Introduction

Chalcophile elements, such as Pb, Sb, Bi, and Zn, are those metals and heavy semimetals which have a low affinity for oxygen and a strong tendency to bond with sulfur and form sulfides and sulfosalts. Endogenic formations with chalcophile elements concentrated predominantly in oxygen-bearing minerals are relatively rare in nature. The most well-known occurrences of this type are Franklin and Sterling Hill Fe–Zn deposits in New Jersey, USA (Tarr 1929; Palache 1929a, b; Wilkerson 1962), Långban, Nordmark (including Jakobsberg) and Pajsberg (including Harstigen) Fe–Mn deposits in the Bergslagen ore province, Sweden (Palache 1929b; Moore 1970; Holtstam and Langhof 1999), metasomatic rocks of the “Mixed series” near Nežilovo in Macedonia (Chukanov et al. 2015), and parts of the Fe–Mn oxide ore units in the Kombat Mine in Namibia (Innes and Chaplin 1986; Dunn 1991). All of these localities are characterized by an extraordinarily wide variety of minerals, many of which are unknown from other localities. In particular, the Kombat Mine is the type locality of 13 mineral species (https://www.mindat.org/loc-2427.html).

The new mineral roymillerite, with the idealized formula Pb24Mg9(Si9AlO28)(SiO4)(BO3)(CO3)10(OH)14O4, was discovered in the course of investigations of mineral associations from the Kombat Mine, situated in the Otavi Valley, 49 km south of Tsumeb, Grootfontein district, Otjozondjupa region, northern Namibia. The material carrying the new mineral was collected by miners during underground mining in the mid to the late 1980s or early 1990s. Based on comparisons with other samples collected at Kombat during this period, it seems likely that they originate from the locally Pb-(Cu)-rich, non-sulfide units situated within the Fe–Mn oxide ores of the Asis West Sector of the mine. Associated minerals in this case are jacobsite, cerussite, hausmannite, sahlinite, rhodochrosite, barite, grootfonteinite (Pb3O(CO3)2, IMA 2015-051: Siidra et al. 2015), Fe–Mn oxides, and melanotekite.

The mineral is named to honor Dr. Roy McG. Miller (b. 1941) who has made important contributions to the knowledge of the geology of Namibia, beginning with his studies of Damara granites in the 1960s and progressing from there to include a wide range of rock types and geological formations. The mineral and its name have been approved by the IMA Commission on New Minerals, Nomenclature and Classification (IMA No. 2015-093). The holotype specimen is catalogued as # 20080176 at the Swedish Museum of Natural History, Stockholm, Sweden.

Properties of roymillerite

General appearance and physical properties

Roymillerite forms platy single-crystal grains and aggregates up to 1.5 mm across and 0.3 mm thick, occurring in a predominantly fine-grained groundmass of rhodochrosite, cerussite and other associated minerals (Fig. 1a, b). The only distinct form is {001}. The studied sample material exhibits a marked mineralogical and textural banding, potentially indicating an original layering; seemingly, roymillerite occurs mainly within a rhodochrosite band, while grootfonteinite occurs in a groundmass dominated by cerussite, with disseminated crystals and subhedral grains of Mn–(Fe) oxides, and locally euhedral to subhedral melanotekite. Granular aggregate of sahlinite platelets constitutes a separate band showing an indistinct foliation.

Fig. 1
figure 1

a Aggregate of roymillerite platelets (R) intergrown with rhodochrosite and cerussite (white). Black crystals are jacobsite. Field of view 2.9 mm. b Platy crystals of roymillerite, showing distinct cleavage traces parallel to elongation, within a granular groundmass dominated by rhodochrosite. Polished section back-scattered electron (BSE) image

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Roymillerite is transparent, colorless, and the streak is white. The lustre is strong, vitreous. Cleavage is perfect on (001); thin platelets are flexible, non-elastic. Fracture across cleavage is uneven. Parting has not been observed. The Mohs hardness is ca. 3. The density could not be measured because of the absence of heavy liquids with D > 5 g/cm3. Density calculated using the empirical formula is equal to 5.973 g/cm3.

