We made a CIF file for olivine in the Pnma space group.

data_Olivine
_audit_creation_method     'manual editing'
_chemical_name_systematic
;
Forsterite
;
_chemical_formula_sum '(Mg2) Si O4'
_cell_length_a 10.1902
_cell_length_b 5.9783
_cell_length_c 4.7534
_cell_angle_alpha 90
_cell_angle_beta 90
_cell_angle_gamma 90
_cell_volume 289.58
_symmetry_cell_setting     'Orthorhombic'
_symmetry_space_group_name_H-M 'P n m a'

loop_
_space_group_symop_operation_xyz
  'x,y,z'
  'x,y,1/2-z'
  '-x,-y,1/2+z'
  '1/2+x,1/2-y,1/2+z'
  '1/2-x,1/2+y,1/2-z'
  '1/2-x,1/2+y,z'
  '1/2+x,1/2-y,-z'
  '-x,-y,-z'

loop_
    _atom_type_symbol
    _atom_type_description 
    'Mg' 'Mg' 
    'Si' 'Si' 
    'O' 'O' 

loop_
_atom_site_label
_atom_site_type_symbol
_atom_site_fract_z
_atom_site_fract_x
_atom_site_fract_y
_atom_site_occupancy
Mg1   'Mg' 0.00000   0.00000   0.00000   1.00000
Mg2   'Mg' 0.99090   0.27650   0.25000   1.00000
Si   'Si' 0.42780   0.09480   0.25000   1.00000
O1   'O' 0.76700   0.09250   0.25000   1.00000
O2   'O' 0.21540   0.44810   0.25000   1.00000
O3   'O' 0.28100   0.16400   0.03350   1.00000

But it do not work right. Some atoms are in double in the unit cell.

Here is the correction :

data_global
_chemical_name_mineral 'Forsterite'
loop_
_publ_author_name
'Miyake M'
'Nakamura H'
'Kojima H'
'Marumo F'
_journal_name_full 'American Mineralogist'
_journal_volume 72 
_journal_year 1987
_journal_page_first 594
_journal_page_last 598
_publ_section_title
;

;
_database_code_amcsd 0001089
_chemical_formula_sum 'Mg2 Si O4'
_cell_length_c 4.779
_cell_length_a 10.277
_cell_length_b 5.995
_cell_angle_alpha 90
_cell_angle_beta 90
_cell_angle_gamma 90
_cell_volume 294.437
_exptl_crystal_density_diffrn      4.271
_symmetry_space_group_name_H-M 'P n m a'
loop_
_space_group_symop_operation_xyz
  'x,y,z'
  'x,1/2-y,z'
  '-x,1/2+y,-z
  '1/2-x,1/2+y,1/2+z'
  '1/2+x,1/2-y,1/2-z'
  '1/2+x,y,1/2-z'
  '1/2-x,-y,1/2+z'
  '-x,-y,-z'
loop_
_atom_site_label
_atom_site_fract_z
_atom_site_fract_x
_atom_site_fract_y
_atom_site_occupancy
Mg1   0.00000   0.00000   0.00000   1.0000
Mg2   0.99090   0.27650   0.25000   1.0000
Si   0.42780   0.09480   0.25000   1.00000
O1   0.76700   0.09250   0.25000   1.00000
O2   0.21540   0.44810   0.25000   1.00000
O3   0.28100   0.16400   0.03350   1.00000

But it has problem to run on PolyXSim. Here is the last correction, which give a correct crystal structure and can be read by PolyXSim.

data_Olivine
_audit_creation_method     'manual editing'
_chemical_name_systematic
;
Forsterite
;
_chemical_formula_sum '(Mg2) Si O4'
_cell_length_a 10.1902
_cell_length_b 5.9783
_cell_length_c 4.7534
_cell_angle_alpha 90
_cell_angle_beta 90
_cell_angle_gamma 90
_cell_volume 289.58
_symmetry_cell_setting     'Orthorhombic'
_symmetry_space_group_name_H-M 'P n m a'
_space_group_IT_number                 62

loop_
_space_group_symop_operation_xyz
   'x, y, z'
   '-x, -y, -z'
   '-x+1/2, -y, z+1/2'
   'x+1/2, y, -z+1/2'
   '-x, y+1/2, -z'
   'x, -y+1/2, z'
   'x+1/2, -y+1/2, -z+1/2'
   '-x+1/2, y+1/2, z+1/2'

loop_
    _atom_type_symbol
    _atom_type_description 
    'Mg' 'Mg' 
    'Si' 'Si' 
    'O' 'O' 

loop_
   _atom_site_label
   _atom_site_occupancy
   _atom_site_fract_x
   _atom_site_fract_y
   _atom_site_fract_z
   _atom_site_B_iso_or_equiv
   _atom_site_type_symbol
   Mg1        1.0     0.000000      0.000000      0.000000       1.000000 Mg
   Mg2        1.0     0.276500      0.250000      0.990900       1.000000 Mg
   Si         1.0     0.094800      0.250000      0.427800       1.000000 Si
   O1         1.0     0.092500      0.250000      0.767000       1.000000 O
   O2         1.0     0.448100      0.250000      0.215400       1.000000 O
   O3         1.0     0.164000      0.033500      0.281000       1.000000 O