Rewitzerite, K(H₂O)Mn₂(Al₂Ti)(PO₄)₄[O(OH)](H₂O)₁₀⋅4H₂O, a new monoclinic paulkerrite-group mineral, from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria, Germany

Abstract

Rewitzerite, K(H₂O)Mn₂(Al₂Ti)(PO₄)₄[O(OH)](H₂O)₁₀⋅4H₂O, is a new monoclinic member of the paulkerrite group, from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria, Germany. It was found in specimens of altered zwieselite, in association with rockbridgeite. Rewitzerite forms clusters of colourless elongated hexagonal-shaped prisms, up to 0.1 mm long. The crystals are flattened on {010} and elongated along [100], with forms {010}, {001}, {111} and {$\bar{1}$11}. The calculated density is 2.33 g⋅cm^–3. Optically, rewitzerite crystals are biaxial (+), with α = 1.585(2), β = 1.586(2), γ = 1.615(2) (measured in white light) and 2V(meas) = 25(2)°. The empirical formula from electron microprobe analyses and structure refinement is ^A1[K_0.77(H₂O)_0.23]^A2[H₂O] ^M1(Mn²⁺_0.82Mg_0.64Fe³⁺_0.43□_0.11)_Σ2.00 ^M2+M3(Al_1.51Ti⁴⁺_1.06Fe³⁺_0.43)_Σ3.00(PO₄)₄ ^X[(OH)_0.54F_0.42O_1.04]_Σ2.00(H₂O)₁₀⋅4H₂O, where □ = vacancy.

Rewitzerite has monoclinic symmetry with space group P2₁/c and unit-cell parameters a = 10.444(2) Å, b = 20.445(2) Å, c = 12.2690(10)Å, β = 90.17(3)°, V = 2619.8(6) ų and Z = 4. The crystal structure was refined using synchrotron single-crystal data to wR_obs = 0.068 for 5894 reflections with I > 3σ(I). The crystal structure has the same topology as that for orthorhombic paulkerrite-group minerals but differs primarily in having an ordering of K⁺ and H₂O molecules in different A sites, whereas they are disordered at a single A site in the orthorhombic members of the group.

Introduction

Rewitzerite was found in specimens of altered zwieselite in the Mineralogical State Collection, Munich, Germany (SNSB). Preliminary energy-dispersive X-ray analyses of rewitzerite crystals indicated that they were close in composition to the recently described mineral pleysteinite, [(H₂O)K]Mn₂Al₃(PO₄)₄F₂(H₂O)₁₀⋅4H₂O (Grey et al., 2023c). Single-crystal diffraction studies showed, however, that the crystals had monoclinic symmetry with space group P2₁/c, a maximal non-isomorphic subgroup of the space group Pbca for pleysteinite. The mineral and its name (symbol Rwz) have been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA2023-005, Grey et al., 2023b). The name honours Christian H.A. Rewitzer (born 1955) of Furth im Wald, Bavaria. Christian has been a passionate and dedicated mineral collector since he was 14 years old and he now has a collection of +20,000 specimens. His collection includes more than 1000 mineral specimens from the Hagendorf Süd phosphate pegmatite mine that he collected during weekly visits from 1972. After the mine closure in 1984, he turned his research interests to other phosphate pegmatites in Bavaria, Spain and Portugal and has published several papers on them. He has recently established his own mineralogical laboratory, equipped with a scanning electron microscope and powder X-ray diffractometer, so he can conduct preliminary characterisation studies on specimens to determine if they are potential new species. He has been involved in the discovery, characterisation and naming of hydroniumpharmacoalumite, fehrite, alcantarillaite, cuprozheshengite, whiteite-(CaMnFe) and pleysteinite. Christian Rewitzer has agreed to the naming of the mineral in his honour.

The holotype specimen is housed in the mineralogical collections of the Natural History Museum of Los Angeles County, catalogue number 76280. Portions of the original holotype specimen, used for the initial characterisation using energy-dispersive X-ray analysis, and for the colour images are registered as cotype specimens at the Mineralogical State Collection Munich (SNSB), registration number MSM38037.

