Formation mechanisms and environmental influences on wulfenite crystal growth

Wulfenite from the Mežica mine is a secondary mineral precipitated from meteoric solutions during and after the oxidation of the primary mineral assemblage in the mine, which mainly consisted of galena (PbS) and sphalerite (ZnS). Galena minerals contain inclusions of molybdenite (MoS₂), while sphalerite minerals have Mo impurities of about 3 ppm^16,17. During the growth of the wulfenite crystals, many inclusions, mainly calcite and dolomite, were incorporated into the crystals themselves. These inclusions probably precipitated out of solution together with the wulfenite and apparently this occurred more often in the early stages of wulfenite crystal growth, immediately after the formation of the tabular base. This increased concentration of inclusions was mainly found in a thinly layered 200 µm segment or an inclusion-rich layer within the wulfenite crystal. These layers or clusters of carbonate inclusions possibly indicate a change in the chemical composition of the solutions from which they precipitated, as shown in Fig. 3 and 5. Furthermore, the presence of these inclusions likely affects the color and light transmission properties of the wulfenite crystals by changing their optical properties. In contrast to the work of Kloprogge et al.¹⁸we have not found a secondary wulfenite phase forming these darker layers. Instead, optically darker layers could result from the scattering of visible light by the aforementioned inclusions. These layers and sets of inclusions in bipyramidal crystals mark the point where the {100} surfaces merge into the {101} surfaces to form the tetragonal pyramid on either side of the table base (see Figure 3).

The diversity of crystal morphologies is due to internal factors such as crystal structure and the presence of dislocations or twins, or to external factors such as temperature, pressure, chemical composition of solutions, their degree of saturation and Eh-pH conditions.⁷.

Twinning or structural defects can be excluded as a possible explanation for the differences in the external morphology of wulfenite, as these could not be detected by SCXRD or HAADF-STEM.

The structure of a crystal can be affected by the presence of impurities resulting from the replacement of ions. This is because the different ionic radii and bond lengths associated with these impurities can change the shape of the ionic polyhedra, leading to significant changes in the lattice parameters. Such changes can lead to a reduction in crystal symmetry and a transition to a space group with lower symmetry, such as \({\text{I}}\overline{4}\). At the same time, changes in the external shape of a crystal are often associated with a process called ‘surface capping’, where the introduction of certain chemicals changes the surface energies of the crystal facets.¹⁹. These changes in surface energies can promote the development of new crystal facets and cause the crystal to adopt a different habit or crystal shape. This process can significantly change the appearance of the crystal. In our study, carbonates formed shortly after the initial development of the tabular wulfenite crystals. These carbonates are the only evidence of external influences in the solution, indicating that they may have influenced the growth of the wulfenite crystals. However, we believe that this was not the case. If surface capping had occurred, we would expect the wulfenite crystals to exhibit alternating growth patterns, alternating tabular and bipyramidal shapes, as in the spinel system (MgAl₂O₄) in the presence of BeO²⁰. We would also expect to be able to detect these agents in the atomic resolution HAADF-STEM images, especially outside the aforementioned thin inclusion-rich layer. However, these images show no defects or impurities, supporting our conclusion that surface coverage did not significantly affect crystal growth. Moreover, the possible presence of impurities in our study does not seem to change the crystal symmetries of the bipyramidal and tabular samples to the extent that they can be considered as different crystallographic phases, as both morphologies belong to space group I4.₁/a, as confirmed by SCXRD analysis.

