HOW THE MINERALS FORM AND CHANGE

     Observations of the weathering zones of ore deposits prove that the rate of the sulphide oxidation depends on many factors among which the following should be mentioned:
- mineral composition of ores (generally marcasite oxidises easier than pynte and polymineral aggregates easier than monomineral);
- size of grains (fine-grained aggregates oxidise easier than coarse-grained);
- composition and structure of the wall-rocks;
- climatic and hydrogeological conditions (fide e.g. Smirnov 1956). Many of these factors take also part in laboratory oxidation of iron sulphides. Coarse-grained, pure pyrite aggregates are the best preserved ones in mineral collections and still remain fresh, while many of fine-crystalline aggregates of marcasite already after few years undergo a complete decay. However, the observations of specimens of the same mineral with very similar development, stored under the same conditions, revealing that one of them is almost unaltered whereas the other is covered by thick efflorescences of secondary sulphates, suggest that an additional factor plays a substantial role in such a case. It seems that the differences in the rate of weathering of FeS2 specimens are connected with the mechanism and local conditions of oxidation.
     The pathways of FeS2 oxidation may be controlled either chemically or organically by microorganisms, especially the bacteria Thiobacillus ferrooxidans (fide Moses et al. 1987; Taylor and Wheeler 1984, 1994). Under favourable conditions the bacterial oxidation rate is accelerated up to few orders of magnitude relative to the abiotic rate. One can notice in the collection under study that the oxidation process runs most intensively in those specimens which contain kaolinite clay impurities or in which iron sulphides crystallized on the lignite base. Kaolinite and lignite play here twofold role. On the one hand they absorb water, an indispensable agent in each oxidation mechanism. Under laboratory conditions, usually with low humidity, the supply of water to the oxidation environment is of fundamental importance (Borek 1994). The possibility of keeping strong acidic environment in the porous matter is of equal importance. Very low pH values are necessary for activity of the oxidation bacteria. These bacteria can intensively develop only in such places where sulphuric acid resulting from the oxidation of sulphides is preserved. If the medium is not porous (i.g. coarse-crystalline sulphides without admixture of other substances), the progress of oxidation is significantly slower. The oxidation of FeS2 encrusting siderite matrix runs more slowly because siderite acts as a neutralizing agent for sulphuric acid. Therefore, the specimens of marcasite covering the siderite matrix have been preserved without sign of oxidation, whereas similar specimens of marcasite on the lignite matrix were subject to the complete decomposition. It seems that marcasite specimens on the carbonate matrix (e.g. marl, limestone, dolomite, siderite) are the only proper specimens for mineral collections. A chance of keeping marcasite specimens on the lignite or clay matrix for a longer period is rather small.
     The observed differences in the rate of sulphide alteration can be explained by different conditions of the growth of oxidation bacteria. The iron sulphides altered very insignificantly in the specimens when the conditions were not favourable for developing of bacteria. But in any place where the originated acid products stabilised the best conditions of the bacteria development, oxidation was significantly accelerated. As a result, pyrite or marcasite altered quickly into powdery assemblages of sulphate minerals. Simultaneously, the originating sulphuric acid destroys labels and cardboard boxes in which such specimens are kept.
     The crystallisation of the sulphate paragenesis results from evaporation of small portion of solutions concentrating the products of FeS2 oxidation and clay alteration in acidic environment. The oxidation occurs not only on the surface but mainly inside the specimens where from the solutions are drawn up through capillary fractures and then concentrate on the surface. The kind of the sulphate minerals is a function of the solution composition and local conditions, e.g. humidity, acidity and redox potential. One can notice the evolution of the mineral composition of the sulphate efflorescences during storage. It is similar to that observed under natural condition and lead to the crystallisation of more stable phases containing ferric iron. An inconspicuous presence of rozenite, commonly found in similar parageneses, is the characteristic feature of the specimens studied. Moreover, the presence of aluminium sulphates, e.g. halotrichite and alunogen, deserves attention, with halotrichite appearing in the earlier stage of weathering and then being replaced by alunogen.

Trends of alteration of mineral composition of the sulphate efflorescences stored under room condition.

REFERENCES

BOREK S.L., 1994: Effect of humidity on pyrite oxidation. In: Environmental Geochemistry of Sulphide Oxidation. C.N. Alpers, D.W. Blowes (eds), ACS Symposium Series 550, 31—44. BUURMAN P., 1975: In vitro weathering products of pyrite. Geologie en Mijnbouw 54, 101—KUBISZ J., 1964: Studies on wupergene sulphate minerals occurring in Poland. Prace Geol. Kom.Nauk Geol PAN 26, 1-76.

MOSES C.O., NORDSTROM D.K., HERMAN S., MILLS A. L., 1987: Aqueous pyrite oxidation by dissolved oxygen and by ferric ion. Geochim. Cosmochim. Acta, 51, 1561-1571.

STEPISIEWICZ M., 1983: Sideronatrite and copiapite from Turoszów, Lower Silesia. Arch. Miner. 34(2), 35-44.

STEPISIEWICZ M., 1987: Mineralogy of the clay rocks of the Turoszów Trough, Lower Silesia. Arch. Miner. 38(2), 109-205.

TAYLOR B.E., WHEELER M.C., 1984: Stable isotope geochemistry of acid mine drainage: experimental oxidation of pyrite. Geochim. Cosmochim. Acta 48 2669-2678.

TAYLOR B.E., WHEELER M.C., 1994: Sulfur- and oxygen- isotope geochemistry of acid mine drainage in the Western United States: field and experimental studies revisited. In Environmental geochemistry of sulphide oxidation. C. N. Alpers, D. W. Blowes (eds). ACS Symposium Series 550,481-514.

WIESE R.G., POWELL M.A., FYFE W.S., 1987: Spontaneous formation of hydrated iron sulphates on laboratory samples of pyrite- and marcasite-bearing coals. Chem. Geol. 63, 29-38.

Ó J. Parafiniuk & M. Stepisiewicz 2000