Comparison of the crystal chemistry of selected MSi₆O₁₅-based silicates

Abstract

The structures of four A 3 MSi 6 O 15 (•nH 2 O) silicates (A ϭ Na or K, M ϭ Nd or Y) recently determined by the authors are compared with one another and with the structures of related silicates. The 2:5 Si:O ratio in these compounds (silicate tetrahedra linked by sharing three O atoms per tetrahedron plus one unshared O atom) permits layer, double-chain, or doublering configurations. In ␣-K 3 NdSi 6 O 15 •2H 2 O, ␤-K 3 NdSi 6 O 15 , and Na 3 NdSi 6 O 15 •2H 2 O, the linkage is found to result in a corrugated layered structure, in ␤-Na 3 YSi 6 O 15 a double-chain structure, and in ␣-Na 3 YSi 6 O 15 a unique double-ring structure. Although the factors that govern the stabilities of ring vs. chain vs. layered structures remain to be completely elucidated, it is apparent in the layered structures that larger M cations (such as Nd and Ce) produce a greater degree of corrugation than do Zr and Ti cations. The more open structures of neodymium and cerium silicates contain large channels that may serve as pathways for fast alkali ion transport.

The Structure and Conductivity of K8Nd3Si12O32(OH): A Layered Silicate with Paths for Possible Fast-Ion Condution

“…In these compounds, all silicate tet-rahedral units contain three bridging and one terminating oxygen atoms and the layers can be described in terms of the condensation of wollastonite-type chains (14). In particular, the layers in -K NdSi O ' 2H O and -K NdSi O , compounds compositionally similar to the one considered here, can both be generated in a two-step condensation process.…”

Sazhinite-(La), Na3LaSi6O15(H2O)2, a new mineral from the Aris phonolite, Namibia: Description and crystal structure

“…The depletion of these elements in sazhinite-(La) from Aris is caused by competition from the concomitant growth of other minerals such as eudialyte and korobitsynite, which deplete the fluid in small-radius cations (<0.8 Å ). Haile and Wuensch (1997) Sokolova et al, 2003], and a framework silicate (K 2 Ce[Si 6 O 15 ]; Karpov et al, 1976). Haile et al (1995) proposed that the topology of the silicate units in [Si 6 O 15 ] compounds is controlled by the electronegativity of the cation at the M site rather than by difference in radius.…”

Hydrothermal synthesis, structure, and property characterization of rare earth silicate compounds: NaBa3Ln3Si6O20 (Ln = Y, Nd, Sm, Eu, Gd)

“…Most of these structures are based on sodium or potassium alkali metal cations. [17][18][19] Given the massive number of corresponding transition metal and main group metal silicates, the number of alkaline earth metal or mixed alkaline/alkaline earth metal containing rare earth silicates surprisingly is somewhat limited. This opens a promising avenue of investigation of a relatively undeveloped class of new single crystals with attractive physical properties.…”