The crystal structure of celsian from Jakobsberg, Sweden, (Ba0.95K0.05Al1.95Si2.05 O8, a = 8.622,b= 13.078, c= 14.411 Å, β = 115.09°, space group I2/c) has been refined using 528 ‘a’ reflections (h+k even, I even) and 272 ‘b’, reflections (h+k odd, l odd) > 1

σ^
. The ‘b’ reflections were measured at one-quarter the rate of ‘a’ reflections to obtain a higher level of significance for these very much weaker ‘difference’ reflections whose intensities are related only to Al/Si ordering in the tetrahedral framework.

The Jakobsberg celsian is very similar to a celsian from Broken Hill, Australia, (Cn~84Or~18) refined by Newnham and Megaw (1960). The mean T-O distances for the symmetrically non-equivalent tetrahedral sites are T1O = 1.639, T2z = 1.636; T1z = 1.719, T2O = 1.718 Å, indicating that by comparison with fully ordered anorthite the Si-rich sites contain ~0.18 Al and the Al-rich sites ~0.79 Al. In the Broken Hill celsian the A1 contents are ~0.17 and ~0.76, respectively, consistent with a somewhat lower Al/(A1+Si) ratio. These partially disordered Al/Si distributions are not in strict accord with the aluminum avoidance principle and may result from anti-phase domain textures as previously observed in bytownite and transitional anorthite.

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