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Mineralogical Magazine; April 2004; v. 68; no. 2; p. 335-341; DOI: 10.1180/0026461046820190
© 2004 Mineralogical Society of Great Britain and Ireland
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Stabilization of trivalent Mn in natural tetragonal hydrogarnets on the join ‘hydrogrossular’–henritermierite, Ca3Mn3+2 [SiO4]2[H4O4]

U. Hålenius*

Department of Mineralogy, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden

* E-mail: ulf.halenius{at}nrm.se


   ABSTRACT
TOP
ABSTRACT
Introduction
 Samples and experimental
 Results and discussion
 Acknowledgements
 References
 
Four relatively intense and broad absorption bands centred at ~12500, ~19500, ~21500 and ~23000 cm–1 were recorded in polarized electronic single-crystal spectra of natural, optically uniaxial and pleochroic hydrogarnets with henritermierite contents ranging from 35 to 97 mol.%. These absorption bands arise from spin-allowed electronic d-d transitions in trivalent Mn located at the axially distorted six-coordinated site of the tetragonal hydrogarnet structure.

The crystal field stabilization energy (CFSE) for trivalent Mn at the Mn site, as derived from band energies, is ~185 kJ/mol. This considerably higher CFSE for Mn3+ in tetragonal hydrogarnets as compared to cubic garnets (130–145 kJ/mol) explains the natural occurrence of close to end-member tetragonal Mn3+-hydrogarnets while only limited Mn3+-substitution is observed in natural cubic garnets.

The fact that incorporation of Mn3+ at intermediate concentrations stabilizes the tetragonal hydrogarnets indicates the potential natural existence of a number of new, partially Mn3+-substituted, hydrogarnets, e.g. tetragonal Mn3+-bearing ‘hydroandradite’.

KEYWORDS: chemical analysis, electronic spectra, trivalent manganese, henritermierite, ‘hydrogrossular’


   Introduction
TOP
ABSTRACT
 Introduction
Samples and experimental
 Results and discussion
 Acknowledgements
 References
 
THE tetragonal hydrogarnet, henritermierite, Ca3Mn3+2 [SiO4]2[H4O4], is a rare mineral, which has so far only been observed in two areas worldwide; the Anti-Atlas Mountains in Morocco (Gaudefroy et al., 1969) and the Kalahari Minefields, Republic of South Africa (e.g. Cairncross et al., 1997). In contrast to henritermierite, natural cubic garnets and cubic hydrogarnets are generally low in trivalent manganese. The reason for this is probably related to the differences in Mn3+ coordination in the two different structure types. Trivalent cations occur at undistorted octahedral sites in the cubic phases while in the tetragonal hydrogarnet structure Mn3+ is located at a six-coordinated site displaying a strong axial distortion (Aubry et al., 1969; Armbruster et al., 2001). With the aim of exploring the extent to which excess stabilization of Mn3+ in the tetragonal hydrogarnet is due to these site symmetry differences, a number of Mn3+-bearing anisotropic hydrogarnets including natural henritermierite were studied by means of optical absorption spectroscopy.


   Samples and experimental
TOP
ABSTRACT
 Introduction
 Samples and experimental
Results and discussion
 Acknowledgements
 References
 
Four single crystals of Mn3+-bearing hydrogarnet were used in the present study. Three of these (#35, #45, #76) came from the Wessels mine and one (#97) originated from the N’Chwaning II mine. Both localities are found in the Kalahari manganese fields, South Africa. The three crystals from the Wessels mine were selected from a sample consisting mainly of Mn3+-bearing vesuvianite. All hydrogarnet crystals in this sample occurred as irregularly shaped grains on vesuvianite and were commonly strongly chemically zoned (Fig. 1Go). A large number of crystal fragments was analysed by SEM/EDS techniques in order to aid selection of crystals representing a wide compositional range. The three crystals finally selected were all optically uniaxial and were pleochroic in orange hues with E>O. The crystals from the N’Chwaning II mine were darker in colour and commonly euhedral. The selected crystal was optically uniaxial and displayed similar pleochroism to those from the Wessels mine.



