THE STRUCTURE OF HARADAITE AND A NOTE ON THE Si-O BOND LENGTHS IN SILICATES

Haradaite, SrVSi2O7, is orthorhombic, space group Amam, with four formula units in the cell, a=7.06, b=14.64, c=5.33Å. The structure contains Si4O12 chains which are joined together by vanadium to form a sheet closely related to those in lamprophyllite, bafertisite and astrophyllite. These minerals may conveniently be called quasi-layer silicates. There are two kinds of bridge bonds in the chain: one is a nonlinear bond (∠Si-O-Si 150°) of length 1.664±0.008Å, the other is a linear bond of length 1.578±0.009Å. The non-bridge bonds have a length 1.598±0.03Å. A general survey on the Si-O bond lengths in silicates shows that, for nonlinear cases, the mean value of bridge bonds is 1.641Å and that of non-bridge bonds 1.605Å, and for linear cases, these values are respectively 1.59Å and 1.63Å.