Occurrence and general appearance

Natromelansonite was found at the Poudrette quarry (Demix quarry), Mont Saint-Hilaire, Quebec, Canada in a highly altered pegmatite in nepheline syenite (Horvath et al., Reference Horvath, Gault, Pfenninger-Horvath and Poirier2019, and references therein). It occurs on rhodochrosite together with polylithionite and beige aggregates of poorly diffracting clay mineral and steacyite microcrystals. The specimen was collected by Gilles Haineault in 2004.

Natromelansonite occurs as an outer zone of tabular crystals up to 0.1 × 0.3 × 1 mm in size (Fig. 1) flattened on (001) where the inner zone is melansonite. The boundary between the two species is sharp and marked by a thin zone of uranium-bearing melansonite (Fig. 2). The observed crystal forms are dominant pinacoid {001} with two other pinacoids {001} and {100}.

Figure 1.

Grey tabular prismatic crystals composed of natromelansonite and melansonite on a beige aggregate of a poorly diffracting clay mineral and steacyite microcrystals. Specimen CMNMC 90813. Photo: François Génier.

Figure 2.

Back-scattered electron image of a crystal composed of melansonite and natromelansonite zones. Bright zone – U-bearing melansonite. Polished section, specimen CMNMC 90813.

Physical and optical properties

Natromelansonite is grey with a white powder colour and vitreous lustre (Fig. 1). The cleavage is parallel to {010}, perfect. The fracture is stepped. The Mohs hardness is 3.5. The mineral has green fluorescence under short-wave ultraviolet light. The density calculated using the empirical formula and unit-cell volume refined from the single-crystal X-ray diffraction (XRD) data is 2.31 g/cm³.

Natromelansonite is optically biaxial (+), α = 1.510(2), β = 1.516(2), γ = 1.524(3), 2V(meas.) = 86(3)° (from a spindle-stage extinction curve) and 2V(calc.) = 82°.

Experimental methods

Chemical data were obtained using a JEOL 8230 SuperProbe electron microscope equipped with five WDS spectrometers (University of Ottawa – Canadian Museum of Nature MicroAnalysis Laboratory, Canada) with an accelerating voltage of 15 kV and a beam current of 10 nA. Both natromelansonite and melansonite are unstable under an electron beam, so a larger beam diameter of 20 μm was used to minimise the damage. The matrix correction method was ZAF or Phi-Rho-Z calculations (Armstrong, Reference Armstrong and Newbury1988). The following reference materials were used: albite (NaKα), sanidine (KKα, AlKα and SiKα), diopside (CaKα), tephroite (MnKα), sanbornite (BaLα), YIG (YLα), YbPO₄ (YbLα), zircon (ZrLα) and UO₂ (UMα). Both the peak and the background count times were 20 s for all elements. The intensity data were corrected for Time Dependent Intensity loss (or gain) using a self-calibrated correction for KKα, SiKα and NaKα. The time/counts analysis showed significant Na migration in natromelansonite (Fig. 3).

Figure 3.

Time dependent intensity loss for NaKα in natromelansonite (8 analyses).

The Fourier-transform infrared (FTIR) spectrum of natromelansonite was obtained at the Canadian Conservation Institute, Canada using a Bruker Hyperion 2000 microscope interfaced to a Tensor 27 spectrometer with a wide-band mercury cadmium telluride (MCT) detector. A small grain of natromelansonite was mounted and compressed on a low-pressure diamond anvil microsample cell and analysed in transmission mode. The spectrum was collected between 400–4000 cm^–1 by averaging 256 scans at a 4 cm^–1 operating resolution.

Powder X-ray diffraction (PXRD) data were collected at the Canadian Museum of Nature, Canada using a Bruker D8 Discover microdiffractometer equipped with a DECTRIS EIGER2 R 500K detector and IμS microfocus X-ray source (CuKα₁, λ = 1.54060 Å) with the Kα₂ contribution removed using the ‘Strip Kα2’ tool in Bruker Diffrac.EVA V4.3. The instrument was calibrated using a statistical calibration method (Rowe, Reference Rowe2009). A powder ball ~200 μm in diameter, mounted on a fibre pin mount, was analysed with continuous Phi rotation and 10° rocking motion along the Psi axis of the Centric Eulerian Cradle stage.

Single-crystal XRD studies were carried out at the Natural History Museum, University of Oslo, Norway on a Rigaku XtaLAB Synergy-S diffractometer equipped with a HyPix 6000HE detector (MoKα, λ = 0.71073 Å) operating at 50 kV and 1 mA at room temperature. The data were collected and processed using CrysAlis Pro software (Rigaku Oxford Diffraction, UK).

