On variety of the Ca coordination in the incommensurate structure of synthetic iron-bearing akermanite, Ca_2(Mg_<0.55>,Fe_<0.45>)Si_2O_7
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The incommensurate modulation in a synthetic iron-bearing åkermanite, Ca<SUB>2</SUB>(Mg, Fe)Si<SUB>2</SUB>O<SUB>7</SUB>, has been determined at room temperature by the five-dimensional refinements of the structure. The basic structure is tetragonal <I>P</I>-42<SUB>1</SUB><I>m</I> with unit-cell dimensions <I>a</I>=7.8679(3), <I>c</I>=5.0144(2)Å, <I>V</I>=310.41(2)Å<SUP>3</SUP>, <I>Z</I>=2, <I>M</I>=286.69, <I>D<SUB>x</SUB></I>=3.067Mg·m<SUP>−3</SUP>, Mo<I>K</I>α with graphite monochromator, μ=3.199mm<SUP>−1</SUP>, <I>F</I>(000)=284.49, Mg/(Mg+Fe)=0.554(6), <I>R</I>=0.065 for 770 unique reflections. The modulated structure is also tetragonal <I>P</I><SUB><I>P</I>4<I>mg</I></SUB><SUP><I>p</I>-42<SUB>1<I>m</I></SUB></SUP>, <B>k</B><SUB><B>1</B></SUB>=0.295(2)×(<B>a</B><SUP>*</SUP>+<B>b</B><SUP>*</SUP>), <B>k</B><SUB><B>2</B></SUB>=0.295(2)×(-<B>a</B><SUP>*</SUP>+<B>b</B><SUP>*</SUP>), where <B>k</B><SUB><B>1</B></SUB>, <B>k</B><SUB><B>2</B></SUB> are the wave vectors and <B>a</B><SUP>*</SUP>, <B>b</B><SUP>*</SUP> the reciprocal lattice vectors of the basic structure. <I>R</I>=0.136 for 3965 unique reflections. The modulation is caused mainly by the shifts of Ca and O atoms, and substitutional modulation at (Mg, Fe) sites was not detected in the present studies. Six-coordinated Ca forming a distorted oxygen octahedra exists in the modulated structure and most of them form clusters together with the flattened (Mg,Fe)O<SUB>4</SUB> tetrahedra. The number of the clusters in the present material is less than that in Co-åkermanite. The reason may be attributed to the disorder of the modulation amplitudes and wavelength.
- Mineralogical Journal
Mineralogical Journal 20(2), 47-58, 1998-04-01
Japan Association of Mineralogical Sciences