The crystal structure 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 in a 4-coordinate geometry to one Si(10), one Si(11), one Si(14), and one Si(17) atom. In the second Mg site, Mg(2) is bonded in a 4-coordinate geometry to one Si(13), one Si(17), one Si(18), and one Si(3) atom. In the third Mg site, Mg(3) is bonded in a 5-coordinate geometry to one Si(10), one Si(14), one Si(17), one Si(18), one Si(3), and one Si(4) 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(9) 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(3), and one Si(8) atom. In the sixth Mg site, Mg(6) is bonded in a 6-coordinate geometry to one Si(10), one Si(13), one Si(17), one Si(18), one Si(3), and one Si(4) atom. In the seventh Mg site, Mg(7) is bonded in a 7-coordinate geometry to one Si(12), one Si(13), one Si(15), one Si(16), one Si(5), one Si(7), and one Si(9) atom. In the eighth Mg site, Mg(8) is bonded in a 5-coordinate geometry to one Si(10), one Si(11), one Si(15), one Si(16), and one Si(8) atom. In the ninth Mg site, Mg(9) is bonded in a 3-coordinate geometry to one Si(12), one Si(15), one Si(16), one Si(2), one Si(5), one Si(6), one Si(7), and one Si(8) atom. In the tenth Mg site, Mg(10) is bonded in a distorted rectangular see-saw-like geometry to one Si(11), one Si(2), one Si(4), and one Si(8) atom. There are eighteen inequivalent Si sites. In the first Si site, Si(1) is bonded in a distorted tetrahedral geometry to one Si(12), one Si(13), one Si(16), and one Si(5) atom. In the second Si site, Si(2) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(9), one Si(11), one Si(14), one Si(15), and one Si(16) atom. In the third Si site, Si(3) 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(13), 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 Mg(6), one Si(10), one Si(14), and one Si(17) atom. In the fifth Si site, Si(5) is bonded in a 6-coordinate geometry to one Mg(4), one Mg(7), one Mg(9), one Si(1), one Si(18), and one Si(7) atom. In the sixth Si site, Si(6) is bonded in a 5-coordinate geometry to one Mg(9), one Si(10), one Si(11), one Si(15), and one Si(8) atom. In the seventh Si site, Si(7) is bonded in a 7-coordinate geometry to one Mg(7), one Mg(9), one Si(13), one Si(15), one Si(18), one Si(5), and one Si(9) atom. In the eighth Si site, Si(8) is bonded in a 7-coordinate geometry to one Mg(10), one Mg(5), one Mg(8), one Mg(9), one Si(14), one Si(17), and one Si(6) atom. In the ninth Si site, Si(9) is bonded in a distorted hexagonal planar geometry to one Mg(4), one Mg(7), one Si(12), one Si(16), one Si(3), and one Si(7) atom. In the tenth Si site, Si(10) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(3), one Mg(5), one Mg(6), one Mg(8), 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(1), one Mg(10), one Mg(8), one Si(12), one Si(2), and one Si(6) atom. In the twelfth Si site, Si(12) is bonded in a 6-coordinate geometry to one Mg(7), one Mg(9), one Si(1), one Si(11), one Si(15), and one Si(9) atom. In the thirteenth Si site, Si(13) is bonded in a 7-coordinate geometry to one Mg(2), one Mg(4), one Mg(6), one Mg(7), one Si(1), one Si(3), and one Si(7) atom. In the fourteenth Si site, Si(14) is bonded in a 6-coordinate geometry to one Mg(1), 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 7-coordinate geometry to one Mg(7), one Mg(8), one Mg(9), one Si(12), one Si(2), one Si(6), 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(9) atom. In the seventeenth Si site, Si(17) is bonded in a 8-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), one Mg(4), one Mg(5), one Mg(6), one Si(4), and one Si(8) 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 CIF file [CIF]