Description and discussion of the crystal structure

Natromelansonite is pseudo-orthorhombic. Its monoclinic unit cell (P2₁/m) is very close to the orthorhombic cell (Pmma) of melansonite (Gore and McDonald, Reference Gore and McDonald2023) and is not doubled as was described in monteregianite-(Y) (Ghose et al., Reference Ghose, Sen Gupta and Campana1987; Table 6). Group P2₁/m is a translationengleiche subgroup of Pmma and such pseudosymmetry in monteregianite-related minerals was predicted by Ghose et al. (Reference Ghose, Sen Gupta and Campana1987).

Table 6.

Comparative data for natromelansonite and related minerals.

The natromelansonite crystal structure consists of two different types of sheets normal to the b axis: (a) a double sheet of tetrahedra (T) (Fig. 5) and (b) a sheet of octahedra (O) (Fig. 6). The T and O sheets alternate along the [010] direction forming a TOT topology (Fig. 7) typical for members of the rhodesite mero-plesiotype series (Ferraris and Gula, Reference Ferraris and Gula2005; Cadoni and Ferraris, Reference Cadoni, Ferraris and Krivovichev2011), including melansonite, KNa₂Zr[Si₇AlO₁₉]⋅4–5H₂O (Gore and McDonald, Reference Gore and McDonald2023), monteregianite-(Y), KNa₂Y[Si₈O₁₉]⋅5H₂O (Chao, Reference Chao1978; Ghose et al., Reference Ghose, Sen Gupta and Campana1987), delhayelite, K₄Na₂Ca₂[AlSi₇O₁₉]₂F₂Cl (Sahama and Hytönen, Reference Sahama and Hytönen1959; Pekov et al., Reference Pekov, Zubkova, Chukanov, Sharygin and Pushcharovsky2009), hydrodelhayelite KCa₂[AlSi₇O₁₇(OH)₂]⋅3H₂O (Dorfman and Chigarov, Reference Dorfman and Chigarov1979; Pekov et al., Reference Pekov, Zubkova, Chukanov, Sharygin and Pushcharovsky2009), and fivegite, K₄Ca₂[AlSi₇O₁₇(O_2–xOH_x)][(H₂O)_2–xOH_x]Cl (Pekov et al., Reference Pekov, Zubkova, Chukanov, Zadov and Pushcharovsky2011). Members of the series are characterised by nearly identical T sheets which may show a different ratio between upwards- and downwards-pointing tetrahedra which is related to the number, charge and coordination number of the cations in the O sheet and to the Si/Al ratio within the silicate sheet. Most of the compounds have a T:O ratio of 8:19 (T = Si and Al). Monoclinic and orthorhombic members are known.

Figure 5.

(a) Double sheet of tetrahedra (T) in the structure of natromelansonite; (b) the single sheet consisting of four- and eight-membered rings. The structure was visualised using CrystalMaker® software.

Figure 6.

Sheet of octahedra (O) in the structure of natromelansonite.

Figure 7.

General view of the crystal structure of natromelansonite. Projection on (100). The unit cell is outlined. T and O sheets are indicated. Yellow and purple spheres are extra-framework Na and K atoms, respectively.

The O sheet in natromelansonite (Fig. 6) consists of alternating edge-sharing fully occupied Zr(1)O₆ and Na(1)O₄(H₂O)₂ octahedra and a highly distorted partially occupied (40%) Na(2)O₄(H₂O)₂ octahedron.

The T sheet is composed of two single silicate sheets of the apophyllite type with four- and eight-membered rings (Fig. 5a). They are formed by Si1-, Si2- and Si3-centred tetrahedra with the <Si–O> distance in the range of 1.608–1.610 Å, and a larger Si4-centred tetrahedron with the <Si4–O> distance of 1.664 Å, with a mixed Si/Al occupancy Si_0.5Al_0.5. The BVS at the Si4 site is 3.65. valence units (vu), which confirms mixed occupancy at the site. The T sheet can be also described using the structure hierarchy concept (Hawthorne et al., Reference Hawthorne, Sokolova and Uvarova2019) as based on the 3-connected plane net 4.82 plus an oikodoméic (up/down) operation (u³d)₁(u⁴du²d)₁ with all d tetrahedra occupied by Al atoms.

The two single sheets are linked via two vertex-sharing Si4-centred tetrahedra (Si4–O10–Si4, Fig. 5b), which resulted in the creation of large channels. Interstitial Na and K atoms occur in the channels in a split site with occupancies of 0.54, 0.21 and 0.25 for the Na3, Na4 and K5 sites, respectively. The Na3–Na4, Na3–K5 and Na4–K5 distances are 2.28 Å, 1.60 Å and 0.84 Å, respectively. Sodium is the dominant extra-framework cation.

