A note on mean distances, R̅[MO₆], in substituted polyhedra, [(M1-xMx′)O6], in the crystal structures of oxygen based solid solutions: local versus crystal averaging methods

The crystal chemistry of Mn3+ in the clino- and orthozoisite structure types, Ca2M3 3+[OH|O|SiO4|Si2O7]: A structural and spectroscopic study of some natural piemontites and “thulites” and their synthetic equivalents

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(Al3–pMp 3+) [OH|O|SiO4|Si2O7] where M3+ = Mn3+ or (Mn1–n 3+Fen 3+) for the synthetic or natural minerals, respectively. Electron microprobe analyses of the single crystals studied revealed substitutional degrees pM3+ = 0.13 or 0.51 in natural and synthetic “thulite”, respectively, and 0.57 ≤ pM3+ ≤ 1.17 or 0.83 ≤ pM3+ ≤ 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 2 v (C 2″) pseudo-symmetry of the M(3) sites, a super-group of their site symmetry Cs . 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. CFSEMn 3+ 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 z 2 or d( x ²- y ²), respectively — may play a role.

Local mean chromium–oxygen distances in Cr3+-centered octahedra of natural grossular-uvarovite garnet solid solutions from electronic absorption spectra

The crystal field parameter 10Dq[cm⁻¹] of octahedral chromium obtained as the energy of the 4A2g → 4T2g band of Cr3+ in the electronic absorption spectra, EAS, of a series of 13 natural grossular-uvarovite garnets with compositions close to the binary join Ca3(Al1– x Cr x )2Si3O12, depend on the chromium fraction xCr of the solid solutions as

10Dq[cm⁻¹] = (–515.51 · x Cr) + 16579 (r = 0.984)

The 10Dq[cm⁻¹]-values were evaluated in terms of local mean octahedral distances, R̅ (Cr–O), [Å] local with the result that

R̅ (Cr–O) local = 0.01262 · x Cr + 1.9812 (r = 0.982) (a)

Equation (a) is in very good agreement with mean 〈Al/Cr—O〉; distances of “individual” octahedra R̅i calculated for the case XCr(individual) = 1.00 from the distance relations obtained in X-ray structure refinements of low symmetry chromian garnets (Wildner and Andrut, 2001). The proves again, that the EAS-method to derive local 3d N-ion–oxygen distances is reliable.

R̅ (Cr–O) local = ƒ(x Cr) and the corresponding R̅ i-function deviate strongly from the Vegard line of the crystal averaged function R̅ (Cr–O) average = ƒ(x Cr), the “virtual crystal model”. This indicates significant structural relaxation around Cr3+[6] in the garnet solid solutions studied. The relaxation coefficient calculated from the data, ∊ lim x Cr→0 = 0.82, is closer to ∊ = 1.0 for full relaxation, i.e. the “hard sphere model”, than to ∊ = 0.0 for absent relaxation in the “virtual crystal” model. This is proposedly related to the uptake of strain around octahedral chromium, in the large (CaO8) polyhedra interconnected with the octahedra of the grossular-uvarovite garnets.