The material described by Ca2S2O9 crystallizes in the monoclinic C2 space group. There are four inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 8-coordinate geometry to two equivalent O(1), two equivalent O(10), two equivalent O(4), and two equivalent O(7) atoms. In the second Ca site, Ca(2) is bonded in a 9-coordinate geometry to one O(10), one O(11), one O(13), one O(3), one O(4), two equivalent O(5), and two equivalent O(8) atoms. In the third Ca site, Ca(3) is bonded to one O(1), one O(2), one O(6), one O(7), two equivalent O(12), and two equivalent O(9) atoms to form distorted CaO8 hexagonal bipyramids that share corners with two equivalent S(1)O4 tetrahedra, corners with two equivalent S(3)O4 tetrahedra, edges with two equivalent Ca(3)O8 hexagonal bipyramids,  an edgeedge with one S(2)O4 tetrahedra, and  an edgeedge with one S(3)O4 tetrahedra. In the fourth Ca site, Ca(4) is bonded in a 9-coordinate geometry to one O(14), two equivalent O(11), two equivalent O(2), two equivalent O(3), and two equivalent O(6) atoms. There are three inequivalent S sites. In the first S site, S(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form SO4 tetrahedra that share corners with two equivalent Ca(3)O8 hexagonal bipyramids. In the second S site, S(2) is bonded to one O(5), one O(6), one O(7), and one O(8) atom to form SO4 tetrahedra that share  an edgeedge with one Ca(3)O8 hexagonal bipyramid. In the third S site, S(3) is bonded to one O(10), one O(11), one O(12), and one O(9) atom to form SO4 tetrahedra that share corners with two equivalent Ca(3)O8 hexagonal bipyramids and  an edgeedge with one Ca(3)O8 hexagonal bipyramid. There are fourteen inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Ca(1), one Ca(3), and one S(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Ca(3), one Ca(4), and one S(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Ca(2), one Ca(4), and one S(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Ca(1), one Ca(2), and one S(1) atom. In the fifth O site, O(5) is bonded in a distorted trigonal planar geometry to two equivalent Ca(2) and one S(2) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to one Ca(3), one Ca(4), and one S(2) atom. In the seventh O site, O(7) is bonded in a 3-coordinate geometry to one Ca(1), one Ca(3), and one S(2) atom. In the eighth O site, O(8) is bonded in a 3-coordinate geometry to two equivalent Ca(2) and one S(2) atom. In the ninth O site, O(9) is bonded in a 3-coordinate geometry to two equivalent Ca(3) and one S(3) atom. In the tenth O site, O(10) is bonded in a 3-coordinate geometry to one Ca(1), one Ca(2), and one S(3) atom. In the eleventh O site, O(11) is bonded in a distorted single-bond geometry to one Ca(2), one Ca(4), and one S(3) atom. In the twelfth O site, O(12) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Ca(3) and one S(3) atom. In the thirteenth O site, O(13) is bonded in a single-bond geometry to one Ca(2) atom. In the fourteenth O site, O(14) is bonded in a single-bond geometry to one Ca(4) atom. is represented by the Crystallographic Information File (CIF) [CIF]
data_Ca2S2O9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   9.504
_cell_length_b   9.504
_cell_length_c   12.201
_cell_angle_alpha   50.137
_cell_angle_beta   50.137
