Siarczanowe produkty wietrzenia na złożu dwusiarczku żelaza Gór Świętokrzyskich

Tadeusz Wieser


Sulphate weathering product on the iron disulphide deposit of the Holy Cross Mts.

On one of his geological excursions in the Holy Cross Mts. during an inspection of a deposit of pyrite and marcasite the author encountered some interesting products of secondary alterations of sulphide minerals. These products, originating indirectly or directly from the weathering of iron disulphides, were discovered there in great quantities in open cut and underground workings and on dumps, from where they were collected, particular attention being paid to the circumstances of their occurrence. Thanks to the advice and kindness of Prof. A. Gaweł, the author was able to carry out optical and chemical determinations of these sulphate minerals, most of which have been hitherto unknown from Polish territory and occur in interesting parageneses. II. Geological and Depositary Conditions Geologically the deposit of pyrite and marcasite at is closely associated with the great torsion of the principal Łysogóry range, produced by a transversal dislocation having the character of a flexure depressing its eastern wing (J. Czarnocki 1947, p. 252). In connection with this so-called Łysogóry dislocation there arose a fissure system; one of these fissures, corresponding to the external surface of Czarnocki’s Łysogóry dislocation, underwent mineralization under the influence of hydrothermal solutions arriving from the depths. The deposit itself, an exhaustive knowledge of which is due to the work of C. Poborski (1947), has the shape of a vein which is steeply dipping and very irregular in its strike. The irregularity of this vein is increased by its numerous apophyses, stringers and adjoining impregnation zones in clay selvages, siderites and in the country rocks. The latter belong to coral dolomites of the Givetian, and in particular to marls and dolomites of the Eifelian and partly of the Couvinian and, furthermore, in the southern part to upper Silurian shales (the so-called Rzepin series) together with subordinate lower Devonian quartzites. The mode in which the iron disulphides occur together with siderite and haematite, as well as the numerous examples of their succession, indicate that the deposit has the character of a vein of the replacement type, subjected to rejuvenation. The earlier metasomatic deposit of siderite also underwent a metasomatic process of haematitization and pyritization, or marcasitization. From among the original minerals the most common is the so-called «powdery» or earthy pyrite, while «rocky» pyrite, haematite and siderite are additional constituents of the deposit. Favourable hydrogeological conditions, produced by the position of the deposit within a strongly dislocated zone, exerted an influence upon the establishment of an extensive oxidized and hydrated part of the deposit. Most of the sulphate minerals studied and constituting the subject of this work, originate from this zone of the oxidation and hydration processes the final effect of their action was the formation of a large gossan, composed chiefly of variously hydrated goethite, nests of halloysite, besides small quantities of allophane, manganese oxides and similar minerals. III. Description of the minerals Apart from the above-mentioned occurrence of the sulphate minerals in the zone of weathering, they are known to occur in the part of the deposit which is unaltered by atmospheric agencies. In the latter case their origin is due only to the agency of man, in underground and open cut workings. Genetically the sulphate minerals can be systematized as follows: 1. minerals originating directly from the decomposition of iron disulphide; 2. minerals originating from wall rocks under influence of decomposition products of pyrite and marcasite; 3. final alteration products; the latter sulphates are the most stable ones under normal physico-chemical conditions prevailing at the surface. 1. Decomposition products of iron disulphides To the category of sulphates originating directly from the decomposition of iron disulphides, belongs in principle only one mineral: melanterite. It is a ferrous sulphate (FeSO(4) 7H(2)O), stable under the conditions prevailing in underground workings in moderate humidity and strong circulation of air. Similar conditions, favourable to the formation of melanterite may also arise, as has been ascertained, within the part of a deposit included in open cut workings. Melanterite is then readily formed in the loosened portions, covered with the diggings which have slid down on to it. M e lan te rite Among the collected samples of melanterite, two varieties were observed to exist. One of these is melanterite of a greenish colour, encountered almost exclusively in underground workings, especially