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The color of rock salt—A review
SEDIMENTARY GEOLOGY ELSEVIER
Sedimentary
Geology
94 (1995) 267-276
The color of rock salt-a
review
Peter Sonnenfeld University of Windsor, Department of Geology, 401 Sunset Avenue, Windsor, Ont. N9B 3P4, Canada Received
14 April 1994; revised version
accepted
25 July 1994
Abstract Pure rock salt is colorless, but natural occurrences are often discolored. Halite may be discolored by: (a) the inclusion of interstitial matter; (b) impurities; or (c) structural defects in the crystal lattice. Blue halite is absent beneath all potash seams and may occur some small distance above sylvite, but not above carnallite zones. It is frequently found in tectonically disturbed zones, in breached anticlines, fault zones, or fracture and crevice fillings. It is easy to induce a blue color in the laboratory by: (a) various methods of irradiation; (b) a soaking in sodium vapors; or (c) the emplacement of various impurities. None of these methods seems to have occurred in nature. Natural blue discoloration is concentrated in oviform clumps along paths of circulating brines. Descending brines quickly saturate with sodium and chlorine, leach, brecciate and recrystallize the halites and preferentially dislodge bromine ions from crystal lattices. Platelets of metallic sodium left behind in lattice defects cause a blue (or rarer yellow) discoloration.
1. Introduction Hitherto the color of halite [NaCl] and sylvite [KCI] has for the most part been studied only in the laboratory, on samples completely divorced from sampling sites and without reference to the stratigraphic or regional position of the sample within an evaporite basin (e.g., Przibram, 1926; Steinmetz, 1932; Wieninger, 1950; Dexter, 1951, 1954). In the early part of this century, the investigation of colored rock salt samples centered on detailed studies of their response to various types of artificial irradiation (e.g., Pieszczek, 1905; Siedentopf, 1908; Przibram and Belar, 1924; Doelter, 1925; Frisch, 1927; Zekert, 1927; Przibram, 1929, 1931, 1953, 1956; Borchert, 1948; 0037.0738/95/$09.50 0 1995 Elsevier SSDI 0037-0738(94)00093-X
Science
Dexter, 1951, 1954), or what traces of impurities they might contain (e.g., Siedentopf, 1905; Prinz, 1908; Valentinov, 1916; Doelter, 1920; Blank and 1927; Blank and Smekal, 1930; Urbach, Savostyanova, 1930; Born, 1934; Repshe, 1936; Friend and Allchin, 1940a,b; Serra, 1949; Wieninger, 1951; Miller and Fisher, 1974; Popov, 1975). Sonnenfeld (1984) summarized some of the data on *discolored salt.
2. Types of rock salt discoloration Both halite and sylvite are essentially colorless in their pristine state. However, they can also be milky white, brownish, grayish, or bluish black,
B.V. All rights reserved
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P. Sonnenfeld / Sedimentary Geology 94 (1995) 267-276
smoky, pink, red, lavender, violet, purple, yellow, green, pale or dark blue (Table 1). Sylvinite zones, i.e., zones of intimate mixtures of halite and
sylvite, may contain smoky, pink, light or dark blue, lavender, violet, purple or greenish halite (Cornu, 1908; Aprodov, 1945).
