The crystal chemistry of Mn³⁺ in the clino- and orthozoisite structure types, Ca₂M₃³⁺[OH|O|SiO₄|Si₂O₇]: A structural and spectroscopic study of some natural piemontites and “thulites” and their synthetic equivalents

Abstract

Six new structure refinements and eleven sets of polarised, single-crystal electronic absorption spectra, E‖X, Y and Z, in the energy range 35000–5000 cm^–1 were obtained on natural and synthetic orthozoisite-type “thulites” and clinozoisite-type piemontites: Ca₂(Al_3–pM_p³⁺) [OH|O|SiO₄|Si₂O₇] where M³⁺ = Mn³⁺ or (Mn_1–n³⁺Fe_n³⁺) for the synthetic or natural minerals, respectively. Electron microprobe analyses of the single crystals studied revealed substitutional degrees p_M3+ = 0.13 or 0.51 in natural and synthetic “thulite”, respectively, and 0.57 ≤ p_M3+ ≤ 1.17 or 0.83 ≤ p_M3+ ≤ 1.47 in the natural or synthetic piemontites, respectively.

Manganese in “thulite” is trivalent, as it is in piemontite. In both structure types, M³⁺ fractionates strongly into the axially compressed [M(3)O₆] polyhedra, and does not enter the M(2) sites. Mean M(3)—O and M(1)—O distances increase in both structures, compared to the M³⁺-free Al end members. Such distance changes in piemontite are +0.47% and +0.53% per 0.1x_M3+, respectively, (x = site fraction). The bending angle of the Si₂O₇-group in cis-configuration, ∢Si(1)—O(9)—Si(2), decreases from 164.4° in clinozoisite to 147.4° in the most Mn³⁺-rich synthetic piemontite with p_Mn³⁺ = 1.47 (emp) or x_Mn³⁺(M3) = 0.931 and x_Mn³⁺(M1) = 0.460 (from structure refinement).

The detailed evaluation of the changes, due to Al→M³⁺ substitution, of individual bond lengths, as well as the quantitative evaluation of the intensities of the strong spinallowed dd bands of Mn³⁺ in M(3), prove that in natural piemontite the preference of Mn³⁺ for M(3) over M(1) is more pronounced than that of Fe³⁺. This is in accord with the Jahn-Teller effect of 3d⁴-configurated Mn³⁺. In addition, evaluation of the individual M(3)–O(i) distances with increasing x_Mn³⁺(M3) in piemontite indicates that the axial compression of the [M(3)O₆] polyhedra increases. This contrasts with the behaviour of Fe³⁺-bearing epidotes and is again in accord with the Jahn-Teller effect of Mn³⁺.

The polarisation behaviour of the three strong spin-allowed dd-bands of Mn³⁺ in M(3), v_I at 13000–12000 cm^–1 (E‖Y), v_II at 19000–18000 cm^–1 (E‖Y and Z, Z > Y) and v_III at 24000–22000 cm^–1 (E‖X) is best interpreted by assuming a C_2v(C₂″) pseudo-symmetry of the M(3) sites, a super-group of their site symmetry C_s. Evaluation of the energies of v_I, v_II and v_III on the basis of the energy level diagram obtained for Mn³⁺ with the above pseudo-symmetry yields the crystal field parameter 10 Dq = 13540 cm^–1 for x_Mn³⁺(M3) = 0:931. 10 Dq increases slightly by 30 cm^–1 per -0.1x_Mn³⁺(M3). Such values and the Jahn-Teller splitting of the octahedral crystalfield ground-state of Mn³⁺, δ = v_I, yield a crystal field stabilisation energy of Mn³⁺(M3) of 14080 cm^–1 for x_Mn³⁺(M3) = 0:931. CFSE_Mn³⁺ increases slightly by 28 cm^–1 per -0.1x_Mn³⁺(M3). Such values are appreciably smaller than those typical of Mn³⁺ substituting for Al in the axially elongated [M(1)O₆] octahedra in the andalusite structure type. This different behaviour of Mn³⁺ in the two structure types is likely due to the smaller deviation of (c=a)_oct in piemontite M(3) compared to andalusite M(1) for the same site fractions of Mn³⁺. In addition, the axial inversion effect — compressed [M(3)O₆] in the clinozoisite-type or elongated [M(1)O₆] in the andalusite-type, involving the electron hole of 3d⁴ in d_z2 or d_(x²-y²), respectively — may play a role.