within the deposit of the so-called powdery pyrite. This melanterite occurs in the form of stalactites (up to one meter long), and crusts of a fine-grained to coarse-grained structure, less often irregularly fibrous. Apart from rare crystal faces and skeleton crystals, occurring here and there, the author did not notice among the collected specimens of melanterite any euhedral crystals suitable for goniometric measurements. For the purpose of identifying the mineral the author was obliged, of necessity, to make use of cleavage flakes and thin slides, oriented and prepared in a plane parallel to the observed actual faces. The orientation of the optical directions and cleavage planes in one of such cleavage flakes, in accordance with (001), is shewn in Fig. 1 on page 448. The direction of the mean refractive index Np is steady, normal to the face (010) and the bisectrix of the angle (110): (110)= 82° of the planes of distinct cleavage, in accordance with (110). Measured on several flakes by the immersion method in sodium light, it amounted to N=1,482. Determinations of the remaining refractive indices gave Nmax=N(beta)= ,486, and Nmin=N(alfa)=1,473. N(gamma)-N(alfa)=0,013. The optic sign is negative. Specific gravity = 1,876 g./c. c. The results of a chemical analysis, carried out with a sample of greenish melanterite, are listed in Table I, included in the Polish text. The water of crystallization was determined by heating in an electric furnace (differential thermal method); in connection therewith it was ascertained that one-seventh of it does not escape until the temperature exceeds 200° C (mostly at c. 270° C), while its last molecules are still discharged at a temperature of 490—500° C. Very instructive is a comparison of the above-mentioned results with the dehydration curve shown in Fig. 2 (page 450), prepared by J. Glogoczowski who kindly supplied it to the author of this publication. As regards the origin and the paragenetic conditions of the mineral described, emphasized ought to be the mass occurrence and the facility with which melanterite is formed by means of the decomposition of iron disulphide. Facilitated by the large area of the earthy, «powdery» pyrite attacked by atmospheric agents, the above-mentioned process is sufficiently rapid for a large part of the still undecomposed sulphide dust to be imprisoned in the melanterite in the form of inclusions. In case of the decomposition of a disulphide containing impurities composed of vein clays = clayey selvages or gouges (German «Gangletten»), melanterite is the first to crystallize, until the moment when there is produced the stechiometrie proportion FeSO(4):Al(2)(SO1)3= 1:1, requisite for the formation of halotrichite. Melanterite is an unstable mineral under the physico-chemical conditions prevailing at the surface; normally it undergoes either leaching out, or else — in case of favourable conditions of humidity and aeration — alteration into copiapite. Magnesium m e lan te rite The other variety of melanterite is externally well characterized by a blue-green to light-blue colour, and by its association with halotrichite and dolomitic rocks. This variety, particularly common in open cut and underground workings carried out in dolomitic rocks or in their neighbourhood, is distinguished by a constant MgO content of several per cent., this being an inducement to accept a separate name: magnesium melanterite. Similarly as in the variety described previously, the chief form of its occurrence are granular aggregates, forming incrustations, stal gates (of spherical, reniform and other shapes) is mostly smooth, corroded, or else rough on account of skeleton crystals and rare scattered faces, the latter belonging mainly to (010), (101), (001), and producing an indistinctly tabular structure of the grains. The interfacial angles between identifiable faces, and the cleavage planes, both as to their direction and the degree of their distinctness, agree with the ones measured in normal melanterite. An easy differentiation of the two varieties is made possible only by means of chemical analysis and microscopic observation of the optic density. Apart from the observations carried out on fragments obtained ,by taking advantage of cleavage in accordance with c (001), many data were also provided by measurements executed on microlites which are formed on a glass slide from the saturated solution consisting of the hygroscopic water adsorbed on the sulphate grains. These microlites, attaining at a relatively rapid rate a diameter of 0,05 mm and more, are characterized — as indicated by Fig. 3. — by strong development of their faces: (010), (001), (110) and (101). It follows from the measured extinction angles and the values of the refractive indices in sodium light that the variety richer in magnesia, preserving the