Table 1 Common
2.1. Colors due to interstitial
colors
in rock salt (for references
see text)
Color
Impurity
milky white
entrained inclusions
pink
finely dispersed hematite in sylvinite zone minor Mn-admixture
needles
red
finely dispersed
needles
orange
traces of sylvite particles 150-180
yellow
minor traces of hematite minor amounts of KC1 light bitumina sulfur as pigment particles 130-150 nm exposure to some radioactivity
green
atacamite [Cu,CI(OH),] chloritic clays particles 110-120 nm trace of KRr + Au traces of sylvite
grayish black
clay admixtures
brownish
organic
black
air bubbles
or fluid
hematite
nm
matter
blue
lattice defects/colour centers admixture of KBr + Au in sylvinite zone particles 90-110 nm Pb, Cu, Ag, Au as pigments by adding metallic Na in Na-vapor Cl- removal by ionization rapid growth
violet
slow growth particles SO-90 nm + 0.5% KCI or RbCl exposure to radioactivity
lavender
in sylvinite zone
purple
by cathode rays in sylvinite zone
black to dark blue
Na-vapor + pressure large FezO, platelets by cathode rays
smoky
in sylvinite zone
admixtures
The inclusion of interstitial admixtures, such as clay particles, bitumina, or minute crystals of other minerals, changes the color of rock salt. The intensity of the color is then a function of the amount of admixture. Milky white halite obtains its color from innumerable tiny air bubbles or fluid inclusions that are entrained during the primary precipitation (Reimer and Utter, 1979). Gray to black rock salt is a common product of clay admixtures (Manecki and Pawlikowski, 19751, e.g., at Chanaral in the Atacama Desert of Chile (L. Darapsky in Przibram, 1956). A black discoloration can be caused by an oriented growth of black hairs of cryptomelane, as in Permian sylvites of New Mexico (Sun, 19621, by an admixture of large platelets of hematite (Gawel, 1947; Richter, 1962, 19641, or by an ample presence of organic matter (Ujueta, 1969). Naturally green rock salt is produced by the admixture of atacamite [Cu,CI(OH),] in Austria (Cornu, 1907), green chloritic clay in Transcarpathia, or by traces of sylvite that contain goldbearing potassium bromide (Blank and Urbach, 1927). Finely dispersed hematite needles produce a red or bright pink rock salt or sylvite (Gawel, 1947; Richter, 1962, 1964). However, a dark pink color can also result from a slight admixture of manganese to halite (Murata and Smith, 1946). N-hydrides, such as urea, complex with MgCl, (Emons, 1967) and render it almost infinitely soluble, thus hindering its coprecipitation in carnallite. In a stratified brine covered by inflow the N-hydrides are preserved and foster the precipitation of the slowly nucleating sylvite. Carnallite precipitates only where the dissolved N-hydrides, that are derived from the decomposition of proteins, are destroyed by sunlight in a brine that is in contact with the atmosphere. The destruction of these N-hydrides produces innumerable fluid inclusions of organic N, (often associated also
P. Sonnenfeld /Sedimentmy
with NH,) in carnallites (the so-called “popcorn salt”). When that happens, the pH of the brine is reduced and the bivalent Fe-compounds in solution (derived from the decomposition of chlorophyll and hemoglobin) are oxidized to goethite [FeO(OH)] and precipitate. Brown goethite soon turns into needles and platelets of red hematite [Fe,O,] in the intensely hygroscopic brine. Consequently, primary carnallite contains hematite and is red. Leaching produces at first a red sylvite and eventually even a red halite (Sonnenfeld, 1992a). However, halite veins also may turn red by leaching ions from adjacent clays. Later recrystallization displaces the hematite needles to crystal boundaries, leaving a colorless halite or sylvite with red crystal outlines. A yellow color is introduced by: (a) Minute traces of hematite (Lupinovich and Kislik, 1965). (b) A small admixture of sylvite, whereby 5.4 ppt KC1 or RbCl suffice (Aprodov, 1945); most tested halites contain half or less of that amount (Przibram, 1956). (c) A bituminous contamination. The inclusion of organic matter may ultimately lead to dark brown to black hues (Ujueta, 1969). (d) Exposure to radiation, insufficient to produce