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

. 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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.