Tetravalent lead in nature – plattnerite crystals from Mine du Pradet (France) and Mount Trevasco (Italy)

1 Introduction

Lead forms three technologically important oxides: divalent Pb^IIO, mixed-valent Pb^II₂Pb^IVO₄ and tetravalent Pb^IVO₂ [1], [2], [3], [4] which are well-known for the following applications. PbO is an important precursor for glass varieties with high refractive index [3], [4] or for transparent conducting films [5]. Pb₃O₄ has long-time been used as long-lasting anti-corrosive paint (as suspension in linseed or turpentine oil), and PbO₂ is the outstanding cathode material of the lead-acid battery. However, since the implementation of the RoHS (Restriction of Hazardous Substances) directive by the European Union in 2002, the use of lead compounds is strongly diminishing. The lead-acid battery is the only lead-based product used on a large scale and with an almost complete recycling rate.

The two modifications of PbO and also Pb₃O₄ are synthetically accessible under ambient pressure conditions, while the synthesis of the two modifications of PbO₂ requires high oxygen pressures, an electrochemical synthesis route or strongly oxidizing agents [6], [7], [8]. Studies of the thermal decomposition reactions of lead dioxide led to the discovery of the mixed-valent phases Pb₂O₃ and Pb₁₂O₁₉.

Several inorganic chemistry textbooks [9], [10] state, that naturally occurring lead compounds are all divalent. However, meanwhile it is known that besides the divalent minerals litharge (α-PbO) and massicot (β-PbO), also minium (Pb₃O₄), scrutinyite (α-PbO₂) and plattnerite (β-PbO₂) with tetravalent lead have been found as minerals [11], [12], [13]. Lead dioxide is formed as an oxidation product during hydrothermal mineralization [12]. Plattnerite crystals can also be generated from Cu-Fe-Pb oxysulfide precursors under suitable Eh-pH conditions [14]. In this context it is interesting to note that plattnerite (β-PbO₂) has also been observed as product of the “black darkening” process (not to be confused with “brown darkening” of paints due to PbS formation with atmospheric hydrogen sulfide) of red lead pigments in artworks [15], [16], [17]. Humidity and hydrogen peroxide resulting from bacteria metabolism are discussed as parameters for the oxidation process.

The present contribution focuses on the structural characterization of natural plattnerite (β-PbO₂). So far, only X-ray and neutron powder diffraction data were reported, mostly for synthetic material [18], [19], [20], [21] and for lead dioxide of charged electrodes of lead-acid batteries [22]. Plattnerite is a rare mineral. The historical background and details for the different deposits were summarized by White Jr. [23]. Several specimens are documented in diverse internet data bases, e.g. www.mineralienatlas.de, www.johnbetts-fineminerals.com or www.webmineral.com. Further mineral localities can be found through a simple internet search for the mineral name. Although most of these samples show well-shaped single crystals, to the best of our knowledge, no single crystal diffraction data of plattnerite have been published. Herein we report on single crystal X-ray and electron diffraction studies of plattnerite crystals from two different deposits.

2 Experimental

2.1 Sample selection

The first sample originated from Cap Garonne (Département du Var, région Provence-Alpes-Côte d’Azur), Mine du Pradet [24], one of the mineralogically most important places in France. Plattnerite crystallized in a small fissure on a quartz matrix. The neighboring phases were mimetite (Pb₅(AsO₄)₃Cl), cerusite (PbCO₃) and duftite (PbCu(AsO₄)OH). The plattnerite specimen was localized in the north mine (mine Nord) on pillar no. 20. A piece of this plattnerite specimen is shown in Fig. 1. The sample consists of thin intergrown platelets and the surface shows a blue black luster. The sample color and habit is similar to the plattnerite from Ojuela mine, Mapimí, Durango, Mexico (RRUFF data base entry R070605) [25].

Fig. 1:

Agglomerated plattnerite crystals of the north mine (mine Nord, pilier 20) of Cap Garonne; FoV 0.6 mm.

The second sample is plattnerite AN12 from Mount Trevasco, Parre, Seriana Valley, Bergamo, Italy, (U07). The thin needles (Fig. 2) grew on a smithsonite (ZnCO₃) matrix. They have a slightly reddish luster. This is similar to plattnerite from Snake Pit, lower Mex-Tex mine, Hansonburg District, Socorro County, New Mexico, USA (RRUFF data base entry 080019) [25].