While Vesselinov’s 1995 article¹³ shows bipyramidal crystals as two tetragonal pyramids whose bases ((00–1) and (001)) are fused together and lack {100} facets, the bipyramidal specimens from Mežica usually show {100} facets, together with {101} and often { 001} at the very top and bottom of the crystals (Fig. 6). Bipyramidal specimens thus have a tabular base, with two tetragonal pyramids on each side ((001), (00–1)). Therefore, we can assume that the wulfenite crystals in Mežica started to grow as crystals with a tabular morphology, meaning that the growth initially started mainly in the directions < 100 > and was oppressed in the directions < 001 >. However, at some point during crystal growth, the major growth directions changed to < 001 > and therefore some crystals began to develop surfaces and eventually acquire a bipyramidal morphology. The others continued to grow as tabular crystals; in some of them growth was suppressed and they remained tabular. Vesselinov explains the change in preferred directions of growth with different relative concentrations of Pb²⁺ and MoO₄²⁻ ions in the solutions from which wulfenite crystals precipitate^5.13. With experimental work, Vesselinov showed that wulfenite crystals mainly form in the directions < 100 > grow and form tabular crystals {001} when the concentration of MoO₄²⁻ ions is greater than the concentration of Pb²⁺ ions in the solution (C_Pb/C_MoO4< 1)¹³. On the other hand, if the concentration of Pb²⁺ ions is higher (C_Pb/C_MoO4> 1), wulfenite crystals grow preferentially in directions < 001 > and form bipyramidal crystals in which two tetragonal pyramids {101} are fused in the 001 plane¹³.

Therefore, we propose a mechanism where the relatively rapid change in the chemical composition of the solutions during the growth of the wulfenite crystals is the main factor determining the morphology of the wulfenite crystals from Mežica, with the above-mentioned work of Vesselinov providing the theoretical basis. The crystallization of wulfenite can be represented by three different phases of crystal development (see Figure 5):

1. During the first stage (Fig. 4: Growth of {001} crystals) of wulfenite formation in the Mežica area, the solutions were rich in Pb²⁺ and MoO₄²⁻ ions from the oxidation of galena (as the main source of Pb)¹⁶sphalerite and molybdenite (as the main source of Mo)^16,17. The solutions penetrated systems of faults and faults in the carbonate rock and quickly became supersaturated, resulting in the precipitation of wulfenite. The specimens studied in this work all started primarily in < 100 > directions to grow and formed small, tabular {001} crystals, indicating a ratio C._Pb /C_MoO4<1^13.21. Inclusions also grew in these crystals, which consisted mainly of calcite, suggesting the presence of Ca and Mg ions in solution. Moreover, the presence of carbonates also allows conclusions to be drawn about the pH of the solutions, which was in the range 7.5–8, as predicted for sulfate-hydrocarbonate solutions at ambient temperatures (<50 °C).¹¹.
2. In the second, relatively short phase (Fig. 4: The period of increased precipitation of carbonate inclusions), the heterogeneous growth of carbonate inclusions increases rapidly. This abrupt increase in carbonate precipitation marks the change in the chemical composition of the solutions, with the ratio C_Pb /C_MoO4ratio becomes greater than 1^13.21. It should be noted that we assume that the carbonate precipitation coincides with the change in the composition of the solutions. It is therefore the result of the change in the chemistry of the solution and not its cause. Therefore, the carbonate precipitation has no influence on the further development of the wulfenite crystals. The shift in C_Pb /C_MoO4The ratio is probably due to the reduced concentration of MoO₄²⁻ in the solutions, which result from extensive oxidation of the sphalerite ore bodies, depleting the primary source of Mo¹⁷. In contrast, the concentration of Pb²⁺ remained stable as long as there was sufficient interaction between galena and meteoric water, as galena ore bodies oxidized less quickly than sphalerite ore bodies¹⁷. The carbonate inclusions formed thin bands covering the surfaces of the wulfenite crystals, with fewer inclusions in the intervening regions. This indicates that the increased precipitation rates of Ca and Mg were not uniform, likely due to fluctuating concentrations of dissolved Ca and Mg over a relatively short geological period. However, the rate of precipitation of carbonate inclusions in this phase was consistently higher than in earlier and later phases, as shown in Figure 3.
3. In the third phase, the ratio of C_Pb/C_MoO4in the solutions is greater than 1, which favors the growth of wulfenite, especially in the directions < 001 > so that tetragonal pyramids {101} began to grow on the tabular base of the crystals, giving them a bipyramidal morphology (Fig. 4: Growth of {001} inhibited, growth of {101} begins).

Some crystals retain their tabular morphology during the third phase. It should be noted that carbonate inclusions are also present in these crystals in the form of bands, suggesting that they precipitated from the same solutions as bipyramidal wulfenite, but did not experience surface growth due to factors such as orientation or spatial constraints that did not allow growth. in the < 001 > directions. This explains why some clusters show both morphologies, although the crystals are spatially very close to each other, separated by distances of up to a few millimeters.

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