Rewitzerite belongs to the Dana mineral classification 42.11.21 of hydrated phosphates containing hydroxyl or halogen. This classification group is called the paulkerrite group in Dana's New Mineralogy, edition 8 (Gaines et al., 1997) and comprises paulkerrite (Peacor et al., 1984), mantienneite (Fransolet et al., 1984) and benyacarite (Demartin et al., 1997) that have all been reported to have orthorhombic symmetry, space group Pbca, with a ≈ 10.5, b ≈ 20.5 and c ≈ 12.4 Å. The group has been added to recently with the characterisation of two new orthorhombic members, pleysteinite (Grey et al., 2023c) and hochleitnerite (Grey et al., 2023d). The paulkerrite group has been approved as a new mineral group by the IMA-CNMNC, proposal 22-K-bis (Grey et al., 2023a).

Occurrence and associated minerals

Rewitzerite occurs in specimens of altered zwieselite that were collected in 1971 by one of the authors (EK) from the 47 m level of the Hagendorf Süd pegmatite mine quarry in the Oberpfalz, northeast Bavaria (49°39′1″N, 12°27′35″E). The zwieselite is bleached to a very light beige colour and contains many millimetre-sized vugs. Colourless tabular crystals of rewitzerite and associated rockbridgeite are located on the walls of the vugs. No other minerals were found in the vugs. The leached zwieselite contains residual grains of older columbite, quartz, sphalerite and zircon which sometimes are protruding into the vugs.

Physical and optical properties

Rewitzerite forms clusters of colourless elongated hexagonal-shaped prisms, up to 0.1 mm long (Fig. 1). The crystals are flattened on {010} and elongated along [100], with forms {010}, {001}, {111} and {$\bar{1}$11} as shown in Fig. 2. The calculated density, for the empirical formula and single-crystal unit-cell volume, is 2.33 g⋅cm^–3.

Figure 1. Rewitzerite crystals, dark green rockbridgeite and fine-grained zwieselite in vug. Field of view is 0.17 mm. Photo by C. Rewitzer, cotype specimen MSM38037.

Figure 2. Crystal drawing of rewitzerite; clinographic projection in nonstandard orientation, a vertical.

Optically, rewitzerite crystals are biaxial (+), with α = 1.585(2), β = 1.586(2) and γ = 1.615(2) (measured in white light). The measured 2V from extinction data analysed with EXCALIBR (Gunter et al., 2004) is 25(2)°, and the calculated 2V is 21.3°. Dispersion and pleochroism were not observed. The optical orientation is X = c, Y = b, Z = a. The Gladstone-Dale compatibility index (Mandarino, 1981) is –0.025 (excellent) based on the empirical formula and the calculated density.

Chemical composition

Crystals of rewitzerite were analysed using wavelength-dispersive electron microprobe (EMP) spectrometry on a JEOL JXA 8500F Hyperprobe operated at an accelerating voltage of 15 kV and a beam current of 2.2 nA. The beam was defocused to 5 μm. Analytical results (average of 12 analyses on 4 crystals) are given in Table 1, where they are compared with the analyses for pleysteinite. There was insufficient material for direct determination of H₂O, so it was based upon the crystal structure. The EMP results show a strong negative correlation between atoms of Fe and atoms of Al (Fig. 3).

Table 1. Analytical data (wt.%).

* All Fe as Fe³⁺ in rewitzerite based on BVS.

** Based on stoichiometry: 4P and 34(K+O+F).

Figure 3. Plot of EMP results, atoms of Fe vs. atoms of Al in rewitzerite.

