Pleysteinite, [(H₂O)_0.5K_0.5]₂Mn₂Al₃(PO₄)₄F₂(H₂O)₁₀  ⋅  4H₂O, the Al analogue of benyacarite, from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria, Germany

Pleysteinite, [(H₂O)_0.5K_0.5]₂Mn₂Al₃(PO₄)₄F₂(H₂O)₁₀ • 4H₂O, is the aluminium analogue of benyacarite, from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria, Germany. It was found in specimens of altered zwieselite, in association with nordgauite, fluellite, rockbridgeite, pyrite and columbite. Pleysteinite occurs as isolated and small aggregates of colourless, stubby prisms that are typically 10 to 30 µm wide and up to 100 µm long. The crystals are flattened on {010} and bounded by {111}, {100} and {001} planes. The calculated density is 2.34 g cm⁻³. Optically, pleysteinite crystals are biaxial (+), with α=1.566(2), β=1.580(2), γ=1.600(2) (measured in white light) and 2V(meas.) = 80(1)^∘. The empirical formula from electron microprobe analyses and structure refinement is [(H₂O)_0.50K_0.50]₂(Mn_1.20Mg_0.49Fe${}_{\mathrm{0.27}}^{\mathrm{2}+}$Zn_0.05)_∑2.01(Al_1.63Fe${}_{\mathrm{0.20}}^{\mathrm{3}+}$Ti${}_{\mathrm{0.19}}^{\mathrm{4}+}$)_∑2.02(Al_0.56Ti${}_{\mathrm{0.44}}^{\mathrm{4}+}$) (PO₄)_4.02[F_0.58O_0.31(OH)_0.11]₂(H₂O)₁₀ • 3.92H₂O. Pleysteinite has orthorhombic symmetry, with space group Pbca and unit-cell parameters a = 10.4133(8) Å, b=20.5242(17) Å, c=12.2651(13) Å, V=2621.4(4) ų and Z=4. The crystal structure was refined using single-crystal data to wR_obs=0.054 for 1692 reflections with I>3σ(I). The crystal structure contains corner-connected linear trimers of Al-centred octahedra that share corners with PO₄ tetrahedra to form 10-member rings parallel to (010). K⁺ cations and water molecules are located in the rings. Additional corner-sharing of the PO₄ tetrahedra with Mn(H₂O)₄O₂ octahedra occurs along [010] to complete the 3D framework structure.

1 Introduction

Pleysteinite was found in specimens of altered zwieselite from the Hagendorf-Süd pegmatite mine, Bavaria. The specimens were collected in the 1970s by one of the authors (Erich Keck) from the 64–67 m level of the mine. Energy-dispersive spectroscopic (EDS) analysis of the small prismatic crystals of the mineral indicated that it was most likely the aluminium analogue of the titanium-bearing phosphate mineral benyacarite, (H₂O,K)₂(Mn²⁺,Fe²⁺)₂(Fe³⁺,Ti)₂Ti(PO₄)₄(O,F)₂• 14H₂O (Demartin et al., 1993, 1997), and this was confirmed from a single-crystal structure determination.

Benyacarite is closely related to two other titanium-bearing phosphate minerals, paulkerrite, KTi(Mg,Mn)₂(Fe³⁺,Mg,Al,Ti)₂(PO₄)₄(OH)₃ • 15H₂O (Peacor et al., 1984) and mantienneite, (K_0.5□_0.5)Ti(Mg_1.5Fe${}_{\mathrm{0.5}}^{\mathrm{3}+}$)_∑2Al₂(PO₄)₄(OH)₃ • 15H₂O, □ = vacancy (Fransolet et al., 1984). Demartin et al. (1993) proposed the general formula (H₂O, K)₂TiM1₂M2₂(PO₄)₄(O,F)₂ • 14H₂O for these minerals, which are distinguished by the dominant cations at the M1 and M2 sites: Mn²⁺ and Fe³⁺ in benyacarite, Mg and Fe³⁺ in paulkerrite and Mg and Al in mantienneite. A proposal for a paulkerrite group, based on the first-described member, has been submitted to the International Mineralogical Association (IMA) Commission on New Minerals, Nomenclature and Classification (CNMNC). Pleysteinite is the fourth member of the group, with dominant Mn²⁺ at the M1 site and dominant Al at the M2 and Ti sites. Full characterisation of the mineral led to its approval as a new species by the IMA CNMNC, proposal IMA2022-077. The name is for Pleystein, a municipality in Bavaria located 18 km east of Weiden in the Oberpfalz. The Hagendorf-Süd pegmatite that provided the specimens of pleysteinite is situated near the municipal area of Pleystein. The holotype specimen is housed in the collections of the Mineralogical State Collection Munich (SNSB), with registration number MSM 38032. A cotype used for optical characterisation and the powder diffraction pattern is housed in the mineralogical collections of the Natural History Museum of Los Angeles County, catalogue number 76271.

