Ebnerite and epiebnerite: NH₄ZnPO₄ dimorphs with zeolite-type frameworks from the Rowley mine, Arizona, USA

Abstract

Ebnerite and epiebnerite, both with the ideal formula NH₄ZnPO₄, are new mineral species from the Rowley mine, Maricopa County, Arizona, USA. They occur in an unusual bat-guano-related, post-mining assemblage of phases. Epiebnerite grows epitactically on ebnerite and replaces it. Ebnerite and epiebnerite are found in intimate association with alunite, halite, mimetite, newberyite, sampleite, struvite and wulfenite on hematite-rich quartz–baryte matrix. Crystals of ebnerite are colourless narrow prisms up to ~0.3 mm in length. The streak is white, lustre is vitreous, Mohs hardness is ~2, tenacity is brittle and fracture is splintery. The density is 2.78(2) g⋅cm^–3. Ebnerite is optically uniaxial (–) with ω = 1.585(2) and ɛ = 1.575(2). Epiebnerite occurs as colourless prisms or blades, up to about 10 × 3 × 2 μm, in parallel growth forming ribs with serrated edges epitactic on ebnerite prisms. The streak is white, lustre is vitreous, Mohs hardness is probably ~2, tenacity is brittle. The calculated density is 2.851 g⋅cm^–3. Epiebnerite is optically biaxial with all indices of refraction near 1.580. Electron microprobe analysis gave the empirical formula [(NH₄)_0.89K_0.06]_Σ0.95(Zn_0.96Cu_0.07)_Σ1.03[(P_0.97Si_0.03)_Σ1.00O₄] for ebnerite and [(NH₄)_0.67K_0.28]_Σ0.95(Zn_0.99Cu_0.02)_Σ1.02(P_1.00O₄) for epiebnerite. Ebnerite is hexagonal, P6₃, with a = 10.67051(16), c = 8.7140(2) Å, V = 859.25(3) ų and Z = 8. Epiebnerite is monoclinic, P2₁, with a = 8.796(16), b = 5.457(16), c = 8.960(16) Å, β = 90.34(6)°, V = 430.1(17) ų and Z = 4. The structures of ebnerite (R₁ = 0.0372 for 1168 I_o > 2σI reflections) and epiebnerite (known from synthetic monoclinic NH₄ZnPO₄) are zeolite-like frameworks based upon corner-sharing linkages between alternating ZnO₄ and PO₄ tetrahedra with channels in the frameworks hosting the NH₄ groups. The two structures are topologically distinct. Ebnerite belongs to the family of ‘stuffed derivatives’ of tridymite, whereas epiebnerite possesses an ABW-type zeolite structure.

Introduction

The NH₄ZnPO₄ dimorphs ebnerite and epiebnerite are new mineral species with zeolite-like framework structures found in an unusual bat guano assemblage in the Rowley mine in southwestern Arizona, USA. The ebnerite structure belongs to the family of ‘stuffed derivatives’ of tridymite (Buerger, 1954). Epiebnerite has an ABW-type zeolite structure (Bu et al., 1997; Kahlenberg et al., 2001).

The name ebnerite honours John Ebner of Tucson, Arizona, USA (b. 1931). Mr. Ebner has been an avid mineral collector for more than 65 years and has focused on micro-minerals for the last 41 years. He has developed one of the most significant micromount collections in the world, now numbering well over 50,000, and has also dedicated himself to documenting and preserving the knowledge and history of micromounting. In 1997, he was inducted into the Micromounts Hall of Fame. Mr. Ebner has contributed immensely to the amateur mineral community, writing articles, presenting programs, organising symposia and serving as an officer in many clubs. He has also contributed significantly to mineralogical science in a variety of ways, such as providing specimens for study and volunteering his services. Mr. Ebner has been a volunteer in the Department of Geosciences and the mineral museum at the University of Arizona since 2016. Mr. Ebner has agreed to the naming of the mineral in his honour. The name epiebnerite is a combination of the prefix ‘epi’ for ‘near’ in Greek and the mineral name ‘ebnerite’ because the mineral is the dimorph of ebnerite. ‘Epi’ also alludes to the fact that the mineral has only been found growing epitaxially on ebnerite or forming epimorphs after ebnerite.

