Occurrence and associated minerals

The hydroxylbenyacarite specimen derives from a granite pegmatite at the El Criollo mine. The pegmatite contains large masses of triplite up to 5 metres. The maroon brown triplite is partially altered to secondary phosphate minerals, especially bluish phosphosiderite, lilac strengite and reddish-brown bermanite. Other alteration products of triplite at the locality are libethenite, dufrenite, red–brown eosphorite and rockbridgeite-group minerals. The new mineral occurs in corrosion cavities in the triplite, associated with strengite, quartz and fine-grained manganese oxides (Fig. 1). It is younger than the other phosphates.

Figure 1. Greenish-yellow crystal of hydroxylbenyacarite associated with lilac strengite and yellowish cryptocrystalline quartz on specimen MSM38036. Minerals below the hydroxylbenyacarite crystal are coated with black manganese oxides. Field of view 1.65 mm. Photo by Christian Rewitzer.

Physical and optical properties

Hydroxylbenyacarite forms isolated and intergrown light greenish-yellow rhombic tablets. The crystals generally have dimensions of 20 to 80 μm, occasionally up to ~400 μm (Fig. 1). The crystals are flattened on {010} and slightly elongated on [001], with the main forms being {010} and {111}. The calculated density for the empirical formula and single-crystal unit cell volume is 2.32 g cm^–3.

Optically, hydroxylbenyacarite crystals are biaxial (+), with α = 1.608(3), β = 1.624(3) and γ = 1.642(3) (measured in white light). The 2V is 88(2)°, measured directly on a spindle stage, compared with 2V (calc.) = 87.5°. Dispersion is moderate with r < v. The optical orientation is X = b, Y = c and Z = a. The mineral is nonpleochroic. The Gladstone–Dale compatibility index (Mandarino, Reference Mandarino1981) is 0.016 (superior) based on the empirical formula and single-crystal unit-cell parameters.

Raman spectroscopy

Raman spectroscopy was conducted on a Horiba XploRA PLUS spectrometer using a 532 nm diode laser, 100 μm slit and 1800gr/mm diffraction grating and a 100× (0.9 NA) objective. The spectrum is shown in Fig. 2. The O–H stretch region has two broad bands with maxima at 3373 and 3101 cm^–1 and shoulders at 3609, 3481, 3212 and 2958 cm^–1. The H–O–H bending mode region for water has a band at 1660 cm^–1. The P–O stretching region has a strong band at 949 cm^–1 with shoulders at 1013 and 980 cm^–1 corresponding to symmetric P–O stretching modes and a weaker band at 1133 cm^–1 corresponding to an antisymmetric P–O stretch. Bending modes of the (PO₄)^3– groups are manifested by a band centred at 597 cm^–1 and a composite band with a maximum at 411 cm^–1 and shoulders at 479 and 445 cm^–1. Peaks at lower wavenumbers are related to lattice vibrations. An intense pair of bands at 832 and 774 cm^–1 is a consistent feature of paulkerrite-group minerals that can be assigned to Ti–O stretching vibrations associated with short Ti–O bonds, as reported for numerous titanates (Bamberger et al., Reference Bamberger, Begun and MacDougall1990; Tu et al., Reference Tu, Guo, Tao, Katiyar, Guo and Bhalla1996). Silva et al. (Reference Silva, Filho, Silva, Balzuweit, Bantiignies, Caetano, Moreira, Freire and Righi2018) 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 and a weaker band at 740 cm^–1 that were assigned as Ti–O stretching modes for Ti–O bonds with distances of 1.76 and 1.85 Å. For comparison, hydroxylbenyacarite has a similar Ti–O distance at the M2 site of 1.81 Å that could be associated with the band at 832 cm^–1. The peak at 774 cm^–1 in the Raman spectrum for hydroxylbenyacarite may be the corresponding Ti–F stretching vibrations for Ti at the M2 site.

Figure 2. Raman spectrum for hydroxylbenyacarite.

Chemical composition

Crystals of hydroxylbenyacarite were difficult to analyse because they underwent severe cracking due to dehydration resulting from beam heating and the high vacuum of the microprobe. The results presented in Table 1 are for regions of crystals that were least affected by cracking. The crystals 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 5 μm. Analytical results (average of 5 analyses on 5 crystals) are given in Table 1. There was insufficient material for direct determination of H₂O, of which the presence is indicated by a low analysis total for oxides and confirmed by Raman spectroscopy, so it was based upon the crystal structure, with 15 H₂O per 4 P. The FeO/Fe₂O₃ proportioning in Table 1 is obtained from the crystal structure, with Fe²⁺ at the M1 site and Fe³⁺ at the M2 and M3 sites.

