Geologiczne warunki występowania soli niebieskiej, ametystu i fiołkowego fluorytu

Antoni Gaweł


Geological conditions of origin of blue salt, ametyst and Violet fluorite

A.) Blue salt The research for the cause of blue colour of rock salt, commenced in the middle of the last century, up to the present time, has been the subject of mostly laboratory investigations- Papers of F. Kreutz (1), published by the Polish Academy of Science in the years 1892—1897 are a precious contribution to the problem. Owing to some simple but consequently carried out experiments, F. Kreutz came to the conclusion, that the blue colour of rock salt, and many other chemical compounds (KCl, KBr, KI, K(2)CO(3), BaCl(2), CaCO(3) is due to iron compounds absorbed in minute quantities by growing crystal. Durability of the colour depends on temperature and degree of oxidation. Samples of blue rock salt, discolorated by F. Kreutz when heated to a temperature of ca. 400°C, did not lose the. colour, if heated in paraffine, without the access of air. In consequence of that observation Kreutz regained the blue colour of discolored crystals, applying during the heating, sodium and potassium vapours as reducing agent. Owing to difficulties of analytical determination of colouring substances, the results of these experiments were interpreted, as if sodium vapours influenced the internal structure of crystal and caused the blue colour. Since then, these experiments have been known as F. Kreutz's method of colouring salt with alkali metals. It is obvious,, that laboratory conditions cannot be supported by the natural conditions in which rock salt occurs. At present the D o e lte r ’s view is generally accepted, that the blue colour is due to emanation of radioactive substances. This view has been supported by the discovery of radioactive isotope of potassium. Experiments with radioactive emanations, and still before with kathode rays (Goldstein) proved, that salt crystals may be coloured blue or violet, after a transition stage of wine-yellow colour. It is accepted that radiation disturbing the normal structure of crystal lattice causes the coloration. This view seems to be supported by spectrographic examination of colourless salt crystals with blue external zone from Stassfurt; it dit not show any differences in chemical composition of contaminations in both parts of examined crystals. Therefere the authors of these experiments (T. G. Kennard, D. H. Howell and M. P. Yaeckel) think, that the blue colour is not due to colouring matter. In the year 1935 in the salt mine Hall in Tirol a wineyellow variety of salt was found, which under influence of day-light becomes colourless. (0. Schauberger (3)). These wine-yellow salt associated at Hall with blue salt is regarded as the first example of salt with natural yellow colour, corresponding to the state, which precedes the blue colour caused by radioactive agents (K. Przibram (4)). Encouraged by the observations in the mine at Hall I have undertaken a research aiming at the solution of this problem, bearing in mind geojogical conditions of occurence of blue salt. During a survey of the Wieliczka mine, in the third level in the cross-cut Ferdinand d’ Este I-st, I have found wineyellow fibrous salt, filling fissures in saline clay, which covers as the younger horizon the northern part of the salt body. Veins are associated with numerous fissures, through which water is dripping in a small amount, depositing iron hydroxydes on the walls of the cross-cut. In this place the blue salt used to be found. The blue salt is very rare in Wieliczka mine. J.N. Hrdina (5) was the first, who in 1842 mentioned its occurence and H. Rose described it in 1863. F. Kreutz states, that during his time, blue salt had not been found ał Wieliczka. The Institute of Mineralogy of the University of Cracow possesses in the Museum a sample of fibrous blue salt found at the fourth level of the Nadachów „chamber“, but no exact gelogical conditions of occurence of this sample are known. The Nadachów-„chamber“ is situated in ,,Spiza“salt (sal spissum lat.), along which the same saline clays occur, in which on the third level blue and yellow salt was found. The sample was probably found in the first half or about the middle of the last century. The presence of oxidised iron compounds in the veins with blue and yellow salt indicates the percolation of surface waters. The occurence of yellow and blue salt with hydroxides iron covers, in the same veins, indicates that they are in a paragenetic association. It should be mentioned, that in no analysis of various Wieliczka salts, potassium has been found; this excludes the possible interpretation of blue colour of vein salt, as being due to potassium. I also have the folloving observations in potassium salt mines at Kałusz, Hotyń and Stebnik, where beautiful crystals of blue salt occur: 1) Salt crystals occuring together with sylvite in sylvinite beds are colourless, or milky-white, and show no traces of blue colour which should be expected here, owing to the presence of large quantities of potassium, if only radioactive radiation were responsible for the blue colour. 