_cell_angle_gamma   43.565
_symmetry_Int_Tables_number   1
_cell_volume   549.536
_cell_formula_units_Z   3
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
  Ca  Ca0  1  0.510  0.490  0.500  1.0
  Ca  Ca1  1  0.991  0.340  0.566  1.0
  Ca  Ca2  1  0.660  0.009  0.434  1.0
  Ca  Ca3  1  0.130  0.539  0.925  1.0
  Ca  Ca4  1  0.461  0.870  0.075  1.0
  Ca  Ca5  1  0.027  0.973  0.000  1.0
  S  S6  1  0.256  0.243  0.751  1.0
  S  S7  1  0.757  0.744  0.249  1.0
  S  S8  1  0.247  0.592  0.310  1.0
  S  S9  1  0.408  0.753  0.690  1.0
  S  S10  1  0.897  0.276  0.186  1.0
  S  S11  1  0.724  0.103  0.814  1.0
  O  O12  1  0.197  0.435  0.754  1.0
  O  O13  1  0.565  0.803  0.246  1.0
  O  O14  1  0.312  0.050  0.887  1.0
  O  O15  1  0.950  0.688  0.113  1.0
  O  O16  1  0.071  0.288  0.750  1.0
  O  O17  1  0.712  0.929  0.250  1.0
  O  O18  1  0.447  0.201  0.611  1.0
  O  O19  1  0.799  0.553  0.389  1.0
  O  O20  1  0.015  0.663  0.410  1.0
  O  O21  1  0.337  0.985  0.590  1.0
  O  O22  1  0.266  0.713  0.147  1.0
  O  O23  1  0.287  0.734  0.853  1.0
  O  O24  1  0.333  0.644  0.339  1.0
  O  O25  1  0.356  0.667  0.661  1.0
  O  O26  1  0.364  0.357  0.346  1.0
  O  O27  1  0.643  0.636  0.654  1.0
  O  O28  1  0.792  0.505  0.088  1.0
  O  O29  1  0.495  0.208  0.912  1.0
  O  O30  1  0.785  0.256  0.350  1.0
  O  O31  1  0.744  0.215  0.650  1.0
  O  O32  1  0.885  0.139  0.173  1.0
  O  O33  1  0.861  0.115  0.827  1.0
  O  O34  1  0.127  0.199  0.135  1.0
  O  O35  1  0.801  0.873  0.865  1.0
  O  O36  1  0.138  0.132  0.424  1.0
  O  O37  1  0.868  0.862  0.576  1.0
  O  O38  1  0.677  0.323  0.000  1.0
[/CIF]
.
The material described by Ca2S2O9 crystallizes in the monoclinic C2 space group. There are four inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 8-coordinate geometry to two equivalent O(1), two equivalent O(10), two equivalent O(4), and two equivalent O(7) atoms. In the second Ca site, Ca(2) is bonded in a 9-coordinate geometry to one O(10), one O(11), one O(13), one O(3), one O(4), two equivalent O(5), and two equivalent O(8) atoms. In the third Ca site, Ca(3) is bonded to one O(1), one O(2), one O(6), one O(7), two equivalent O(12), and two equivalent O(9) atoms to form distorted CaO8 hexagonal bipyramids that share corners with two equivalent S(1)O4 tetrahedra, corners with two equivalent S(3)O4 tetrahedra, edges with two equivalent Ca(3)O8 hexagonal bipyramids,  an edgeedge with one S(2)O4 tetrahedra, and  an edgeedge with one S(3)O4 tetrahedra. In the fourth Ca site, Ca(4) is bonded in a 9-coordinate geometry to one O(14), two equivalent O(11), two equivalent O(2), two equivalent O(3), and two equivalent O(6) atoms. There are three inequivalent S sites. In the first S site, S(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form SO4 tetrahedra that share corners with two equivalent Ca(3)O8 hexagonal bipyramids. In the second S site, S(2) is bonded to one O(5), one O(6), one O(7), and one O(8) atom to form SO4 tetrahedra that share  an edgeedge with one Ca(3)O8 hexagonal bipyramid. In the third S site, S(3) is bonded to one O(10), one O(11), one O(12), and one O(9) atom to form SO4 tetrahedra that share corners with two equivalent Ca(3)O8 hexagonal bipyramids and  an edgeedge with one Ca(3)O8 hexagonal