The atomic proportions, normalised to 4P and 34(K+O+F) atoms per formula unit are:

$${\rm K}_{0.77}{\rm M}{\rm n}_{0.82}{\rm M}{\rm g}_{0.64}{\rm F}{\rm e}^{3 + }_{0.86} {\rm A}{\rm l}_{1.51}{\rm T}{\rm i}^{4 + }_{1.06} {\rm P}_{4.00}{\rm F}_{0.42}{\rm O}_{32.81}{\rm H}_{31.00}$$

The general structural formula for monoclinic paulkerrite-group minerals is A1A2M1₂M2₂M3(PO₄)₄X ₂(H₂O)₁₀⋅4H₂O, where A1 and A2 = K and H₂O and X = F, OH and O (see Discussion section). Expressing the atomic proportions in this form and taking into account the refined site occupancies from the crystal structure, the empirical formula for rewitzerite is: ^{A 1}[K_0.77(H₂O)_0.23]^{A 2}[H₂O] ^{M 1}(Mn²⁺_0.82Mg_0.64Fe³⁺_0.43□_0.11)_Σ2.00 ^{M 2}(Al_1.00Ti⁴⁺_0.67Fe³⁺_0.33)_Σ2.00 ^{M 3}(Al³⁺_0.51Ti⁴⁺_0.39Fe³⁺_0.10)_Σ1.00 (PO₄)₄ ^X[(OH)_0.54F_0.42O_1.04]_Σ2.00 (H₂O)₁₀⋅4H₂O, where □ = vacancy, corresponding to the end-member formula K(H₂O)Mn₂Al₃(PO₄)₄(OH)₂(H₂O)₁₀⋅4H₂O.

Although this formula was approved by the CNMNC, it is in conflict with the empirical formula that has divalent O^2– dominant at the X site. The problem arises because of the high degree of mixing of Ti and Al at the M2 and M3 sites, so that small changes in the distribution of the minor constituent Fe³⁺ between the two sites can result in different dominant cations at the sites. This is a problem common to all paulkerrite-group minerals, and to overcome it the compositions at the M2 and M3 sites are merged, (M2)₂M3, and the composition is plotted on a ternary plot, Al₃–Ti₃–Fe₃, that is divided into different composition fields based on different end-member compositions, Ti₃, Al₂Ti, Ti₂Al etc. The details of this approach will be reported separately, but the outcome for rewitzerite is shown in the ternary plot in Fig. 4. This shows that the (M2)₂M3 empirical composition for rewitzerite is located in the Al₂Ti end-member compositional field. When this is combined with the dominant A, M1 and X site constituents the end-member formula for rewitzerite is K(H₂O)Mn₂(Al₂Ti)(PO₄)₄[O(OH)](H₂O)₁₀⋅4H₂O, which requires K₂O 5.04, MnO 15.20, Al₂O₃ 10.92, TiO₂ 8.56, P₂O₅ 30.40, H₂O 29.88, total 100.00 wt.%. The revised end-member formula for rewitzerite has been approved by the IMA-CNMNC (proposal revised 22-K-bis, Grey et al., 2023a).

Figure 4. Ternary diagram for (M2)₂M3 site Al–Ti–Fe³⁺ compositions, showing end-member compositions (Al₂Ti, Ti₂Al etc.) and location of the empirical compositions for rewitzerite and pleysteinite.

Raman spectroscopy

Raman spectroscopy was conducted on a Horiba XploRA PLUS spectrometer using a 532 nm diode laser, 100 μm slit and 1800 gr/mm diffraction grating and a 100× (0.9 NA) objective. The spectrometer was calibrated using the 520.7 cm^–1 band of silicon. The sample was susceptible to thermal damage at high laser power, so the spectrum was recorded at a laser power of 4mW. The sample was examined after the spectrum was recorded to verify that no thermal damage had occurred.

