Introduction
Oxalate minerals are the largest subclass of organic minerals. In the current list of minerals approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA-CNMNC, Pasero, Reference Pasero2025), those derived from organic acids include three acetates, one citrate, two formates, seven glycolates, one mellitate, one methanesulfonate and 37 oxalates. While some oxalate minerals may form abiotically, most are found to occur in connection with biological systems. They are known to form when sources of oxalic acid interact with metal cations leached from primary minerals (e.g. Baran and Monje, Reference Baran and Monje2008; Baran, Reference Baran2014; Frank-Kamenetskaya et al., Reference Frank-Kamenetskaya, Zelenskaya, Izatulina, Vereshchagin, Vlasov, Himelbrant and Pankin2021). In biologically induced mineralisation, acid-producing microorganisms, such as fungi, lichens and bacteria, as well as guano, are common sources of oxalic acid (e.g. Baran, Reference Baran2014; Gadd et al., Reference Gadd, Bahri-Esfahani, Li, Rhee, Wei, Fomina and Liang2014). Moreover, oxalates represent the only ionic organic minerals known to be stable over geological timescales on Earth (Hoffman and Bernasconi, Reference Hofmann and Bernasconi1998; Hazen et al., Reference Hazen, Downs, Jones and Kah2013). Thus, their presence may be an indicator of plant life either current or in pre-existing life forms.
Oxalate materials have been the subject of various investigations owing to their diverse applications, such as hydrogen storage, carbon sequestration, catalysis, gas separation and photovoltaics (e.g. de Faria et al., Reference de Faria, Yoshida, Pinheiro, Guedes, Krambrock, Diniz and Machado2007; Androš et al., Reference Androš, Jurić, Planinić, Žilić, Rakvin and Molčanov2010; Furukawa et al., Reference Furukawa, Cordova, O’Keeffe and Yaghi2013; Garcia-Teran et al., Reference Garcia-Teran, Beobide, Castillo, Cepeda, Luque, Perez-Yanez and Roman2019). In particular, the study of the crystallisation of copper oxalate is very interesting and promising, because copper is a toxic element and the formation of insoluble copper oxalate can be used in bioremediation technologies for copper-contaminated environments with the help of oxalate-producing microorganisms (e.g. Fomina et al., Reference Fomina, Hillier, Charnock, Melville, Alexander and Gadd2005; Ren et al., Reference Ren, Li, Geng and Li2009; Tsekova et al., Reference Tsekova, Todorova and Ganeva2010; Gadd et al., Reference Gadd, Bahri-Esfahani, Li, Rhee, Wei, Fomina and Liang2014; Glukhova et al., Reference Glukhova, Frank, Danilova, Avakyan, Banks, Tuovinen and Karnachuk2018). For example, Tsekova et al. (Reference Tsekova, Todorova and Ganeva2010) described the recovery of copper and other metals from industrial wastewater using Aspergillus niger, which is one of the most commonly used oxalate-producing micromycetes (Gadd et al., Reference Gadd, Bahri-Esfahani, Li, Rhee, Wei, Fomina and Liang2014; Zelenskaya et al., Reference Zelenskaya, Izatulina, Frank-Kamenetskaya and Vlasov2021; Frank-Kamenetskaya et al., Reference Frank-Kamenetskaya, Zelenskaya, Izatulina, Gurzhiy, Rusakov and Vlasov2022). Moreover, there have been numerous publications on the application of Cu-oxalates in industry for their interesting physical and chemical properties, including use as antiferromagnets (e.g. Donkova and Mehandjiev, Reference Donkova and Mehandjiev2005; Behnoudnia and Dehghani, Reference Behnoudnia and Dehghani2013), and for their acting as a precursors to the formation of a number of widely used nanoparticles, such as Cu, CuO, Cu2O and Cu(OH)2 (e.g. Aimable et al., Reference Aimable, Puentes and Bowen2011; Singh et al., Reference Singh, Kapoor, Dubey and Srivastava2012; Gadd et al., Reference Gadd, Bahri-Esfahani, Li, Rhee, Wei, Fomina and Liang2014; Wu and Huang, Reference Wu and Huang2016).