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  		FIG. 1. BSE image of a zoned Mn3+-bearing ‘hydrogrossular’ from the Wessels mine. The grain displays a core (light grey) richer in Mn than the broad rim (dark grey). In addition, a thin complex intermediate zone (white) slightly enriched in Fe is observed.

 
Orientation of these crystals was aided by crystal morphology and optical examination under the polarizing microscope using conoscopic illumination. One of the crystals from the Wessels mine and the crystal from the N’Chwaning II mine were oriented with the optic axis parallel to the glass slide bearer, which enabled preparation of absorbers allowing optical recordings in E||O as well as E||E. The two remaining crystals from the Wessels mine were highly irregular in shape and the orientation of the principal optical plane was imperfect. Consequently, only E||O could be recorded from these two crystals. The oriented crystals were embedded in a thermoplastic resin with retained orientation. After resin hardening, the absorbers were ground and polished on two parallel surfaces producing slabs of single-crystal absorbers. The final thickness of the double-sided, polished single crystals was determined by means of a digital micrometer to be in the range 19–43 µm. Subsequent to the optical absorption measurements, the single-crystal absorbers were removed through dissolution of the thermoplastic and embedded in epoxy resin, which, after hardening, was polished and coated with carbon to allow electron microprobe analyses of the crystal areas used for measurements of the optical absorption spectra.

Concentrations of heavier elements (Z ≥9) were determined by electron microprobe techniques, using a Cameca SX50 instrument operated at 20 kV accelerating potential and 12 nA sample current. Standard samples comprised albite (Na and Si), Al2O3 (Al), MgO (Mg), MnTiO3 (Mn, Ti), Fe2O3 (Fe) and wollastonite (Ca). Wavelength dispersive scans revealed no detectable amounts of any additional elements. Raw data were reduced by means of the PAP routine (Pouchou and Pichoir, 1984). Hydrogarnet formulae (Table 1Go) were calculated on the basis of 24 negative charges and H2O contents were calculated assuming that [SiO4] + [H4O4] = 3.


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  		TABLE 1. Microprobe analyses of the hydrogarnet samples.
 
Room-temperature polarized optical absorption spectra were obtained with a Zeiss MPM 800 single beam microscope spectrometer using a 75 W Xenon arc and a 100 W tungsten lamp as light source in the UV-VIS region (350–800 nm) and the NIR region (800–2100 nm), respectively. A photomultiplier and a photoconductive PbS cell served as detectors in the two respective spectral regions. Concave holographic gratings were used as monochromators and an UV-transparent Glan-Thompson prism served as polarizer. For the UV-VIS range, the spectral resolution was set at 1 nm and spectra were recorded at 1 nm steps during three cycles, while resolution and step size were both 5 nm in the NIR region. During all recordings, air served as a reference medium and UV-transparent (Zeiss Ultrafluar 10x) lenses were used as objective and condenser. The diameter of the measuring spot was 40 µm. The areas measured for optical absorption spectra were identical to those analysed by electron microprobe. All recorded spectra were analysed by means of peak fitting using the Jandel PeakFit 4 program assuming Gaussian shaped absorption bands and UV-background absorption.


   Results and discussion
TOP
ABSTRACT
 Introduction
 Samples and experimental
 Results and discussion
Acknowledgements
 References
 
Chemistry
All the tetragonal hydrogarnets studied are members of the ternary system ‘hydrogrossular’ (HGr)-‘hydroandradite’ (HAnd)-henritermierite (Hen), and their henritermierite component content varies from 35 to 97 mol.% (Table 1Go). However, the ‘hydroandradite’ component of the crystals studied is very small (0–8 mol.%) and therefore they may be considered as members on the binary join ‘hydrogrossular’–henritermierite.

The crystals from the Wessels mine sample are always chemically zoned. One representative crystal from the present sample showing a Mn3+-rich ‘hydrogrossular’ core and rim enriched in Al (analysis #35 in Table 1Go) is illustrated in Fig. 1Go. The core and rim compositions of this crystal correspond to HGr0.49Hen0.45HAnd0.06 and HGr0.61Hen0.35HAnd0.04, respectively. All the analysed crystals in the sample from Wessels mine are low in ‘hydroandradite’ component. With the exception of a few crystals displaying higher henritermierite contents (e.g. crystal #76 in Table 1Go), the analysed crystal fragments revealed compositions within the range displayed by the zoned crystal illustrated in Fig. 1Go.

