Mimetite and polymineralic mimetite-pyromorphite-vanadinite single crystals from the Sowie Mts, Poland

A few apparent vanadinite crystals together with crystals of pyromorphite and mimetite were collected from dumps at the abandoned “Friedrich von Thielau” galena and baryte mine near the village Grodziszcze (former Lampersdorf) (50°37’32” N 16°39’16” E) in the Sowie Mts, SW Poland. The quartz-baryte vein, ca. 120 m long and up to 1.5 m thick was exploited for argentiferous galena in the 19^th century and for baryte at the beginning of the 20^th century (Stysz & Mączka, 2007). Hydrothermal quartz-baryte veins are hosted by biotite-oligoclase paragneisses and migmatites dated at 385-360 Ma (Breemen et al., 1988; Jastrzębski et al., 2021).

Galena crystals up to 2 cm in size occur sparsely in the vein with no physical contact with the secondary Pb minerals. Mimetite is the most abundant secondary Pb-mineral in the collected samples. Mimetite from Grodziszcze was first observed by Traube (1888) as yellow-greyish barrel-shaped crystals on galena. Mimetite occurs as single crystals (Fig. 1) or radial aggregates of crystals. Pyromorphite pale-green acicular crystals, up to 0.5 mm long, occur on platy baryte crystals either as radial aggregates or bundles of crystals (Fig. 2).

Figure 1.

Figure 2.

Spindle-shaped grey to dark brown crystals composed of subparallel individuals split at their terminations (Fig. 3), up to 4 mm long, with adamantine luster occur on baryte in association with cerussite. X-ray energy-dispersive (EDX) spectra of those crystals show their chemical composition typical of vanadinite. However, inspection of the crystals interiors revealed their polymineralic nature with multiple zoning and overgrowths of mimetite, pyromorphite, and vanadinite. The crystals are brittle and easy to crumble. Due to technical problems with sample preparation only one of those crystals was suitable for the detailed examination.

Figure 3.

Samples were investigated by optical microscopy, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and Raman microspectroscopy. Mineral morphology and spatial distribution of relevant elements within observed minerals were examined using a Philips XL 30 ESEM/TMP scanning electron microscope coupled to an energy-dispersive spectrometer (EDAX type Sapphire) at the Institute of Earth Sciences, University of Silesia. Operating conditions were: accelerating voltage 15 kV; working distance ca. 10 mm; counting time 40 s. The same conditions were applied for back-scattered electron (BSE) imaging.

Chemical compositions of minerals were determined using a CAMECA SX-100 microprobe at the Laboratory of Electron Microscopy, Microanalysis and X-ray Diffraction at the University of Warsaw Faculty of Geology, Warsaw, Poland. The microprobe operated in the wavelength-dispersive (WDS) mode at an accelerating voltage of 15 kV and a beam current of 10 nA. Counting time for each element was 40 s at the peak position and 20 s for background. The electron beam diameter was 5 μm. La lines were measured for As, Cd, Pb and Mα lines for Bi. Kα lines were measured for other elements. The following standards were used: GaAs for As; YPO₄ for P; V₂O₅ for V; sphalerite for Zn, crocoite for Pb, Bi₂Te₃ for Bi; diopside for Si and Ca, hematite for Fe. The number of atoms in a formula unit (apfu) were calculated from the obtained EPMA data expressed in weight percent (wt%) based on 12.5 oxygen equivalents as recommended by Ketcham (2015).

Raman spectra of vanadinite and mimetite were recorded with the WITec alpha 300R Confocal Raman Microscope (Institute of Earth Sciences, Faculty of Natural Sciences, University of Silesia, Sosnowiec, Poland) equipped with an air-cooled solid laser 532 nm and a CCD camera operating at -61°C. The laser radiation was coupled to a microscope through a single-mode optical fibre with a diameter of 3.5 μm. An air Zeiss (LD EC Epiplan-Neofluan DIC-100/0.75NA) objective was used. Raman scattered light was focused by an effective Pinhole size of about 30 μm and a monochromator with a 600 mm⁻¹ grating. The power of the laser at the sample position was 42 mW. Integration times of 10 s with an accumulation of 15 scans were chosen and a resolution of 3 cm⁻¹. Spectra processing was performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH, USA). The Raman bands were fitted using a Gauss-Lorentz cross-product function with the minimum number of component bands used for the fitting process.