general optical orientation, has a slighty higher extinction angle: ZAc= c 80°, or ZAa=24°, as well as slightly lower corresponding refractive indices: N(gamma) =1,483; N(beta)=1,480; N(alfa)=1,472-1,470. Birefringence N(gamma)—N(alfa)= 0,011—0,013. The optic axial plane is parallel to (010). The optic sign is negative. The interfacial angles measured on the microlites — (110): (110)= 98°, and (101): (001)—45° — are close to the ones quoted for melanterite by Schaller (97°47'; cf. N. J. f. Min., 1904, II, p. 40) and Zepharovich (43°44'; 1879, p. 189; cf. Hintze, 1930, p. 4359). The results of the chemical analysis concerning the sample of blue-green melanterite are shown in Table II on page 452. Furthermore, the partial analyses of the magnesia content gave 2,95% and 10,10% by weight in the samples from the Holy Cross Mts., and 3,40% in the sample from Olkusz. The quantity of water of crystallization and the mode of its combining seem to be analogical to the ones quoted in the description of the greenish melanterite. The blue-green variety of melanterite is considerably rarer in comparison with the green variety. An unquestionable association exists between the distribution of magnesium melanterite and the occurrence of dolomites in the wall rocks. This association explains sufficiently the high magnesia content in the ferrous sulphates and the constant presence of epsomite and halotrichite in the paragenetic complex arising during the oxidation and hydration of pyrite, and also during the dissociation of ferrous sulphate in dilute solutions: the free anion of sulphuric acid attacked the adjacent country rocks — such as dolomites, marls, shales and gouges — giving in consequence epsomite originating from the dolomite, and halotrichite from the marls, shales and gouges (vein days). Part of the mobile Mg ion, released during the above-mentioned alterations, occupied the place of Fe" during the precipitation of Fe and Al. compounds, both in melanterite and halotrichite. The quoted example is one more instance of the substitution, known to exist in the mineral world, of Fe'* ions by Mg" ions, and the production of the proper isomorphous mixtures or atomic substitution series (cf. amphiboles, pyroxenes, spinels, garnets, etc.). The possibility of the existence of similar mix-crystals within heptahydrous ferrous sulphates was ascertained relatively long ago, during the attempts to reproduce artificial mix-crystals of Fe" and other bivalent metals (Basilius Valentinus, J. W. Retgers). Considering the discovery, by means of analysis, of a large magnesia admixture in the specimens of natural melanterite from the deposit at of the Holy Cross Mts. and of Cracow area and in two earlier analyses, and in view of the fact that separate names are used for mixcrystals of Fe, Mn, Zn, and Cu (luckyite, sommairite, pisanite), the author proposes the use of the name magnesium m e lan te rite for minerals belonging to the isomorphous series (Fe, Mg)SO(4).7 H(2)O. 2. Alteration products of country and gangue rocks The SO(4) anion produced during the weathering processes of pyrite, and partly combined with free sulphuric acid, attacked first of all the country rocks of low chemical resistance, contributing to the production of various secondary sulphates. Depending upon the lithological composition of the above-mentioned rocks, from dolomite were produced the mineral epsomite and small quantities of halotrichite, while from marls, shales, and vein clays were formed halotrichite and alunogen. Epsomite It is one of the very common alteration minerals forming crusts and stalactites in underground workings, and efflorescences within open cut workings. The habit of the individual anhedral grains is distinctly tabular and, furthermore, prismatic to fibrous. The orientation of the most common crystal faces, cleavage planes and optic directions is illustrated in Fig. 4 on page 454. The optical properties of the epsomite (N(gamma) =1,461; N(beta)= 1,455; N(alfa)= 1,434; N(gamma)—N(alfa)= 0,027), similarly as the specific gravity (1.684 g./c. c.) and other properties, do not differ from those generally accepted for this mineral. It is only the chemical analyses of the compact and acicular varieties of the epsomite which impart to it a certain distinctive position, on account of the contents of ferrous oxide which are higher than the ones normally encountered (cf. Table III). The lower manesia contents, in comparison with those calculated theoretically, are further explainable by the great quantity of liquid inclusions, visible even under a magnifying lens (solution of H(2)SO(4)). The above-mentioned admixture of FeO, proceeding from contamination of the parent solution by decomposition products of the ferrugineous impurities in dolomite, or else of iron disulphides or siderite, entered into the