Algebraic Varieties with Rational Dissections.

a set of parallel lathlike strips which extend indefinitely in the direction of the b-axis and which have their common planar surfaces parallel to (101). The co-ordination of the Ca3 (or Na in pectolite) is curious. Its nearest neighbors are the three oxygen atoms 02, 03, and 04. All other oxygen atoms are at distances much greater than the sum of the ionic radii of Ca (or Na) and 0. The Ca3 (or Na) atoms thus have trigonal pyramidal co-ordination, and this pyramid shares edges with the edges of the Ca, and Ca2 octahedra. The lengths of the aand c-axes of pectolite and wollastonite are quite close, but pectolite has a considerably shorter b-axis than wollastonite. In the structure this is reflected in the close approach of the 03 and 04 atoms of neighboring silicon tetrahedra. Evidently a hydrogen bond occurs between these atoms which results in a relative contraction of the b-axis of pectolite. More detailed accounts of the determination of the structures of pectolite and wollastonite and more detailed descriptions of these structures are published elsewhere.