data_Mg5Si9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.516
_cell_length_b   7.398
_cell_length_c   12.158
_cell_angle_alpha   89.946
_cell_angle_beta   98.246
_cell_angle_gamma   113.204
_symmetry_Int_Tables_number   1
_cell_volume   532.046
_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  0.596  0.975  0.681  1.0
  Mg  Mg1  1  0.719  0.019  0.421  1.0
  Mg  Mg2  1  0.190  0.650  0.530  1.0
  Mg  Mg3  1  0.732  0.437  0.344  1.0
  Mg  Mg4  1  0.764  0.688  0.571  1.0
  Mg  Mg5  1  0.348  0.230  0.484  1.0
  Mg  Mg6  1  0.692  0.139  0.134  1.0
  Mg  Mg7  1  0.489  0.175  0.875  1.0
  Mg  Mg8  1  0.963  0.529  0.028  1.0
  Mg  Mg9  1  0.032  0.263  0.857  1.0
  Si  Si10  1  0.238  0.247  0.165  1.0
  Si  Si11  1  0.400  0.611  0.928  1.0
  Si  Si12  1  0.993  0.848  0.368  1.0
  Si  Si13  1  0.137  0.261  0.648  1.0
  Si  Si14  1  0.104  0.454  0.256  1.0
  Si  Si15  1  0.002  0.803  0.831  1.0
  Si  Si16  1  0.368  0.768  0.197  1.0
  Si  Si17  1  0.759  0.465  0.766  1.0
  Si  Si18  1  0.739  0.763  0.195  1.0
  Si  Si19  1  0.170  0.954  0.674  1.0
  Si  Si20  1  0.727  0.908  0.898  1.0
  Si  Si21  1  0.972  0.954  0.067  1.0
  Si  Si22  1  0.348  0.042  0.292  1.0
  Si  Si23  1  0.436  0.554  0.734  1.0
  Si  Si24  1  0.283  0.875  0.001  1.0
  Si  Si25  1  0.543  0.455  0.076  1.0
  Si  Si26  1  0.811  0.332  0.589  1.0
  Si  Si27  1  0.444  0.599  0.390  1.0
[/CIF]
.
The crystal structure 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 in a 4-coordinate geometry to one Si(10), one Si(11), one Si(14), and one Si(17) atom. In the second Mg site, Mg(2) is bonded in a 4-coordinate geometry to one Si(13), one Si(17), one Si(18), and one Si(3) atom. In the third Mg site, Mg(3) is bonded in a 5-coordinate geometry to one Si(10), one Si(14), one Si(17), one Si(18), one Si(3), and one Si(4) 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(9) 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(3), and one Si(8) atom. In the sixth Mg site, Mg(6) is bonded in a 6-coordinate geometry to one Si(10), one Si(13), one Si(17), one Si(18), one Si(3), and one Si(4) atom. In the seventh Mg site, Mg(7) is bonded in a 7-coordinate geometry to one Si(12), one Si(13), one Si(15), one Si(16), one Si(5), one Si(7), and one Si(9) atom. In the eighth Mg site, Mg(8) is bonded in a 5-coordinate geometry to one Si(10), one Si(11), one Si(15), one Si(16), and one Si(8) atom. In the ninth Mg site, Mg(9) is bonded in a 3-coordinate geometry to one Si(12), one Si(15), one Si(16), one Si(2), one Si(5), one Si(6), one Si(7), and one Si(8) atom. In the tenth Mg site, Mg(10) is bonded in a distorted rectangular see-saw-like geometry to one Si(11), one Si(2), one Si(4), and one Si(8) atom. There are eighteen inequivalent Si sites. In the first Si site, Si(1) is bonded in a distorted tetrahedral geometry to one Si(12), one Si(13), one Si(16), and one Si(5) atom. In the second Si site, Si(2) is bonded in a 6-coordinate geometry to one Mg(10), one Mg(9), one Si(11), one Si(14), one Si(15), and one Si(16) atom. In the third Si site, Si(3) 