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, Reference Buerger1954). Epiebnerite has an ABW-type zeolite structure (Bu et al., Reference Bu, Feng, Gier and Stucky1997; Kahlenberg et al., Reference Kahlenberg, Fischer and Baur2001).
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., Reference Kampf, Gu, Yang and Marty2023; and epiebnerite (Epb): IMA 2023-066, Kampf et al., Reference Kampf, Ma and Marty2024c]. 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 (Reference Wilson2020).
The new minerals were found in a hot and humid area of the mine (see fig. 26 in Wilson, Reference Wilson2020) 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., Reference Kampf, Celestian, Nash and Marty2021a), alunite, ammineite, antipinite, aphthitalite, bassanite, biphosphammite, carboferriphoxite (Kampf et al., Reference Kampf, Ma, Hawthorne and Marty2024a), cerussite, davidbrownite-(NH4) (Kampf et al., Reference Kampf, Cooper, Rossman, Nash, Hawthorne and Marty2019a), dendoraite-(NH4) (Kampf et al., Reference Kampf, Cooper, Celestian, Ma and Marty2022a), edwindavisite (Yang et al., Reference Yang, Gu, Kampf, Marty, Gibbs. and Downs.2023a), ferriphoxite (Kampf et al., Reference Kampf, Ma, Hawthorne and Marty2024b), fluorite, halite, hydroglauberite, mimetite, mottramite, natrosulfatourea (Kampf et al., Reference Kampf, Celestian, Nash and Marty2021a), newberyite, perite, phoxite (Kampf et al., Reference Kampf, Celestian, Nash and Marty2019b), relianceite-(K) (Kampf et al., Reference Kampf, Cooper, Celestian, Ma and Marty2022b), rowleyite (Kampf et al., Reference Kampf, Cooper, Nash, Cerling, Marty, Hummer, Celestian, Rose and Trebisky2017), salammoniac, sampleite, struvite, thebaite-(NH4) (Kampf et al., Reference Kampf, Cooper, Celestian, Nash and Marty2021b), 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, Reference Donnay and Harker1937) 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, Reference Mandarino2007), 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, Reference Donnay and Harker1937) 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., Reference Bu, Feng, Gier and Stucky1997). 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, Reference Mandarino2007), 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. (Reference Frost, Palmer and Pogson2011), Sergeeva et al. (Reference Sergeeva, Zhitova and Bocharov2019), Števko et al. (Reference Števko, Sejkora, Uher, Cámara, Škoda and Vaculovič2018) and Yakovenchuk et al. (Reference Yakovenchuk, Pakhomovsky, Konopleva, Panikorovskii, Bazai, Mikhailova, Bocharov, Ivanyuk and Krivovichev2018).
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)0.89K0.06]Σ0.95(Zn0.96Cu0.07)Σ1.03[(P0.97Si0.03)Σ1.00O4] and [(NH4)0.67K0.28]Σ0.95(Zn0.99Cu0.02)Σ1.02(P1.00O4), 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., Reference Harrison, Sobolev and Phillips2001) and the PXRD pattern for epiebnerite closely matched that calculated from the crystal structure of synthetic monoclinic (ABW-type) NH4ZnPO4 (Bu et al., Reference Bu, Feng, Gier and Stucky1997). 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) Å3 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) Å3 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., Reference Bu, Feng, Gier and Stucky1997) 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., Reference Bu, Feng, Gier and Stucky1997).
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, Reference Sheldrick2015a). Refinement proceeded by full-matrix least-squares on F 2 using SHELXL-2016 (Sheldrick, Reference Sheldrick2015b). 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., Reference García-Rodríguez, Rute-Pérez, Piñero and González-Silgo2000). 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. (Reference García-Rodríguez, Rute-Pérez, Piñero and González-Silgo2000); all others are from Gagné and Hawthorne (Reference Gagné and Hawthorne2015). 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., Reference Harrison, Sobolev and Phillips2001) and epiebnerite is the natural counterpart of synthetic monoclinic NH4ZnPO4 (Bu et al., Reference Bu, Feng, Gier and Stucky1997). Harrison et al. (Reference Harrison, Sobolev and Phillips2001) 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, Reference Buerger1954). 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., Reference Pekov, Yapaskurt, Belakovskiy, Vigasina, Zubkova, Sidorov and Della Ventura2017), belongs to this family and is isostructural with megakalsilite, KAlSiO4 (Khomyakov et al., Reference Khomyakov, Nechelyustov, Sokolova, Bonaccorsi, Merlino and Pasero2002). 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., Reference Yang, Gu, Gibbs and Downs2023b).
Le and Navrotsky (Reference Le and Navrotsky2008) 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.
Acknowledgements
Structures Editor Peter Leverett and two anonymous reviewers are thanked for constructive comments, which improved the manuscript. Ed Davis, current contractor of the Rowley mine, is thanked for allowing underground access for the study of the occurrence and the collecting of specimens. This study was funded, in part, by the John Jago Trelawney Endowment to the Mineral Sciences Department of the Natural History Museum of Los Angeles County.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1180/mgm.2024.15.
Competing interests
The authors declare none.
