Both beryl (Be3Al2Si6O18) and cordierite (Al3(Mg,Fe)2Si5AlO18) have six-membered rings of tetrahedra with ideal formulas Si6O18, and used to be classified as cyclosilicates. However, more recent classifications sometimes treat them as tectosilicates.
The reason is that silicate minerals often contain other tetrahedra with ions like Be and Al. It is particularly common for Al to substitute for Si. In some silicates the other ions are ordered and occur in only specific sites, but in others the ions are mixed randomly. In such a case, it makes sense to consider all tetrahedra in describing the structure. In the case of beryl and cordierite, the tetrahedral rings are cross-linked by additional tetrahedra.
The case for calling beryl a cyclosilicate is a bit stronger than cordierite, since the Be and Si tetrahedra are distinct. The picture below shows a single unit cell of beryl. The yellow Si tetrahedra are in the upper layer, the green ones in the lower layer. The outer points of the tetrahedra are actually edges viewed along their length. The purple tetrahedra contain Be and are viewed down their four-fold inversion axes. They connect the bottom corners of the tetrahedra in the upper ring with the top corners of the tetrahedra in the lower ring. The solitary blue atoms are Al, in 6-fold coordination with the adjacent tetrahedral oxygens.
In the drawing below, we see the rings from the side, showing how they are connected by the solitary tetrahedra.
Below is a drawing of the complete lattice, showing also the octahedral coordination of Al in blue. These are somewhat idealized drawings since the real structures have slightly tilted and rotated polyhedra.
In the case of cordierite, the purple tetrahedra contain Al, but so do some of the ring tetrahedra. Trying to distinguish between the tetrahedra becomes pretty artificial, so small wonder many mineralogists regard it as a tectosilicate. At high temperatures cordierite is hexagonal, implying that the ordering of Al and Si in the rings must be completely random, but on cooling it distorts to orthorhombic, pseudohexagonal symmetry, probably because the Al settle into an ordered arrangement. The Mg and iron are in the octahedra. The unit cell dimensions, 1.71 by 0.97 nanometers, are in the ratio 1.76, very close to the square root of three. Thus a unit cell split diagonally is very close to a 30-60 right triangle.
Created 19 December 2001, Last Update 14 December 2009
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