The material described by Mg5Si9 crystallizes in the triclinic P1 space group. There are ten inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one Si(10), one Si(14), one Si(17), one Si(2), and one Si(8) atom to form distorted edge-sharing MgSi5 trigonal bipyramids. In the second Mg site, Mg(2) is bonded in a 4-coordinate geometry to one Si(10), one Si(13), one Si(17), one Si(18), one Si(5), and one Si(7) atom. In the third Mg site, Mg(3) is bonded in a 6-coordinate geometry to one Si(14), one Si(17), one Si(18), one Si(3), one Si(4), and one Si(5) atom. In the fourth Mg site, Mg(4) is bonded in a 6-coordinate geometry to one Si(13), one Si(17), one Si(18), one Si(3), one Si(5), and one Si(7) atom. In the fifth Mg site, Mg(5) is bonded in a 6-coordinate geometry to one Si(10), one Si(14), one Si(17), one Si(18), one Si(6), and one Si(8) atom. In the sixth Mg site, Mg(6) is bonded to one Si(10), one Si(17), one Si(18), one Si(3), and one Si(5) atom to form distorted MgSi5 tetrahedra that share  a cornercorner with one Si(7)Mg3Si4 hexagonal pyramid and  an edgeedge with one Mg(1)Si5 trigonal bipyramid. In the seventh Mg site, Mg(7) is bonded in a 4-coordinate geometry to one Si(12), one Si(15), one Si(16), one Si(2), one Si(7), and one Si(9) atom. In the eighth Mg site, Mg(8) is bonded in a 7-coordinate geometry to one Si(11), one Si(12), one Si(15), one Si(16), one Si(2), one Si(6), and one Si(8) atom. In the ninth Mg site, Mg(9) is bonded in a 5-coordinate geometry to one Si(12), one Si(15), one Si(16), one Si(3), and one Si(5) atom. In the tenth Mg site, Mg(10) is bonded in a 8-coordinate geometry to one Si(1), one Si(11), one Si(12), one Si(14), one Si(15), one Si(2), one Si(4), and one Si(6) atom. There are eighteen inequivalent Si sites. In the first Si site, Si(1) is bonded in a 5-coordinate geometry to one Mg(10), one Si(12), one Si(13), one Si(15), and one Si(16) atom. In the second Si site, Si(2) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(10), one Mg(7), one Mg(8), one Si(11), one Si(14), and one Si(16) atom. In the third Si site, Si(3) is bonded in a 7-coordinate geometry to one Mg(3), one Mg(4), one Mg(6), one Mg(9), one Si(13), one Si(5), and one Si(9) atom. In the fourth Si site, Si(4) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(3), one Si(10), one Si(11), one Si(14), and one Si(17) atom. In the fifth Si site, Si(5) is bonded in a 8-coordinate geometry to one Mg(2), one Mg(3), one Mg(4), one Mg(6), one Mg(9), one Si(18), one Si(3), and one Si(9) atom. In the sixth Si site, Si(6) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(5), one Mg(8), one Si(10), one Si(11), and one Si(8) atom. In the seventh Si site, Si(7) is bonded to one Mg(2), one Mg(4), one Mg(7), one Si(13), one Si(15), one Si(18), and one Si(9) atom to form distorted SiMg3Si4 hexagonal pyramids that share  a cornercorner with one Mg(6)Si5 tetrahedra. In the eighth Si site, Si(8) is bonded in a 6-coordinate geometry to one Mg(1), one Mg(5), one Mg(8), one Si(14), one Si(16), and one Si(6) atom. In the ninth Si site, Si(9) is bonded in a 1-coordinate geometry to one Mg(7), one Si(3), one Si(5), and one Si(7) atom. In the tenth Si site, Si(10) is bonded in a 6-coordinate geometry to one Mg(1), one Mg(2), one Mg(5), one Mg(6), one Si(4), and one Si(6) atom. In the eleventh Si site, Si(11) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(8), one Si(12), one Si(2), one Si(4), and one Si(6) atom. In the twelfth Si site, Si(12) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Mg(9), one Si(1), and one Si(11) atom. In the thirteenth Si site, Si(13) is bonded in a 5-coordinate geometry to one Mg(2), one Mg(4), one Si(1), one Si(3), and one Si(7) atom. In the fourteenth Si site, Si(14) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(10), one Mg(3), one Mg(5), one Si(2), one Si(4), and one Si(8) atom. In the fifteenth Si site, Si(15) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Mg(9), one Si(1), and one Si(7) atom. In the sixteenth Si site, Si(16) is bonded in a 6-coordinate geometry to one Mg(7), one Mg(8), one Mg(9), one Si(1), one Si(2), and one Si(8) atom. In the seventeenth Si site, Si(17) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), one Mg(4), one Mg(5), one Mg(6), and one Si(4) atom. In the eighteenth Si site, Si(18) is bonded in a 7-coordinate geometry to one Mg(2), one Mg(3), one Mg(4), one Mg(5), one Mg(6), one Si(5), and one Si(7) atom. is represented by the Crystallographic Information File (CIF) [CIF]