The new minerals and their names were approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association [ebnerite (Ebr): IMA2022-123, Kampf et al., 2023; and epiebnerite (Epb): IMA 2023-066, Kampf et al., 2024c]. Two cotype specimens of ebnerite are deposited in the collections of the Natural History Museum of Los Angeles County, Los Angeles, California, USA, catalogue numbers 76275 and 76276. One cotype specimen of ebnerite is deposited at the University of Arizona Alfie Norville Gem and Mineral Museum, catalogue number 22729 and one cotype specimen is deposited at the RRUFF Project (https://rruff.info/), deposition number R220032. Four cotype specimens of epiebnerite are deposited in the collections of the Natural History Museum of Los Angeles County, Los Angeles, California, USA, catalogue numbers 76294, 76295, 76296 and 76297.

Occurrence

Ebnerite and epiebnerite were collected by one of the authors (JM) on the 125-foot level of the Rowley mine, ~20 km NW of Theba (small settlement and railroad depot), Maricopa County, Arizona, USA (33°2'57''N 113°1'49.59''W). The Rowley mine is on the western slope of the Painted Rock Mountains (in the Painted Rock mining district) and overlooks the Dendora Valley, immediately to the west. It is a former Cu–Pb–Au–Ag–Mo–V–baryte–fluorspar mine that exploited veins presumed to be related to the intrusion of an andesite porphyry dyke into Tertiary volcanic rocks. Although the mine has not been operated for ore since 1923, collectors took notice of the mine as a source of fine wulfenite crystals around 1945. An up-to-date account of the history, geology and mineralogy of the mine has been published recently by Wilson (2020).

The new minerals were found in a hot and humid area of the mine (see fig. 26 in Wilson, 2020) in an unusual bat guano-related, post-mining assemblage of phases that include a variety of vanadates, phosphates, oxalates and chlorides, some containing (NH₄)⁺. This secondary mineral assemblage is found growing on baryte–quartz-rich matrix and, besides ebnerite and epiebnerite, includes allantoin (Kampf et al., 2021a), alunite, ammineite, antipinite, aphthitalite, bassanite, biphosphammite, carboferriphoxite (Kampf et al., 2024a), cerussite, davidbrownite-(NH₄) (Kampf et al., 2019a), dendoraite-(NH₄) (Kampf et al., 2022a), edwindavisite (Yang et al., 2023a), ferriphoxite (Kampf et al., 2024b), fluorite, halite, hydroglauberite, mimetite, mottramite, natrosulfatourea (Kampf et al., 2021a), newberyite, perite, phoxite (Kampf et al., 2019b), relianceite-(K) (Kampf et al., 2022b), rowleyite (Kampf et al., 2017), salammoniac, sampleite, struvite, thebaite-(NH₄) (Kampf et al., 2021b), thenardite, urea, vanadinite, weddellite, willemite, wulfenite and several other potentially new minerals. Ebnerite and epiebnerite are both rare and have been found on only a handful of specimens. Ebnerite has been found without epiebnerite; however, epiebnerite has only been found growing on and/or replacing ebnerite. Because of the apparently consistent orientation of epiebnerite crystals on ebnerite, we interpret them to be epitactic overgrowths and/or epimorphs. Both minerals are found in intimate association with alunite, halite, mimetite, newberyite, sampleite, struvite and wulfenite on hematite-rich quartz–baryte matrix. It is worth noting that groups of ebnerite crystals without epiebnerite overgrowths have been found immediately adjacent to ebnerite with epiebnerite overgrowths.

Physical and optical properties

Ebnerite

Crystals of ebnerite are colourless narrow prisms (often tapering), up to ~0.3 mm in length, typically forming fan- and bowtie-like sprays (Fig. 1). Although terminations appear irregular under moderate magnification, under high magnification, they are observed to consist of a hexagonal pyramid and/or a basal pedion. The Bravais–Friedel–Donnay–Harker principle (Donnay and Harker, 1937) predicts the {100} hexagonal prism and {101} hexagonal pyramid as the prominent forms, which is consistent with scanning electron microscopy (SEM) observations of ebnerite crystals. The {001} basal pedion is also clearly present based on SEM observations (Figs 2 and 3). The non-centrosymmetric space group suggests hemimorphic morphology; however, doubly terminated crystals have not been observed and the Bravais–Friedel–Donnay–Harker principle indicates positive and negative forms to be equally probable. Therefore, to the above forms can be added {10$\bar{1}$} and {00$\bar{1}$}.