The sample from the N’Chwaning II mine contains crystals approaching the henritermierite end-member composition, and they are all homogeneous and very similar in composition. The analysis of crystal #97 (Table 1Go), which was used for measurements of the optical spectra of henritermierite, is representative of the composition of these crystals.

Optical absorption spectra and spectrum fitting
The polarized optical absorption spectra of all single-crystal absorbers showed two relatively strong and broad absorption features at ~21,500 and ~12,500 cm–1. The absorption at the higher energy is distinctly pleochroic with E >O, while the low-energy band is weakly dichroic showing E >O (Fig. 2Go). In addition, the absorption feature at the higher energy is distinctly skewed. On close inspection two absorption shoulders at ~19,500 and 23,100 cm–1 are evident on either side of the absorption maximum at ~21,500 cm–1 (Fig. 2Go). These spectra differ considerably in character from those recorded on Mn3+-bearing cubic garnets, in which only two absorption bands caused by spin-allowed d-d transitions in octahedrally coordinated Mn3+ occur at ~20,000 and ~17,000 cm–1 (Frentrup and Langer, 1981; Langer and Lattard, 1984; Geiger et al., 1999). The linear absorption coefficient of the two absorption regions at ~21,500 and ~12,500 cm–1 in the present hydrogarnet-spectra increase systematically with increasing Mn3+ content of the crystal (Fig. 3Go).



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  		FIG. 2. Polarized optical absorption single-crystal spectra of the close to end-member henritermierite (sample #97) from the N’Chwaning II mine. Positions {nu}1–{nu}4 indicate band peaks and shoulders. The absorber thickness was 19 µm.

 


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  		FIG. 3. E||O optical spectra of the four investigated tetragonal hydrogarnet samples. Numbers attached to each spectrum indicate the molar fraction of henritermierite in the corresponding sample. Absorber thickness was 43 µm for all samples except for crystal #97, for which it was 19 µm.

 
Different fitting models were applied to deconvolute the spectra obtained. Fair fits were produced by using a simple model comprising three Gaussian functions, of which two were allocated to the two respective absorption areas in the visible-NIR region and one to the region of the UV-absorption edge. However, this simple model failed to fit the skewed shape of the absorption peak at ~21,500 cm–1 in a satisfactory manner.

Considering the Mn3+ coordination in henritermierite it is obvious why the initial simple fitting model failed. Trivalent Mn forms two long (~2.2 Å) apical bonds with oxygen ligands and four shorter (~1.9 Å) equatorial bonds (Aubry et al., 1969; Armbruster et al., 2001) in this mineral structure. However, the equatorial bonds are slightly different in length and the ligands in the equatorial plane are also of two different types. Two bonds are directed towards OH ligands in trans-configuration at a distance of 1.903 Å from the central Mn3+ cation, while the remaining two bonds with oxygen ligands are slightly longer, at 1.950 Å. The true symmetry of the Mn3+ octahedron is thus reduced to C2v. Four spin-allowed electronic d-d transitions are expected for Mn3+ in an electronic field of this symmetry. In view of this, a more complex model comprising three Gaussian functions to fit the absorption feature at ~21,500 cm–1 and one to fit the band at ~12,500 cm–1 was adopted.

Considerably better fits of the recorded spectra were obtained by using this physically more realistic model (Fig. 4Go). Due to closeness to the UV-absorption edge as well as dense peak spacing it became necessary to constrain the widths of the three bands in the ~21,500 cm–1 region to be equal when using this model. Applying this fitting model, four absorption bands at 12,400 (230), 19,910 (190), 21,660 (140) and 23,220 (110) cm–1 were detected. The band at the lowest wavenumber, {nu}1, displays a large band width, 5680 (230) cm–1, while the remaining three absorption bands, {nu}–{nu}, are less broad with {omega}1/2 = 2460 (200) cm–1. The consistency of the band parameters, as indicated by relatively small standard deviations, suggests that the data retrieved from the computer-aided fits may be considered as reliable and useful for extracting information on the Mn3+ stabilization in the present minerals.