Optical properties

Roymillerite is nonpleochroic, colorless. It is optically biaxial, negative, α = 1.86(1), β ≈ γ = 1.94(1) (λ = 589 nm). 2V (meas.) = 5(5)°. Dispersion of optical axes was not observed. The orientation is: X ≈ c.

The mineral is non-fluorescent under both long- and short-wave UV radiation.

Infrared absorption spectroscopy

To obtain infrared (IR) absorption spectra, powdered samples were mixed with anhydrous KBr, pelletized, and analyzed using an ALPHA FTIR spectrometer (Bruker Optics) with the resolution of 4 cm−1 in the wavenumber range from 360 to 3800 cm−1; 16 scans were obtained. The IR spectrum of an analogous pellet of pure KBr was used as a reference.

Absorption bands in the IR spectrum of roymillerite (Fig. 2a) and their assignments are (cm−1; s—strong band, w—weak band, sh—shoulder): 3700, 3583, 3513, 3352 (O–H stretching vibrations of OH− groups), 2396w (possibly, acid Si–OH group), 1726w (combination mode), 1385 s (asymmetric stretching vibrations of \({\text{CO}}_{3}^{2 - }\) groups), 1231, 1204 (asymmetric stretching vibrations of \({\text{BO}}_{3}^{3 - }\) groups), 1083, 1050sh, 1042, 999 s (Si–O stretching vibrations of the Si5O14 sheets), 915, 898, 875sh (Si–O stretching vibrations of isolated SiO4 4− groups), 842w, 806w, 780w (out-of-plane vibrations of \({\text{CO}}_{3}^{2 - }\) groups, possibly combined with Mg···O–H and Pb···O–H bending vibrations), 725w, 688, 679 (in-plane vibrations of \({\text{CO}}_{3}^{2 - }\) groups, possibly combined with O–Si–O bending vibrations of the Si5O14 sheets), 635w, 605w, 580w, 548w, 467 s, 420sh, 400sh (combination of Si–O–Si bending and Mg···O stretching vibrations).

Fig. 2
figure 2

Powder IR spectra of a roymillerite and b britvinite from Långban, Värmland, Sweden

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The IR spectrum of roymillerite is similar to that of the related mineral britvinite (Chukanov et al. 2008; Fig. 2b). As compared with the latter, roymillerite is characterized by much stronger bands of \({\text{CO}}_{3}^{2 - }\) groups, their shifts, and the presence of an additional O–H-stretching band at 3513 cm−1.

Chemical composition

Seven chemical analyses were carried out for Mg, Mn, Fe, Pb, Al, and Si using a digital scanning electron microscope Tescan VEGA-II XMU equipped with an Oxford INCA Wave 700 spectrometer (WDS mode, with an accelerating voltage of 20 kV, electron beam current of 20 nA, beam diameter of 3 μm, and a counting time of 100 s for each element). Calculations of the results of the X-ray microanalysis were carried out by means of the INCA Energy 300 software package. Contents of other elements with atomic numbers >8 are below their respective detection limits.

H2O and CO2 were determined by gas chromatography of products of ignition at 1200 °C. The content of B2O3 was derived from structural data. The presence of \({\text{BO}}_{3}^{3 - }\) groups is confirmed by the IR spectrum. Analytical data are given in Table 1.

Table 1 Chemical composition (in wt%) of roymillerite
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The empirical formula of roymillerite (based on 83 O atoms per formula unit, i.e., Z = 1) is Pb24.12Mg8.74Mn1.25Fe0.94B1.03Al1.04C9.46Si9.39H14.27O83. The simplified formula written in accordance with structural data (see below) is Pb24Mg9(Si9AlO28)(SiO4)(BO3)(CO3)10(OH)14O4.

