Bull. Minéral. (1985). 108, 1-8

The crystal structure of katoite and implications within the hydrogrossular group of minerals

La structure cristalline de la katoite et ses implications dans le groupe de /' hydrogrossulaire .

Introduction

In the continuous isomorphous series Ca3Al2 (Si04)3 (grossular) — Ca3Al2(OH),2 (synthetic compound), katoite with the schematical for¬ mula Ca3Al2(SiO)4 (OH)8, has been recently described (Passaglia and Rinaldi, 1984) as a new mineral being the first natural member in which more than 50 % of tetrahedral Si is sub¬ stituted by 4H atoms. Katoite was found as white transparent octahedra associated with to-bermorite, afwillite and hydrocalumite in a pho-nolite at Campomorto, Montalto di Castro, Vi-terbo, Italy.

Structural data on members of this series are available for : anhydrous end-member grossular (Menzer, 1926, 1928 ; Abrahams and Geller, 1958 ; Prandl, 1966 ; Novak and Gibbs, 1971), natural plazolite (Si/4H ratio slightly higher than one) (Pabst, 1937 ; Basso et al., 1983) and syn¬ thetic compounds Ca3Al2(Si04)2 16(OH)3 36

(Cohen-Addad et al., 1967) and Ca3Al2(OH)l2 (Cohen-Addad et al., 1967 ; Bartl, 1969).

Aim of this work is to determine the crystal structure of katoite and to define the structural effects of the Si substitution by 4H in the gros¬ sular framework.

Experimental

A katoite crystal with an almost perfect oc¬ tahedral morphology (size about 0. 1*2 mm in the [111] ) of the holotype of Passaglia and Rinaldi (1984) was used for data collection. Electron microprobe analysis of the same crystal was obtained by Passaglia and Rinaldi (1984). The space group was confirmed to be laid, by long exposure Weissenberg and precession photo¬ graphs. X-ray diffraction data were collected with a Siemens AED diffractometer with Ni-filtered CuÀa radiation in the to/20 scan mode,