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  		FIG. 4. Fitted optical absorption spectra of henritermierite (sample #97). The thicker solid line represents the experimental data and the thinner lines show the four fitted Mn3+ bands as well as the fitted UV-background absorption.

 
Band assignments and crystal f|eld parameters
As noted above, four spin-allowed d-d bands due to electronic transitions in Mn3+ at the distorted octahedral site are predicted to occur in spectra of Mn-bearing tetragonal hydrogarnets. One of these transitions, representing an electron transfer between the two eg orbitals is expected to occur at comparatively low energy, while three transitions involving electron transfer from each of the three t2g orbitals to the energetically highest eg orbital are expected to be closely spaced at higher energies (e.g. Burns, 1993). This general scheme has been experimentally confirmed from spectra of various oxygen-based minerals, in which Mn3+ occurs at axially distorted six-coordinated sites. For instance, four spin-allowed Mn3+ d-d bands occur at ~12,200, ~17,600, ~18,500 and 22,000 cm–1 in the spectra of piemontite (e.g. Smith et al., 1982). In an octahedral crystal field of C2v symmetry, the four spin-allowed Mn3+ bands are caused by electronic transitions between five split energy levels, derived from the spectroscopic 5D state. A pure point charge calculation based on the Mn-site geometry in henritermierite (Armbruster et al., 2001) indicates that the ground state is 5A1 and the four excited states are, in order of increasing energy, 5A1, 5A2 5B2 and 5B1. In addition, this simple calculation indicates that the relative energy of the four transitions is 0.48:0.80:0.91:1.00, which compares very well with the experimentally observed band-energy ratios of 0.53:0.86:0.93:1.00. As observed in optical absorption spectra of other Mn3+-bearing oxygen-based minerals, the bandwidth of the low-energy band, which is caused by electron promotion between the eg orbitals, is considerably larger than for the remaining three bands. This is considered to be an effect of strongly vibrating apical ligands, towards which the dz2 orbital of the central Mn3+ ion is directed.

As for polarization effects, group theory (e.g. Cotton, 1971) predicts that the high-energy transitions 5A1 -> 5B2 and 5A1 -> 5B1 will occur when the electronic vector of the incident light is perpendicular to the symmetry axis (C2) of the Mn site. Furthermore, it predicts that the transition 5A1 -> 5A2 is symmetry forbidden, and finally, the low-energy transition 5A1 -> 5A1 is expected to occur when the electronic vector of the incident light coincides with the symmetry axis of the site. The C2 symmetry axis, which coincides with the elongation direction of the Mn3+ site in henritermierite is at an angle of ~70° to the crystallographic c axis (Armbruster et al., 2001), which in turn coincides with the optical E direction. This implies that the polarization of the high-energy bands assigned to the 5A1 -> 5B2 and 5A1 -> 5B1 transitions is expected to be E >O, while the low-energy band due to the 5A1 -> 5A1 is predicted to show E <O polarization. The observed polarization (Table 2Go) of the two high-energy bands at ~21,650 ({nu}3) and ~23,200 cm–1 ({nu}4) is in agreement with these predictions, but theory and experiment disagree for the two remaining bands at lower energies. The 5A1 -> 5A2 transition, which, theoretically, is forbidden, is observed in the present spectra as an almost isotropic band ({nu}2 ) at ~19,900 cm–1. Furthermore, the low-energy band ({nu}1) at ~12,400 cm–1 (5A1 -> 5A1 transition) shows a weak polarization opposite to that predicted. A possible explanation of these limited disagreements between predictions and experiments is that coupling between vibronic and electronic states partially invalidates the predictions, which are based purely on an electrical dipole concept.


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  		TABLE 2. Parameters of the spin-allowed Mn+ d-d bands in the present hydrogarnet spectra.
 