Figure 1. A specimen (R230005) on which the new mineral edwindavisite (indicated by red arrows), was found.
Figure 2. A microscopic view of Fig. 1 showing fans of green bladed edwindavisite crystals, associated with a blue sampleite-like phase and ammineite.
Figure 3. Back-scattered electron images (a and b), showing fans of prismatic or bladed edwindavisite crystals.
This study describes a new oxalate mineral, edwindavisite, ideally Cu(C2O4)(NH3), discovered from the Rowley mine, Maricopa County, Arizona, USA. The new mineral name honours Mr. F. Edwin (Ed) Davis, Jr., an Arizona-based mineral collector and the current contractor operator of the Rowley mine, who has gladly accepted the proposed mineral name. Ed Davis graduated from Miami University in Oxford, Ohio, USA, with degrees in chemical engineering, mathematics and chemistry. He is credited for the discovery of the Purple Passion deposit near Wickenburg, Arizona, USA, which is world-renowned among fluorescent mineral collectors. Ed Davis has helped in the collection of multiple new minerals from the Rowley mine, such as ebnerite and epiebnerite (Kampf et al., Reference Kampf, Gu, Yang, Ma and Marty2024), as well as edwindavisite described here. In particular, he provided access to the mine, helped in exploration, and helped in removing specimens from difficult to negotiate underground workings. The new mineral and its name (symbol Ewd) have been approved by the IMA (IMA 2023-056, Yang et al., Reference Yang, Gu, Kampf, Marty, Gibbs and Downs2023). Parts of the cotype samples have been deposited at the University of Arizona Alfie Norville Gem and Mineral Museum (Catalogue # 22732) and the RRUFF Project (deposition # R230003). In addition, two cotype specimens are deposited in the collections of the Natural History Museum of Los Angeles County, Los Angeles, California, USA (catalogue numbers 76287 and 76288).
Sample description and experimental methods
Occurrence
Edwindavisite was discovered on specimens collected on the 125-foot level of the Rowley mine, ∼20 km NW of Theba, Maricopa County, Arizona, USA (33°257’N, 113°149.59’W). This mine 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. The mine has not been operated for ore since 1923, but it has been a rich source of fine wulfenite crystals for mineral collectors in the past 70 years. A detailed description of the history, geology and mineralogy of the Rowley mine has been presented by Wilson (Reference Wilson2020). Edwindavisite was found on a quartz matrix, in intimate association with ammineite, a sampleite-like mineral, baryte, ebnerite and wulfenite (Fig. 1). The area where the edwindavisite specimens were collected has an unusual bat guano-related, secondary mineral assemblage, from which a number of new mineral species have been discovered, including rowleyite (Kampf et al., Reference Kampf, Cooper, Nash, Cerling, Marty, Hummer, Celestian, Rose and Trebisky2017), phoxite (Kampf et al., Reference Kampf, Celestian, Nash and Marty2019b), davidbrownite-(NH4) (Kampf et al., Reference Kampf, Cooper, Rossman, Nash, Hawthorne and Marty2019a), natrosulfatourea (Kampf et al., Reference Kampf, Celestian, Nash and Marty2021a), allantoin (Kampf et al., Reference Kampf, Celestian, Nash and Marty2021a), thebaite-(NH4) (Kampf et al., Reference Kampf, Cooper, Celestian, Nash and Marty2021b), relianceite-(K) (Kampf et al., Reference Kampf, Cooper, Celestian, Ma and Marty2022a), dendoraite-(NH4) (Kampf et al., Reference Kampf, Cooper, Celestian, Ma and Marty2022a), ebnerite, epiebnerite (Kampf et al. Reference Kampf, Gu, Yang, Ma and Marty2024), ferriphoxite and carboferriphoxite (Kampf et al., Reference Kampf, Ma, Hawthorne and Marty2025a).