Table 1. Chemical data (wt.%) for hydroxylbenyacarite.

*Fe²⁺/Fe³⁺ based on the crystal structure, with Fe²⁺ assigned with all Mn²⁺ and Mg at the M1 site and the remaining iron as Fe³⁺ assigned to the M2 and M3 sites.

S.D. – standard deviation

The microprobe analyses showed a small departure (1.222) from the stoichiometric ratio of ΣM/P from the crystal structure of 5/4 = 1.25, and the structure refinement confirmed that metal atom vacancies occurred at the M1 site, so the normalisation of the formula was based on 4 P per formula unit, giving the atoms per formula unit as:

$${\rm K}_{0.46}{\rm C}{\rm a}_{0.06}{\rm M}{\rm n}_{1.52}{\rm M}{\rm g}_{0.02}{\rm F}{\rm e}^{2 + }_{0.34} {\rm F}{\rm e}^{3 + }_{1.22} {\rm A}{\rm l}_{0.02}{\rm T}{\rm i}_{1.77}{\rm P}_4{\rm F}_{0.16}{\rm O}_{32.48}{\rm H}_{30}.$$

Expressing the empirical results in structural form gives the empirical formula:

Ca_0.06 ^A[K_0.46(H₂O)_0.88□_0.66]_Σ2.00 ^{M 1}(Mn_1.52Mg_0.02Fe²⁺_0.35□_0.11)_Σ2.00 ^{M 2+M3}(Fe³⁺_1.21Al_0.02Ti_1.77)_Σ3.00(PO₄)₄ ^X[F_0.16(OH)_0.70O_1.14]_Σ2.00(H₂O)₁₀⋅3.77H₂O.

The compositions at the M2 and M3 sites have been merged so that the end-member composition can be determined using the merged-sites procedure recently approved for paulkerrite-group minerals by the IMA-CNMNC, proposal revised 22-K-bis (Grey et al., Reference Grey, Boer, MacRae, Wilson, Mumme and Bosi2023a). The merged (M2₂M3) sites approach is illustrated graphically in Fig. 3, showing a ternary Al₃–Fe³⁺₃–Ti₃ diagram with the possible end-member compositions designated (e.g. Ti₃, Al₂Ti, TiFe₂). The empirical (M2₂M3) composition for hydroxylbenyacarite is shown in Fig. 3 to be located in the end-member (Ti₂Fe) composition field. Combining with the dominant constituents at M1, A and X sites gives the end-member formula (H₂O)₂Mn₂(Ti₂Fe)(PO₄)₄[O(OH)](H₂O)₁₀⋅4H₂O, which requires MnO 14.74, Fe₂O₃ 8.30, TiO₂ 16.60, P₂O₅ 29.50, H₂O 30.86, total 100 wt.%. Note that charge neutrality in the end-member formula requires mixed-valency O^2– and OH^–/F^– and OH^– is dominant over F^– (in accord with Hatert and Burke, Reference Hatert and Burke2008 and Bosi et al., Reference Bosi, Hatert, Halenius, Pasero, Ritsuro and Mills2019).

Figure 3. Ternary diagram for (M2₂M3) site Al–Ti–Fe³⁺ compositions, showing end-member compositions (e.g. Al₂Ti, AlTi₂) and location of the experimental composition for hydroxylbenyacarite and for other published paulkerrite-group minerals. Blue crosses have Mg at M1 and red crosses have Mn at M1.

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 orthorhombic unit-cell parameters (space group Pbca (#61)) are a = 10.580(14), b = 20.78(3), c = 12.529(17) Å, V = 2755(7) ų and Z = 4.

Table 2. Powder X-ray diffraction data (d in Å) for hydroxylbenyacarite (I _calc > 1.5), Pbca model.*

*Strongest reflections shown in bold font.

Single-crystal diffraction data were collected on a crystal measuring 0.071 × 0.131 × 0.179 mm. Data were collected at 299 K using a XtaLab Synergy 4-circle diffractometer equipped with a Dualflex Hypix detector and using MoKα radiation, λ = 0.71073 Å. Refined unit-cell parameters and other data collection details are given in Table 3.

Table 3. Crystal data and Pbca crystal structure refinement for hydroxylbenyacarite.