blue or violet discoloration (Przibram, 1936a, 1950). The yellow color is more unstable than the violet or the blue (Przibram, 1956) and may turn to blue under overburden pressure (Shcherbina, 1958). A yellow discoloration in salt occurs at Solikamsk (Aprodov, 1945) and at Hallstatt, Austria, where daylight bleaches it in minutes, but it remains stable in the dark to 160-180°C (Schauberger, 1935; Przibram 1936b). In contrast, a blue discoloration induced by cathode rays decreases after irradiation ceases (Borchert, 1948), even if the sample is kept in the dark (Doelter, 1925). (e) Clear rock salt subjected to both beta-rays and to pressure quickly turns yellow in response to the stress (Ludewig and Reuther, 1923; Przibram and Belar, 1924; Zekert, 1927; Przibram, 1927a; Wieninger, 1950). It turns yellow faster under application of one-sided pressure of at least 1 kPa and turns brown at pressures in excess of 3.9 MPa. Either way it changes to blue after exposure to daylight or some heat (Przibram,
Geology 94 (1995) 267-276
269
1927b; Wieninger and Adler, 1950). A gradual recrystallization over time of either halite or sylvite causes a loss of radiation-induced color, and can be arrested by applying a pressure of almost 1 GPa (Przibram, 1929, 1932). (f) Blue halite turns yellow when it is powdered and compressed again (Przibram, 1927b). 2.2. Blue colors due to crystal defect structures Halite crystals harbor a great variety of crystal lattice defects, which act as color centers. Shlichta (1968) and Gmelin (1973) have tabulated the halite crystal defect structures, that potentially lead to discoloration. Lattice spacings and Schottky defects reach a maximum near a 30% concentration of KC1 (Barrett and Wallace, 1954), causing an optimum number of structural disturbances and an optimum concentration of impurities (Przibram, 1951, 1953, 1956). A blue discoloration in halites or a violet one in sylvites is almost always due to the emplacement of various colloidal metals into defects in crystal lattices. This is true of gold, silver or copper or even bivalent alkali earth ions (Siedentopf, 1905; Serra, 1949; Seitz, 1951). Colloidal gold discolors both halite and sylvite (Blank and Urbach, 1927; Repshe, 1936; Friend and Allchin, 1940a, b). Bromine concentrations in the brine and bromide substitutions in the crystal lattice plays an important role, because bromine in brines catalyzes the mobilization of gold. Gold-bromide complexes are stable (Miller and Fisher, 1974; Popov, 1975). Depending on the amount of the gold-bearing KBr the rock turns green or blue (Blank and Urbach, 1927). Most of these metals do not occur naturally in sufficient quantities in halite lattices to cause any sizable discoloration. Halite crystals occasionally absorb light at wave lengths typical of sulfur ions and yield a blue discoloration. This feature indicates both an increase in the number of color centers and the presence of colloidal sulfur, which in some instances discharges upon heating as fine globules (Prinz, 1908; Doelter, 1920; Wieninger, 1951). Some samples of blue halite discharge organic liquids upon heating (Prinz, 1908) others evolve
270
P. Sonnenfeld /Sedimentary
gas upon solution (Cornu, 1908), some cloud-over upon heating because hydrated cations have been built into the precipitating halite (Yamamoto, 1938; Truesdell and Jones, 1969). By far the most common cause of a blue discoloration of rock salt is the occurrence of a stoichiometric excess of metallic sodium in crystal lattices (Cornu, 1910; Seitz, 1946, 1954). Overall, a rapid rate of crystal growth fosters the occurrence of lattice errors and results in a deep blue color, slower growth in a clearer, more violet color (Przibram, 1929). The precise hue and the intensity of the color depend not only on the particle size, but also on the enrichment of submicroscopic sodium subchloride Na,Cl-crystals (Przibram, 1927b; Kahanowicz, 19321, or of colloidal sodium particles (Pustyl’nikov, 1975). A discoloration by the formation of the Na,Cl due to a beta-particle bombardment was suggested by Pieszczek (1905) and disputed by Siedentopf (1905). The existence of the subchloride, first postulated by Rose (1863) has since been questioned (Gmelin, 1964). In addition to colorless sodium chloride [NaCll and sodium subchloride [Na,Cl] there is also a rock salt of peppery taste, which could be an admixture in a ratio of 1: 18 of NaCl of a specific gravity 2.165 and Na-Cl = Cl-Na (or Na,Cl,) of a specific gravity of 4.328 (Lopez-Rubio and de la Rubia-Pacheco, 1950). The transparent variety has a specific gravity of 2.2779 with some bromine substituting for chlorine, the blue variety has a specific gravity of 2.262 with empty lattice spaces instead of bromine substitutions. 