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Titanium incorporation and ^VITi³⁺ -^IVTi⁴⁺ charge transfer in synthetic diopside

¹ Department of Mineralogy, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden
² Department of Nuclear Physics, Lund Institute of Technology, Lund University, Box 118, SE-221 00 Lund, Sweden
³ Hashi Institute for Silicate Science, Nishinakanobu 1-9-25, Shinagawa, Tokyo 142-0054, Japan

Correspondence: ^* E-mail: henrik.skogby{at}nrm.se

A series of Ti-doped diopside samples synthesized by fluxgrowth methods under reducing conditions were investigated by microanalytical methods and optical absorption spectroscopy, to assess Ti incorporation mechanisms, valence states, and related electronic transitions. Chemical characterization show that Ti occurs both in the tri-and tetravalent states, with Ti⁴⁺ preferentially ordered to the tetrahedral position whereas Ti³⁺ is restricted to the M1 position. Charge-balance is maintained by incorporation of Na and minor B, stemming from the flux compound. Polarized optical absorption spectra reveal three major absorption features. Two relatively narrow bands centered at 18 500 and 15 700 cm^–ly polarized in the crystallographic c-direction, display absorbance values that correlate with the calculated Ti³⁺ (M1) concentration, and are assigned to spin-allowed d-d transitions in Ti³⁺. A broader band centered around 24 500 cm^–1ys absorbance values that are well correlated with the product of ^VITi³⁺ and ^IVTi⁴⁺. Based on the spectral characteristics of this band, including a strong polarization in the crystallographic b-direction, we assign this band to an ^M1Ti³⁺-^TTi⁴⁺ intervalence charge transfer process.

Key Words: Crystal synthesis • diopside • optical spectroscopy • pyroxene • major and minor elements • Ti in pyroxene • order-disorder