The spectrum is shown in Fig. 5. The O–H stretch region has a broad band that can be assigned to H-bonded water, with a maximum at 3345 cm^–1 and a shoulder at 3110 cm^–1. Hydroxyl ion stretching is evident by a sharp shoulder at 3560 cm^–1. The H–O–H bending region for water has a band at 1665 cm^–1 and a shoulder at 1595 cm^–1. Two strong bands at 955 and 1012 cm^–1 in the P–O stretching region can be assigned to symmetric stretching modes whereas weaker bands at 1090 and 1135 cm^–1 correspond to antisymmetric P–O stretching modes. Bending mode vibrations of the (PO₄)^3– groups are located at 605 and 455 cm^–1. Bands at lower wavenumbers are related to lattice vibrations. A strong band at 845 cm^–1 with a shoulder at 780 cm^–1 can be assigned to metal–oxygen stretch vibrations for short Ti–O bonds that occur in linear trimers of corner-connected octahedra M2–M3–M2 in the structure, Fig. 6, by analogy with published Raman spectra for titanates containing short Ti–O distances (Tu et al., 1996; Bamberger et al., 1990, Stassen et al., 1998; Su et al., 2000). We have observed similar strong bands in other paulkerrite-group minerals, pleysteinite and hochleitnerite that have in common a short M2–X distance of ~1.8 Å. Silva et al. (2018) have recently reported the modelling of the Raman spectrum of Na₂Ti₃O₇ using first-principles calculations based on density functional theory. The spectrum has a strong band at 849 cm^–1 that was assigned as a Ti–O stretching mode for Ti3–O2 and Ti1–O4 bonds with distances of 1.76 and 1.85 Å, respectively (Yakubovich and Kireev, (2003). For comparison, rewitzerite has similar M2–O bond distances of 1.77 and 1.82 Å. The shoulder at 780 cm^–1 in the Raman spectrum for rewitzerite may be the corresponding Ti–F stretching vibrations for Ti at the M2 sites.

Figure 5. Raman spectrum of rewitzerite.

Figure 6. (010) slice of the rewitzerite structure. Prepared using ATOMS (Dowty, 2004).

Crystallography

Powder X-ray diffraction data were recorded using a Rigaku R-Axis Rapid II curved imaging plate microdiffractometer with monochromatised MoKα radiation. A Gandolfi-like motion on the φ and ω axes was used to randomise the sample. Observed d values and intensities were derived by profile fitting using JADE Pro software (Materials Data, Inc.). Data are given in Table 2. Refined monoclinic unit-cell parameters (space group P2₁/c (#14)) are a = 10.44(2) Å, b = 20.44(2) Å, c = 12.27(2) Å, β = 90.17(16)°, V = 2620(7) ų and Z = 4.

Table 2. Powder X-ray diffraction data (d in Å) for rewitzerite (I _calc > 1.5).

Single-crystal diffraction data were collected at the Australian Synchrotron microfocus beamline MX2 (Aragao et al., 2018). Intensity data were collected using a Dectris Eiger 16M detector and monochromatic radiation with a wavelength of 0.7107 Å. The crystal was maintained at 100 K in an open-flow nitrogen cryostream during data collections. The diffraction data were collected using a single 36 second sweep of 360° rotation around phi. The resulting dataset consists of 3600 individual images with an approximate phi angle of each image being 0.1 degrees. The raw intensity dataset was processed using XDS software to produce data files that were analysed using SHELXT (Sheldrick, 2015) and JANA2006 (Petříček et al., 2014). Refined unit-cell parameters and other data collection details are given in Table 3.

Table 3. Crystal data and structure refinement for rewitzerite.

