The crystal structure described by K2CaS2O9 crystallizes in the monoclinic P2_1 space group. There are two inequivalent K sites. In the first K site, K(1) is bonded in a 8-coordinate geometry to one O(2), one O(3), one O(6), one O(9), two equivalent O(4), and two equivalent O(8) atoms. In the second K site, K(2) is bonded to one O(2), one O(6), one O(7), one O(9), two equivalent O(4), and two equivalent O(8) atoms to form distorted KO8 hexagonal bipyramids that share corners with four equivalent K(2)O8 hexagonal bipyramids,  a cornercorner with one S(2)O4 tetrahedra, corners with two equivalent S(1)O4 tetrahedra, and edges with two equivalent S(2)O4 tetrahedra. Ca(1) is bonded in a 9-coordinate geometry to one O(2), one O(3), one O(6), one O(7), one O(9), two equivalent O(1), and two equivalent O(5) atoms. There are two inequivalent S sites. In the first S site, S(1) is bonded to one O(1), one O(2), one O(5), and one O(6) atom to form SO4 tetrahedra that share corners with two equivalent K(2)O8 hexagonal bipyramids. In the second S site, S(2) is bonded to one O(3), one O(4), one O(7), and one O(8) atom to form SO4 tetrahedra that share  a cornercorner with one K(2)O8 hexagonal bipyramid and edges with two equivalent K(2)O8 hexagonal bipyramids. There are nine inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Ca(1) and one S(1) atom. In the second O site, O(2) is bonded in a distorted tetrahedral geometry to one K(1), one K(2), one Ca(1), and one S(1) atom. In the third O site, O(3) is bonded in a distorted single-bond geometry to one K(1), one Ca(1), and one S(2) atom. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to two equivalent K(1), two equivalent K(2), and one S(2) atom. In the fifth O site, O(5) is bonded in a 3-coordinate geometry to two equivalent Ca(1) and one S(1) atom. In the sixth O site, O(6) is bonded in a distorted single-bond geometry to one K(1), one K(2), one Ca(1), and one S(1) atom. In the seventh O site, O(7) is bonded in a distorted single-bond geometry to one K(2), one Ca(1), and one S(2) atom. In the eighth O site, O(8) is bonded in a distorted single-bond geometry to two equivalent K(1), two equivalent K(2), and one S(2) atom. In the ninth O site, O(9) is bonded in a 3-coordinate geometry to one K(1), one K(2), and one Ca(1) atom. is represented by the Crystallographic Information File (CIF) [CIF]
data_K2CaS2O9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   7.170
_cell_length_b   6.233
_cell_length_c   10.010
_cell_angle_alpha   74.755
_cell_angle_beta   90.000
_cell_angle_gamma   90.000
_symmetry_Int_Tables_number   1
_cell_volume   431.618
_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
  K  K0  1  0.489  0.689  0.657  1.0
  K  K1  1  0.989  0.311  0.343  1.0
  K  K2  1  0.000  0.691  0.665  1.0
  K  K3  1  0.500  0.309  0.335  1.0
  Ca  Ca4  1  0.744  0.158  0.963  1.0
  Ca  Ca5  1  0.244  0.842  0.037  1.0
  S  S6  1  0.742  0.652  0.992  1.0
  S  S7  1  0.242  0.348  0.008  1.0
  S  S8  1  0.747  0.220  0.642  1.0
  S  S9  1  0.247  0.780  0.358  1.0
  O  O10  1  0.576  0.797  0.991  1.0
  O  O11  1  0.076  0.203  0.009  1.0
  O  O12  1  0.736  0.555  0.871  1.0
  O  O13  1  0.236  0.445  0.129  1.0
  O  O14  1  0.582  0.184  0.737  1.0
  O  O15  1  0.082  0.816  0.263  1.0
  O  O16  1  0.730  0.438  0.536  1.0
  O  O17  1  0.230  0.562  0.464  1.0
  O  O18  1  0.909  0.800  0.979  1.0
  O  O19  1  0.409  0.200  0.021  1.0
  O  O20  1  0.248  0.534  0.880  1.0
  O  O21  1  0.748  0.466  0.120  1.0
  O  O22  1  0.916  0.223  0.731  1.0
  O  O23  1  0.416  0.777  0.269  1.0
  O  O24  1  0.762  0.040  0.570  1.0
  O  O25  1  0.262  0.960  0.430  1.0
  O  O26  1  0.221  0.985  0.779  1.0
  O  O27  1  0.721  0.015  0.221  1.0
[/CIF]
.
