Surface, cristobalite. hydroxyl groups

Infrared absorption data obtained on silica SB (8) have been interpreted in terms of the completely random association of tetrahedra. This is consistent with the absence of any high temperature treatment for this material. If true, the association of this surface with geometrical considerations based on crystalline materials of comparable density is of doubtful validity. If such association is made, these data indicate that the surface structure is more comparable to tridymite than to cristobalite. The tridymite structure has been shown (4) to accommodate 4.6 hydroxyl groups and 4.6 water molecules per 100 sq. A. 

Figure 15. Top Octahedral face of (3-cristobalite. Bottom Rhombohedral face of (3-tridymite. The large open circles represent surface oxygen atoms. The small black circles represent surface silicon atomsy each of which carries one hydroxyl group when the surface is fully hydrated. The small crosses represent second-layer silicon atoms. The second-layer oxygen atoms are not shown, being directly under the second-layer silicon atoms. The partly visible third-layer oxygen atoms of /3-cristobalite are shown as shaded circles. The third-layer oxygen atoms of (3-tridymite are hidden because they are directly under the first-layer oxygen atoms. (Reproduced with permission from reference 65.

Atomic arrangement of the <111> octahedral face of cristobalite. Large circles oxygen atoms. Small shaded-in circles silicon atoms at surface. Dashed circles position of hydroxyl groups on surface attached to underlying silicon atoms. Hydroxyl surface coverage equals 4.55 OH/nm. Atomic sizes not to scale [1]. 

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