data_Mg5Si9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.676
_cell_length_b   6.927
_cell_length_c   13.643
_cell_angle_alpha   75.462
_cell_angle_beta   76.368
_cell_angle_gamma   66.221
_symmetry_Int_Tables_number   1
_cell_volume   552.372
_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  0.262  0.434  0.302  1.0
  Mg  Mg1  1  0.759  0.732  0.553  1.0
  Mg  Mg2  1  0.999  0.136  0.477  1.0
  Mg  Mg3  1  0.168  0.700  0.634  1.0
  Mg  Mg4  1  0.585  0.604  0.402  1.0
  Mg  Mg5  1  0.317  0.284  0.538  1.0
  Mg  Mg6  1  0.385  0.746  0.940  1.0
  Mg  Mg7  1  0.576  0.303  0.074  1.0
  Mg  Mg8  1  0.715  0.013  0.871  1.0
  Mg  Mg9  1  0.194  0.121  0.108  1.0
  Si  Si10  1  0.015  0.382  0.918  1.0
  Si  Si11  1  0.068  0.548  0.118  1.0
  Si  Si12  1  0.065  0.102  0.692  1.0
  Si  Si13  1  0.911  0.991  0.313  1.0
  Si  Si14  1  0.736  0.031  0.658  1.0
  Si  Si15  1  0.510  0.947  0.222  1.0
  Si  Si16  1  0.552  0.537  0.752  1.0
  Si  Si17  1  0.586  0.582  0.192  1.0
  Si  Si18  1  0.428  0.917  0.738  1.0
  Si  Si19  1  0.533  0.018  0.385  1.0
  Si  Si20  1  0.892  0.897  0.160  1.0
  Si  Si21  1  0.997  0.036  0.982  1.0
  Si  Si22  1  0.941  0.438  0.747  1.0
  Si  Si23  1  0.906  0.352  0.277  1.0
  Si  Si24  1  0.384  0.374  0.908  1.0
  Si  Si25  1  0.773  0.652  0.015  1.0
  Si  Si26  1  0.145  0.722  0.429  1.0
  Si  Si27  1  0.599  0.401  0.598  1.0
[/CIF]
.
The material described by Mg5Si9 crystallizes in the triclinic P1 space group. There are ten inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one Si(10), one Si(14), one Si(17), one Si(2), and one Si(8) atom to form distorted edge-sharing MgSi5 trigonal bipyramids. In the second Mg site, Mg(2) is bonded in a 4-coordinate geometry to one Si(10), one Si(13), one Si(17), one Si(18), one Si(5), and one Si(7) atom. In the third Mg site, Mg(3) is bonded in a 6-coordinate geometry to one Si(14), one Si(17), one Si(18), one Si(3), one Si(4), and one Si(5) atom. In the fourth Mg site, Mg(4) is bonded in a 6-coordinate geometry to one Si(13), one Si(17), one Si(18), one Si(3), one Si(5), and one Si(7) atom. In the fifth Mg site, Mg(5) is bonded in a 6-coordinate geometry to one Si(10), one Si(14), one Si(17), one Si(18), one Si(6), and one Si(8) atom. In the sixth Mg site, Mg(6) is bonded to one Si(10), one Si(17), one Si(18), one Si(3), and one Si(5) atom to form distorted MgSi5 tetrahedra that share  a cornercorner with one Si(7)Mg3Si4 hexagonal pyramid and  an edgeedge with one Mg(1)Si5 trigonal bipyramid. In the seventh Mg site, Mg(7) is bonded in a 4-coordinate geometry to one Si(12), one Si(15), one Si(16), one Si(2), one Si(7), and one Si(9) atom. In the eighth Mg site, Mg(8) is bonded in a 7-coordinate geometry to one Si(11), one Si(12), one Si(15), one Si(16), one Si(2), one Si(6), and one Si(8) atom. In the ninth Mg site, Mg(9) is bonded in a 5-coordinate geometry to one Si(12), one Si(15), one Si(16), one Si(3), and one Si(5) atom. In the tenth Mg site, Mg(10) is bonded in a 8-coordinate geometry to one Si(1), one Si(11), one Si(12), one Si(14), one Si(15), one Si(2), one Si(4), and one Si(6) atom. There are eighteen inequivalent Si sites. In the first Si site, Si(1) is bonded in a 5-coordinate geometry to one Mg(10), one Si(12), one Si(13), one Si(15), and one Si(16) atom. In the second Si site, Si(2) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(10), one