data_global
_chemical_name_mineral 'Akermanite'
loop_
_publ_author_name
'Yang H'
'Hazen R M'
'Downs R T'
'Finger L W'
_journal_name_full 'Physics and Chemistry of Minerals'
_journal_volume 24 
_journal_year 1997
_journal_page_first 510
_journal_page_last 519
_publ_section_title
;
 Structural change associated with the incommensurate-normal phase transition
 in akermanite, Ca2MgSi2O7, at high pressure
 Sample: P = 1.80 GPa
;
_database_code_amcsd 0008034
_chemical_formula_sum 'Ca2 Mg Si2 O7'
_cell_length_a 7.7937
_cell_length_b 7.7937
_cell_length_c 4.9696
_cell_angle_alpha 90
_cell_angle_beta 90
_cell_angle_gamma 90
_cell_volume 301.862
_exptl_crystal_density_diffrn      2.999
_symmetry_space_group_name_H-M 'P -4 21 m'
loop_
_space_group_symop_operation_xyz
  'x,y,z'
  '1/2-y,1/2-x,z'
  'y,-x,-z'
  '1/2-x,1/2+y,-z'
  '-x,-y,z'
  '1/2+y,1/2+x,z'
  '-y,x,-z'
  '1/2+x,1/2-y,-z'
loop_
_atom_site_label
_atom_site_fract_x
_atom_site_fract_y
_atom_site_fract_z
_atom_site_U_iso_or_equiv
Ca   0.33310   0.16690   0.50530   0.01596
Mg   0.00000   0.00000   0.00000   0.01051
Si   0.14040   0.35960   0.93460   0.00988
O1   0.50000   0.00000   0.18380   0.01900
O2   0.14040   0.35960   0.25580   0.01900
O3   0.08130   0.18530   0.78190   0.01646
loop_
_atom_site_aniso_label
_atom_site_aniso_U_11
_atom_site_aniso_U_22
_atom_site_aniso_U_33
_atom_site_aniso_U_12
_atom_site_aniso_U_13
_atom_site_aniso_U_23
Ca 0.01692 0.01692 0.01376 0.00646 0.00196 0.00196
Mg 0.00739 0.00739 0.01702 0.00000 0.00000 0.00000
Si 0.00985 0.00985 0.01001 -0.00031 -0.00020 -0.00020
O1 0.02216 0.02216 0.01151 -0.01292 0.00000 0.00000
O2 0.01816 0.01816 0.01902 0.00646 -0.00255 -0.00255
O3 0.02493 0.00985 0.01314 -0.00369 0.00471 -0.00432