316  0.474  1.0
  Mg  Mg6  1  0.794  0.125  0.119  1.0
  Mg  Mg7  1  0.356  0.987  0.940  1.0
  Mg  Mg8  1  0.832  0.575  0.036  1.0
  Mg  Mg9  1  0.819  0.217  0.892  1.0
  Mg  Mg10  1  0.404  0.231  0.157  1.0
  Mg  Mg11  1  0.282  0.488  0.014  1.0
  Mg  Mg12  1  0.313  0.922  0.333  1.0
  Mg  Mg13  1  0.958  0.146  0.676  1.0
  Mg  Mg14  1  0.103  0.285  0.325  1.0
  Mg  Mg15  1  0.117  0.593  0.781  1.0
  Mg  Mg16  1  0.531  0.692  0.210  1.0
  Mg  Mg17  1  0.614  0.496  0.806  1.0
  Si  Si18  1  0.990  0.907  0.201  1.0
  Si  Si19  1  0.087  0.991  0.507  1.0
  Si  Si20  1  0.969  0.877  0.840  1.0
  Si  Si21  1  0.663  0.820  0.952  1.0
  Si  Si22  1  0.706  0.268  0.307  1.0
  Si  Si23  1  0.556  0.849  0.770  1.0
  Si  Si24  1  0.252  0.812  0.118  1.0
  Si  Si25  1  0.211  0.237  0.844  1.0
  Si  Si26  1  0.768  0.377  0.630  1.0
  Si  Si27  1  0.437  0.648  0.417  1.0
[/CIF]
.
The crystal structure described by Mg9Si5 crystallizes in the triclinic P1 space group. There are eighteen inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one Mg(14), one Si(2), one Si(6), one Si(8), and one Si(9) atom to form distorted MgMgSi4 tetrahedra that share  a cornercorner with one Si(9)Mg8 hexagonal bipyramid, corners with two equivalent Mg(5)MgSi4 tetrahedra,  an edgeedge with one Mg(5)MgSi4 tetrahedra, and  an edgeedge with one Mg(6)Si4 tetrahedra. In the second Mg site, Mg(2) is bonded in a 4-coordinate geometry to one Mg(14), one Si(10), one Si(2), one Si(5), and one Si(9) atom. In the third Mg site, Mg(3) is bonded in a 3-coordinate geometry to one Si(10), one Si(2), and one Si(9) atom. In the fourth Mg site, Mg(4) is bonded in a 4-coordinate geometry to one Si(1), one Si(10), one Si(2), and one Si(5) atom. In the fifth Mg site, Mg(5) is bonded to one Mg(14), one Si(10), one Si(2), one Si(6), and one Si(9) atom to form distorted MgMgSi4 tetrahedra that share  a cornercorner with one Si(9)Mg8 hexagonal bipyramid,  a cornercorner with one Mg(6)Si4 tetrahedra, corners with two equivalent Mg(1)MgSi4 tetrahedra,  an edgeedge with one Mg(1)MgSi4 tetrahedra, and  an edgeedge with one Mg(6)Si4 tetrahedra. In the sixth Mg site, Mg(6) is bonded to one Si(10), one Si(2), one Si(5), and one Si(9) atom to form a mixture of distorted corner and edge-sharing MgSi4 tetrahedra. In the seventh Mg site, Mg(7) is bonded in a distorted T-shaped geometry to one Si(1), one Si(4), and one Si(5) atom. In the eighth Mg site, Mg(8) is bonded in a 5-coordinate geometry to one Si(3), one Si(4), one Si(6), one Si(7), and one Si(8) atom. In the ninth Mg site, Mg(9) is bonded in a 3-coordinate geometry to one Si(1), one Si(4), and one Si(7) atom. In the tenth Mg site, Mg(10) is bonded in a 3-coordinate geometry to one Mg(14), one Si(4), one Si(6), and one Si(8) atom. In the eleventh Mg site, Mg(11) is bonded in a 3-coordinate geometry to one Si(1), one Si(5), and one Si(7) atom. In the twelfth Mg site, Mg(12) is bonded in a 3-coordinate geometry to one Si(4), one Si(7), and one Si(8) atom. In the thirteenth Mg site, Mg(13) is bonded in a 5-coordinate geometry to one Si(1), one Si(10), one Si(2), one Si(5), and one Si(7) atom. In the fourteenth Mg site, Mg(14) is bonded in a 10-coordinate geometry to one Mg(1), one Mg(10), one Mg(16), one Mg(2), one Mg(5), one Si(2), one Si(3), one Si(6), one Si(8), and one Si(9) atom. In the fifteenth Mg site, Mg(15) is bonded in a 3-coordinate geometry to one Si(1), one Si(10), and one Si(5) atom. In the sixteenth Mg site, Mg(16) is bonded in a 3-coordinate geometry to one Mg(14), one Si(3), one Si(6), one Si(8), and one Si(9) atom. In the seventeenth Mg site, Mg(17) is bonded in a distorted