composition of epsomite by means of ionic substitution, Fe” taking the place of Mg” . Taking into consideration the different structural type in the two terminal links (rhombic epsomite and monoclinic melanterite), the mix-crystals of (Fe, Mg)S04.7H20 , therefore, are an example of an isodimorphous series in which still unknown are all the intermediate stages. The stability of epsomite is dependent upon the vapour pressure of water in the surrounding atmosphere. When the pressure is too low, epsomite undergoes dehydration, in consequence of which is produced the observed variety which is probably sexihydrate. From the admixture of ferrous sulphate in epsomite is produced the mineral copiapite. Halotrichite It is a common alteration product of wall rocks, particularly Devonian marls, Silurian shales, and vein clays (gouges). The only and very characteristic habit of individuals of halotrichite is a fibrous habit with a high proportion of elongation. The separate fibres are assembled in bundles arranged in a pinnate, radiating or divergent manner (criss-cross felted aggregates). The surface of the aggregates is spheroidal, globular, etc. It is also known in the shape of very small microliths (0,05 mm long on the average), forming alone or together with copiapite aggregates of grease-like consistence, commonly called «rock butter», strongly saturated with concentrated sulphuric acid. The maximum angle of extinction in relation to the direction of elongation (Nmin.A c =3 1 ° ; see Fig. 5), the refractive indices (Nmax. = 1,494; Nmax=1,483), and the birefringence (Nmax.—N min. = 0,011) make possible an easy differentiation from epsomite which occasionally has a similar habit. The specific gravity is 1,840 g./c. c. It follows from the tabulation of two chemical analyses (Table IV on page 458) of the parallel fibrous and criss-cross felted varieties of halotrichite that the halotrichites are generally mix-crystals poor in pickeringite molecules (4,45% and 9,56% by weight of MgAl(2)(SO4)4). Their content as regards water of crystallization approximates the theoretical one (22 molecules of H20) and amounts to 23,2 and 22,5 molecules of H20 in the fibrous and criss-cross felted varieties respectively. Release of water of crystallization takes place in a temporature range from 40° to 360° C. A requisite condition for the production of halotrichite, apart from the presence of clayey substances in the surroundings of the decomposing disulphide, is also high acidity of the solutions; this lowers the oxidation of FeSO(4) and makes hydrolysis of MgAl(2)(SO4)4). impossible. Halotrichite readily occurs in the association halotrichitemelanterite, while the association melanterite-alunogen has never been discovered. The iron-aluminium alum described is as a generally unstable compound under the conditions of surface weathering. The chief alteration product is the mineral copiapite. Keramohalite (alunogen) In a like manner as halotrichite, alunogen is a mineral which is genetically associated with the occurrence of clayey rocks. The morphological properties of this- mineral make possible its easy differentiation from the associated and intergrowing halotrichite. The habit of the examined fragments of alunogen is exclusively laminated, in accordance with 6 (010). The separate laminae are assembled in aggregates with a parallel, radiated or divergent structure, forming coatings and crusts on the halotrichite aggregates. Fig. 6 on page 460 shows the relation of the habit of individuals or their fragments to the directions of cleavage planes and the optic directions. Calculated by the immersion method in Na— light, the refractive indices amounted to N(gamma)=1,472; N(beta)= 1,464; N(alfa)=1,461. Birefringence is weak, amounting to 0,011. The optic sign is positive. Alunogen is probably produced from slightly acid solutions which — in the presence of a suitable PH concentration — impeded the hydrolysis of aluminium sulphate, but frequently did not prevent the release of goethite, produced from the hydrolysis of ferrous sulphate uncombined in halotrichite. As a rule the alunogen occurs in association with halotrichite as the last in succession, but it has never been discovered in company with melanterite. In general, it is a quite stable mineral, subject to leaching at the most. 3. Final sulphate alteration products Copiapite Basic ferric sulphate, the mineral copiapite, belongs undoubtedly to the most stable sulphate alteration products under normal surface conditions. Being common on the surface, forming, for instance, nests of several centimetre dimensions within open cut workings, or else the common efflorescences on dumps, it is at the same time almost unknown, in underground workings. The most common form in which copiapite occurs are earthy aggregates or mealy efflorescences, composed of