Fig. 2:

Plattnerite needles (up to 0.8 mm length) from Mount Trevasco grown on a zinc carbonate matrix.

2.2 X-ray diffraction

The Cap Garonne sample was studied first. Plattnerite pieces were mechanically broken from the quartz matrix. A light-microscopic study of these pieces showed the strong crystal intergrowth and a slate-like structure. Several pieces were fixed on glass fibers using varnish and their quality was checked using a Buerger precession camera with white Mo radiation and an image plate technique. The Laue photographs showed split and elongated reflections, already pointing to multi-domain crystals. Intensity data of one of these pieces was subsequently collected at room temperature by using a STOE IPDS-II image plate system (graphite-monochromatized Mo radiation; λ=71.073 pm) in oscillation mode. The image plate data confirmed the multi-domain character. A reasonable integration of the data was not possible. Nevertheless, the refined lattice parameters of the domains of a=494.7 and c=339.3 pm confirmed the rutile type unit cell and were in good agreement with all previous literature data [18], [19], [20], [21], [22]. These multi-domain pieces were also used for an electron-microscopic study (vide infra).

Subsequently thin needles of the Mount Trevasco sample were broken from the zinc carbonate matrix, mounted to glass fibers and tested by Laue photographs. These needles (Fig. 3) were of good quality and showed sharp reflections in the Laue photographs. A first data set was collected at room temperature on the same STOE IPDS-II image plate diffractometer. A numerical absorption correction was applied to the data set.

Fig. 3:

Scanning electron microscopic picture (secondary electron detector) of a plattnerite needle (8×8×100 μm) from the Mount Trevasco sample, mounted on a glass fiber.

Since the oxygen atoms showed a slight anisotropy at room temperature, we also collected data of the same crystal at T=90 K using a STOE StadiVari diffractometer (Mo microfocus source; Pilatus detection system). Due to a Gaussian-shaped profile of the StadiVari X-ray source scaling was applied along with the numerical absorption correction. Details of both collections and the crystallographic parameters are listed in Table 1.

Table 1:

Crystal data and structure refinement parameters for PbO₂ (Mount Trevasco specimen), space group P4₂/mnm, Z=2 and 239.2 g mol⁻¹.

Temperature, K	293(1)	90(1)
Lattice parameters (single crystal data)
a, pm	495.81(10)	495.33(5)
c, pm	338.66(7)	338.67(4)
Unit cell volume V, nm3	0.0833	0.0831
Calculated density, g cm−3	9.54	9.56
Crystal size, μm3	10×10×190	10×10×190
Diffractometer type	IPDS-II (STOE)	StadiVari (STOE)
Transmission (min/max)	0.062/0.629	0.350/0.669
Detector distance, mm	60	40
Exposure time, min	7	53
ω range/increment, deg.	0–180/1.0	0–180/0.3
Integr. Param. (A/B/EMS)	14.0/−1.0/0.030	5.0/−2.0/0.030
Absorption coefficient, mm−1	100.8	101.0
F(000), e	196	196
θ range, deg	5.8–34.9	5.8–32.2
Range in hkl	±7, ±7, ±5	±7, ±7, ±4
Total no. reflections	1025	2654
Independent reflections/Rint	117/0.0245	96/0.0208
Reflections with I>3 σ(I)/Rs	72/0.0102	69/0.0031
Data/ref. parameters	117/9	96/9
R/wR for I>3σ(I)	0.0149/0.0503	0.0136/0.0491
R/wR for all data	0.0303/0.0513	0.0181/0.0499
Extinction coefficient	52(11)	17(9)
Goodness-of-fit on F2	1.85	2.01
Largest diff. peak/hole, e Å−3	1.42/−1.43	1.00/−0.73

2.5 Transmission electron microscopy (TEM)

Selected-area electron diffraction (SEAD) patterns and EDX analysis data of a small plattnerite single crystal (Cap Garonne sample; see Fig. 4) were recorded using a Philips CM-200 STEM transmission electron microscope (super twin objective lens, point resolution 0.23 nm, U₀=200 kV) combined with a double-tilt low-background sample holder (Gatan) and an EDAX system. To obtain electron transparent particles a polycrystalline piece of plattnerite (appox. 50×50×20 μm³) was crushed on a microscope slide under one drop of ethanol. The obtained crystal fragments were transferred as a suspension on a copper grid coated with holey carbon film (Plano GmbH, 200 mesh). Simulations of the experimental SAED patterns were done (kinematical intensities) using the JEMS software [27], [28]. TEM-EDX investigations confirmed the presence of Pb and O and small amounts of Fe up to 0.9 at% in accordance with previous SEM-EDX measurements (vide supra).

