|
Pyrite, a substantial component of most of the metallic ores is presented in nearly all ore deposits, in the altered wall-rocks and also in most operating and abandoned exploration and mining sites and in the mining wastes. Formed in reductive conditions, when exposed on the Earth surface under the influence of the active atmospheric and biological agents, it is put to intensive alterations.
Pyrite is known with its well developed polyhedral crystals and various aggregates. Widespread in the ore deposits however are and formations with peculiar textures some of them with high porosity. It was established, that such are the colloform pyrite aggregates with radiated and concentric-zonal texture. They consist of numerous spherulites, composed of radial pyrite straight needles of micrometer (e.g. 1-15 m) width. SEM observations show that the needles have well developed euhedral side faces with intensive oblique and cross striations. The X-ray and morphological data show that the individual needles are elongated along the [001] direction, with subparallel orientation of the other two main axes. The separate spherulites are formed by splitting of single seeds.
The specific texture of such radial spheroidal aggregates determines their high porosity, due to the -sized interstitial radial open spaces and characteristic very large interface areas and surface energy. Our evaluation shows that a typical spherulite, with radius 1 mm compared to a massive crystal with the same volume, has up to 500 times larger total open interface. This characteristic predetermines the high reactivity of these aggregates in supergene conditions, significantly higher than those of massive pyrite. Mostly, such colloform and other similar highly porous aggregates determine the exceptional reactivity and bad environmental influence of pyrite-bearing mine wastes in mining regions. Exposed to weathering conditions even for a short time such pyrite aggregates because of the strong mineral-water interaction undergo alterations resulting in formation of highly soluble phases. Varied Fe sulphates were identified (melanterite, rozenite, szomolnokite), gradually transforming in dependence of the humidity of the environment. Found were also kornelite, hohmannite, copiapite and others.
Often, pyrite includes increased content of minor elements like As. As was found, its presence is a characteristic feature of many colloform pyrites. Their transformation in weathering proceeds to highly soluble sulphate-arsenate minerals (zykaite, sarmientite and other) and is important additional source of contamination.
Similar behavior is characteristic also and for other highly porous aggregates with specific textures: framboidal pyrite composed of ordered or isometric microcrystals, as well as skeletal pyrite and marcasite, replacing pyrrhotite. Thus, when evaluating the environmental impact of pyrite in mining wastes besides its quantity important is to know its textural and chemical characteristics.
|