microscopic scales of copiapite. The dimensions of these scales do not exceed, as a rule, 0,07 mm in diameter, and 0,002 mm in thickness. Fig, 7 and 8 show the shape of these so-called (by Zirkel) microplakites and of the considerably rarer belonites, as well as the optic orientation and the cleavage- plane orientation. The optical properties of copiapite distinguish it among the above-discussed sulphates by its uncommonly high refractive indices (N(gamma)=1,597; N(beta)=1,548; N(alfa)=1,530), strong birefringence (N(gamma)-N(alfa)= 0,067), and pleochroism, distinct even in very small scales, in the colours: X — light sulphur-yellow; Y — light yellow to colorless; Z — light greenish-yellow. The absorption scheme is X > Z > Y . The optic sign is positive. The specific gravity, determined picnometrically, amounted to 2,056 g./c. c. Hardness 2,5. Chemical analyses were carried out with two samples of copiapite; their results are listed in Table V on page 464. On the basis of the quoted molecular relations, it may be stated that the copiapites are almost pure basic ferric sulphates with the formula: 2Fe(2)O(3).5SO(3).16—18 H(2)O. Water of crystallization is released in the interval from 50° to 110°C in a quantity of c. 7—9 molecules; above 110°C in a quantity of 8 to 9 molecules; its last molecules still escape at a temperature of c. 500° C. The course of dehydration in an electric furnace (differential thermal method) is shown in Fig. 9 (on page 463), prepared kindly by J. Glogoczewski. A comparison of the curve shape for copiapite with the corresponding curve for melanterite reveals, particularly in the interval from 85° to 130° C; the influence exerted by the dissociation of copiapite (S02 being removed in temp. 100—270° C). Genetically copiapite is undoubtedly associated with the oxidation and hydration of iron disulphide, or of the ferrous salts produced therefrom (melanterite, halotrichite). In spite of its chemical stability (according to Posnjak and Merwin it is stable in relation to its saturated solution only at a temperature below 90° C), the abovementioned mineral undergoes, in a relatively easy manner, leaching out, or in the presence of a decreasing concentration of S04” ions it is further subjected to hydrolysis, forming the final alteration product of iron sulphates, i. e. the mineral goethite. IV. Conclusions A comparison of the results of observations carried out on the sulphate minerals on the deposit suggests, first of all, the conclusion concerning the close association between the chemical composition of parent rocks and the kind of sulphate minerals produced, exhausting all possible sequences of deposition, under the local conditions of occurrence. The absence of many minerals produced by weathering, associated genetically with other depositary and «hydrogeological conditions (absent are neutral iron sulphates, such as coquimbite and quenstedtite, in view of the deficiency of reduction agents), is compensated by their unprecedented, as a rule, quantity and purity. The production of such a great abundance of sulphate minerals was due, in the first place, to the specific mineral and water conditions. The high degree of dispersion of the chief mineral in the deposit (i. e., pyrite or marcasite), the exceptionally favourable water conditions, and the relatively great lithological diversity of the country rocks — all this contributed to the origin of numerous manifestations of alteration processes. The appended Fig. 10 has been prepared for the purpose -of illustrating the course of the above-mentioned processes, with special reference to the migrations of ions and compounds arising during these transformations. In this table the existence of the four principal mineral associations is shown graphically. The first association, composed of melanterite and copiapite (the latter sometimes absent, depending upon conditions), is closely connected with weathering changes within the sulphide minerals themselves. The second and third association, arising in accordance with the chemical composition of the gangue or country rocks and the Pw of the solutions, were strongly contaminated by Fe" ions produced during the decomposition of, sulphides, siderite and rocks containing ferrugineous impurities. The latter associations are characterized by the greatest and most diversified accumulations of sulphate minerals. The fourth association, on the other hand, is the final «insoluble» product of the oxidation and hydration of supergene minerals and almost the only constituent of the gossan, deprived of Mg and Ca salts which have been leached out. Thus ends the cycle of alteration processes of iron disulphides on the deposit in which the intermediate stages are formed by the sulphate minerals discussed in this work.

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