2.3. Color hues The hues are caused by different degrees of dispersion and depend on the diameter of the colloids: particles 80-90 nm in size discolor to a bluish violet hue; particles 90-110 nm in diameter to a blue hue; particles 110-120 nm in size to a greenish hue; particles 130-150 nm in diameter to a yellowish hue; and particles 150-180 nm in size to an orange hue (Doelter, 1910, 1920, 1925, 1929). Purple rock salt may contain dispersed small particles of metallic sodium, blue rock salt large ones (Savostyanova, 1930; Kahanowicz,
Geology 94 (1995) 267-27ti
1932; Liermann and Rexer, 1932; Przibram, 1936a, 1947; Holdoway, 1974; Tret’yakov, 1990). In Cambrian blue halites of Siberia Pustyl’nikov (1975) found colloidal particles S-100 nm in size arranged either in parallel bands and strings of nodular shapes or dispersed in patches around solution cavities.
3. Properties
of blue rock salt
Compared to ordinary rock salt, blue salt: (1) has a conchoidal fracture; (2) is harder (Pustyl’nikov, 1975); (3) dissolves more rapidly in water (Vinokurov, 1958); (4) may give an alkaline reaction when dissolved (Cornu, 1908; Guthrie, 19291, disputed by Spezia (1909); (5) has a lower index of refraction (Pustyl’nikov, 1975); (6) shows birefringence and pleochroism (Pustyl’nikov, 1975); pleochroic halos of irregular shape are unevenly distributed; (7) may have a higher pH (Vinokurov, 1958); (8) has a lesser magnetic susceptibility Winokurov, 1958); (9) requires an increase in the number of color centers per unit of volume (Siedentopf, 1905); (10) at times offers a distinct glow at 350°C (Guthrie, 19291.
4. Radiation
as a cause of discoloration?
4.1. Arguments in favor of radiation as a cause of discoloration Because it is easy to create a blue discoloration by exposing halite to either metallic sodium vapor or strong irradiation with various energy sources, it has been generally accepted that radiation is the cause of discoloration, as naturally occurring rock salt has never been exposed to sodium vapors. Blue rock salt can be produced artificially: (a) in about 10% of all crystallization runs from a solution saturated for both KC1 and NaCl and containing 320 g/l of MgCl, (Kirchheimer, 1976); (b) by adding sodium vapor or a mother liquor containing vaporized sodium (Siedentopf, 1905); (c) by electric sparks (Steinmetz, 1932); the blue color decreases even in the dark after the irradia-
P. Sonnenfeld/SedimentaryGeology 94 (1995) 267-276 tion ceases; (d) by ultraviolet irradiation (Dexter, 1951); (e) by subtracting chlorine ions through ionization (Siedentopf, 1905; Hoffmann, 1934); (f) by irradiation with gamma-rays (Dexter, 1954); (g) by irradiation with beta-rays from a Co-60 or a cathode-ray source (Przibram, 1953, 1956). Cathode rays used for ionization produce a metallic coating of sodium on halite, the sodium being the sputtered cathode material (Siedentopf, 1908; Frisch, 1927). The hues always grade from blue-black closest to the radiation source to pale blue and purple farther from it, the darkening of the crystal being caused by ionic diffusion (Seitz, 1946). The discoloration by a beta-particle bombardment occurs by splitting off chlorine to leave a sodium-enriched sodium chloride complex (Hoffmann, 1934; Hacskaylo and Otterson, 1953) or by adding an electron to a sodium ion and neutralizing it (Przibram, 1929, 1931, 1953, 1956; Apollonov and Koshchug, 1986). X-ray radiation produced free chlorine in halites only in minutest quantities (Hacskaylo et al., 1953). Nevertheless, it is easier to separate chloride ions and leave a sodium enrichment, than to achieve the opposite. The