*w = [σ²(ǀF_oǀ)+(uF_o)²]^–1, u = instability factor

Structure refinement in Pbca

A structural model was obtained in space group Pbca using SHELXT (Sheldrick, Reference Sheldrick2015). It was found to conform to the general structural formula for orthorhombic paulkerrite-group members, A ₂M1₂M2₂M3(PO₄)₄X ₂(H₂O)₁₀⋅4H₂O, where divalent cations (Mn²⁺, Mg and Fe²⁺) are located at M1 and Fe³⁺, Al and Ti are located at the M2 and M3 sites (Grey et al., Reference Grey, Boer, MacRae, Wilson, Mumme and Bosi2023a). Using the EMPA as a guide, Mn, Mg and Fe were assigned to M1, with the Mn and Mg contents fixed at the empirical formula values and the Fe content refined. Fe and Ti were assigned to M2 and M3 and the site occupancies of the pair of atoms refined. K and O (for H₂O) were assigned to the A site, with the K content fixed at the empirical formula value and the O content refined. For both the M1 and A sites, less than full occupancy was obtained, corresponding to vacancies at these sites. A difference-Fourier map showed a relatively large peak (2.9 e ^– Å^–3) at a site coordinated to 8 oxygens at distances in the range 2.23 to 2.72 Å, mean value 2.52 Å. The coordination number and bond lengths are consistent with Ca which was present as a minor constituent in the chemical analyses. With Ca located at the site, its occupancy refined to a value within the spread of CaO analyses (Table 1). The refinement was conducted using anisotropic displacement parameters (ADPs) for all atoms except the partially occupied Ca site. Two oxygen atoms (O4 and O5) were found to have slightly non-positive definite ADPs, and the displacement parameters were made isotropic for these atoms, 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, Reference Gagné and Hawthorne2015) are reported in Table 4. Selected interatomic distances are reported in Table 5. Although H atoms were not located in the refinement, the BVS values in Table 4 show clearly the presence of seven independent H₂O groups, O9 to O15, as well as an anion site, X, having a low BVS of 1.66 due to partial incorporation of OH as indicated in the empirical formula. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Table 4. Atomic coordinates, equivalent isotropic displacement parameters (Ų) and bond valence sums (BVS, in valence units) for hydroxylbenyacarite, Pbca average structure.

*U _iso

Table 5. Polyhedral bond lengths [Å] for the Pbca model for hydroxylbenyacarite.

Structure refinement in P2₁/c

In the processing of the single-crystal data using CrysAlisPro (Rigaku OD, Reference Rigaku2022), automatic data reduction chose the orthorhombic cell given in the previous section, with parameters obtained from the fitting of 18,367 reflections, but the program also provided an unconstrained triclinic unit cell with parameters a = 10.5512(2), b = 20.7276(5), c = 12.4988(3) Å, α = 90.027(2), β = 90.065(2) and γ = 89.99(2)°. Although a satisfactory refinement of the crystal structure was obtained in an orthorhombic cell, space group Pbca, as for benyacarite (Demartin et al., Reference Demartin, Pilati, Gay and Gramaccioli1993) the β value of 90.065° prompted an exploration of the possibility of the structure having monoclinic symmetry, analogous to the P2₁/c structures for other paulkerrite-group minerals (Grey et al., Reference Grey, Boer, MacRae, Wilson, Mumme and Bosi2023a). It was possible to obtain a constrained monoclinic cell in CrysAlisPro, with a = 10.5467(3), b = 20.7222(5), c = 12.5031(3) Å, α = 90, β = 90.068(2) and γ = 90°. To further check on the possibility of lower symmetry, simulated precession images were generated from the complete diffraction data using the UNWARP facility in the CrysAlisPro software. The a*b* zone is shown in Fig. 4 for two different scalings. Fig. 4a shows that the pattern contains diffuse streaking parallel to b*, suggestive of (010) stacking faults, as has been reported for paulkerrite (Peacor et al., Reference Peacor, Dunn and Simmons1984). In Fig. 4b the scale was increased to emphasise weaker diffraction effects. This image shows diffuse reflections with h = 2n+1 that violate the a-glide extinction conditions for Pbca. Similarly, the b*c* zone showed very weak diffuse reflections with k = 2n + 1 that are not allowed in Pbca. On the basis of these observations, we converted the Pbca model coordinates to those for the P2₁/c subgroup in JANA2006 (Petříček et al., Reference Petříček, Dušek and Palatinus2014) and refined the monoclinic model. The refinement was problematic, in giving numerous non-positive ADPs and relatively wide ranges of P–O distances, 1.48 to 1.58 Å. Nevertheless, an important result from the P2₁/c refinement was that not only does some K/H₂O ordering occur in split A sites as reported for other monoclinic paulkerrite-group members (Grey et al., Reference Grey, Boer, MacRae, Wilson, Mumme and Bosi2023a), but Ca was also ordered in only one of the two sites generated by transformation of the Pbca structure.