The crystal structure described by K2CaS2O9 crystallizes in the monoclinic P2_1 space group. There are two inequivalent K sites. In the first K site, K(1) is bonded in a 8-coordinate geometry to one O(2), one O(3), one O(6), one O(9), two equivalent O(4), and two equivalent O(8) atoms. In the second K site, K(2) is bonded to one O(2), one O(6), one O(7), one O(9), two equivalent O(4), and two equivalent O(8) atoms to form distorted KO8 hexagonal bipyramids that share corners with four equivalent K(2)O8 hexagonal bipyramids,  a cornercorner with one S(2)O4 tetrahedra, corners with two equivalent S(1)O4 tetrahedra, and edges with two equivalent S(2)O4 tetrahedra. Ca(1) is bonded in a 9-coordinate geometry to one O(2), one O(3), one O(6), one O(7), one O(9), two equivalent O(1), and two equivalent O(5) atoms. There are two inequivalent S sites. In the first S site, S(1) is bonded to one O(1), one O(2), one O(5), and one O(6) atom to form SO4 tetrahedra that share corners with two equivalent K(2)O8 hexagonal bipyramids. In the second S site, S(2) is bonded to one O(3), one O(4), one O(7), and one O(8) atom to form SO4 tetrahedra that share  a cornercorner with one K(2)O8 hexagonal bipyramid and edges with two equivalent K(2)O8 hexagonal bipyramids. There are nine inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Ca(1) and one S(1) atom. In the second O site, O(2) is bonded in a distorted tetrahedral geometry to one K(1), one K(2), one Ca(1), and one S(1) atom. In the third O site, O(3) is bonded in a distorted single-bond geometry to one K(1), one Ca(1), and one S(2) atom. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to two equivalent K(1), two equivalent K(2), and one S(2) atom. In the fifth O site, O(5) is bonded in a 3-coordinate geometry to two equivalent Ca(1) and one S(1) atom. In the sixth O site, O(6) is bonded in a distorted single-bond geometry to one K(1), one K(2), one Ca(1), and one S(1) atom. In the seventh O site, O(7) is bonded in a distorted single-bond geometry to one K(2), one Ca(1), and one S(2) atom. In the eighth O site, O(8) is bonded in a distorted single-bond geometry to two equivalent K(1), two equivalent K(2), and one S(2) atom. In the ninth O site, O(9) is bonded in a 3-coordinate geometry to one K(1), one K(2), and one Ca(1) atom. is represented by the Crystallographic Information File (CIF) [CIF]
data_K2CaS2O9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   7.170
_cell_length_b   6.233
_cell_length_c   10.010
_cell_angle_alpha   74.755
_cell_angle_beta   90.000
_cell_angle_gamma   90.000
_symmetry_Int_Tables_number   1
_cell_volume   431.618
_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
  K  K0  1  0.489  0.689  0.657  1.0
  K  K1  1  0.989  0.311  0.343  1.0
  K  K2  1  0.000  0.691  0.665  1.0
  K  K3  1  0.500  0.309  0.335  1.0
  Ca  Ca4  1  0.744  0.158  0.963  1.0
  Ca  Ca5  1  0.244  0.842  0.037  1.0
  S  S6  1  0.742  0.652  0.992  1.0
  S  S7  1  0.242  0.348  0.008  1.0
  S  S8  1  0.747  0.220  0.642  1.0
  S  S9  1  0.247  0.780  0.358  1.0
  O  O10  1  0.576  0.797  0.991  1.0
  O  O11  1  0.076  0.203  0.009  1.0
  O  O12  1  0.736  0.555  0.871  1.0
  O  O13  1  0.236  0.445  0.129  1.0
  O  O14  1  0.582  0.184  0.737  1.0
  O  O15  1  0.082  0.816  0.263  1.0
  O  O16  1  0.730  0.438  0.536  1.0
  O  O17  1  0.230  0.562  0.464  1.0
  O  O18  1  0.909  0.800  0.979  1.0
  O  O19  1  0.409  0.200  0.021  1.0
  O  O20  1  0.248  0.534  0.880  1.0
  O  O21  1  0.748  0.466  0.120  1.0
  O  O22  1  0.916  0.223  0.731  1.0
  O  O23  1  0.416  0.777  0.269  1.0
  O  O24  1  0.762  0.040  0.570  1.0
  O  O25  1  0.262  0.960  0.430  1.0
  O  O26  1  0.221  0.985  0.779  1.0
  O  O27  1  0.721  0.015  0.221  1.0
[/CIF]
.