Figure 1. Sprays of ebnerite crystal with blue balls of sampleite. (Specimen #76275; Field of view 1.1 mm across).

Figure 2. Scanning electron microscopy image of the upper portion of a group of ebnerite crystals.

Figure 3. Crystal drawing of ebnerite; clinographic projection in standard orientation. The drawing was created using SHAPE, version 7.4 (Shape Software, Kingsport, Tennessee, USA).

Ebnerite has a white streak, vitreous lustre, brittle tenacity and splintery fracture. The Mohs hardness is ~2 based on scratch tests. Although no cleavage direction could be determined, the splintery fracture suggests at least one good cleavage in the [001] zone – possibly {100}. The mineral does not fluoresce in either long- or short-wave ultraviolet illumination. The density measured by flotation in a mixture of methylene–iodide and toluene is 2.78(2) g⋅cm^–3. At room temperature, ebnerite is insoluble in H₂O, but easily soluble in dilute HCl.

Ebnerite is optically uniaxial (–) with ω = 1.585(2) and ɛ = 1.575(2) and it is nonpleochroic. The Gladstone–Dale compatibility (Mandarino, 2007), 1 – (K _p/K _c), is 0.010 in the range of superior compatibility for the empirical formula.

Epiebnerite

Epiebnerite occurs as colourless prisms or blades, up to about 10 × 3 × 2 μm, in parallel growth forming ribs with serrated edges epitactic on ebnerite prisms or forming epimorphs after ebnerite (Fig. 4). No forms could be measured because of the minute size of crystals. The Bravais–Friedel–Donnay–Harker principle (Donnay and Harker, 1937) predicts that prisms are likely to be elongated on [010] with the prism the forms {100} and {001} and terminations consisting of the forms {110} and {011}. Synthetic crystals are reported to exhibit both racemic and rotational twinning (Bu et al., 1997). Note that we have been unable to define crystallographically the apparent epitactic relation between epiebnerite and ebnerite.

Figure 4. Epiebnerite prisms in parallel growth forming serrated ribs growing epitaxially on ebnerite needles. The blue balls are sampleite and the green–yellow crystal with growth hillocks in the background is wulfenite. (Specimen #76294; FOV 0.5 mm across).

The streak is white, the lustre is vitreous and the tenacity is brittle. The Mohs hardness is probably ~2, but crystals are too small to test. The density could not be measured because crystals are too small to see in density liquids. The calculated density is 2.851 g⋅cm^–3. At room temperature, epiebnerite is insoluble in H₂O, but easily soluble in dilute HCl.

Complete optical determinations were impossible because of the minute size of the crystals; however, the average index of refraction is very close to 1.580 because crystals are virtually invisible in all orientations in the 1.580 immersion liquid. The Gladstone–Dale compatibility (Mandarino, 2007), 1 – (K _p/K _c), is 0.012 in the range of superior compatibility for the empirical formula using n _av = 1.580.

Raman spectroscopy

Raman spectroscopy was conducted on a Horiba XploRA PLUS spectrometer using a 532 nm diode laser, 1800 gr/mm diffraction grating and a 100× (0.9 NA) objective. A 100 μm slit was used for ebnerite and a 200 μm slit was used for epiebnerite. Because crystals of epiebnerite are very sensitive to the laser, the spectrum was recorded at 2 mW power. Consequently, the epiebnerite spectrum exhibits significant noise, which makes it difficult to assign precise wavenumbers to the weaker bands. The spectra from 3700 to 60 cm^–1 are shown in Fig. 5 including labelled mode assignments based on several references: Frost et al. (2011), Sergeeva et al. (2019), Števko et al. (2018) and Yakovenchuk et al. (2018).

