Hydrothermal Synthesis and Crystal Structure of Vesuvianite Compounds, Ca₁₉Al₁₃Si₁₈O₇₁(OH)₇ and Sr₁₉Fe₁₂Ge₁₉O₇₂(OH)₆

New compositions of synthetic vesuvianite were investigated using hydrothermal synthesis. High quality single crystals with the formula Ca₁₉Al₁₃Si₁₈O₇₁(OH)₇ (I) having the vesuvianite-type structure were crystallized during a high temperature hydrothermal growth reaction. Starting materials of Al₂O₃ and CaSiO₃ reacted at 670 °C and 2 kbar in 0.5 M aqueous alkali hydroxide mineralizer to form single crystals up to 0.25 mm per edge. Similar reactions employing SrO, Fe₂O₃, and GeO₂ reacting at 580 °C and 2 kbar in 6 M aqueous alkali hydroxide mineralizers led to the formation of the analogous Sr₁₉Fe₁₂Ge₁₉O₇₂(OH)₆ (II). These crystals were obtained in sizes up to 0.5 mm per edge. The structures of both compounds were refined in space group P4/nnc after careful evaluation of the diffraction data and subsequent test refinements. Elemental analysis indicated only the presence of Ca²⁺, Al³⁺, and Si⁴⁺ cations in I and only the presence of Sr²⁺, Fe³⁺, and Ge⁴⁺ cations in II, representing synthetic vesuvianite comprising the minimum number of unique cations. The use of larger cations than are typically found in natural vesuvianite, such as Sr²⁺, Fe³⁺, and Ge⁴⁺, resulted in an expanded crystalline lattice and extended the vesuvianite analogs to include an increasing variety of elements.