Figure 4. Reconstructed a*b* precession images generated at two different scale values (a) shows diffuse streaking parallel to b*, indicative of (010) stacking faults. (b) A higher scale was employed to show weak diffraction effects. Weak diffuse reflections with h = 2n + 1, as indicated by the red arrow, violate the a-glide extinction condition for Pbca but are consistent with subgroup P2₁/c.

The refinement difficulties encountered in the P2₁/c refinement are most likely linked to the structural disorder that is indicated by the streaking parallel to b* and to the diffuse nature of the weak reflections. Based on the lengths of the diffuse streaks, the Ca and K/H₂O ordering that is associated with the symmetry lowering is restricted to domains on the scale of the unit cell, 1 to 2 nm. In the absence of better-quality crystals of hydroxylbenyacarite, we restrict our discussion of the crystal structure to the orthorhombic representation in Pbca, which corresponds to the average structure. Further analysis of the structural disorder evidenced by the streaked reflections shown in Fig. 4 may benefit from the modular approach described by Aksenov et al. (Reference Aksenov, Charkin, Banaru, Banaru, Volkov, Deineko, Kuznetsov, Rastsvetaeva, Chukanov, Shkurskii and Yamnova2023).

Discussion

The hydroxylbenyacarite average crystal structure in Pbca is built from an alternation of two types of (001) slabs, centred at z = ¼, ¾ and 0, ½. These are shown in Figs 5 and 6, respectively. The heteropolyhedral layers at z = ¼, ¾ contain [100] kröhnkite-type chains (Hawthorne, Reference Hawthorne1985) of four-member rings of corner-connected PO₄ tetrahedra and M2O₄X(H₂O) octahedra. Each PO₄ tetrahedron also shares a corner with M1O₂(H₂O)₄ octahedra along [010] to complete the 2D network of polyhedra. The corner-shared linkages form eight-member rings of alternating octahedra and tetrahedra. The (001) slabs at z = 0, ½ comprise isolated M3O₄X ₂ octahedra together with Ca, the A site constituents (K, H₂O) and the zeolitic-type H₂O groups at sites O14 and O15. The Ca site has not been previously reported in crystal structure studies of other paulkerrite-group members. As seen in Fig. 6, one of the Ca bonds is to H₂O at the A site (labelled as K). The distance Ca–A is only 2.62 Å, so occupations of the Ca site and of K at the A site are mutually exclusive. Locally, when Ca is present, the A site is occupied by H₂O. The Ca-centred polyhedron has 12 triangular faces in the form of a snub disphenoid deltahedron.

Figure 5. (001) section of the hydroxylbenyacarite structure at z = ¼. Drawn using ATOMS (Dowty, Reference Dowty2004).

Figure 6. (001) section of the hydroxylbenyacarite structure at z = 0, showing in-section bonds to Ca and K. Drawn using ATOMS (Dowty, Reference Dowty2004).

The M3O₄X ₂ octahedra share their trans-X vertices with M2O₄X(H₂O) octahedra in the layers above and below, giving linear trimers that correspond to short segments of the 7 Å chains that are common to many phosphate minerals (Moore, Reference Moore1970). The M2–M3–M2 trimers are illustrated in Fig. 7. A feature of the trimers is a short M2–X distance (1.81 Å), due to displacement of the M2-site atoms from the centres of the octahedra towards the bridging X-site anions with the M3-centred octahedra. This type of displacement is a common feature of chain structures containing Ti (Bamberger et al., Reference Bamberger, Begun and MacDougall1990) and is the origin of the strong Raman bands in the region 770–850 cm^–1 (Fig. 2). The corner-shared connectivity between the kröhnkite-type chains and the M3O₄X ₂ octahedra (Fig. 7) generates 10-member rings, elongated along [100]. The water molecules at O14 and O15 and the A-site constituents, K and H₂O, are located at interstitial sites within the ring whereas the Ca site is at the periphery.

Figure 7. (010) section through the structure of hydroxylbenyacarite at y = 0. Drawn using ATOMS (Dowty, Reference Dowty2004).

