Crystal chemistry of kimzeyite from Anguillara, Mts. Sabatini, Italy
Schingaro, Emanuela; Scordari, Fernando; Capitanio, Flavio; Parodi, Giancarlo; Smith, David C.; Mottana, Annibale
European Journal of Mineralogy Volume 13 Number 4 (2001), p. 749 - 759
published: Jul 18, 2001
ArtNo. ESP147051304011, Price: 29.00 €
A single garnet crystal of kimzeyite from Anguillara Sabazia, Sabatini Volcanic District (Rome), has been studied by electron probe microanalysis (EPMA), single crystal X-ray diffraction (XRD) and Raman spectroscopy (RS). X-ray intensities for the structure refinement were collected by means of both a conventional four circle diffractometer (CD) and an area detector (AD) diffractometer. Salient crystallographic data from AD measurements are: a = 12.397(4) Å, V = 1905.24 Å3, μ = 25 cm-1, crystal dimension ~ 40 μm, Dx = 3.83 g cm-3. The chemical analyses allowed to ascertain that Ca, Ba, Fe, Al, Mg, REE, Zr, Si and Ti are present in the structure. The transition element distribution is: Fe3+(Y) << fe3+(Z), and Ti as Ti4+ occurs only at Y. No hydrogen atoms were detected. The problem of the correct distribution of Ba and REE (essentially Eu and Gd) is dealt with, but not fully solved in the present study. Because of the very small amount of sample available and of the strong dilution of Ba and REE (~0.03 and 0.02 atoms per formula unit respectively) the correct distribution of these atoms over the X and Y sites is problematical. The radius ratio rule and electrostatic requirements suggest Ba at X and REE at Y. From these principles, the following site distribution can be deduced: Ca2+ and Ba2+ at X; Mg2+, REE3+, Zr4+, Ti4+, Fe3+ at Y; Si4+, Al3+, Fe3+ at Z. On the other hand, structural details and the best fit between EPMA and XRD site populations indicate REE at X and Ba at Y. Bond strength theory arguments support this hypothesis. The main substitution mechanisms can be summed up as follows: 1) Ti4+(Y) + Fe3+(Z) ↔ Si4+(Z) + M3+(Y) [schorlomite substitution]; 2) Zr4+(Y) + M3+(Z) ↔ Si4+(Z) + M3+(Y) [kimzeyite substitution]; 3) Mg2+(Y) + Zr4+(Y) ↔ 2M3+(Y) [morimotoite-like substitution], where M3+ = (Al, Fe). In addition, following the bond strength theory indications, another mechanism which possibly involves the X and Y sites is also taken into account: 4) REE3+(X) + Ba2+(Y) ↔ Ca2+(X) + M3+(Y).