Celsian - Structure

Structure

The symmetry in celsian is somewhat different from the symmetry normally found in feldspars. It is monoclinic with a body centered lattice similar to those of anorthite. Insufficient evidence has been found to suggest that celsian lacks a center of symmetry, so its space group is I 2/c (Newnham and Megaw, 1960). The space group differs from others of its group like orthoclase, albite and body center anorthite are C2/m, C1bar and I1bar.

X-ray analysis shows that the values for the lattice parameters a, b, c axes and angles are approximately a=863 pm, b=131.0 pm, c=1400 pm and β=116°, θ=90° (Gay, 1956).

There are 8 formula units per cell, and the general position is eightfold, so all atoms can lie in general positions (Newnham and Megaw,1960). This structure is very similar to that of orthoclase and sanidine but differs in a couple of ways:

1. The distribution of Si and Al.

2. The coordinates of all the atoms.

The distribution of silicon and aluminium along the tetrahedral sites mixed with the nature of the barium atom makes an impact on the surrounding silicate framework (Newham and Megaw,1960). The Si-Al bonds are partially ordered, and in some cases the aluminium substitute’s silicon.

The order in celsian is very simple, each aluminium tetrahedron is surrounded by four silicon tetrahedra, and vice versa (Newham and Megaw,1960). Also there is another type of transformation besides aluminium-silicon, where silicon-poor goes into a silicon-rich network that involves having to simultaneously be a replacement of Al, and Si at other sites.

The barium ion has an irregular configuration close to the one in potassium in the feldspars. Each barium has an oxygen close, and thanks to this configuration it has a strong effect on the silicon-oxygen-silicon bond angles.