* weight = 1/(σ(F₀)²

Structure refinement

A structural model for rewitzerite was obtained in space group P2₁/c using SHELXT (Sheldrick, 2015). Inspection of the model showed that it had the same topology as the Pbca model for pleysteinite (Grey et al., 2023c) but with the Pbca sites all split into pairs of sites at x,y,z and ½+x, ½–y, –z. To ensure that the same atom labelling was retained as for pleysteinite for ease of comparison, the pleysteinite atom coordinates were imported to JANA2006 and then transformed to the non-isomorphous subgroup P2₁/c. Twinning was implemented with 2-fold rotation about c*. Initial assignment of cations to different sites was based on the published site occupancies in the related minerals benyacarite (Demartin et al., 1993) and pleysteinite (Grey et al., 2023c). Manganese and Mg were incorporated at the M1 sites, Al and Ti at the M2 and M3 sites and K and O (for H₂O) at the A sites and their occupancies were refined to obtain the site scattering values. At a later stage of the refinements, a fixed amount of Fe was included at the M2 and M3 sites, to take account of the Al-for-Fe substitution shown in Fig. 3. The amount of Fe in the two sites was adjusted in incremental steps and the Al and Ti occupancies were refined in a series of runs to optimise the agreement between the refined site occupancies and the EMP results. The resulting refined site occupancies and site-scattering values are given in Table 4. Refinement with anisotropic displacement parameters for all atoms in JANA2006 converged at wR _obs = 0.068 for 5894 reflections with I > 3σ(I). Details of the data collection and refinement are given in Table 3. The refined coordinates, equivalent isotropic displacement parameters and bond-valence sums (BVS) (using Gagné and Hawthorne, 2015) from the refinement are reported in Table 5. Selected interatomic distances are reported in Table 6. Although the H atoms were not located, the low BVS in Table 5 for O9 through to O15 are consistent with them being H₂O molecules. Possible H-bonded O⋅⋅⋅O pairs involving the water molecules at O9 to O15 and at the A1 and A2 sites are listed in Table 7. Most of the distances are greater than 2.7 Å and correspond to weak H-bonds according to Libowitzky (1999), consistent with the broad Raman band extending from 3200 to 3600 cm^–1. Only a few of the distances in Table 7 are below 2.7 Å (2.67–2.68 Å) corresponding to strong H-bonds, consistent with the shoulder at 3110 cm^–1 in the Raman spectrum. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Table 4. Refined site-occupation factors and site scattering for rewitzerite.

Table 5. Refined atom coordinates, equivalent isotropic displacement parameters (Ų) and bond valence sums (BVS, in valence units) for rewitzerite.

Table 6. Polyhedral bond lengths [Å] for rewitzerite.

Table 7. Possible H-bonded O⋅⋅⋅O distances (< 3 Å) in rewitzerite.

Discussion

The monoclinic crystal structure of rewitzerite is closely related to the orthorhombic, Pbca, structures of benyacarite (Demartin et al., 1993) and pleysteinite (Grey et al., 2023c). The structures are based on heteropolyhedral layers of corner-connected M2(Op)₄X(H₂O) and M3(Op)₄X₂ octahedra and PO₄ tetrahedra parallel to (010) (Fig. 6), that are connected into 3D frameworks by corner-sharing between the PO₄ tetrahedra and M1(Op)₂(H₂O)₄ octahedra along [010] (Fig. 7). A feature of the (010) layers is linear trimers of corner-connected octahedra M2–M3–M2 oriented along ~ [102] and [$\bar{1}$02]. The M2-site cations are strongly displaced from the centres of the octahedra along <001> towards the bridging X anions to give short M2–X distances of ~1.8 Å (Table 6), manifested by strong Raman M–O stretching modes (Fig. 5). The corner-connected octahedra and tetrahedra form 10-member rings, elongated along [100] (Fig. 6) in which are located the A-site species (K⁺ and H₂O) and zeolitic H₂O molecules (O14 and O15). The A1 sites, containing dominant K, and A2 sites, containing dominant H₂O, are located in different 10-member rings, centred at 0,y,0 and ½,y,½, respectively in Fig. 6.

Figure 7. [100] projection of the structure of rewitzerite. Prepared using ATOMS (Dowty, 2004).

Figure 8 shows an (001) slice through the crystal structure centred at z = ¼. It illustrates a different aspect of the structure, viz. the corner-shared linkage of M2-centred octahedra and PO₄ tetrahedra into 4-member rings that are fused into kröhnkite-type ribbons (Hawthorne, 1985) along [100]. The bridging between adjacent kröhnkite ribbons via the M1(Op)₂(H₂O)₄ octahedra gives 8-member rings that bound [001] channels. The heteropolyhedral (001) layer in rewitzerite (Fig. 8) has the same topology as (001) layers of kröhnkite ribbons interconnected via M1(Op)₂(H₂O)₄ octahedra in laueite-group minerals (Mills and Grey, 2015). Adjacent (001) layers in rewitzerite at z = ¼ and z = ¾ (Fig. 8) are interconnected via corner sharing of the M2-centred octahedra with isolated M3-centred octahedra located in (001) planes at z = 0 and ½.