2 Occurrence and associated minerals

The holotype specimen, Hag500, containing pleysteinite crystals on altered zwieselite (Fig. 1) comes from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria, Germany (49^∘39^′1^′′ N, 12^∘27^′35^′′ E). The zwieselite specimens are bleached to a very light maroon colour and contain many vugs up to some millimetres in diameter. The fine-grained zwieselite is very soft and sometimes shows a silky lustre. The larger holes contain rockbridgeite, Fe²⁺Fe${}_{\mathrm{4}}^{\mathrm{3}+}$(PO₄)₃(OH)₅, and crystals of fluellite, Al₂(PO₄)F₂OH • 7H₂O, up to 1 mm. The smaller vugs contain silky needles of white nordgauite, MnAl₂(PO₄)₂(F,OH)₂ • 5H₂O, and are lined with colourless equant crystals of pleysteinite up to 0.1 mm. Nordgauite is younger than pleysteinite. Embedded and sometimes protruding into vugs are cubes of pyrite and tiny tabular crystals of columbite which are older than the zwieselite.

Figure 1 Pleysteinite crystals on fine-grained zwieselite. Field of view is 0.5 mm. Photo by Christian Rewitzer.

3 Physical and optical properties

Pleysteinite forms isolated and small aggregates of colourless, stubby prisms on fine-grained zwieselite (Fig. 1). The crystals are typically 10 to 30 µm wide and up to 100 µm long and often display pyramidal terminations (Fig. 2). A feature of the crystals is that they readily cleave into thin tablets parallel to {010} in a vacuum due to dehydration (Fig. 3). The crystals are flattened on {010} and bounded by {111}, {100} and {001} planes (Fig. 2). The calculated density, for the empirical formula and single-crystal unit-cell volume, is 2.34 g cm⁻³.

Optically, pleysteinite crystals are biaxial (+), with α=1.566(2), β=1.580(2), γ=1.600(2) (measured in white light). The measured 2V from extinction data analysed with EXCALIBR (Gunter et al., 2004) is 80(1)^∘, and the calculated 2V is 80.7^∘. Dispersion and pleochroism were not observed. The optical orientation is X=c, Y=b, Z=a. The Gladstone–Dale compatibility index (Mandarino, 1981) is −0.019 (superior) based on the empirical formula and the calculated density.

Figure 2 Pleysteinite crystal (photo Christian Rewitzer) with crystal drawing (clinographic projection in nonstandard orientation, a vertical) showing face indexing.

Figure 3 Back-scattered electron image of pleysteinite crystals, showing dehydration cleavage in the vacuum of the electron microscope.

4 Chemical composition

Crystals of pleysteinite were analysed using wavelength-dispersive 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 10 µm. Analytical results (average of analyses on seven crystals) are given in Table 1. There was insufficient material for direct determination of H₂O, so it was based upon the crystal structure analysis. The atomic proportions, normalised to 33 anions and with ${\mathrm{Fe}}^{\mathrm{2}+}/{\mathrm{Fe}}^{\mathrm{3}+}$ based on the crystal structure ((Fe²⁺ with other divalent cations at the M1 site and remaining iron as Fe³⁺ at the M2 site), are K_1.00Mn_1.20Mg_0.49Fe${}_{\mathrm{0.27}}^{\mathrm{2}+}$Zn_0.05Al_2.19Fe${}_{\mathrm{0.20}}^{\mathrm{3}+}$Ti${}_{\mathrm{0.63}}^{\mathrm{4}+}$P_4.02F_1.16O_31.84H_30.06.

Table 1 Analytical data (wt %) for pleysteinite.

^* Based on the structural formula.