Based on the presence of weak diffuse diffraction effects such as shown in Fig. 4, hydroxylbenyacarite has monoclinic symmetry, space group P2₁/c, with ordering of the interstitial K⁺ and Ca²⁺ cations, though the ordering is restricted to very small regions on the scale of the unit cell. Locally, hydroxylbenyacarite has the same symmetry as the monoclinic paulkerrite-group minerals rewitzerite (Grey et al., Reference Grey, Hochleitner, Kampf, Boer, MacRae, Mumme and Keck2023b), paulkerrite (Grey et al., Reference Grey, Boer, MacRae, Wilson, Mumme and Bosi2023a) and macraeite (Bosi et al., Reference Bosi, Hatert, Pasero and Mills2023). The other group members are the orthorhombic minerals benyacarite (Demartin et al., Reference Demartin, Pilati, Gay and Gramaccioli1993, Reference Demartin, Gay, Gramaccioli and Pilati1997) and mantienneite (Fransolet et al., Reference Fransolet, Oustriere, Fontan and Pillard1984), and the minerals pleysteinite (Grey et al., Reference Grey, Hochleitner, Rewitzer, Kampf, MacRae, Gable, Mumme, Keck and Davidson2023c) and hochleitnerite (Grey et al., Reference Grey, Keck, Kampf, MacRae, Gable, Mumme, Glenn and Davidson2023d). The latter two minerals were reported as orthorhombic, Pbca, isostructural with benyacarite, based on laboratory single-crystal diffraction data. However the diffraction patterns of these minerals are affected by sectoral twinning, and new (Rewitzer et al., Reference Rewitzer, Hochleitner, Grey, Macrae, Mumme, Boer, Kampf and Gable2024) data collections using a synchrotron microfocus source to obtain data from a single sector, have shown that both pleysteinite and hochleitnerite are monoclinic, P2₁/c, isostructural with paulkerrite.

The general formulae are A ₂M1₂M2₂M3(PO₄)₄X ₂(H₂O)₁₀⋅4H₂O for orthorhombic members and A1A2M1₂M2₂M3(PO₄)₄X ₂(H₂O)₁₀⋅4H₂O for monoclinic members, where A = K, H₂O and □ (= vacancy); M1 = Mn²⁺, Mg, Fe²⁺, Zn and Ca (rarely Fe³⁺); M2 and M3 = Fe³⁺, Al and Ti⁴⁺; and X = O, OH and F. In monoclinic species, K and H₂O show an ordering at the A1 and A2 sites. The end-member formulae for the members, and their unit-cell parameters are reported in Table 6. The formulae are based on the merged (M2₂M3) site compositions approach that has been approved by the IMA-CNMNC, nomenclature proposal revised 22_K-bis (Grey et al., Reference Grey, Boer, MacRae, Wilson, Mumme and Bosi2023a). As seen from Table 6, hydroxylbenyacarite is the hydroxyl analogue of benyacarite. It is also closely related to hochleitnerite, having the same dominant constituents at M1 (Mn) and at the merged (M2₂M3) site (Ti₂Fe) but different dominant constituents at the A and X sites.

Table 6. Paulkerrite-group members*.

*Formulae are based on the merged (M2₂M3) site compositions approach as approved by the IMA–CNMNC, nomenclature proposal revised 22-K-bis.

**Monoclinic unit cells for pleysteinite and hochleitnerite obtained from synchrotron microfocus beam diffraction studies (Rewitzer et al., Reference Rewitzer, Hochleitner, Grey, Macrae, Mumme, Boer, Kampf and Gable2024).

References: [1] This work; [1av] average local structure; [2] Demartin et al. (Reference Demartin, Pilati, Gay and Gramaccioli1993); [3] Fransolet et al. (Reference Fransolet, Oustriere, Fontan and Pillard1984); [4] Grey et al. (Reference Grey, Hochleitner, Kampf, Boer, MacRae, Mumme and Keck2023b); [5] Grey et al. (Reference Grey, Boer, MacRae, Wilson, Mumme and Bosi2023a); [6] Grey et al. (Reference Grey, Keck, Kampf, MacRae, Gable, Mumme, Glenn and Davidson2023d); [7] Grey et al. (Reference Grey, Hochleitner, Rewitzer, Kampf, MacRae, Gable, Mumme, Keck and Davidson2023c); [8] Bosi et al. (Reference Bosi, Hatert, Pasero and Mills2023)]