trigonal non-coplanar geometry to one Si(1), one Si(10), and one Si(7) atom. In the eighteenth Mg site, Mg(18) is bonded in a 5-coordinate geometry to one Si(3), one Si(4), one Si(6), one Si(8), and one Si(9) atom. There are ten inequivalent Si sites. In the first Si site, Si(1) is bonded in a 8-coordinate geometry to one Mg(11), one Mg(13), one Mg(15), one Mg(17), one Mg(4), one Mg(7), one Mg(9), and one Si(7) atom. In the second Si site, Si(2) is bonded in a 8-coordinate geometry to one Mg(1), one Mg(13), one Mg(14), one Mg(2), one Mg(3), one Mg(4), one Mg(5), and one Mg(6) atom. In the third Si site, Si(3) is bonded in a 6-coordinate geometry to one Mg(14), one Mg(16), one Mg(18), one Mg(8), one Si(4), and one Si(8) atom. In the fourth Si site, Si(4) is bonded in a 8-coordinate geometry to one Mg(10), one Mg(12), one Mg(18), one Mg(7), one Mg(8), one Mg(9), one Si(3), and one Si(6) atom. In the fifth Si site, Si(5) is bonded in a 7-coordinate geometry to one Mg(11), one Mg(13), one Mg(15), one Mg(2), one Mg(4), one Mg(6), and one Mg(7) atom. In the sixth Si site, Si(6) is bonded in a 8-coordinate geometry to one Mg(1), one Mg(10), one Mg(14), one Mg(16), one Mg(18), one Mg(5), one Mg(8), and one Si(4) atom. In the seventh Si site, Si(7) is bonded in a 7-coordinate geometry to one Mg(11), one Mg(12), one Mg(13), one Mg(17), one Mg(8), one Mg(9), and one Si(1) atom. In the eighth Si site, Si(8) is bonded in a 8-coordinate geometry to one Mg(1), one Mg(10), one Mg(12), one Mg(14), one Mg(16), one Mg(18), one Mg(8), and one Si(3) atom. In the ninth Si site, Si(9) is bonded to one Mg(1), one Mg(14), one Mg(16), one Mg(18), one Mg(2), one Mg(3), one Mg(5), and one Mg(6) atom to form distorted SiMg8 hexagonal bipyramids that share corners with two equivalent Si(10)Mg8 hexagonal bipyramids,  a cornercorner with one Mg(1)MgSi4 tetrahedra,  a cornercorner with one Mg(5)MgSi4 tetrahedra, and  an edgeedge with one Si(10)Mg8 hexagonal bipyramid. In the tenth Si site, Si(10) is bonded to one Mg(13), one Mg(15), one Mg(17), one Mg(2), one Mg(3), one Mg(4), one Mg(5), and one Mg(6) atom to form a mixture of distorted corner and edge-sharing SiMg8 hexagonal bipyramids. is represented by the CIF file [CIF]
data_Mg9Si5
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.885
_cell_length_b   7.359
_cell_length_c   13.015
_cell_angle_alpha   94.646
_cell_angle_beta   89.240
_cell_angle_gamma   116.673
_symmetry_Int_Tables_number   1
_cell_volume   587.100
_cell_formula_units_Z   2
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  0.369  0.065  0.681  1.0
  Mg  Mg1  1  0.728  0.059  0.467  1.0
  Mg  Mg2  1  0.129  0.641  0.539  1.0
  Mg  Mg3  1  0.833  0.666  0.362  1.0
  Mg  Mg4  1  0.710  0.712  0.600  1.0
  Mg  Mg5  1  0.470  0.316  0.474  1.0
  Mg  Mg6  1  0.794  0.125  0.119  1.0
  Mg  Mg7  1  0.356  0.987  0.940  1.0
  Mg  Mg8  1  0.832  0.575  0.036  1.0
  Mg  Mg9  1  0.819  0.217  0.892  1.0
  Mg  Mg10  1  0.404  0.231  0.157  1.0
  Mg  Mg11  1  0.282  0.488  0.014  1.0
  Mg  Mg12  1  0.313  0.922  0.333  1.0
  Mg  Mg13  1  0.958  0.146  0.676  1.0
  Mg  Mg14  1  0.103  0.285  0.325  1.0
  Mg  Mg15  1  0.117  0.593  0.781  1.0
  Mg  Mg16  1  0.531  0.692  0.210  1.0
  Mg  Mg17  1  0.614  0.496  0.806  1.0
  Si  Si18  1  0.990  0.907  0.201  1.0
  Si  Si19  1  0.087  0.991  0.507  1.0
  Si  Si20  1  0.969  0.877  0.840  1.0
  Si  Si21  1  0.663  0.820  0.952  1.0
  Si  Si22  1  0.706  0.268  0.307  1.0
  Si  Si23  1  0.556  0.849  0.770  1.0
  Si  Si24  1  0.252  0.812  0.118  1.0
  Si  Si25  1  0.211  0.237  0.844  1.0
  Si  Si26  1  0.768  0.377  0.630  1.0
  Si  Si27  1  0.437  0.648  0.417  1.0
[/CIF]
.