Katoite, a new member of the Ca3Al2(SiO4)3-Ca3Al2(OH)12 series and a new nomenclature for the hydrogrossular group of minerals

The new mineral katoite, Ca3Al2(SiO4)(OH)8, occurs in Pietramassa near Montalto di Castro (Viterbo, Italy), associated with many other hydrated calcium silicates and aluminates in the vugs of a phonolite which effused through argillaceous marls of Pliocene age. Microprobe and TG analyses yielded the empirical formula Ca2.96(Al1.85Mg0.01)(Si0.69S0.11)O2.93(OH)9.07 ; X-ray study showed it to be cubic, Ia3d with a 12.358(2) Å. Physical properties (thermal behaviour, refractive index, infrared spectrum, and X-ray powder diffraction) are consistent with its chemical composition and with its pertinence to the solid solution series Ca3Al2Si3O12 (grossular) and Ca3Al2(OH)]2 (synthetic phase) with an approximate content of 27 % of the former and 73 % of the latter. This makes it the first natural compound of the series with a grossular content lower than 50 % and confirms the existence, also in nature, of a complete series with a SiO4 ⇌ (OH)4 isomorphous replacement within the grossular structure. By analogy with other isomorphous mineral series, two names, hibschite and katoite, are appropriate to represent the two solid solution fields with more and less than 50 % Si in the tetrahedra, respectively. The name hydrogrossular remains applicable as a group name for members of the whole series with an appreciable OH content but otherwise undetermined SiO4/(OH)4 ratio. The paragenetic sequence found in nature and laboratory experiments reported in earlier literature, indicate for katoite a crystallization temperature of approximately 300 °C with a relatively high water vapour pressure and a limited Si supply.