The Gladstone–Dale compatibility index (Mandarino 1981) calculated from the empirical formula and single-crystal unit-cell parameters is 1 − (K P/K C) = 0.000 (superior).

X-ray diffraction data and crystal structure

Powder X-ray diffraction data for roymillerite were collected with a Rigaku R-AXIS Rapid II single-crystal diffractometer equipped with cylindrical image plate detector using Debye–Scherrer geometry (d = 127.4 mm), at an accelerating voltage of 40 kV and a current of 15 mA and 15 min exposure time. Data (in Å for CoKα) are given in Table 2.

Table 2 Powder X-ray diffraction data (d in Å) for roymillerite
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Unit-cell parameters of the triclinic unit cell refined from the powder data are: a = 9.309(9) Å, b = 9.331(9) Å, c = 26.44(3) Å, α = 83.31(3)°, β = 83.30(3)°, γ = 59.87(4)°, V = 1968(2) Å3.

Single-crystal X-ray diffraction patterns of roymillerite show low quality of crystals and are typically characterized by streaking (indicating staking disorder) and ill-defined reflections. Twinning was not observed. A colorless grain with the sizes 0.11 × 0.12 × 0.14 mm was used for single-crystal X-ray data collection. The data were collected at room temperature on a Bruker “Kappa Apex Duo” diffractometer with graphite-monochromatized MoKα radiation (λ = 0.71073 Å) and a CCD detector using the ω − θ scanning mode. Raw data were integrated and then scaled, merged, and corrected for Lorentz-polarization effects (APEX2 2009). A semiempirical absorption correction based upon the intensities of equivalent reflections was applied using SADABS (Sheldrick, 2008). A total of 29,590 reflections within the sphere limited by θ = 30.64° were measured. Based on R int values, the primitive triclinic unit cell, space group P \(\bar{1}\) (no. 2) was chosen with the following parameters: a = 9.3152(15), b = 9.3164(15), c = 26.463(4) Å, α = 83.295(3)°, β = 83.308(3)°, γ = 60.023(2)°; V = 1971.2(6) Å3. This cell can be transformed via the matrix [−1 −1 0/1 −1 0/0 0 1] to a pseudomonoclinic C-centered pseudocell with a = 16.134, b = 9.319, c = 26.463, β = 97.74°; V = 3942.4 Å3, R int ~ 45%.

A structure model was determined by the “charge flipping” method, using the SUPERFLIP computer program (Palatinus and Chapuis 2007). The structure determination and refinement were carried out using the Jana2006 program package (Petřiček et al. 2006). Illustrations were produced with the JANA2006 program package in combination with the program DIAMOND (Brandenburg and Putz 2005). Atomic scattering factors for neutral atoms together with anomalous dispersion corrections were taken from International Tables for X-Ray Crystallography (Ibers and Hamilton 1974). The experimental details of the data collection and refinement results are listed in Table 3.

Table 3 Crystal parameters, data collection and structure refinement details for roymillerite
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The final refinement cycles converged with R 1 = 5.71, wR 2 = 9.08, GOF = 1.02 for 11,532 independent reflections with I > 3σ(I). The highest positive peak and deepest minimum in the final residual electron-density map are 6.91 and −2.55 eÅ−3, respectively. The rather large final values of Δρ min and Δρ max could be explained by the disordering of lead atoms. However, attempts to split sites failed because displacement parameters of additional sites became negative. Like in the structure of molybdophyllite (Kolitsch et al. 2012), the largest residual peaks were located at the levels of Pb layers which indicates the OD character of roymillerite structure.

Table 4 shows the fractional atomic coordinates, atomic displacement parameters and bond-valence sums. Selected interatomic distances are given in Table 5.