Three morphologically different types of mimetite crystals have been observed: (I) well developed, yellow and transparent hexagonal prismatic crystals double terminated by bipyramids (Fig. 1A); (II) barrel-shaped, dark grey and opaque prismatic crystals with rough surface and dull appearance (Fig. 1C); (III) poorly-developed white prismatic crystals with etched faces (Fig. 1E). Type-I crystals show smooth prismatic faces and rough bipyramidal faces (Fig. 1A,B). Type-II crystals under SEM show features indicative of autoepitaxial overgrowths (Fig. 1C,D). All crystal faces of the type-III crystals are rough and porous with features typical of etching (Fig. 1E,F).

Type-I crystals appear homogeneous in reflected light under the optical microscope; whereas, type-II crystals are oscillatory zoned. Type-III crystals show sector zoning, whereas, oscillatory zoning is faintly expressed. Back-scattered electrons revealed zoning in apparently homogenous type-I crystals and confirmed oscillatory zoning in type-II crystals and sectorial zoning in type-III crystals.

Electron microprobe data for all three types of mimetite crystals are given in Table 1. Despite zoning, the P/As in the type-I crystal is low and uniform throughout the crystal, ranging from 0.01 to 0.02 (1.4 mol% of pyromorphite component) except for the sharp increase in P within the 15 μm-wide outermost rim with the pyromorphite molecule ranging from 11.5 to 23.2 mol% (Table 1, analyses 1-3), i.e. an order of magnitude higher than in the interior of the crystal. The type-II mimetite crystal has slightly higher overall content of P compared to the type-I crystal (Table 1); however, likewise the type-I crystal, the mole fraction of pyromorphite is rather uniform throughout the crystal ranging from 6 to 7 mol% and increases sharply to 14.3 mol% in the outermost rim. In the type-III crystal, pyramidal sectors are slightly enriched in pyromorphite molecule (ca. 12 mol%) compared to prismatic sectors (ca. 8 mol%) (Table 1). In all of the types, V has either been undetected by EPMA or it is at most 0.01 apfu.

Raman spectrum of the mimetite type I crystal (Fig. 4) is characterised by very strong band at 805 cm⁻¹ and two shoulder bands at 761 and 779 cm⁻¹, which, according to Frost et al. (2007) can be related to the v1 symmetric and v₃ asymmetric stretching vibrations of (AsO₄)^3-. The lack of OH is in accord with EPMA data (Table 2) and is further confirmed by the absence of OH vibration bands in the Raman spectrum (the Raman spectrum above 3000 cm⁻¹ is featureless and as such is not reproduced in Fig. 4). Bending vibrations are observed in the range of 300-430 cm⁻¹. Two bands at 307 and 334 cm⁻¹ are assigned to v₂ symmetric bending vibrations of (AsO₄)^3-. Bands at 362, 388, and 418 cm⁻¹ are ascribed to the v₄ asymmetric bending (AsO₄)^3- vibrations. Bands below 200 cm⁻¹ are assigned to the lattice modes.

Figure 4.

The Raman spectrum of mimetite from Grodziszcze was compared to that of mimetite from Tsumeb, Namibia (a reference sample) (Fig. 4). The band at 917 cm⁻¹ seen in the Raman spectrum of mimetite from Grodziszcze is absent in the Tsumeb sample. That band is assigned to the v₁ symmetric stretching mode of (PO₄)^3- resulting from the phosphate anions substitution for arsenate anions in mimetite (Bajda et al., 2011) in accord with the EPMA data for the outermost zones in the investigated mimetite (Table 1).

