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
Authors: Langer K. , Tillmanns E. , Kersten M. , Almen H. , Arni R. K. 
Published: December 2003

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.