Tetrahedrally coordinated Ti⁴⁺ in synthetic Ti-rich potassic richterite: Evidence from XRD, FTIR, and Raman studies

Samples have been prepared at 1 kbar P_H2O for the join richterite-titanium richterite in the system K₂O-Na₂O-CaO-MgO-TiO₂-SiO₂-H₂O. Experimental results indicate that the Ti solubility strongly depends on temperature: it increases from 0.2 to about 0.8 atom per formula unit between 600 °C and 800 °C; no further increase is observed from 800 °C to 900 °C. Both FTIR and Raman spectroscopic data indicate that Ti4+ is entirely incorporated in tetrahedral sites. No significant change is observed in the OH-stretching region, indicating that Ti⁴⁺ is not bonded to OH groups. Only the Si-O stretching region is afected by Ti⁴⁺, with band shifts closely corresponding to the calculated shift factor (0.92) for a ^[4]Si → ^[4]Ti substitution. The substitutional mechanism responsible for Ti incorporation is the direct isovalent substitution Si⁴⁺ → Ti⁴⁺. The progressive decrease in the tremolitetype band in the OH-stretching region with increasing Ti content confirms that the A-site cation splitting over A subsites is a function of the geometry of the A cavity, controlled by the major cationic substitutions. Powder XRD data for synthetic single-phase Ti-rich richterite reflect a regular increase in a, b, c, and β as a function of the Ti content, in response to the replacement of the small Si⁴⁺ ion (0.26 Å) by the larger Ti⁴⁺ ion (0.42 Å).