The intensities of the recorded absorption bands show a linear increase with increasing Mn3+ content of the hydrogarnet crystals (Fig. 5Go). The recorded absorption bands display molar absorption coefficients ({varepsilon} values) in the range 5–50 l mol–1 cm–1, which is in agreement with band assignments to spin-allowed d-d transitions in a 3d cation. In detail, these values are distinctly lower than recorded for corresponding Mn3+-bands in room-temperature spectra of, for example, piemontite (Smith et al., 1982) and andalusite-type structures (Abs-Wurmbach et al., 1981), for which observed {varepsilon}-values are in the range 20–300 l mol–1 cm–1. A reason for the higher values observed for corresponding absorption bands in spectra of epidote- and andalusite-type structures is that the Mn3+-bearing sites in these minerals lack a centre of symmetry (Abs-Wurmbach et al., 1981). This weakens the Laport selection rule for electronic transitions and aids d-p orbital mixing, which promotes intensity enhancement of spin-allowed d-d bands. In contrast, Mn3+ is located at a centrosymmetric site in henritermierite (Armbruster et al., 2001) and consequently the degree of d-p orbital mixing remains small.



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  		FIG. 5. Correlation plot of the net integral absorption of the absorption envelope at ~21500 cm–1 (A2+A3+A4; filled squares) and the absorption band at ~12500 cm–1 (A1; filled circles) in E||O spectra with the Mn3+ content.

The lines represent linear least-square fits.

 
On the basis of the assignments of the observed spin-allowed d-d bands a crystal-field splitting ({Delta}o) of 15,400 (150) cm –1 and a crystal field stabilization energy (CFSE) of 185 (1) kJ/mol is calculated for Mn3+ in tetragonal hydrogarnets. This CFSE value is comparable to those recorded for Mn3+ located in strongly Jahn-Teller-distorted sites in andalusite- and epidote-type minerals (Abs-Wurmbach et al., 1981; Kersten et al., 1987). However, it is substantially higher than the CFSE values (130–145 kJ/mol) determined for Mn3+ in cubic garnets (Frentrup and Langer, 1981; Langer and Lattard, 1984). This considerably higher CFSE for Mn3+ in tetragonal hydrogarnets as compared to cubic garnets presents an explanation for the fact that close to end-member tetragonal Mn3+-hydrogarnets occur in nature while only limited Mn3+-substitution is observed in natural cubic garnets.

Potential for new natural tetragonal hydrogarnets
An additional aspect of the present study is that several of the crystals from the Wessels mine are tetragonal hydrogarnets with subordinate amounts of henritermierite component. For instance, hydrogarnet #35 of Table 1Go is dominated by a ‘hydrogrossular’ component (61 mol.%) and it should, by existing nomenclature rules, be considered as a new tetragonal ‘hydrogrossular’ mineral1. Obviously, trivalent Mn, even at intermediate concentrations, has the capability to stabilize the tetragonal hydrogarnet structure and consequently there exists a potential for natural occurrences of additional new tetragonal hydrogarnet species as e.g. Mn3+-bearing tetragonal ‘hydroandradite’. How much Mn3+ is required to achieve stabilization of the tetragonal phase still needs to be defined. An indication of the minimum Mn3+-substitution threshold is the occurrence of cubic Mn3+-bearing (10% of the octahedral sites) ‘hydroandradite’ at Wessels mine (Armbruster, 1995).


   Acknowledgements
TOP
ABSTRACT
 Introduction
 Samples and experimental
 Results and discussion
 Acknowledgements
References
 
Jens Gutzmer and Bruce Cairncross are thanked for generously sharing material from the Wessels and N’Chwaning II mines, respectively. Hans Harryson is thanked for microprobe analytical work. My thanks also go to Chris Stanley for helpful comments. Financial support from the Swedish Research Council (VR) is gratefully acknowledged.


  
TOP
ABSTRACT
 Introduction
 Samples and experimental
 Results and discussion
 Acknowledgements
 References
 
Dedicated to the memory of Dr A. J. Criddle, Natural History Museum, London, who died in May 2002

1 The Al-dominant analogue to henritermierite was recognized by the IMA Commission on New Minerals and Mineral Names (CNMMN) as a new mineral, holtstamite, in December 2003. Back

[Manuscript received 1 February 2003: revised 4 August 2003]


   References
TOP
ABSTRACT
 Introduction
 Samples and experimental
 Results and discussion
 Acknowledgements
 References
 