Figure 4. Raman spectra of edwindavisite, whewellite and ammineite.
Appearance, physical and chemical properties
Edwindavisite occurs as radial fans or sprays of bladed or prismatic crystals, elongated on [001]. Individual crystals are up to 0.50 × 0.08 × 0.06 mm (Figs 2 and 3), with the common forms being {100}, {010} and {001}. It is green, transparent with a pale green streak and vitreous lustre. Edwindavisite is brittle and has a Mohs hardness of ∼2 (based on scratch tests); cleavage is perfect on {100}. No parting was observed. The density, measured by flotation in heavy liquids, is 2.55(2) g/cm3 and the calculated density is 2.53 g/cm3 on the basis of the empirical chemical formula and the unit-cell volume determined from single-crystal X-ray diffraction data. Optically, edwindavisite is biaxial (+), with α = 1.550(2), β = 1.559(2), γ = 1.755(5) (white light), 2Vmeas. = 26(2)° and 2Vcal. = 26.4°. The dispersion is distinct with r > v. The optical orientation is X = a, Y = c, Z = b, and the pleochroism is X = light green, Y = pale green and Z = deep green, with Y < X ≪ Z. The compatibility index was not calculated, because of the lack of a k-value for the NH3 group. Based on the measured optical data and density, along with an ideal chemical formula, we derived an estimated k-value of 0.495 for the NH3 molecule. Given this k-value for NH3, we obtained a compatibility index of –0.005 (superior) for edwindavisite and –0.018 (superior) for shilovite, Cu(NH3)4(NO3)2, with the data reported by Chukanov et al. (Reference Chukanov, Britvin, Möhn, Pekov, Zubkova, Nestola, Kasatkin and Dini2015).
Table 5. Fractional atom coordinates and isotropic or equivalent isotropic displacement parameters (Å2) for edwindavisite
Table 6. Anisotropic displacement parameters (Å2) for edwindavisite
Table 7. Selected bond distances (Å) for edwindavisite
Table 8. Hydrogen-bond geometry (Å, °) in edwindavisite
Notes: Symmetry codes: (ii) x, −y+½, z+½; (v) x+½, y, −z+½; (vi) x+½, −y+½, −z; (vii) −x+½, y+½, z. D = donor; A = acceptor.
Table 9. Bond-valence sums (vu) for edwindavisite
Note: The significant deviation of bond-valence sums from the ideal values for N3+ and H+ ions is due to the fact that the bond-valence parameters (R 0 = 0.935 Å and B = 0.572 Å) given by Gagné (Reference Gagné2021) were derived from the neutron diffraction data based on the average N–H distance of 0.999 Å (with a range of 0.915–1.025 Å). This N–H distance is remarkably longer than any reported N–H distances determined from the X-ray structure analyses for the NH3 group (0.7–0.9 Å). If we constrain our N–H distance to be 0.95 Å, then we have the bond valence sums of 3.48 vu for N and 0.97 vu for H.
Figure 5. The configuration of the Cu atom coordinated octahedrally by (5O + N) in edwindavisite. The structures were drawn using VESTA (Momma and Izumi, Reference Momma and Izumi2011).
Figure 6. A corner-sharing Cu-octahedral chain in (a) edwindavisite and (b) triazolite.
Figure 7. A layer of corner-sharing Cu-octahedral chains linked together by oxalate groups in edwindavisite. The figure legends are the same as in Fig. 5.
Figure 8. Crystal structure of edwindavisite, showing the Cu(C2O4)(NH3) layers stacked along [100], which are interconnected by N–H···O hydrogen bonds provided by NH3 ammonia molecules. For clarity and simplicity, N–H···O hydrogen bonds between Cu(C2O4)(NH3) layers are not drawn. The figure legends are the same as in Fig. 5.