Figure 5. Raman spectra of ebnerite (red) and epiebnerite (blue).

Chemical analysis

Ebnerite analyses (4 points) were done on a Shimadzu EPMA-1720 electron microprobe in wavelength dispersive spectroscopy (WDS) mode. Analytical conditions were 15 kV accelerating voltage, 10 nA beam current and 2 μm beam diameter. Analytical data for ebnerite are given in Table 1. Epiebnerite analyses (4 points) were performed on a JEOL JXA-iHP200F electron microprobe in WDS mode. Analytical conditions were 15 kV accelerating voltage, 10 nA beam current and 10 μm beam diameter. It was impossible to polish epiebnerite crystals, so analyses were done on unpolished crystal faces. Because of the uneven surfaces of epiebnerite crystals and their thinness, analytical values were significantly low, requiring normalisation. The results are given in Table 2.

Table 1. Analytical data (wt.%) for ebnerite.

S.D. – standard deviation

Table 2. Analytical data (wt.%) for epiebnerite.

S.D. – standard deviation; Norm. – normalised

The empirical formulas for ebnerite and epiebnerite (based on O = 4 atoms per formula unit) are [(NH₄)_0.89K_0.06]_Σ0.95(Zn_0.96Cu_0.07)_Σ1.03[(P_0.97Si_0.03)_Σ1.00O₄] and [(NH₄)_0.67K_0.28]_Σ0.95(Zn_0.99Cu_0.02)_Σ1.02(P_1.00O₄), respectively. The ideal formula for both is NH₄ZnPO₄, which requires (NH₄)₂O 14.60, ZnO 45.62, P₂O₅ 39.78, total 100 wt.%.

X-ray crystallography

Powder X-ray studies were done 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. The powder X-ray diffraction (PXRD) pattern for ebnerite closely matched that calculated from the crystal structure of synthetic hexagonal (HEX-type) NH₄ZnPO₄ (Harrison et al., 2001) and the PXRD pattern for epiebnerite closely matched that calculated from the crystal structure of synthetic monoclinic (ABW-type) NH₄ZnPO₄ (Bu et al., 1997). Observed d-values and intensities were derived by profile fitting using JADE Pro software (Materials Data, Inc.). The powder data for ebnerite and epiebnerite are presented in Supplementary Tables 1 and 2, respectively. Unit-cell parameters refined from the powder data using JADE Pro with whole pattern fitting are a = 10.6690(3), c = 8.727(3) Å, V = 863.7(5) ų and Z = 8 for ebnerite (space group P6₃) and a = 8.796(16), b = 5.457(16), c = 8.960(16) Å, β = 90.34(6)°, V = 430.1(17) ų and Z = 4 for epiebnerite (space group P2₁).

A single-crystal study of epiebnerite was not attempted because of the small size of crystals and their occurrence in subparallel intergrowths. Figure 6 shows the close match between the epiebnerite PXRD and that calculated from the structure of synthetic monoclinic (ABW-type) NH₄ZnPO₄ (Bu et al., 1997) following the whole-pattern-fitting cell refinement.

Figure 6. Recorded PXRD of epiebnerite compared with the calculated lines and simulated pattern based on the structure of the synthetic ABW-type NH₄ZnPO₄ (Bu et al., 1997).

Single-crystal X-ray studies for ebnerite were done using a Rigaku Xtalab Synergy D/S 4-circle diffractometer equipped with CuKα radiation. The structure was solved using SHELXT (Sheldrick, 2015a). Refinement proceeded by full-matrix least-squares on F ² using SHELXL-2016 (Sheldrick, 2015b). The two N sites were refined with joint occupancies by N and K. The occupancies of the two Zn and two P sites were refined, but joint occupancies were not employed because of the close correspondence in scattering power of Zn and Cu and of P and Si. The less-than-full refined occupancies obtained for the Zn and P sites are consistent with some substitution of Cu for Zn and Si for P, as indicated by the electron microprobe analysis (EPMA). All H atoms associated with the NH₄ groups were located through difference-Fourier syntheses. The H sites were refined with soft restraints of 0.90(3) Å on the N–H distances and 1.45(3) Å on the H–H distances and with the U _eq of each H set to 1.2 times that of the associated N atom; the occupancies of the H sites were tied to those of the N sites. Data collection and refinement details are given in Table 3, atom coordinates and displacement parameters in Table 4, selected bond distances and angles in Table 5 and a bond-valence analysis in Table 6. Note that, even though the location of H sites allowed the determination of the hydrogen bonding scheme thereby helping to understand how the NH₄ groups are bound in the channels, bond-valence calculations are more straightforward if the NH₄ groups are treated as spherical cations (see García-Rodriguez et al., 2000). The crystallographic information file for ebnerite has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Table 3. Data collection and structure refinement details for ebnerite.