2) Blue colour occurs only in these parts of mines where secondary recrystallisation is visible. 3) In recrystallized parts besides colourless crystals, also yellow and blue crystals of variable intensity of colour appear. The blue colouring is often zonal, or appears along pyramides of growth and it is sometimes limited to the colourless part along the walls of pyramidal cube, or forms irregular nebulous centres These salts are not always accompanied by sylvine but in fissures and nests with blue salt, rusty films of iron hydroxides can be observed. At Stebnik two exposures may be traced in the level S/III where percolating surface waters have deposited aggregates of yellow, blue and colourless crystals among iron hydroxides. Another proof, that blue salt occurs only in recrystallized salts, is furnished by a sample, provening from a vein from Solno near Inowrocław. The vein cutting a potassium salts bed, regarded by ]. Poborski (6) as a sylvite vein occuring on the fifth level of the Solno mine, is filled with secondary crystals, some of which are uniformly and entirely coloured in .light-blue or possesses tiny blue spots disorderly scattered. The association of blue salt with yellow salt and with iron hydroxides compounds, must represent the natural conditions to which the blue colour is due. Therefore I am inclined to return to F. Kreutz’s views, that the blue colour is due to admixtures of iron compounds. Colloidal iron hydroxides, forming the dispersed phase in the crystal, may cause the colour, which depending on their quantity and particularly on the dimensions of particles, may be orange, yellow or blue, For comparison the measurements of colloidal particles of gold in sylvite, executed by ]. Ch. Repsche (7) should be reminded; he showed that paie-rose, blue or greenish- blue colour of sylvite is caused by the different dimensions of particles. As colloidal particles possess an electric charge, they influence iones or atoms of the crystal lattice, in which they are dispersed, and deforming their external zone of electrons, they cause distortions in normal structure of the lattice, what is manifested in colour. This action of colloidal particles, probably operating from a certain diameter, may be compared to the mutual influence of molecules in mixed crystals of coloured compounds, or in alloys. The colour of such mixed crystals is due not to the summing up of colours of their compouds, but according to T. Barth, G. Lunde, J.A. M. van Liempt (8) to distortion of the electrons position at the surface; of atoms and ions. The heating of blue crystals restores in a way the balance of the lattice thus causing the discoloration. The action of sodium vapours, kathode radiation and radioactive agencies may be interpreted by their influence on the distribution of electrons, and the state of electric charge of the substances, dispersed in the crystal sensitive to colouring. B) Amethysts from the Cracow resion Amethysts occuring in fissures and nests in melaphyres, were mentionad for the first time in 1787 by J. Jaśkiewicz (9), Professor of mineralogy at the University of Cracow and by Ph. Carosi (10) in 1783. They are not very intensely coloured in violet. The colour is distributed sometimes in crystals of quartz in form of turbid local concentrations. In the quartz geodes there are also crystals of lemon-yellow colour. Some crystals are colourless to a depth of a few mm, but their interior with sharp contours, is violet or yellow coloured. In these colourless parts of crystals overmantling the coloured ones, inclusions of goethite may be observed. They are directed to the rhombohedral walls, thinner and threadlike toward the centre and becoming thicker toward the crystal surface. These inclusions usually terminate with crystallographic forms of goethite. Often goethite crystals project over the quartz surface in form of tiny wedges. The paragenesis of quartz '3nd goethite is mentioned by E. Liesegang (11), who presumes, that bluish-violet colour of amethyst is connected with iron compounds dissolved in the crystal. This may be proved by the facts observed in the Cracow district. The origin of the amethysts of the Cracow district is connected with the action of hydrous low-temperature solutions on the volcanic rocks. From these solutions, calcite was the first mineral which crystallized on the walls of getfdes, with the dominating form (2131). On the calcite, quartz was deposited, partly replacing it. This quartz was fine-grained or compact, "coloured in green by delessite. Sometimes silica was precipitated as feebly banded jasper or achate. Continuous, influx of silica solutions through the walls of cavities associated with simultaneous dissolution of calcite caused the crystallisation of large (several cm) crystals of quartz, at first yellow, becoming later more and more violet; the violet colour becomes the most intense at the convergence of terminal rhombohedral edges („Spitzen-Amethyste“ from Rio Grande and Uruguay). In the most intensively coloured amethysts