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

Abstract

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.