Abs-Wurmbach, I., Langer, K., Seifert, F. and Tillmanns, E. (1981) The crystal chemistry of (Mn3+, Fe3+)-substituted andalusites (viridines and kanonaite), (Al1–x–yMn3+ xFe3+y)2(O|SiO4): crystal structure refinements, Mössbauer and polarized optical absorption spectra. Zeitschrift für Kristallographie, 155, 81–113.[ISI][GeoRef]

Armbruster, T. (1995) Structure refinement of hydrous andradite Ca3Fe1.54Mn0.20Al0.26(SiO4)1.65(O4H4)1.35 from the Wessels mine, Kalahari manganese field, South Africa. European Journal of Mineralogy, 7, 1221–1225.[Abstract/Free Full Text][ISI][GeoRef]

Armbruster, T., Kohler, T., Libowitzky, E., Friedrich, A., Miletich, R., Kunz, M., Medenbach, O. and Gutzmer, J. (2001) Structure, compressibility, hydrogen bonding, and dehydration of the tetragonal hydrogarnet, henritermierite. American Mineralogist, 86, 147–158.[Abstract/Free Full Text][ISI][GeoRef]

Aubry, A., Dusausoy, Y., Laffaille, A. and Protas, J. (1969) Détermination éttude de la structure cristalline de l’henritermierite, hydrogrenat de symétrie quadratique. Bullétin de la Société Française de Minéralogie et de Cristallographie, 92, 126–133.[GeoRef]

Burns, R.G. (1993) Mineralogical Applications of Crystal Field Theory, 2nd edition. Cambridge Topics in Mineral Physics and Chemistry, 5 (A. Putnis and R.C. Lieberman, editors). Cambridge University Press, Cambridge, UK, 551 pp.

Cairncross, B., Beukes, N. and Gutzmer, J. (1997) The Manganese Adventure: The South African Manganese Fields. Associated Ore and Metal Corporation Limited. Marshalltown, Johannesburg, Republic of South Africa, 236 pp.

Cotton, F.A. (1971) Chemical Applications of Group Theory. Wiley-Interscience, New York, Chichester, UK, Brisbane, Australia, Toronto, Canada, Singapore.

Frentrup, K.R. and Langer, K. (1981) Mn3+ in garnets: Optical absorption spectrum of a synthetic Mn3+-bearing silicate garnet. Neues Jahrbuch für Mineralogie Monatshefte, 245–256.

Gaudefroy, C., Orliac, M., Permingeat, F. and Parfenoff, A. (1969) L’henritermierite, une nouvelle espèce minérale. Bullétin de la Société Française de Minéralogie et de Cristallographie, 92, 185–190.[GeoRef]

Geiger, C.A., Stahl, A. and Rossman, G.R. (1999) Raspberry-red grossular from Sierra de Cruces Range, Coahuila, Mexico. European Journal of Mineralogy, 11, 1109–1113.[Abstract/Free Full Text][ISI][GeoRef]

Kersten, M., Langer, K., Almen, H. and Tillmanns, E. (1987) Kristallchemie von Piemontiten: Strukturverfeinerungen und polarisierte Einkristallspektren. Zeitschrift für Kristallographie, 178, 212.

Langer, K. and Lattard, D. (1984) Mn3+ in garnets II: Optical absorption spectra of blythite-bearing, synthetic calderites, Mn2+3 [8](Fe3+1–nMn3+n)2[6][SiO4]3.Neues Jahrbuch für Mineralogie Abhandlungen 149, 129–141.

Pouchou, J.L. and Pichoir, F. (1984) A new model for quantitative X-ray micro-analysis. I. Application to the analysis of homogeneous samples. La Recherche Aérospatiale, 3, 13–36.

Smith, G., Hålenius, U. and Langer, K. (1982) Low temperature spectral studies of Mn3+-bearing andalusite and epidote type minerals in the range 30000–5000 cm–1. Physics and Chemistry of Minerals, 8, 136–142.[CrossRef][ISI][GeoRef]


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U. HALENIUS, U. HAUSSERMANN, and H. HARRYSON
Holtstamite, Ca3(Al,Mn3+)2(SiO4)3-x(H4O4)x, a new tetragonal hydrogarnet from Wessels Mine, South Africa
European Journal of Mineralogy, April 1, 2005; 17(2): 375 - 382.
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