The chemical composition of edwindavisite was determined using a Shimadzu EPMA-1720 electron microprobe (WDS mode, 15 kV, 10 nA and 5 μm beam diameter). The analytical data (average of 7 analysis points) are given in Table 1, along with the standards used for the probe analyses. The resultant empirical chemical formula, calculated on the basis of 1 Cu apfu, is Cu1.00(C2O4)(NH3)0.99. The hydrogen content was calculated by stoichiometry. The ideal formula, Cu(C2O4)(NH3), requires (wt.%) CuO 47.18, C2O3 42.72, NH3 10.10 (total = 100%). At room temperature, edwindavisite is insoluble in H2O, but easily soluble in dilute HCl.
Table 1. Analytical chemical data (in wt.%) for edwindavisite
Raman spectra
The Raman spectrum of edwindavisite (Fig. 4) was collected from a randomly orientated crystal on a Thermo Almega microRaman system, using a solid-state laser with a frequency of 532 nm at 50 mW power (1/3 of the full power to avoid the burning by the laser) and a thermoelectric cooled CCD detector. The laser is partially polarised with 4 cm–1 resolution and a spot size of 1 µm.
The tentative assignments of major Raman bands for edwindavisite were made based on spectroscopic studies on materials containing oxalate (C2O4)2– and/or ammine groups, such as synthetic edwindavisite Cu(C2O4)(NH3) (Cavalca et al., Reference Cavalca, Villa, Manfredotti, Mangia and Tomlinson1972), ammineite CuCl2(NH3)2 (Bojar et al., Reference Bojar, Walter, Baumgartner and Färber2010), humboldtine Fe2+(C2O4)·2H2O (Frost and Weier, Reference Frost and Weier2003; Echigo and Kimata, Reference Echigo and Kimata2008), whewellite Ca(C2O4)·H2O (Shippey, Reference Shippey1980; Frost and Weier, Reference Frost and Weier2003), and shilovite Cu(NH3)4(NO3)2 (Chukanov et al., Reference Chukanov, Britvin, Möhn, Pekov, Zubkova, Nestola, Kasatkin and Dini2015). Specifically, the Raman spectrum of edwindavisite can be divided into five regions (Table 2). Region 1 ranges from 3160 to 3450 cm–1, with a relatively strong peak centred at 3287 cm–1. The bands in this region are due to the N–H stretching modes in NH3 groups. Region 2 includes the bands between 1120 and 1735 cm–1, which are ascribable to the C=O and C–O stretching vibrations within the C2O4 groups, as well as the H–N–H bending modes within the NH3 groups. In particular, the two sharp peaks, one at 1508 cm–1 and the other at 1454 cm–1, correspond to the symmetric stretching vibrations of C=O and C–O bonds within C2O4. It is interesting to note that whewellite only shows one strong peak at 1493 cm–1 in this region, whereas both oxammite, (NH4)2(C2O4)·H2O and moolooite, Cu(C2O4)·nH2O, exhibit two relatively strong peaks (Frost and Weier, Reference Frost and Weier2003). Region 3 spans the region from 840 to 950 cm–1. The sharp peak at 908 cm–1 in this region is assigned to the C–C stretching vibrations and the weak bands are attributed to the O–C–O antisymmetric bending modes within the C2O4 groups. In comparison, the band corresponding to the C–C stretching vibrations is observed at 909 cm–1 for weddellite and 921 cm–1 for moolooite (Frost and Weier, Reference Frost and Weier2003). Region 4 ranges from 440 to 620 cm–1. The strong peak at 518 cm–1 in the region corresponds to the C–C–O bending mode and the rest of the bands can be attributed to the Cu–O stretching modes involving the shortest Cu–O bonds. Region 5 includes the bands below 350 cm–1, which are associated mainly with the rotational and translational modes of C2O4 and NH3 groups, as well as the Cu2+–(O,N) interactions and lattice vibrational modes. For comparison, the Raman spectra of ammineite and whewellite from the RRUFF Project (Lafuente et al., Reference Lafuente, Downs, Yang, Stone, Armbruster and Danisi2015) are also plotted in Fig. 4.