Figure 8. (001) slice through the rewitzerite structure, at z = ¼. Prepared using ATOMS (Dowty, 2004).

The refined site-occupation factors for the A sites in Table 5 show that there is a high degree of ordering of the K and H₂O in different A sites, with 96% of the K at site A1 while the site A2 is occupied 97% by H₂O. In related minerals benyacarite and pleysteinite, with orthorhombic symmetry, the A1 and A2 sites are merged into a single A site containing approximately equal amounts of K and H₂O. In contrast to the ordering at the A1 and A2 sites, the pairs of M sites in the monoclinic structure show only minor variations in the site scattering (Table 4), bond distances (Table 6) and BVS values (Table 5). This situation is similar to that for jahnsite-group minerals, with general formula XM1M2₂M3₂(H₂O)₈(OH)₂(PO₄)₄ (Kampf et al., 2019). The M2 and M3 sites in jahnsites each comprise pairs of crystallographically independent sites, but in all jahnsite-group minerals, the pairs of sites have the same or almost the same occupancy, and so in the general formula they are not distinguished. Applying the same reasoning to monoclinic paulkerrite-group minerals the general formula can be written as A1A2M1₂M2₂M3(PO₄)₄X ₂(H₂O)₁₀⋅4H₂O, where the K/H₂O-containing sites are separated as A1 and A2, but the pairs of M1, M2 and M3 sites are merged.

Based on their crystal structure refinement for benyacarite, Demartin et al. (1993) considered that the M1 site was occupied by divalent cations, Mn²⁺, Mg²⁺ and Fe²⁺ and in the refinement of pleysteinite (Grey et al., 2023c) the BVS value for site M1 was consistent with divalent cations at this site. In contrast, rewitzerite has elevated BVS values for sites M1a (2.22 vu) and M1b (2.20 vu) that require the Fe at the site (after allocation of all Mn²⁺ and Mg to the site) to be in the trivalent state. This situation is common in mineral groups such as schoonerites and rockbridgeites at Hagendorf Süd, where both reduced species (green Fe²⁺-bearing) and oxidised (red, Fe³⁺-bearing) species occur (Grey et al., 2018, 2019) in close spatial association. The oxidised species generally have M to P ratios that are lower than required by the structural formulae, due to cation vacancies. The EMP analyses for rewitzerite also had lower M:P atomic ratios than the 5:4 required by the structure, corresponding to just over 2% cation vacancies. In the empirical formula, these have been assigned to the M1 sites, but this assignment is somewhat arbitrary – it being difficult to confirm with multiple cations at each site.

Rewitzerite and pleysteinite occupy adjacent (M2)₂M3 composition fields corresponding to Al₂Ti and Al₃, respectively, as shown in Fig. 4, and they have somewhat similar analyses (Table 1), although rewitzerite has only about one third the F content of pleysteinite. There are also significant differences in the contents of M-site cations, with rewitzerite containing 50% less Al and Mn and up to 80% more Fe and Ti than pleysteinite. The EMP data for rewitzerite shows a weak positive correlation between F and Al (R ² = 0.32) but no correlation of F with Fe or Ti. The crystal-chemical properties of the two minerals are compared in Table 8. Despite the minerals having different symmetries and space groups, their crystallographic parameters (unit-cell parameters and powder diffraction peaks) are closely matched, with the only significant difference being a larger b parameter for pleysteinite due to a higher percentage of large cations (Mn²⁺ and Fe²⁺) at the M2 sites. The main crystal-chemical difference associated with the lowering of symmetry in rewitzerite is an ordering of K⁺ and H₂O at the A sites (Fig. 6).

Table 8. Comparison of rewitzerite and pleysteinite.