Schorlomite and morimotoite: what's in a name?

Abstract

The crystal structure of an isotropic, single phase, Ti-rich schorlomite garnet, ideally Ca3Ti4+2(Fe3+2Si)O12, from Magnet Cove, Arkansas was refined with the Rietveld method, space group $Ia\overline 3 d$, and monochromatic synchrotron high-resolution powder X-ray diffraction data. Electron-microprobe analysis gave an average composition {Ca2.92Na0.04Mn2+0.03Mg0.01}Σ3[Ti1.04Fe3+0.46Mg0.18Fe2+0.16Zr4+0.13V3+0.04]Σ2(Si2.21Fe3+0.71Al0.07)Σ3O12, and corresponds to the general garnet formula of [8]X3[6]Y2[4]Z3[4]O12. Schorlomite is the dominant component, but the composition contains significant amounts (>15 mol.%) of andradite, Ca3(Fe3+2)Si3O12, morimotoite, Ca3(Ti4+Fe2+)Si3O12, and morimotoite-(Mg), Ca3(Ti4+Mg)Si3O12. The crystal structure model was refined well using isotropic and anisotropic displacement parameters. Using isotropic displacement parameters, the χ2 and R (F2) Rietveld refinement values are 1.148 and 0.0742, respectively. The unit-cell parameter, a = 12.18599(1) (Å), is large for a natural schorlomite for which complete structural data are available. The bond distances are average <Ca–O> = 2.4461, Ti–O = 2.0085(5), Si–O = 1.7022(5) Å, and site occupancy factors (sofs) for Ca(sof) = 0.963(1), Ti(sof) = 1.045(1), and Si(sof) = 1.150(2). Comparison of schorlomite data from the type locality in Magnet Cove with morimotoite from Ice River, Canada and the type locality in Japan show that they are quite similar and cast doubts as to morimotoite being different from schorlomite.