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 in a 4-coordinate geometry to two equivalent Mg(3), one Si(3), one Si(4), one Si(8), and two equivalent Si(6) atoms. In the second Mg site, Mg(2) is bonded in a 4-coordinate geometry to one Mg(17), one Mg(3), one Si(10), one Si(2), and two equivalent Si(3) atoms. In the third Mg site, Mg(3) is bonded in a 12-coordinate geometry to one Mg(18), one Mg(2), one Mg(6), two equivalent Mg(1), two equivalent Mg(5), one Si(3), one Si(4), one Si(6), one Si(8), and one Si(9) atom. In the fourth Mg site, Mg(4) is bonded in a 3-coordinate geometry to one Mg(17), one Si(2), one Si(3), one Si(6), and one Si(9) atom. In the fifth Mg site, Mg(5) is bonded in a 4-coordinate geometry to two equivalent Mg(3), one Si(3), one Si(4), and two equivalent Si(9) atoms. In the sixth Mg site, Mg(6) is bonded in a 4-coordinate geometry to one Mg(3), one Si(10), one Si(2), one Si(3), and one Si(9) atom. In the seventh Mg site, Mg(7) is bonded in a 4-coordinate geometry to two equivalent Mg(17), one Si(1), one Si(10), one Si(2), and one Si(5) atom. In the eighth Mg site, Mg(8) is bonded to one Si(4), one Si(5), and two equivalent Si(7) atoms to form distorted MgSi4 trigonal pyramids that share  a cornercorner with one Mg(13)MgSi4 tetrahedra and corners with two equivalent Mg(8)Si4 trigonal pyramids. In the ninth Mg site, Mg(9) is bonded in a 4-coordinate geometry to one Mg(17), one Si(5), one Si(7), and two equivalent Si(1) atoms. In the tenth Mg site, Mg(10) is bonded in a distorted T-shaped geometry to one Si(1), one Si(7), 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(10), and one Si(7) atom. In the twelfth Mg site, Mg(12) is bonded in a 3-coordinate geometry to one Si(5), one Si(7), and one Si(8) atom. In the thirteenth Mg site, Mg(13) is bonded to one Mg(17), one Si(1), one Si(5), and two equivalent Si(10) atoms to form distorted MgMgSi4 tetrahedra that share corners with two equivalent Mg(13)MgSi4 tetrahedra and  a cornercorner with one Mg(8)Si4 trigonal pyramid. In the fourteenth Mg site, Mg(14) is bonded in a 4-coordinate geometry to one Si(4), one Si(6), one Si(7), one Si(8), and one Si(9) atom. In the fifteenth Mg site, Mg(15) is bonded in a 4-coordinate geometry to one Mg(17), one Si(1), one Si(10), and two equivalent Si(2) atoms. In the sixteenth Mg site, Mg(16) is bonded in a 5-coordinate geometry to one Si(4), one Si(6), one Si(7), and two equivalent Si(8) atoms. In the seventeenth Mg site, Mg(17) is bonded in a 11-coordinate geometry to one Mg(13), one Mg(15), one Mg(2), one Mg(4), one Mg(9), two equivalent Mg(7), one Si(10), one Si(3), and two equivalent Si(2) atoms. In the eighteenth Mg site, Mg(18) is bonded in a 3-coordinate geometry to one Mg(3), one Si(6), one Si(7), one Si(9), and two equivalent Si(4) atoms. There are ten inequivalent Si sites. In the first Si site, Si(1) is bonded in a 8-coordinate geometry to one Mg(10), one Mg(11), one Mg(13), one Mg(15), one Mg(7), two equivalent Mg(9), and one Si(2) atom. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to one Mg(2), one Mg(4), one Mg(6), one Mg(7), two equivalent Mg(15), two equivalent Mg(17), and one Si(1) atom. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to one Mg(1), one Mg(17), one Mg(3), one Mg(4), one Mg(5), one Mg(6), two equivalent Mg(2), and one Si(10) atom. In the fourth Si site, Si(4) is bonded in a 9-coordinate geometry to one Mg(1), one Mg(14), one Mg(16), one Mg(3), one Mg(5), one Mg(8), two equivalent Mg(18), and one Si(8) atom. In the fifth Si site, Si(5) is bonded in a 5-coordinate geometry to one Mg(12), one Mg(13), one Mg(7), one Mg(8), and one Mg(9) atom. In the sixth Si site, Si(6) is bonded in a 8-coordinate geometry to one Mg(14), one Mg(16), one Mg(18), one Mg(3), one Mg(4), two equivalent Mg(1), one Si(8), and one Si(9) atom. In the seventh Si site, Si(7) is bonded in a 9-coordinate geometry to one Mg(10), one Mg(11), one Mg(12), one Mg(14), one Mg(16), one Mg(18), one Mg(9), and two equivalent Mg(8) atoms. In the eighth Si site, Si(8) is bonded in a 9-coordinate geometry to one Mg(1), one Mg(10), one Mg(12), one Mg(14), one Mg(3), two equivalent Mg(16), one Si(4), and one Si(6) atom. In the ninth Si site, Si(9) is bonded in a 8-coordinate geometry to one Mg(14), one Mg(18), one Mg(3), one Mg(4), one Mg(6), two equivalent Mg(5), and one Si(6) atom. In the tenth Si site, Si(10) is bonded in a 9-coordinate geometry to one Mg(11), one Mg(15), one Mg(17), one Mg(2), one Mg(6), one Mg(7), two equivalent Mg(13), and one Si(3) atom. is represented by the Crystallographic Information File (CIF) [CIF]
data_Mg9Si5
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   5.047
_cell_length_b   7.879
_cell_length_c   14.120
_cell_angle_alpha   83.680
_cell_angle_beta   80.272
_cell_angle_gamma   80.754
_symmetry_Int_Tables_number   1
_cell_volume   544.229
_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  0.873  0.910  0.320  1.0
  Mg  Mg1  1  0.475  0.923  0.501  1.0
  Mg  Mg2  1  0.436  0.705  0.323  1.0
  Mg  Mg3  1  0.967  0.154  0.462  1.0
  Mg  Mg4  1  0.965  0.509  0.375  1.0
  Mg  Mg5  1  0.440  0.503  0.529  1.0
  Mg  Mg6  1  0.454  0.904  0.797  1.0
  Mg  Mg7  1  0.078  0.475  0.990  1.0
  Mg  Mg8  1  0.803  0.138  0.857  1.0
  Mg  Mg9  1  0.237  0.090  0.985  1.0
  Mg  Mg10  1  0.655  0.506  0.838  1.0
  Mg  Mg11  1  0.503  0.695  0.011  1.0
  Mg  Mg12  1  0.171  0.619  0.736  1.0
  Mg  Mg13  1  0.924  0.244  0.193  1.0
  Mg  Mg14  1  0.909  0.352  0.646  1.0
  Mg  Mg15  1  0.712  0.964  0.099  1.0
  Mg  Mg16  1  0.981  0.970  0.661  1.0
  Mg  Mg17  1  0.389  0.439  0.192  1.0
  Si  Si18  1  0.294  0.271  0.794  1.0
  Si  Si19  1  0.457  0.180  0.627  1.0
  Si  Si20  1  0.005  0.769  0.499  1.0
  Si  Si21  1  0.903  0.632  0.179  1.0
  Si  Si22  1  0.973  0.799  0.902  1.0
  Si  Si23  1  0.360  0.074  0.284  1.0
  Si  Si24  1  0.614  0.310  0.023  1.0
  Si  Si25  1  0.206  0.869  0.155  1.0
  Si  Si26  1  0.522  0.309  0.370  1.0