formation of color centers by proton irradiation depends on the dosage, rate, temperature, exposure to light, degree of strain, and the distribution of any impurities (Bird et al., 1981). However, even if the color appears to be identical in the visible spectrum, there occur important differences in the ultraviolet part, if X- or cathode-rays are used, or the sample is heated in sodium vapors (Seitz, 1946, 1954). Sylvite turns violet if subjected to radiation, but the color is likewise not permanent at room temperature, and fades rapidly in sunlight (Ludewig and Reuther, 1923). Minute traces of violet sylvite suffice to discolor halite. Some samples of blue rock salt are enriched in helium Walentinov, 19161, but the helium content is still about four times smaller than that of sylvite and slightly smaller than that of carnallite or kieserite (Strutt, 1909). The presence of helium may thus be merely accidental. Together with helium, lead chloride incorporated into halites may have radium as progenitor and may, therefore, be a potential cause of some blue
271
discoloration (Born, 1934); however, it merely increases the elastic limit about threefold (Blank and Smekal, 1930). Much lead is precipitated with primary halite and gypsum, and is then mobilized very early during recrystallization (Murata and Smith, 1946; Sonnenfeld, 19841, leaving a very insignificant amount in ancient halites. 4.2. Arguments against radiation as cause of discoloration The evidence for radiation damage is not unequivocal. After an exhaustive evaluation of all the data pertaining to light-sensitive blue and purple halite, Kirchheimer (1976, 1978) cautioned that radioactivity may not be the exclusive explanation for the discoloration. Most blue rock salt obtained no more radiation than adjacent undiscolored crystals. If radiation were the prime cause, all halite in contact with sylvite, superjacent or subjacent to it, should over time have turned blue, as the radiation is not particularly focussed. The discoloration should he most intense in direct contact with the potash seams and not occur in discrete bodies some distance from them. Moreover, the blue discoloration occurs only above, but not below potash horizons (Chirvinskiy, 1943). Naturally occurring blue salt shows several significant differences in properties compared to rock salt turned blue by irradiation or by exposure to sodium vapor in the laboratory. Natural blue halite displays thermoluminescence, blue halite due to a soaking in sodium vapors does not (Gmelin, 1964). Higher temperatures or friction are required to bleach natural blue halite, but even more for halite colored by sodium vapor. It has been known since 1775 that grinding makes the blue color disappear, and since 1864 that heating to 275°C has the same effect (Kirchheimer, 1976). Preheating the sample to 150°C inhibits the discoloration (Przibram, 1926). However, a discoloration induced by heating the sample in sodium vapor is retained even beyond 4Oo”C, especially if the specimen has first been exposed to lateral compression (Spezia, 1909; Przibram, 1929; Ivanov, 1953). Gmelin (1964) places the loss of discol-
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P. Sonnenfeld / Sedimentary Geology 94 (1995) 267-276
oration at 570°C. The sample eventually turns black, if it becomes deformed under pressure (Przibram, 1927a; Tammann, 1931). Colorless salt next to blue salt thereby discolors faster than other colorless salt (Ludewig and Reuther, 1923). This happens probably because such halite contains more lattice defects than a halite that precipitated at a greater vertical distance from a sylvinite deposit from less concentrated brines. However, the greater the degree of deformation, the more rapidly the color disappears (Haberfeld, 1933). Peaks in optical absorption centers of blue halites, discolored by radiation, are shifted compared to natural blue crystals (Kirchheimer, 1976). Illumination by electric light bleaches irradiated blue halite within a few hours, but fails to bleach the natural variety (Howard and Kerr, 1960). Most artificial radiation sources that are utilized to produce a color change, are several orders of