Abstract
Orange greenockite (CdS) aggregates were found in a small fumarole at a burned coal dump near Bytom, Upper Silesia, Poland and were studied using a variety of techniques in order to determine their chemistry, morphology, and most importantly, the mechanism of crystal growth. Greenockite rods, wires, and whiskers with bismuth drops on crystal tops are predominant in these aggregates. Greenockite rods oriented sub-perpendicular to the substrate surface. The rod thickness reaches 5–6 μm and about 10 μm in length. The catalyst bismuth drop has a diameter comparable to the rod thickness. Fiber forms (wires and whiskers) are sub-parallel to the substrate surface. The thickness of these forms is usually less than 2 μm, and the length can be close to 1 mm. The bismuth drop diameter can show a large excess over the fiber thickness. Catalyst drops on the tops of whiskers began to change their form dynamically and exploded, spraying bismuth under the electron beam effect. Rods grow along the [01–10] direction, and whiskers and wires (axial forms) along the [0001] direction. Greenockite rod crystals, carrying on top a relatively homogenous bismuth catalyst drop, were formed on the heated substrate according to the VLS (vapor–liquid–solid) mechanism at temperatures not lower than 270 °C. Greenockite whiskers and wires grew just above of the substrate surface according to the VQS (vapor–quasiliquid–solid) mechanism at temperatures lower than 200 °C. These mechanisms of growth have very rarely been recorded to occur in nature and even less so in burning coal dumps. The cooperative growth effects of the fiber greenockite crystals were also described.
Highlights
Burned coal dumps, being the objects of numerous applied geological, ecological, and technological investigations, are specific “technogenic laboratories”, in which, in real-time processes of mineral phase formation, analogical ones to those in volcanic and metamorphic processes take place
We showed that in nature, whiskers can form by the VQS crystal growth mechanism, more complicated than the known VLS mechanism
Greenockite crystals, the formation of which is connected with active fumaroles at burned coal dumps, are represented by diverse morphological forms corresponding to different temperatures and mechanisms of growth
Summary
Burned coal dumps, being the objects of numerous applied geological, ecological, and technological investigations, are specific “technogenic laboratories”, in which, in real-time processes of mineral phase formation, analogical ones to those in volcanic and metamorphic processes take place. Results of the investigation of these processes and their products can be used in different fields of knowledge from the science of meteorites to the growth of crystals. Samples containing orange crusts with a few mm area were collected on a burned coal dump near Bytom town in the Upper Silesia, Poland (Figure 1). These crusts are represented by greenockite, CdS, which forms various morphological types, among which rods, wires, and whiskers [1] with bismuth drops on crystal tops are predominant (Figure 2).
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