The investigated polymineralic crystal is built up of alternating mimetite, pyromorphite, and vanadinite zones as revealed by BSE imaging (Fig. 5), X-ray elemental maps (Fig. 6), and EPMA data (Table 2).

Figure 5.

Figure 6.

Cross-section of the crystal along c-axis shows complex pattern of zonation and overgrowths of multigeneration PyGM (Figs. 5 and 6): (a) pyromorphite + vanadinite porous and oscillatory-zoned core (Pym I + Vna I), (b) narrow band of pyromorphite (Pym II), (c) two mimetite zones (Mim I and Mim II) intercalated by a thin band of pyromorphite and vanadinite (Pym III + Vna II), and (d) vanadinite mantle (Vna III).

The core (Pym I + Vna I) consists of predominant pyromorphite alternating with narrow (<2 μm thick) zones of vanadinite. Acicular crystals of pyromorphite are aligned along the longer axis of the crystal. The curved shape of the zones seen in Figs. 5 and 6 resulted from the split crystal growth. Outer portions of the core are enriched in V (analyses 3 to 5 in Table 2). The distribution of As in the core is random and patchy (Fig. 6). EPMA data were obtained with the electron beam of ca. 5 μm diameter. Therefore, elevated concentrations of V in analyses of the pyromorphite zones (analyses 3 and 4 in Table 2) may have resulted from the contaminations from the adjacent vanadinite zones rather than from the extended pyromorphitevanadinite solid solution.

The Pym II zone is poorly preserved and highly fractured, which precluded reliable EPM analyses. Pyromorphite was identified based on X-ray elemental maps (Table 1) and EDS spectra (not shown here).

The intermediate portion of the crystal consists of the first generation mimetite (Mim I in Fig. 5) overgrown by alternating pyromorphite-vanadinite narrow zone (Pym III + Vna II). Mimetite I precipitated preferentially on the bipyramidal sectors of the growing crystals. Second generation mimetite (Mim II) continued to preferentially grow on bipyramidal faces. Mimetites I and II lack vanadinite component. The content of pyromorphite molecule in mimetite ranges from 5 to 15% (Table 2).

The outermost portions of the crystal (Vna III) consists of almost pure vanadinite (83 – 97 mol%) (analyses 14-18 and 20 in Table 2). The Raman spectrum of the vanadinite overgrowth (Fig. 7) is featured by sharp and intense band at 823 cm⁻¹, which, according to Frost et al. (2003), results the v₁ symmetric stretching vibrations of (VO₄)^3- (Fig. 7). Shoulder bands at 789 and 726 cm⁻¹ are related to the v₃ asymmetric stretching vibrations of (VO₄)^3-. Bending vibrations are observed in the spectral range 280-420 cm⁻¹. Two Raman bands at 316 and 343 cm⁻¹ are ascribed to the v₂ symmetric bending O-V-O vibrations, while band at 408 cm⁻¹ is correlated with the v₄ asymmetric bending O-V-O vibrations. Band below 200 cm⁻¹ are assigned to the lattice modes. The Raman spectrum of the vanadinite overgrowth is similar to the reference vanadinite from Mibladen, Morocco (Fig. 7).

Figure 7.

The distribution of Pb and Bi is uniform throughout the crystal varying within 0.27 apfu with only insignificant substitution of Pb by Ca (Table 2). The Ca/Pb ratio does not exceed 0.03.

X-ray elemental maps reveal sharp boundaries between compositionally contrasting zones (Fig. 6). Each zone represents a single mineral with limited solid solution as suggested by EPMA. In mimetite zones, the substitution of As by P does not exceed 0.43 apfu. Vanadinite is even purer, with As + P = 0.30 apfu, at most. The EPMA data show limited miscibility between all three minerals in a composite crystal (Fig. 8). The maximum extent of solid solutions is not larger than 30 mol%. Most analyzes, however, cluster around 10 mol% of solid solution.

Figure 8.