Table 2. Tentative assignments of major Raman bands for edwindavisite
X-ray crystallography
Powder X-ray diffraction data for edwindavisite were collected on a Rigaku Xtalab Synerg D/S 4-circle diffractometer equipped with CuK radiation and operated at 50 kV and 1 mA. Powder X-ray diffraction data (Table 3) were collected in the Gandolfi powder mode and the unit-cell parameters refined using the program by Holland and Redfern (Reference Holland and Redfern1997) are: a = 11.2011(4), b = 9.4260(3), c = 8.4042(3) Å and V = 887.33(3) Å3.
Table 3. Powder X-ray diffraction data (d in Å, I in %) for edwindavisite
Single-crystal X-ray diffraction data of edwindavisite were collected on a Bruker APEX2 4-circle diffractometer equipped with MoK radiation from a 0.06 × 0.05 × 0.05 mm fragment. All reflections were indexed on the basis of an orthorhombic unit cell (Table 4). The systematic absences of reflections suggest the unique space group Pbca. The structure was solved with SHELXT (Sheldrick, Reference Sheldrick2015a) and refined using SHELXL2019 (Sheldrick, Reference Sheldrick2015b). All H atoms were located through the difference-Fourier syntheses. The positions of all atoms were refined with anisotropic displacement parameters, except for H atoms, which were refined with a fixed isotropic displacement parameter (U iso = 0.05 Å2) and soft restraints of 0.85(3) Å on the N–H distances. The final refinement statistics are listed in Table 3. Atomic coordinates and displacement parameters are given in Tables 5 and 6, respectively. Selected bond distances are presented in Table 7 and hydrogen bonding geometries in Table 8. The bond-valence sums (BVS) were calculated using the parameters given by Brown (Reference Brown2009) for Cu–O and Cu–N bonds, Harris and Hardcastle (Reference Harris and Hardcastle2015) for C–C and C–O bonds, Gagné (Reference Gagné2021) for N–H bonds, and by Ferraris and Ivaldi (Reference Ferraris and Ivaldi1988) for O···H bonds (Table 9). The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).
Table 4. Crystallographic data for natural and synthetic edwindavisite
Crystal structure description and discussion
Edwindavisite is the natural analogue of synthetic catena-μ-oxalato-ammine-copper(II), Cu(C2O4)(NH3) (Cavalca et al., Reference Cavalca, Villa, Manfredotti, Mangia and Tomlinson1972) (Table 4). In its structure, each Cu2+ cation is coordinated by (5O + N), forming a rather distorted and elongated octahedron (Fig. 5) due to the Jahn-Teller effect. The five O atoms coordinated to a Cu atom are from three different (C2O4)2– groups, two being bidentately bonded and one monodentately bonded, with the Cu–O bond lengths ranging from 1.983 to 2.399 Å. The Cu–N distance (1.966 Å) is shorter than any of the Cu–O distances, but agrees well with the values reported for Cu that is bonded to NH3 (Bojar et al., Reference Bojar, Walter, Baumgartner and Färber2010 and references therein).
The Cu-octahedra in the edwindavisite structure share corners with one another to form chains extending along [001] (Fig. 6), which are joined together by oxalate (C2O4)2– groups, giving rise to layers parallel to (100) (Fig. 7). Such layers are interconnected by N–H···O hydrogen bonds (Table 8, Fig. 8). Similar chains of corner-sharing Cu-octahedra have also been observed in triazolite, NaCu2(N3C2H2)2(NH3)2Cl3·4H2O (Chukanov et al., Reference Chukanov, Zubkova, Möhn, Pekov, Belakovskiy, Van and Pushcharovsky2018) (Fig. 6), and pabellóndepicaite, Cu2+2(N3C2H2)2(NH3)2(NO3)Cl·2H2O (Kampf et al., Reference Kampf, Möhn, Ma and Désor2025b), both being Cu-bearing organic minerals containing 1,2,4-triazolate anions, as well as NH3 molecules.