The empirical formula is calculated based on the crystal structure, with divalent cations located at M1 and with the site assignment at M3 from the site occupancy refinement, and the remaining small cations from the EMP analyses located at M2: [(H₂O)_0.5K_0.5]₂^M1(Mn_1.20Mg_0.49Fe${}_{\mathrm{0.27}}^{\mathrm{2}+}$Zn_0.05)_∑2.01^M2(Al_1.63Fe${}_{\mathrm{0.20}}^{\mathrm{3}+}$Ti${}_{\mathrm{0.19}}^{\mathrm{4}+}$)_∑2.02^M3(Al_0.56Ti${}_{\mathrm{0.44}}^{\mathrm{4}+}$)(PO₄)_4.02[F_0.58O_0.31(OH)_0.11]₂(H₂O)₁₀ • 3.92H₂O.

The simplified formula is [(H₂O)_0.5K_0.5]₂(Mn,Mg,Fe²⁺,Zn)₂(Al,Fe³⁺,Ti⁴⁺)₂(Al,Ti⁴⁺)(PO₄)₄(F,O,OH)₂(H₂O)₁₀• 4H₂O.

The ideal formula is [(H₂O)_0.5K_0.5]₂Mn₂Al₃(PO₄)₄F₂(H₂O)₁₀ • 4H₂O, which requires K₂O 5.13, MnO 15.46, Al₂O₃ 16.66, P₂O₅ 30.93, F 4.14, H₂O 29.42, -O≡F −1.74, total 100 wt %. Note that the •14H₂O in the Demartin et al. (1993) general formula has been separated into (H₂O)₁₀ coordinated at the M1 and M2 sites and •4H₂O molecules occupying cavities, to better reflect the crystal structure.

5 Raman spectroscopy

Raman spectroscopy was conducted on a Horiba XploRA PLUS spectrometer using a 532 nm diode laser, 100 µm slit and 1800 g mm⁻¹ diffraction grating and a 100× (0.9 NA) objective. The spectrum is shown in Fig. 4. The O–H stretch region has a broad peak that can be assigned to H-bonded water, with a maximum at 3335 cm⁻¹ and a shoulder at 3115 cm⁻¹. A sharp shoulder at 3560 cm⁻¹ is most likely due to hydroxyls. The H–O–H bending mode region for water has a peak at 1665 cm⁻¹. The P–O stretching region has strong sharp peaks at 960 and 1015 cm⁻¹ and a weaker peak at 1130 cm⁻¹ with a shoulder at 1095 cm⁻¹. The two strong peaks correspond to antisymmetric stretching modes, and the weaker peaks correspond to symmetric P–O stretching modes. Bending modes of the (PO₄)³⁻ groups are manifested by two bands centred at 610 and 460 cm⁻¹. Peaks at lower wavenumbers are related to lattice vibrations. A strong peak at 850 cm⁻¹ with a shoulder at 780 cm⁻¹ is an interesting feature of the spectrum. A tentative assignment is to stretching vibrations for short M–(O,F) bonds that occur in linear trimers of corner-connected octahedra M2–M3–M2 in the structure (Fig. 5). The potassium titanyl phosphate, KTiOPO₄, has chains of corner-connected octahedra with short (1.81–1.83 Å)Ti–O bonds, and the Raman spectrum has an intense band at 770 cm⁻¹ that has been assigned to the symmetric Ti–O stretching vibration (Tu et al., 1996). Strong Raman bands in the range 800 to 900 cm⁻¹ have been reported and assigned to Ti–O stretch vibrations for several potassium titanates, which have corner-connected TiO₆ octahedra involving short (∼ 1.8 A) bridging Ti–O bonds (Bamberger et al., 1990). The M2 site in pleysteinite similarly has the M2 atom displaced from the centre of the octahedron towards the corner-sharing anion with the M3 octahedron giving a short (1.825 A) M2–(O,F) bond.

Figure 4 Raman spectrum for pleysteinite.

Figure 5 {010} slice of the pleysteinite structure.

6 Crystallography

X-ray powder diffraction data were recorded using a Rigaku R-Axis Rapid II curved imaging plate microdiffractometer with monochromatised Mo Kα 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 orthorhombic unit-cell parameters (space group Pbca (#61)) are a=10.407(4) Å, b=20.557(8) Å, c=12.247(5) Å, V=2619.9(7) ų and Z=4.

Table 2 Powder X-ray diffraction data (d in Å) for pleysteinite (I_calc > 1.5).

Table 3 Crystal data and structure refinement for pleysteinite.

Single-crystal diffraction data were collected at room temperature using a Rigaku Oxford Diffraction Supernova four-circle diffractometer equipped with an Atlas CCD detector and using MoKα radiation (λ=0.71073 Å). Refined unit-cell parameters and other data collection details are given in Table 3.