Mg(7), one Mg(8), one Si(11), one Si(14), and one Si(16) atom. In the third Si site, Si(3) is bonded in a 7-coordinate geometry to one Mg(3), one Mg(4), one Mg(6), one Mg(9), one Si(13), one Si(5), and one Si(9) atom. In the fourth Si site, Si(4) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(3), one Si(10), one Si(11), one Si(14), and one Si(17) atom. In the fifth Si site, Si(5) is bonded in a 8-coordinate geometry to one Mg(2), one Mg(3), one Mg(4), one Mg(6), one Mg(9), one Si(18), one Si(3), and one Si(9) atom. In the sixth Si site, Si(6) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(5), one Mg(8), one Si(10), one Si(11), and one Si(8) atom. In the seventh Si site, Si(7) is bonded to one Mg(2), one Mg(4), one Mg(7), one Si(13), one Si(15), one Si(18), and one Si(9) atom to form distorted SiMg3Si4 hexagonal pyramids that share  a cornercorner with one Mg(6)Si5 tetrahedra. In the eighth Si site, Si(8) is bonded in a 6-coordinate geometry to one Mg(1), one Mg(5), one Mg(8), one Si(14), one Si(16), and one Si(6) atom. In the ninth Si site, Si(9) is bonded in a 1-coordinate geometry to one Mg(7), one Si(3), one Si(5), and one Si(7) atom. In the tenth Si site, Si(10) is bonded in a 6-coordinate geometry to one Mg(1), one Mg(2), one Mg(5), one Mg(6), one Si(4), and one Si(6) atom. In the eleventh Si site, Si(11) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(8), one Si(12), one Si(2), one Si(4), and one Si(6) atom. In the twelfth Si site, Si(12) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Mg(9), one Si(1), and one Si(11) atom. In the thirteenth Si site, Si(13) is bonded in a 5-coordinate geometry to one Mg(2), one Mg(4), one Si(1), one Si(3), and one Si(7) atom. In the fourteenth Si site, Si(14) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(10), one Mg(3), one Mg(5), one Si(2), one Si(4), and one Si(8) atom. In the fifteenth Si site, Si(15) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(7), one Mg(8), one Mg(9), one Si(1), and one Si(7) atom. In the sixteenth Si site, Si(16) is bonded in a 6-coordinate geometry to one Mg(7), one Mg(8), one Mg(9), one Si(1), one Si(2), and one Si(8) atom. In the seventeenth Si site, Si(17) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), one Mg(4), one Mg(5), one Mg(6), and one Si(4) atom. In the eighteenth Si site, Si(18) is bonded in a 7-coordinate geometry to one Mg(2), one Mg(3), one Mg(4), one Mg(5), one Mg(6), one Si(5), and one Si(7) atom. is represented by the Crystallographic Information File (CIF) [CIF]
data_Mg5Si9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.676
_cell_length_b   6.927
_cell_length_c   13.643
_cell_angle_alpha   75.462
_cell_angle_beta   76.368
_cell_angle_gamma   66.221
_symmetry_Int_Tables_number   1
_cell_volume   552.372
_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  0.262  0.434  0.302  1.0
  Mg  Mg1  1  0.759  0.732  0.553  1.0
  Mg  Mg2  1  0.999  0.136  0.477  1.0
  Mg  Mg3  1  0.168  0.700  0.634  1.0
  Mg  Mg4  1  0.585  0.604  0.402  1.0
  Mg  Mg5  1  0.317  0.284  0.538  1.0
  Mg  Mg6  1  0.385  0.746  0.940  1.0
  Mg  Mg7  1  0.576  0.303  0.074  1.0
  Mg  Mg8  1  0.715  0.013  0.871  1.0
  Mg  Mg9  1  0.194  0.121  0.108  1.0
  Si  Si10  1  0.015  0.382  0.918  1.0
  Si  Si11  1  0.068  0.548  0.118  1.0
  Si  Si12  1  0.065  0.102  0.692  1.0
  Si  Si13  1  0.911  0.991  0.313  1.0
  Si  Si14  1  0.736  0.031  0.658  1.0
  Si  Si15  1  0.510  0.947  0.222  1.0
  Si  Si16  1  0.552  0.537  0.752  1.0
  Si  Si17  1  0.586  0.582  0.192  1.0
  Si  Si18  1  0.428  0.917  0.738  1.0
  Si  Si19  1  0.533  0.018  0.385  1.0
  Si  Si20  1  0.892  0.897  0.160  1.0
  Si  Si21  1  0.997  0.036  0.982  1.0
  Si  Si22  1  0.941  0.438  0.747  1.0
  Si  Si23  1  0.906  0.352  0.277  1.0
  Si  Si24  1  0.384  0.374  0.908  1.0
  Si  Si25  1  0.773  0.652  0.015  1.0
  Si  Si26  1  0.145  0.722  0.429  1.0
  Si  Si27  1  0.599  0.401  0.598  1.0
[/CIF]
.