bipyramid. There are fourteen inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Ca(1), one Ca(3), and one S(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Ca(3), one Ca(4), and one S(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Ca(2), one Ca(4), and one S(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Ca(1), one Ca(2), and one S(1) atom. In the fifth O site, O(5) is bonded in a distorted trigonal planar geometry to two equivalent Ca(2) and one S(2) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to one Ca(3), one Ca(4), and one S(2) atom. In the seventh O site, O(7) is bonded in a 3-coordinate geometry to one Ca(1), one Ca(3), and one S(2) atom. In the eighth O site, O(8) is bonded in a 3-coordinate geometry to two equivalent Ca(2) and one S(2) atom. In the ninth O site, O(9) is bonded in a 3-coordinate geometry to two equivalent Ca(3) and one S(3) atom. In the tenth O site, O(10) is bonded in a 3-coordinate geometry to one Ca(1), one Ca(2), and one S(3) atom. In the eleventh O site, O(11) is bonded in a distorted single-bond geometry to one Ca(2), one Ca(4), and one S(3) atom. In the twelfth O site, O(12) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Ca(3) and one S(3) atom. In the thirteenth O site, O(13) is bonded in a single-bond geometry to one Ca(2) atom. In the fourteenth O site, O(14) is bonded in a single-bond geometry to one Ca(4) atom. is represented by the Crystallographic Information File (CIF) [CIF]
data_Ca2S2O9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   9.504
_cell_length_b   9.504
_cell_length_c   12.201
_cell_angle_alpha   50.137
_cell_angle_beta   50.137
_cell_angle_gamma   43.565
_symmetry_Int_Tables_number   1
_cell_volume   549.536
_cell_formula_units_Z   3
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
  Ca  Ca0  1  0.510  0.490  0.500  1.0
  Ca  Ca1  1  0.991  0.340  0.566  1.0
  Ca  Ca2  1  0.660  0.009  0.434  1.0
  Ca  Ca3  1  0.130  0.539  0.925  1.0
  Ca  Ca4  1  0.461  0.870  0.075  1.0
  Ca  Ca5  1  0.027  0.973  0.000  1.0
  S  S6  1  0.256  0.243  0.751  1.0
  S  S7  1  0.757  0.744  0.249  1.0
  S  S8  1  0.247  0.592  0.310  1.0
  S  S9  1  0.408  0.753  0.690  1.0
  S  S10  1  0.897  0.276  0.186  1.0
  S  S11  1  0.724  0.103  0.814  1.0
  O  O12  1  0.197  0.435  0.754  1.0
  O  O13  1  0.565  0.803  0.246  1.0
  O  O14  1  0.312  0.050  0.887  1.0
  O  O15  1  0.950  0.688  0.113  1.0
  O  O16  1  0.071  0.288  0.750  1.0
  O  O17  1  0.712  0.929  0.250  1.0
  O  O18  1  0.447  0.201  0.611  1.0
  O  O19  1  0.799  0.553  0.389  1.0
  O  O20  1  0.015  0.663  0.410  1.0
  O  O21  1  0.337  0.985  0.590  1.0
  O  O22  1  0.266  0.713  0.147  1.0
  O  O23  1  0.287  0.734  0.853  1.0
  O  O24  1  0.333  0.644  0.339  1.0
  O  O25  1  0.356  0.667  0.661  1.0
  O  O26  1  0.364  0.357  0.346  1.0
  O  O27  1  0.643  0.636  0.654  1.0
  O  O28  1  0.792  0.505  0.088  1.0
  O  O29  1  0.495  0.208  0.912  1.0
  O  O30  1  0.785  0.256  0.350  1.0
  O  O31  1  0.744  0.215  0.650  1.0
  O  O32  1  0.885  0.139  0.173  1.0
  O  O33  1  0.861  0.115  0.827  1.0
  O  O34  1  0.127  0.199  0.135  1.0
  O  O35  1  0.801  0.873  0.865  1.0
  O  O36  1  0.138  0.132  0.424  1.0
  O  O37  1  0.868  0.862  0.576  1.0
  O  O38  1  0.677  0.323  0.000  1.0
[/CIF]
.