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
- Author(s)
- Klaus Langer, Ekkehart Tillmanns, Michael Kersten, Hartmut Almen, R K Arni
- 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(Al3-pMp3+) [OH | O | SiO4 | Si2O7] where M3+ = Mn3+ or (Mn1-n3+Fen3+) 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 0.1xM3+, 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 pMn3+ = 1.47 (emp) or xMn3+(M3) = 0.931 and xMn3+(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 spin-allowed 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 xMn3+(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), vI at 13000-12000 cm-1 (E ? Y), vII at 19000-18000 cm-1 (E ? Y and Z, Z > Y) and vIII at 24000-22000 cm-1 (E ? X) is best interpreted by assuming a C2v(C2?) pseudo-symmetry of the M(3) sites, a super-group of their site symmetry Cs. Evaluation of the energies of vI, vII and VIII on the basis of the energy level diagram obtained for Mn3+ with the above pseudosymmetry yields the crystal field parameter 10 Dq = 13540 cm-1 for xMn3+(M3) = 0.931. 10 Dq increases slightly by 30 cm-1 per -0.1xMn3+(M3). Such values and the Jahn-Teller splitting of the octahedral crystal-field ground-state of Mn3+, d = vI, yield a crystal field stabilisation energy of Mn3+(M3) of 14080 cm-1 for xMn3+(M3) = 0.931. CFSEMn3+ increases slightly by 28 cm-1 per -0.1xMn3+(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 dz2 or d(x2-y2), respectively - may play a role.
- Organisation(s)
- Department of Mineralogy and Crystallography
- External organisation(s)
- Johann Wolfgang Goethe-Universität Frankfurt am Main, Johannes Gutenberg-Universität Mainz, Julius-Maximilians-Universität Würzburg, Technische Universität Berlin
- Journal
- Zeitschrift für Kristallographie : international journal for structural, physical, and chemical aspects of crystalline materials
- Volume
- 217
- Pages
- 563-580
- No. of pages
- 18
- ISSN
- 0044-2968
- Publication date
- 2002
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 1051 Geology, Mineralogy
- Portal url
- https://ucris.univie.ac.at/portal/en/publications/the-crystal-chemistry-of-mn3-in-the-clino-and-orthozoisite-structure-types-ca2m33oh--o--sio4--si2o7-a-structural-and-spectroscopic-study-of-some-natural-piemontites-and-thulites-and-their-synthetic-equivalents(2dd74278-28db-488b-b7ee-4fbbade6a98e).html