Rhönite: structural and microstructural features, crystal chemistry and polysomatic relationships
Bonaccorsi, Elena; Merlino, Stefano; Pasero, Marco
European Journal of Mineralogy Volume 2 Number 2 (1990), p. 203 - 218
published: Apr 17, 1990
manuscript accepted: Nov 24, 1989
manuscript received: Aug 28, 1989
ArtNo. ESP147050202010, Price: 29.00 €
Abstract Rhönite, Ca2(Mg,Fe2+,Fe3+,Ti)6(Si,Al)6O20, is an uncommon mineral belonging to the aenigmatite group. The crystal structure of two chemically different rhonites, from the volcanic breccias of Scharnhausen (P1̅, a = 10.428, b = 10.807, c = 8.925 Å, α = 105.91, β= 96.13, γ= 124.80°) and from the Allende meteorite (PI, a = 10.367, b = 10.756, c = 8.895 Å, α = 105.98, β = 96.04, γ = 124.72°), were both refined to R = 0.066. The structure presents the common features of all the aenigmatite-like minerals: "octahedral" walls connected through tetrahedral chains with appendices and additional insular octahedra. Distinct ordering schemes were proposed for the two crystals on the basis of the chemical and structural data. The crystal chemistry of rhonite is assessed and discussed and the limitations to solid solution between rhonite and aenigmatite are explained. The OD character of rhonite and aenigmatite-like minerals is briefly reviewed and HRTEM data are presented which give evidence of polytypism phenomena in rhonite from Scharnhausen. Finally the position of these minerals as polysomes within the polysomatic series having pyroxene and spinel as end-members is stressed and discussed.