Fig. 4:

TEM-BF (bright-field) image of the investigated plattnerite crystal from Cap Garonne. The area used to record selected-area electron diffraction patterns is highlighted by a dotted circle.

3 Crystal chemistry

The single crystal and electron diffraction data (see Figs. 5 and 6) confirm the rutile type structure for the naturally grown plattnerite crystals from Cap Garonne and Mount Trevasco. SAED patterns of the Cap Garonne sample showed no additional reflections hence excluding superstructure formation. This clearly manifests that contrary to some textbook opinions [9], [10] tetravalent lead minerals occur in nature, although rarely. Our lattice and positional parameters agree well with all previous work on synthetic lead dioxide [18], [19], [20], [21], [22]. The 293 and 90 K data sets gave no hints for additional reflections or structural distortions. This is in agreement with synthetic PbO₂ which shows only pressure-induced phase transitions [29], [30]. The slight anisotropy of the oxygen displacement parameters at 293 K vanishes for the 90 K data and this is in line with the general observations for rutile-type oxides [31].

Fig. 5:

Experimental and calculated SAED patterns of plattnerite (Cap Garonne sample) recorded along different zone axes. The calculation (kinematical intensities) was carried out by using crystal structure data from ref. [22].

Fig. 6:

Goniometer positions and comparison of experimental (black font) and calculated (red font) tilt angles between the zone axes obtained by the recorded SAED series of the investigated plattnerite crystal (Cap Garonne sample).

The plattnerite unit cell is presented in Fig. 7. Each lead cation has distorted octahedral oxygen coordination with 2×215.8 and 4×216.5 pm Pb–O. The compression of the octahedra in the unit cell c direction leads to O–Pb–O angles of 77.1 and 102.9°. As is well-known for this basic structure type, the PbO₆ octahedra show a trans-edge condensation along the c axis to form rows which are condensed with neighboring rows (which are shifted by half the translation period c) through common corners. This leads to a slightly distorted trigonal planar lead coordination for the oxygen atoms (1×102.9 and 2×128.5° Pb–O–Pb). The plattnerite structure nicely fits into the rutile family of AB₂ oxides [32], [33].

Fig. 7:

The crystal structure of PbO₂ (room temperature data). Lead and oxygen atoms are drawn as blue and magenta ellipsoids (90% probability level). The network of edge- and corner-sharing PbO₆ octahedra is emphasized.

In the Experimental Section we mentioned the blue black, respectively slightly reddish luster of the dark samples. Crushed pieces of the minerals are similar to the dark brown-black synthetic PbO₂ powders; however, the color cannot be described with the valence precise formulation Pb⁴⁺2O²⁻. Recent high resolution valence band X-ray photoelectron and electron energy loss spectra showed that the metallic state (and thus the dark color) of β-PbO₂ result from partial occupation of a conduction band that corresponds to strongly hybridized Pb6s-O2p states [34].

Finally we comment on minerals with tetravalent lead. Besides plattnerite (β-PbO₂) reported herein, and scrutinyite (α-PbO₂) the mineral murdochite also contains tetravalent lead. Murdochite is a secondary mineral that formed in oxidized zones of lead-copper deposits. Originally, murdochite was reported with the composition Cu₆PbO₈ [35], [36]. This would result in an electron-precise formula 6Cu²⁺Pb⁴⁺8O²⁻. Subsequently presented analytical data revealed that natural murdochite contains small amounts of chlorine and bromine, and the composition is in fact Cu₆PbO_8−x(Cl, Br)_2x (x≤0.5) [37], [38], [39]. With the highest halide (X⁻) content the formula is 12Cu²⁺2Pb⁴⁺15O²⁻2X⁻.