Most crystalline silica hydrates are layer structures. They are yielded by the leaching in acid of phyllosilicates lacking tetrahedral Al and in which the sheets are bounded on both sides by apical oxygens shared with both Si and cations. This permits cross-linking of the leached sheets by hydrogen bonds. Solid silica residues amorphous to X-rays are afforded by phyllosilicates in which the sheets are bounded on one side or on both sides by oxygens wholly bonded to Si, by inosilicates, and by tectosilicates with tetrahedral Si/Al ≤2. Silica gels and sols are yielded by silicates containing isolated Si-О motifs and by tectosilicates with tetrahedral Si/Al <2.

Two new crystalline silica hydrates afforded by the leaching of apophyllite (Silica-AP) and sanbornite (Silica-SN) are described. Supplementary data are given for Silica-X and Silica-GL. The supposed silica hydrates reported by leaching of hemimorphite, pectolite, scolecite, antigorite, chrysotile, heulandite, and glauconite are X-ray amorphous.

Silica residues amorphous to X-rays may contain sufficient order residual from the leached framework to influence their optical properties and their response when recrystallized hydrothermally at 175°C or by heating in air to 1000°C.

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