R _int = Σ|F _o²–F _o²(mean)|/Σ[F _o²]. GoF = S = {Σ[w(F _o²–F _c²)²]/(n–p)}^1/2. R ₁ = Σ||F _o|–|F _c||/Σ|F _o|. wR ₂ = {Σ[w(F _o²–F _c²)²]/Σ[w(F _o²)²]}^1/2; w = 1/[σ²(F _o²) + (aP)² + bP] where a is 0.0789, b is 0 and P is [2F _c² + Max(F _o²,0)]/3.

Table 4. Atom positions, site occupancies and displacement parameters (Å)² for ebnerite.

Table 5. Selected bond lengths (Å) and angles (°) for ebnerite.

Table 6. Bond-valences analysis for ebnerite. Values are in valence units (vu).*

* Bond-valence parameters for NH₄⁺–O are from García-Rodriguez et al. (2000); all others are from Gagné and Hawthorne (2015). The cation sites were modelled using full occupancies by their dominant cations.

Discussion of the structures

Both ebnerite and epiebnerite have zeolite-like framework structures based upon corner-sharing linkages between alternating ZnO₄ and PO₄ tetrahedra, with channels in the frameworks hosting the NH₄ groups. Their structures are similar to those of aluminosilicate zeolites, although neither mineral is isostructural with any known mineral. As noted above, ebnerite is the natural counterpart of synthetic hexagonal NH₄ZnPO₄ (Harrison et al., 2001) and epiebnerite is the natural counterpart of synthetic monoclinic NH₄ZnPO₄ (Bu et al., 1997). Harrison et al. (2001) referred to hexagonal NH₄ZnPO₄ as NH₄ZnPO₄–HEX and monoclinic NH₄ZnPO₄ as NH₄ZnPO₄–ABW. The ABW suffix denotes the fact that the monoclinic phase has an ABW-type zeolite structure. The two structures are topologically distinct; the channels in the HEX (ebnerite) structure are defined by six-member rings of tetrahedra, whereas the channels in the ABW (epiebnerite) structure are defined by eight-member rings of tetrahedra (see Fig. 7).

Figure 7. The ebnerite and epiebnerite structures viewed down their channel directions, [001] for ebnerite and [010] for epiebnerite. The unit cell outlines are shown with dashed lines. The figures were created using ATOMS, version 6.5 (Shape Software, Kingsport, Tennessee, USA).

The ebnerite structure belongs to the family of ‘stuffed derivatives’ of tridymite (Buerger, 1954). Members of this family exhibit polymorphic variations in configuration and stacking of the tridymite-type layers of tetrahedra. There are many synthetic compounds in this family and some minerals. Pharmazincite, KZnAsO₄ (Pekov et al., 2017), belongs to this family and is isostructural with megakalsilite, KAlSiO₄ (Khomyakov et al., 2002). Ebnerite has a cell similar to that of nepheline, (Na,K)AlSiO₄, but it differs in polyhedral configuration. The only other mineral with an ABW-type zeolite structure is loomisite, Ba[Be₂P₂O₈]⋅H₂O (Yang et al., 2023b).

Le and Navrotsky (2008) commented on the small difference between the enthalpy of formation of these phases, noting that HEX phases of NH₄MPO₄ (M = Co, Zn) are 3 kJ mol^–1 more enthalpically stable than ABW phases. This may, in part, explain why epiebnerite has only been found as epitaxial overgrowths on ebnerite.