Abstract

The crystal structure of SrAl2Si2O8 with a celsian-type framework structure has been refined to R = 0.047 by full-matrix least-squares methods using 1058 ‘a’-type (h + k even, l even) and 258 much weaker ‘b’-type reflections (h + k odd, l odd).The cell parameters a = 8.388 (3), b = 12.947 (4), c = 14.263 (6) Å, β = 115.2 (1)°; the space groupi is I2/c

This compound was synthesized in a Verneuil furnace from a stoichiometric mixture of SrCO3, SiO2, and Al2O3 and shows a partially disordered Al/Si distribution. The mean T-o distances, T1O = 1.626, T2Z = 1.630, T1Z = 1.732, and T2O = l.735 Å, when compared with the grand mean Si-O bond length of 1.614 Å and Al-O bond length of 1.747 Å in ordered primitive anorthite, indicate approximately 10 percent Al in the Si-rich and 10 percent Si in the Al-rich sites of this compound.

The Sr atom can be considered to be 7-coordinated, (Sr-O) = 2.691 Å, or 9-coordinated, (Sr-O) = 2.786 Å. These values are both 0.03 Å shorter than those reported by Grundy and Ito (1974) for a highly disordered, synthetic (Sr0.84Na0.030.13)A11.7Si2.3O8, (space group C2/m).

Using the 7-coordinated model, there is a positive correlation of mean T-O distances to 2-, 3-, and 4-coordinated oxygen atoms and the parameter, ∑[1/(Sr-O)2], which is the sum of the inverse squares of the Sr-O distances to these oxygens:

C.N.(O)Mean
∑ [1/Sr-O)2]
Mean Si-OMean Al-O
1.610 Å 1.724 Å 
0.137 1.632 Å 1.734 Å 
0.287 1.644 Å 1.755 Å 
C.N.(O)Mean
∑ [1/Sr-O)2]
Mean Si-OMean Al-O
1.610 Å 1.724 Å 
0.137 1.632 Å 1.734 Å 
0.287 1.644 Å 1.755 Å 

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