Oxalic acid is ubiquitous in natural environments. It can be produced by some plants, fungi and lichens (e.g. Dutton and Evans, Reference Dutton and Evans1996; Prieto et al., Reference Prieto, Silva, Rivas, Wierzchos and Ascaso1997; Chen et al., Reference Chen, Blume and Beyer2000; Grąz, Reference Grąz2024). Lichens and fungi living on mineral surfaces have been found to facilitate the dissolution of heavy metals (e.g. Chisholm et al., Reference Chisholm, Jones and Purvis1987; Fomina et al., Reference Fomina, Hillier, Charnock, Melville, Alexander and Gadd2005; Studenroth et al., Reference Studenroth, Huber, Kotte and H.F2013; Schuler et al., Reference Schuler, Demetriou, Shiju and Gruter2021; Amenaghawon et al., Reference Amenaghawon, Ayere, Amune, Otuya, Abuga, Anyalewechi and Darmokoesoemo2024). Yet, only 37 oxalate minerals have been documented to date. Among them, six contain Cu [antipinite, KNa3Cu2(C2O4)4, edwindavisite, fiemmeite, Cu2(C2O4)(OH)2·2H2O, middlebackite, Cu2C2O4(OH)2, moolooite, Cu(C2O4)·nH2O, and wheatleyite, Na2Cu(C2O4)2·2H2O], and seven contain ammonium (NH4)+ [davidbrownite-(NH4), (NH4)5(V4+O)2(C2O4)[PO2.75(OH)1.25]4·3H2O, dendoraite-(NH4), (NH4)2NaAl(C2O4)(PO3OH)2(H2O)2, oxammite, (NH4)2(C2O4)·H2O, phoxite (NH4)2Mg2(C2O4)(PO3OH)2(H2O)4, thebaite-(NH4), (NH4)3Al(C2O4)(PO3OH)2(H2O), ferriphoxite, [(NH4)2K(H2O)][Fe3+(HPO4)2(C2O4)], and carboferriphoxite, [(NH4)K(H2CO3)][Fe3+(HPO4)(H2PO4)(C2O4)]]. Edwindavisite represents the first oxalate mineral that contains neutral ammonia (NH3).
Nitrogen is usually present in minerals as (NO3)– or (NH4)+ ionic groups. In the current IMA-approved mineral list (Pasero, Reference Pasero2025), only seven minerals contain the neutral ammonia molecule (NH3) as a species-defining component: ammineite, CuCl2·2NH3, chanabayaite, Cu2Cl(N3C2H2)2(NH3,Cl,H2O,◻)4, joanneumite, Cu(C3N3O3H2)2(NH3)2, pabellóndepicaite, Cu2+2(N3C2H2)2(NH3)2(NO3)Cl·2H2O, shilovite, Cu(NH3)4(NO3)2, triazolite, NaCu2(N3C2H2)2(NH3)2Cl3·4H2O, and edwindavisite. Interestingly, all these minerals have only been described in the past 20 years. Among them, all but edwindavisite were discovered at Pabellón de Pica, Iquique Province, Chile. Moreover, all of them have Cu2+ as the essential component and bonded to NH3. Nonetheless, it is unclear how the presence of Cu2+ is related to the formation of NH3-bearing minerals.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1180/mgm.2025.16
Acknowledgements
We are very grateful to Prof Peter Leverett and two anonymous reviewers for their constructive comments.
Competing interests
The authors declare none.
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