Twinning of aragonite—the OD approach

Abstract

Twinning process in aragonite and its isotypes on (110) may be described by OD (order –disorder) mechanism. In the Pmcn orientation of aragonite, the OD layer has layer group P(c)mb, with the direction perpendicular to the layer indicated by parentheses. The lattice parameters of the OD layer are b 9.39 A (the [110] diagonal of the aragonite lattice) and c 5.74 A, its thickness a is d(110) of the aragonite lattice, equal to 4.21 A. The interlayer σ-PO’s are \( \left\{ {\left( {{{{{{2}_{{2}}}}} \left/ {{{n_{{{{1} \left/ {{2,\,{1}}} \right.}}}}}} \right.}} \right){{{{{2}_{{{ -_{{{{1} \left/ {2} \right.}}}}}}}}} \left/ {{{a_{{2}}}}} \right.}{{{{{2}_{{1}}}}} \left/ {{{n_{{{{{{2}, - {1}}} \left/ {2} \right.}}}}}} \right.}} \right\} \), brackets denote the direction perpendicular to the OD layer. Two polytypes with maximum degree of order can be derived. MDO1 is Pmcn aragonite itself, MDO2 is a Pcab polytype with a 8.42 A, b 9.39 A and c 5.74 A, unknown as an independent structure. Desymmetrization of the aragonite structure appears to limit occurrence of the MDO2 principle to occasional twin boundaries.