Structure refinement

A structural model was obtained in space group Pbca using SHELXT (Sheldrick, 2015). It was found to be the same as that reported for benyacarite (Demartin et al., 1993). The scattering curve for Mn was used to represent (Mn + Mg + Fe + Zn) at the M1 site, and Fe and Al were located at the M2 site with their occupancies refined. The K + O (for H₂O) occupancies at the K site were also refined. The mean bond distance for the Ti site (≡M3) was found to be only 1.91 Å, which is too short for Ti⁴⁺–O (expected 1.96 Å from bond valence sum (BVS) for an ideal octahedron), and the isotropic displacement parameter was elevated (U=0.03), suggesting replacement by a lighter cation. Al was added to the M3 site and the occupancies of Al/Ti refined, giving dominant Al in the site [Al_0.56(1)Ti_0.44(1)].

Table 4 Atom coordinates, equivalent isotropic displacement parameters (Ų) and bond-valence sums (BVSs) for pleysteinite.

Site scattering: M1 = 0.950(4)Mn; M2 = 0.685(6)Al+0.315Fe; M3 = 0.56(1)Al + 0.44Ti; K/O = 0.473(9)K + 0.527O.

Figure 6 [100] projection of the structure of pleysteinite, made using ATOMS (Dowty, 2006).

Refinement with anisotropic displacement parameters for all atoms in JANA2006 (Petříček et al., 2014) converged at wR_obs=0.054 for 1692 reflections with I>3σ(I). Unambiguous location of H atoms in correct configurations for H₂O molecules in difference Fourier maps was not achieved. Partial H positions have been previously reported for benyacarite (Demartin et al., 1993). Details of the data collection and refinement are given in Table 3. The refined coordinates, equivalent isotropic displacement parameters and bond valence sum (BVS) values (Gagné and Hawthorne, 2015) from the single-crystal refinement are reported in Table 4. Selected interatomic distances are reported in Table 5. Although the H atoms were not located, the BVS values in Table 4 show clearly the presence of seven independent H₂O molecules, O9 to O15. An indication of H₂O at the K/O site was obtained by pairs of weak peaks in a difference Fourier map at 0.8 Å from the central atom and making a H–O–H angle of 100^∘.

7 Discussion

The structure comprises [100] chains of alternating corner-connected M2O₄F(H₂O) octahedra and PO₄ tetrahedra that are connected along [001] by corner-sharing with M3O₄F₂ octahedra into (010) layers shown in Fig. 5. The corner-connected polyhedra form 10-member rings that are occupied by K⁺ ions and water molecules, O14 and O15. These layers are interconnected into a 3D framework structure via corner sharing of the PO₄ tetrahedra with M1O₂(H₂O)₄ octahedra, shown in Fig. 6. The K site contains approximately equal amounts of K⁺ ions and H₂O molecules, as found also for benyacarite (Demartin et al., 1993). The ${\mathrm{K}}^{+}/{\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ coordination is irregular, with six distances in the range 2.72 to 3.09 Å (Table 5) and two longer distances at ∼ 3.3 Å.

Table 5 Polyhedral bond lengths [Å] for pleysteinite.

Pleysteinite is isostructural with benyacarite (Demartin et al., 1993). The published orthorhombic unit-cell parameters for paulkerrite and mantienneite suggest that these two minerals also have the same crystal structure as benyacarite. Demartin et al. (1997) proposed a general formula for the three minerals, (H₂O,K)₂TiM1₂M2₂(PO₄)₄(O,F)₂ • 14H₂O, with divalent cations, Mn²⁺ and Mg²⁺ at the M1 site and trivalent cations, Fe³⁺ and Al³⁺ at the M2 site. All three minerals have approximately one Ti⁴⁺ cation per formula unit, and this was allocated to the special site (1/2,1/2,0) in Pbca in the refinement of benyacarite. Our refinement of the pleysteinite crystal structure, however, shows that Al, rather than Ti, is dominant at the Ti site, and the Ti is partitioned between the Ti and M2 sites. On this basis the formula proposed by Demartin et al. (1997) needs modifying to replace the Ti site with a more general M3 site. We have also separated the •14H₂O in the Demartin formula into (H₂O)₁₀ coordinated at the M1 and M2 sites and •4H₂O molecules occupying cavities, to better reflect the crystal structure. The resulting formula is A₂M1₂M2₂M3(PO₄)₄X₂(H₂O)₁₀ • 4H₂O, where A = K, H₂O and X = O, F, OH.