Table 4 Fractional coordinates, equivalent/isotropic displacement parameters of atoms (U, Å2) and bond-valence sums (BVS, v.u.) in the structure of roymillerite
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Table 5 Selected interatomic distances in the structure of roymillerite
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Discussion

Crystal structure

The crystal structure of roymillerite is related to those of molybdophyllite (Kolitsch et al. 2012) and britvinite (Yakubovich et al. 2008), and is based on a pyrophyllite-type TOT-module, which alternates with the lead-oxo-carbonate I-block (Fig. 3). As a result, the crystal-chemical formula of roymillerite can be written as follows (Z = 1): I{Pb24(OH)6O4(CO3)10[(BO3)0.5(SiO4)0.5]2}TOT{Mg9(OH)8[(Si,Al)10O28]}.

Fig. 3
figure 3

General view of the crystal structure of roymillerite

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The TOT-module in the structure of roymillerite is similar to that in the structures of molybdophyllite and britvinite and contains a central O (octahedral) sheet of edge-sharing MgØ6-octahedra (where Ø = O, OH) linked to two sheets of (Si,Al)O4-tetrahedra (T-sheets). The tetrahedral (Si5O14) sheet is represented by a britvinite-type net with large 12-membered rings (Fig. 4).

Fig. 4
figure 4

TOT-module in the structure of roymillerite

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The U iso values for the Mg atoms are quite low which indicates partial substitution of Mg with Mn/Fe. However, the <Mg–O> distances (2.10–2.09 Å) do not indicate any preference of Mn/Fe for any of the Mg sites. In accordance with Kolitsch et al. (2012) and Yakubovich et al. (2008), small amounts of Al are assumed to substitute for Si.

The I-block in the structure of roymillerite contains ten “lead” layers and four “carbonate” layers, whereas in the structures of britvinite and molybdophyllite there are seven “lead” layers and two “carbonate” layers (Yakubovich et al. 2008), and four “lead” layers and one “carbonate” layer (Kolitsch et al. 2012), respectively.

The Pb atoms in the structure of roymillerite are ordered and occupy polyhedra with coordination numbers from five to seven, average distances Pb–O from 2.47 to 2.68 Å, and long weak bonds with Pb–O >3 Å (Table 5). Thus, these Pb2+ cations are characterized by the stereoactivity of their lone electron pairs.

Adjacent pairs of “lead” layers are separated by “carbonate” layer built by isolated CO3-groups. Two central “carbonate” layers contain heteropolyhedral [(SiO4)0.5(BO3)0.5] cluster (Fig. 3) formed by half-occupied Si6O4-tetrahedra and BO3-triangle (the Si6–B distance is 0.95 Å]. This cluster is similar to that in the structure of britvinite (Yakubovich et al. 2008). One bond of the half-occupied Si6O4-tetrahedra is quite long (Si6–O42 = 1.83 Å) which could be explained by partial protonation of the O42 oxygen atom. The <B–O> distance of 1.30 Å is rather small, consequently a partial CO3-for-BO3 substitution seems possible.

The I-block contains oxygen atoms and hydroxyl groups with the environment formed only by Pb atoms, which could be described in the terms of anion-centered crystal chemistry (Krivovichev et al. 2013). The O12- and O31-atoms are tetrahedrally coordinated forming [OPb4]-tetrahedra with the average distances <O12–Pb> = 2.257 Å and <O31–Pb> = 2.335 Å, whereas O24, O32, and O37 sites of O atoms of OH groups are triangularly coordinated and form distorted anion-centered [(OH)Pb3]-triangles (Fig. 5). The triangles are linked via common Pb–Pb edges with the [O31Pb4]-tetrahedron forming a heteropolyhedral [O(OH)3Pb7]-cluster. Such a cluster was previously revealed in the structures of synthetic plumbonacrite Pb5O(OH)2(CO3)3 (Krivovichev and Burns 2000) and in a lead-exchanged variety of zeolite A (Kolitsch and Tillmanns 2003).