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(13), 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 Mg(6), one Si(10), one Si(14), and one Si(17) atom. In the fifth Si site, Si(5) is bonded in a 6-coordinate geometry to one Mg(4), one Mg(7), one Mg(9), one Si(1), one Si(18), and one Si(7) atom. In the sixth Si site, Si(6) is bonded in a 5-coordinate geometry to one Mg(9), one Si(10), one Si(11), one Si(15), and one Si(8) atom. In the seventh Si site, Si(7) is bonded in a 7-coordinate geometry to one Mg(7), one Mg(9), one Si(13), one Si(15), one Si(18), one Si(5), and one Si(9) atom. In the eighth Si site, Si(8) is bonded in a 7-coordinate geometry to one Mg(10), one Mg(5), one Mg(8), one Mg(9), one Si(14), one Si(17), and one Si(6) atom. In the ninth Si site, Si(9) is bonded in a distorted hexagonal planar geometry to one Mg(4), one Mg(7), one Si(12), one Si(16), one Si(3), and one Si(7) atom. In the tenth Si site, Si(10) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(3), one Mg(5), one Mg(6), one Mg(8), 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(1), one Mg(10), one Mg(8), one Si(12), one Si(2), and one Si(6) atom. In the twelfth Si site, Si(12) is bonded in a 6-coordinate geometry to one Mg(7), one Mg(9), one Si(1), one Si(11), one Si(15), and one Si(9) atom. In the thirteenth Si site, Si(13) is bonded in a 7-coordinate geometry to one Mg(2), one Mg(4), one Mg(6), one Mg(7), one Si(1), one Si(3), and one Si(7) atom. In the fourteenth Si site, Si(14) is bonded in a 6-coordinate geometry to one Mg(1), 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 7-coordinate geometry to one Mg(7), one Mg(8), one Mg(9), one Si(12), one Si(2), one Si(6), 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(9) atom. In the seventeenth Si site, Si(17) is bonded in a 8-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), one Mg(4), one Mg(5), one Mg(6), one Si(4), and one Si(8) 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 CIF file [CIF]
data_Mg5Si9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.516
_cell_length_b   7.398
_cell_length_c   12.158
_cell_angle_alpha   89.946
_cell_angle_beta   98.246
_cell_angle_gamma   113.204
_symmetry_Int_Tables_number   1
_cell_volume   532.046
_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  0.596  0.975  0.681  1.0
  Mg  Mg1  1  0.719  0.019  0.421  1.0
  Mg  Mg2  1  0.190  0.650  0.530  1.0
  Mg  Mg3  1  0.732  0.437  0.344  1.0
  Mg  Mg4  1  0.764  0.688  0.571  1.0
  Mg  Mg5  1  0.348  0.230  0.484  1.0
  Mg  Mg6  1  0.692  0.139  0.134  1.0
  Mg  Mg7  1  0.489  0.175  0.875  1.0
  Mg  Mg8  1  0.963  0.529  0.028  1.0
  Mg  Mg9  1  0.032  0.263  0.857  1.0
  Si  Si10  1  0.238  0.247  0.165  1.0
  Si  Si11  1  0.400  0.611  0.928  1.0
  Si  Si12  1  0.993  0.848  0.368  1.0
  Si  Si13  1  0.137  0.261  0.648  1.0
  Si  Si14  1  0.104  0.454  0.256  1.0
  Si  Si15  1  0.002  0.803  0.831  1.0
  Si  Si16  1  0.368  0.768  0.197  1.0
  Si  Si17  1  0.759  0.465  0.766  1.0
  Si  Si18  1  0.739  0.763  0.195  1.0
  Si  Si19  1  0.170  0.954  0.674  1.0
  Si  Si20  1  0.727  0.908  0.898  1.0
  Si  Si21  1  0.972  0.954  0.067  1.0
  Si  Si22  1  0.348  0.042  0.292  1.0
  Si  Si23  1  0.436  0.554  0.734  1.0
  Si  Si24  1  0.283  0.875  0.001  1.0
  Si  Si25  1  0.543  0.455  0.076  1.0
  Si  Si26  1  0.811  0.332  0.589  1.0
  Si  Si27  1  0.444  0.599  0.390  1.0
[/CIF]
.