  Si  Si27  1  0.694  0.686  0.651  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 in a 4-coordinate geometry to two equivalent Mg(3), one Si(3), one Si(4), one Si(8), and two equivalent Si(6) atoms. In the second Mg site, Mg(2) is bonded in a 4-coordinate geometry to one Mg(17), one Mg(3), one Si(10), one Si(2), and two equivalent Si(3) atoms. In the third Mg site, Mg(3) is bonded in a 12-coordinate geometry to one Mg(18), one Mg(2), one Mg(6), two equivalent Mg(1), two equivalent Mg(5), one Si(3), one Si(4), one Si(6), one Si(8), and one Si(9) atom. In the fourth Mg site, Mg(4) is bonded in a 3-coordinate geometry to one Mg(17), one Si(2), one Si(3), one Si(6), and one Si(9) atom. In the fifth Mg site, Mg(5) is bonded in a 4-coordinate geometry to two equivalent Mg(3), one Si(3), one Si(4), and two equivalent Si(9) atoms. In the sixth Mg site, Mg(6) is bonded in a 4-coordinate geometry to one Mg(3), one Si(10), one Si(2), one Si(3), and one Si(9) atom. In the seventh Mg site, Mg(7) is bonded in a 4-coordinate geometry to two equivalent Mg(17), one Si(1), one Si(10), one Si(2), and one Si(5) atom. In the eighth Mg site, Mg(8) is bonded to one Si(4), one Si(5), and two equivalent Si(7) atoms to form distorted MgSi4 trigonal pyramids that share  a cornercorner with one Mg(13)MgSi4 tetrahedra and corners with two equivalent Mg(8)Si4 trigonal pyramids. In the ninth Mg site, Mg(9) is bonded in a 4-coordinate geometry to one Mg(17), one Si(5), one Si(7), and two equivalent Si(1) atoms. In the tenth Mg site, Mg(10) is bonded in a distorted T-shaped geometry to one Si(1), one Si(7), 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(10), and one Si(7) atom. In the twelfth Mg site, Mg(12) is bonded in a 3-coordinate geometry to one Si(5), one Si(7), and one Si(8) atom. In the thirteenth Mg site, Mg(13) is bonded to one Mg(17), one Si(1), one Si(5), and two equivalent Si(10) atoms to form distorted MgMgSi4 tetrahedra that share corners with two equivalent Mg(13)MgSi4 tetrahedra and  a cornercorner with one Mg(8)Si4 trigonal pyramid. In the fourteenth Mg site, Mg(14) is bonded in a 4-coordinate geometry to one Si(4), one Si(6), one Si(7), one Si(8), and one Si(9) atom. In the fifteenth Mg site, Mg(15) is bonded in a 4-coordinate geometry to one Mg(17), one Si(1), one Si(10), and two equivalent Si(2) atoms. In the sixteenth Mg site, Mg(16) is bonded in a 5-coordinate geometry to one Si(4), one Si(6), one Si(7), and two equivalent Si(8) atoms. In the seventeenth Mg site, Mg(17) is bonded in a 11-coordinate geometry to one Mg(13), one Mg(15), one Mg(2), one Mg(4), one Mg(9), two equivalent Mg(7), one Si(10), one Si(3), and two equivalent Si(2) atoms. In the eighteenth Mg site, Mg(18) is bonded in a 3-coordinate geometry to one Mg(3), one Si(6), one Si(7), one Si(9), and two equivalent Si(4) atoms. There are ten inequivalent Si sites. In the first Si site, Si(1) is bonded in a 8-coordinate geometry to one Mg(10), one Mg(11), one Mg(13), one Mg(15), one Mg(7), two equivalent Mg(9), and one Si(2) atom. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to one Mg(2), one Mg(4), one Mg(6), one Mg(7), two equivalent Mg(15), two equivalent Mg(17), and one Si(1) atom. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to one Mg(1), one Mg(17), one Mg(3), one Mg(4), one Mg(5), one Mg(6), two equivalent Mg(2), and one Si(10) atom. In the fourth Si site, Si(4) is bonded in a 9-coordinate geometry to one Mg(1), one Mg(14), one Mg(16), one Mg(3), one Mg(5), one Mg(8), two equivalent Mg(18), and one Si(8) atom. In the fifth Si site, Si(5) is bonded in a 5-coordinate geometry to one Mg(12), one Mg(13), one Mg(7), one Mg(8), and one Mg(9) atom. In the sixth Si site, Si(6) is bonded in a 8-coordinate geometry to one Mg(14), one Mg(16), one Mg(18), one Mg(3), one Mg(4), two equivalent Mg(1), one Si(8), and one Si(9) atom. In the seventh Si site, Si(7) is bonded in a 9-coordinate geometry to one Mg(10), one Mg(11), one Mg(12), one Mg(14), one Mg(16), one Mg(18), one Mg(9), and two equivalent Mg(8) atoms. In the eighth Si site, Si(8) is bonded in a 9-coordinate geometry to one Mg(1), one Mg(10), one Mg(12), one Mg(14), one Mg(3), two equivalent Mg(16), one Si(4), and one Si(6) atom. In the ninth Si site, Si(9) is bonded in a 8-coordinate geometry to one Mg(14), one Mg(18), one Mg(3), one Mg(4), one Mg(6), two equivalent Mg(5), and one Si(6) atom. In the tenth Si site, Si(10) is bonded in a 9-coordinate geometry to one Mg(11), one Mg(15), one Mg(17), one Mg(2), one Mg(6), one Mg(7), two equivalent Mg(13), and one Si(3) atom. is represented by the Crystallographic Information File (CIF) [CIF]
data_Mg9Si5
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   5.047
_cell_length_b   7.879
_cell_length_c   14.120
_cell_angle_alpha   83.680
_cell_angle_beta   80.272
_cell_angle_gamma   80.754
_symmetry_Int_Tables_number   1
_cell_volume   544.229
_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  0.873  0.910  0.320  1.0
  Mg  Mg1  1  0.475  0.923  0.501  1.0
  Mg  Mg2  1  0.436  0.705  0.323  1.0
  Mg  Mg3  1  0.967  0.154  0.462  1.0
  Mg  Mg4  1  0.965  0.509  0.375  1.0
  Mg  Mg5  1  0.440  0.503  0.529  1.0
  Mg  Mg6  1  0.454  0.904  0.797  1.0
  Mg  Mg7  1  0.078  0.475  0.990  1.0
  Mg  Mg8  1  0.803  0.138  0.857  1.0
  Mg  Mg9  1  0.237  0.090  0.985  1.0
  Mg  Mg10  1  0.655  0.506  0.838  1.0
  Mg  Mg11  1  0.503  0.695  0.011  1.0
  Mg  Mg12  1  0.171  0.619  0.736  1.0
  Mg  Mg13  1  0.924  0.244  0.193  1.0
  Mg  Mg14  1  0.909  0.352  0.646  1.0
  Mg  Mg15  1  0.712  0.964  0.099  1.0
  Mg  Mg16  1  0.981  0.970  0.661  1.0
  Mg  Mg17  1  0.389  0.439  0.192  1.0
  Si  Si18  1  0.294  0.271  0.794  1.0
  Si  Si19  1  0.457  0.180  0.627  1.0
  Si  Si20  1  0.005  0.769  0.499  1.0
  Si  Si21  1  0.903  0.632  0.179  1.0
  Si  Si22  1  0.973  0.799  0.902  1.0
  Si  Si23  1  0.360  0.074  0.284  1.0
  Si  Si24  1  0.614  0.310  0.023  1.0
  Si  Si25  1  0.206  0.869  0.155  1.0
  Si  Si26  1  0.522  0.309  0.370  1.0
  Si  Si27  1  0.694  0.686  0.651  1.0
[/CIF]
.