Fig. 5
figure 5

Coordination environment of a the O12 atom and b cluster formed by anion-centered polyhedra

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Modularity of the molybdophyllite–britvinite–roymillerite series

The modular relationship between molybdophyllite and britvinite was described for the first time by Kolitsch et al. (2012). These authors showed that in terms of modular crystallography (Makovicky 1997), the two minerals belong to a common merotypic series and are characterized by the presence of a five-layer module with the composition {Mg9[Si10O28(OH)8](OPb4)}. This module alternates with an I-block, having the composition [(CO3)3∙H2O] in molybdophyllite, and {[(OH)3OPb7][(BO3)3|(CO3)3]} in britvinite.

Based on the data on the roymillerite structure, we have developed this approach and proposed a new view on modularity of the molybdophyllite–britvinite–roymillerite series. We consider only the three-layer TOT-module as a stable part of the structure. This is stable not only in the crystal-chemical meaning, but also taking into account chemical bonding. The whole intermodular part containing differently alternating simple lead-oxo-carbonate modules is considered as the I-block (Table 6).

Table 6 Composition of the modules in the molybdophyllite–britvinite–roymillerite series
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In the structure of molybdophyllite, the I-block, with the composition [(OPb4)2(H2O)(CO3)3(BO3)3], can be divided into two “molybdophyllite” modules (Fig. 6). Each of them contains anion-centered (OPb4)-tetrahedra and a “carbonate” layer which, being involved in the “molybdophyllite” module, is present also in the structures of britvinite and roymillerite (Fig. 6). The structure of molybdophyllite contains only one “carbonate” layer which is common for two adjacent “molybdophyllite” modules. Along with (OPb4)-tetrahedra and (TO3)-triangles, each module contain an additional A-site, which is occupied by a water molecule (in molybdophyllite) or Pb (in britvinite and roymillerite). Thus, the idealized formula of the “molybdophyllite” module could be written as follows: {(OPb4)A(TO3)3}.

Fig. 6
figure 6

Modular crystal structure of a molybdophyllite, b britvinite, and c roymillerite

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In the I-block of the britvinite structure (Yakubovich et al. 2008), “molybdophyllite” modules alternate with a quite disordered block of lead and oxygen atoms with the composition {(O,OH)4Pb4} (Fig. 6). Kolitsch et al. (2012) refined the crystal structure of britvinite and reported the composition of the central block as {O(OH)3Pb5} (excluding two Pb atoms of the A site). In the case of roymillerite, “molybdophyllite” modules alternate with “plumbonacrite” modules [(O(OH)3Pb7)(TO3)3] which combine heteropolyhedral clusters and (TO3)-groups (Fig. 7). At present, the molybdophyllite–britvinite–roymillerite series contains only three members, however, one can expect to find additional members belonging to this series, with different types of modules.

Fig. 7
figure 7

General view of the a “molybdophyllite” and b “plumbonacrite” modules in roymillerite

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Concluding remarks

As one can see from the above discussion, roymillerite, molybdophyllite and britvinite are related mineral species which have layered modular structures and form a merotypic series. The structures of these minerals are based on the T-layer Si10O28 (the britvinite-type net with large 12-membered rings) and the “molybdophyllite” module (OPb4)(H2O,Pb)(CO3)3. Molybdophyllite contains two adjacent “molybdophyllite” modules. In britvinite, “molybdophyllite” modules alternate with Pb-hydroxide blocks. In roymillerite, “molybdophyllite” modules alternate with “plumbonacrite” modules [(O(OH)3Pb7)(TO3)3]. Hypothetically, other representative of this merotypic series may exist with the similar building principles and various interleaves between “molybdophyllite” modules.

Comparative data for roymillerite, molybdophyllite and britvinite are given in Table 7. Despite roymillerite and britvinite are triclinic, and molybdophyllite is monoclinic, the atomic arrangement in all these minerals is roughly pseudohexagonal.

Table 7 Comparative data for roymillerite and related minerals
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