data_global _chemical_name_mineral 'Akermanite' loop_ _publ_author_name 'Kusaka K' 'Hagiya K' 'Ohmasa M' 'Okano Y' 'Mukai M' 'Iishi K' 'Haga N' _journal_name_full 'Physics and Chemistry of Minerals' _journal_volume 28 _journal_year 2001 _journal_page_first 150 _journal_page_last 166 _publ_section_title ; Determination of structures of Ca2CoSi2O7, Ca2MgSi2O7, and Ca2(Mg.55Fe.45)Si2O7 in incommensurate and normal phases and observation of diffuse streaks at high temperature Sample: T = 359.7 K Melilite group ; _database_code_amcsd 0008520 _chemical_formula_sum 'Mg Ca2 Si2 O7' _cell_length_a 7.8398 _cell_length_b 7.8398 _cell_length_c 5.0058 _cell_angle_alpha 90 _cell_angle_beta 90 _cell_angle_gamma 90 _cell_volume 307.669 _exptl_crystal_density_diffrn 2.943 _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 Mg 0.00000 0.00000 0.00000 0.00798 Ca 0.33219 0.16781 0.50590 0.01494 Si 0.13980 0.36020 0.93380 0.00659 O1 0.00000 0.50000 0.81900 0.01900 O2 0.14040 0.35960 0.25460 0.01760 O3 0.08260 0.18630 0.78710 0.01798