The presence of H₂O⁰ molecules is confirmed by the BVS at the O11, O12, O13 and O14 sites (0.10, 0.04, 0.32 and 0.01 vu, respectively; Table 4) and by the presence of O–H stretching bands and H–O–H bending bands in the IR spectrum of natromelansonite (Fig. 4). Some of these sites (O12, O14) are partially occupied, as it was observed in the crystal structures of monteregianite-(Y) (Ghose et al., Reference Ghose, Sen Gupta and Campana1987) and melansonite (Gore and McDonald, Reference Gore and McDonald2023). In general, water content varies significantly in members of the rhodesite mero-plesiotype series (Ferraris and Gula, Reference Ferraris and Gula2005; Cadoni and Ferraris, Reference Cadoni, Ferraris and Krivovichev2011). In addition, the variable partial occupancy of multiple sites means that the water content varies from grain to grain in one mineral. Furthermore, the anisotropic thermal vibrations of ligand water molecules are very pronounced which complicates proper refinement of these sites in the structure. Thus, the H content is estimated to be 8 pfu in the ideal end-member formula for simplicity, and that number is used in the formula calculations. But the actual water content seems to be variable in the range 4–5 H₂O pfu in both melansonite and natromelansonite.

The resulting structural formula of natromelansonite is ^Na1Na^Na2(□_0.60Na_0.40)₂ ^Na3+Na4+K5(Na_0.75K_0.25)Zr(Si₇Al)O_18.8OH_0.2⋅4.46H₂O.

The idealised formulae of melansonite and natromelansonite

The idealised formula of melansonite was given as (Na,□)□₂KZrSi₈O₁₉⋅5H₂O in the original publication (Gore and McDonald, Reference Gore and McDonald2023). However, detailed examination of the published data combined with our data shows that the formula should be reconsidered by taking into account the behaviour of Al and Na in melansonite. One Si-centred tetrahedron (Si(2)O₄) in the structure of melansonite is significantly larger than the others with the <Si–O> distance of 1.66 Å, indicating that Al atoms are ordered at this site as they are in natromelansonite, and the idealised occupancy at the site should be Si_0.5Al_0.5. The published chemical data showing ~1.0 atoms per formula unit (apfu) of Al (6.32 wt.% Al₂O₃) corroborate this. The Si2 site in melansonite is analogous to the Si4 site in the structure of natromelansonite where Si(2)O₄ tetrahedra link single silicate sheets together. Similar ordering was observed in delhayelite, hydrodelhayelite (Pekov et al., Reference Pekov, Zubkova, Chukanov, Sharygin and Pushcharovsky2009) and fivegite (Pekov et al., Reference Pekov, Zubkova, Chukanov, Zadov and Pushcharovsky2011) – three other members of the rhodesite mero-plesiotype series (Cadoni and Ferraris, Reference Cadoni, Ferraris and Krivovichev2011).

The Na content (0.71 apfu, 2.82 wt.% Na₂O) in melansonite from the original find from a marble xenolith at Mont Saint-Hilaire seems to be unexpectedly low which could be explained by migration of Na under the electron bean as was suggested by Gore and McDonald (Reference Gore and McDonald2023) and the very high count time of 100 seconds. As we have shown, Na migration in melansonite and natromelansonite is significant even with a 20 μm beam (Fig. 3). Additionally, the crystals are very thin, only ~2 μm, which further complicated obtaining realisable data on Na content in the mineral. Melansonite from our find is characterised by a significantly higher Na content (8.20 wt.% Na₂O, Table 1) both due to (1) a larger crystal size which allowed us to reduce Na migration by using a larger beam diameter (20 μm) compared to a 10 μm beam used by Gore and McDonald (Reference Gore and McDonald2023) and (2) lower contents of admixed cations Ca (0.92 wt.% CaO) and Ba (0.59 wt.% BaO) in contrast to 1.75 wt.% CaO and 1.35 wt.% BaO in melansonite from the original find (Gore and McDonald, Reference Gore and McDonald2023).

Furthermore, the occupancy at the Na site in the O sheet of melansonite was fixed at 28% and the refined site-scattering value at the site is unknown (Gore and McDonald, Reference Gore and McDonald2023). The full dataset including the cif was not provided by the authors of the description so the reasoning behind this value is unclear, but it is probable that it could be somewhat higher as it was observed in both natromelansonite and monteregianite-(Y) (Ghose et al., Reference Ghose, Sen Gupta and Campana1987).

Considering all the above, (□Na) instead of □₂ in the formula would much better represent the actual distribution of the cations at the Na2 site. Furthermore, the water content in both minerals is variable in the range 4–5 H₂O pfu. Thus, the melansonite formula should be written as Na(□Na)KZr(Si₇Al)O₁₉⋅4–5H₂O or, in a simplified form, KNa₂Zr(Si₇Al)O₁₉⋅4–5H₂O and the formula of its Na-analogue natromelansonite as Na(□Na)NaZr(Si₇Al)O₁₉⋅4–5H₂O or Na₃Zr(Si₇Al)O₁₉⋅4–5H₂O.