and in colourless quartz hoods (Kappenquartze) on amethysts there are needle-like crystals of goethite, from what one may infer, that goethite crystallisation began together with amethysts. Owing to the speedy growth of goethite needle-like crystals were formed, subsequently overmantled by coloureless quartz in the last stage of silica solution influx. The presence of iron in the solutions from which quartz crystals were formed is due to the weathering of volcanic rocks. The solutions, acid at first, were neutralized by calcite originally filling cavities and afterwards as neutral or feebly alkaline precipitated silica in the form of quartz. Iron hydroxides formed during tne neutralisation of solutions, had been kept as hydrosols particles which were getting into the growing crystals as dispersed phase causing yellow or violet colour, until the concentration became sufficient to permit crystalisation of goethite. The violet colour of amethysts is due to the dispersed phase of iron hydroxides. The dependence of colour on the dimension of colloidal particles of dispersed phase has been described by St. ]. Thugutt (12) by his ultramicroscopic research, who admitted that the growing crystal may embrace, colloidal particles of the same mineral. The heating of amethysts up to 450° — 500° C leads to their discoloration (W. N. Andreew (13)), what reminds of similar behaviour of blue salt. C.) Fluorite of Kopaliny, Lower Silesia South of Lądek, district Bystrzyca a vein 0,5 km long and 40 m thick occurs, composed of mainly milky quartz and fluorite. Fractures and joints divide the vein in rectangular, or rhombohedric blocks; the surface of joints is covered with aggregates of quartz crystals, fibrous and perpendicular to the surface. The quartz - fibres do not occur in hexagonal prisms but show on their ends rhombohedral walls, along which crystal growth took place zonally, what is marked by alternating milky-white and translucent, or slightly bluish laminae. In the last stage of growth the ends of quartz fibres are pink from hematite powder deposited rhytmically on rhombohedral walls. Thin (a few mm) platy crystals of hematite, grown into the most external zonal laminae at the ends of quartz crystals may also be observed. During the last stage of crystallisation fluorite was deposited in the form of opaque, dark-violet fine-grained aggregates in which only exceptionally octahedral crystals, or their casts are visible. The paragenesis of quartz and fluorite with hematite, indicates high temperature solutions. Also octahedral form of fluorite according to G. Kalb (14) is appropriate for high temperature crystallisation, during which walls, parallel to the walls of greatest lattice density, i. e. cleavage ones dominate *. According to G. Berg the dark-violet colour of fluorite is also characteristic of high temperature solutions. In the vein at Kopaliny (Klessengrund) very infrequently small cubic crystals of fluorite occur; they are of secondary origin, growing in empty spaces, not entirely filled by darkviolet fluorite. These crystals are translucid, or feebly violet, in form of nebulae or little spots. They correspond to the younger low-temperature generation of fluorite. The paragenesis of quartz and fluorite with hematite seems to indicate that also in this instance the cause of violet colour should be sought in iron compounds. However, while in the case of blue salt and amethyst the presence of colloidal dispersed phase of iron hydroxides is beyond doubt, in this instance the presence of such a phase would seem problematic, concerning the high temperature origin of fluorite. But experiments of St. Thugutt (16) with solubility of quartz, cassiterite and other hardly soluble minerals indicate, that high temperature solutions are mostly colloidal; their origin is facilitated not only by high temperature but also by friction caused by orogenic movements. Therefore one may assume that in fluorite crystals from Kopaliny there are small amounts of oxide or hydroxides in dispersed state. The absence of luminescence of these crystals may be regarded as another proof; this fact communicated to me by Prof L. Chrobak from Wrocław, has also been confirmed by my observations. St. Kreutz (17) proved that, the absence of luminescence of calcite crystals is due to a larger amount of iron. Conclusions: 1) The blue salt from Wieliczka (South Poland) is a fibrous salt occuring in veins. Blue salts from Kałusz, Hołyń and Stebnik form aggregates and nests in fissures. They are recrystallized and occur together with colourless, yellow and orange salts and iron hydroxides. 2) Amethysts from the Cracow district possess numerous inclusions of goethite. 3) The fluorite from Kopaliny in Lower Silesia form the last link of the successive deposition in a quartz vein. Small amounts of hematite deposited in quartz crystals are associated paragenetic with the quartz and fluorite. 4) In all solutions iron hydroxides formed a dispersed phase during the crystallisation and depending on the dimension of their particles caused the blue coloration.

Full Text: