A Clathrate Crystalline Form of Silica

Abstract

The low-density cubic form of silica (impure), melanophlogite, has a clathrate-type structure that incorporates organic molecules, including long straight-chain hydrocarbons. The structure is analogous to that of the clathrate crystalline hydrates of composition 6X.46H(2)O or 8X.46H(2)O. This is the first known example of a silicate framework structure with the pentagonal dodecahedron as a framework element.

X-ray studies of structural changes in melanophlogite with varying temperature

“…So far, many types of silica clathrates containing different types of polyhedral voids have been reported, mainly in relation to their synthesis with various guest molecules (Gies and Marler, 1992 4 and N 2 , and the M 14 voids contain molecules such as CO 2 , and N 2 . Since the first description of MEP as a new mineral from Sicilian sulfur deposits, many structural studies have been carried out using microscopic, spectroscopic, and X ray diffraction techniques (Skinner and Appleman, 1963;Kamb, 1965;Žák, 1972;Gies, 1983;Kortus et al, 2000;Nakagawa et al, 2001;Nakagawa, 2002). However, some important structural details of this mineral and corresponding synthetic materials remain to be determined, particularly the details of the temperature dependency of the structure, the structural behavior of guest molecules, and their effect on the stability of these low density (e.g., 2.0 × 10 3 kg/m 3 ) frameworks.…”

Properties of methyl radical trapped in amorphous SiO2 and in natural SiO2-clathrate Melanophlogite

“…This latter material is a naturally occurring SiO 2 -based clathrate structure which shares the same crystalline structure of type I gas hydrates [13]. Two types of voids are involved in the structure of Melanophlogite (Cage I and Cage II), which are typically occupied by guest CH 4 , CO 2 and N 2 molecules, weakly interacting with the host structure by van der Waals interaction [13][14][15][16].…”

“…Typically, each cage is occupied by just one guest molecule. The free space within Cage I can be approximately described by a sphere-like void with diameter d~5.7 Å and a volume V~97 Å 3 , whereas that within Cage II can be approximated by an ellipsoid with d 1 =d 2 = 5.8 Å, d 3~7 .7 Å and a volume of 136 Å 3 [13][14][15][16].…”

New silica clathrate minerals that are isostructural with natural gas hydrates

“…When the main diffraction peaks are considered to be those from the cubic chibaite phase with the space group symmetry Fd3 _ (ref. 7), the unit cell parameters are refined as a = 1.93742(3) nm and V = 7.2723(2) nm 3 . A small diffraction peak of the 402 reflection was observed (Supplementary Table S2).…”