magnitude stronger than the radiation naturally occurring in evaporite beds. The 40K radiation normally attenuates in a few millimeters of rock salt, since evaporites contain only 119 ppm of 40K in potash seams (Kirchheimer, 1976). Fluid or sylvite inclusions in blue halite are always surrounded by a halo of colorless halite approximating the thickness of any potential radiation influences. Przibram (1953, 19561, Shcherbina (1958) and Pustyl’nikov (1975) argued that the dosage of beta-radiation is adequate to create radiation damage in halites over time, but Kirchheimer (1976) countered that the halo of colorless halite around sylvite inclusions should present the deepest blue colors, if discoloration were introduced by radiation. 4.3. Blue salt in structural
settings
And&e (1912) and Kennard et al. (1931) took issue with the idea of coloring pigments, and ascribed the discoloration to purely structural causes. This is supported by a preferential occurrence of blue salt near faults, fractures, shear zones or contorted strata (Schultzky, 1926; Howard and Kerr, 1960; Gmelin, 1973), or in domal parts of positive structures near the sur-
face, where the halite beds were subject to deformation, recrystallization or brecciation. The blue discoloration of halite in fracture fillings is deemed to have occurred during or after the application of pressure, judging by the orientation of the blue color (Cornu, 1908; AndrCe, 1912). Blue halite is absent on deeper flanks (Pustyl’nikov, 1975). While linking blue halite to zones of recrystallization, Tret’yakov (1990) noticed the discoloration to be rare in anticlines, except where deformation had opened paths for percolating solutions. Atmospheric oxidation is then able to bleach plastically deformed crystals (Maisch and Drickamer, 1958). Plasticized portions of the rock salt show a lasting, more intense coloration due to an increase in sodium sites where the lattice was loosened (Smekal, 19271; the degree of disarrangement of the crystal lattice determines the hue, with the deepest one occurring at points of strongly disturbed crystal structure (Blank and Urbach, 1927). These are the very points where percolating brines find the easiest entry. Frequently, open channels with mobile intercrystalline fluids occur in the Zechstein evaporites of Germany (Hartwig, 1954). The occurrence of undiscolored fibrous halite inside blue halite crystals (Tret’yakov, 1990), normally a mark of fracture fillings, supports this concept. Fibrous or columnal halite occurs also at Hallein and Hallstatt in Austria (Przibram, 1956). 4.4. Association
of blue salt with sylvinite
While blue halite is often restricted to the contact zone between halite and sylvinite (Przibram, 1929) and thus to potash salt sites (Chirvinskiy, 1945), it is not always associated with bedded pure sylvite (Gawel, 1947; Pustyl’nikov, 1975). Sylvinites that are not affected by circulating waters preserve their content of fluid inclusions and develop only a weak blue discoloration (Tret’yakov, 1990). In Oligocene evaporites of the Upper Rhine valley, blue halite occurs together with swept-in variegated marls and clays, more than 15 cm above a sylvinite horizon (Kirchheimer, 1976).
P. Sonnenfeld /Sedimentary
Very coarse halites in the Middle Devonian Prairie Evaporite Formation of Saskatchewan, Canada, can be light blue and carry abundant air bubbles (Wardlaw, 1968). Sapphire-blue halite crystals, up to 50 mm long and with interctystalline void spaces up to 15 mm in diameter, occur in a near-surface position on top of Lower Cretaceous sylvites in Thailand and represent a dissolution front that has left a porosity of about 40% (Hite, 1982). A secondary sulfatization by percolating meteoric waters that affected all Permian evaporite deposits worldwide (Sonnenfeld, 1992a, b) fostered a blue discoloration in Upper Kama halites (Vakhremeeva, 1964) and in Werra halites (Zechstein Z-1) (Hartwig, 19541, both superjacent to red sylvites and KMg-sulfates. 5. Bromine in the crystal lattice The bromine content of halite, precipitated from increasingly concentrated brines, increases from the onset of halite precipitation very gradually until potash saturation. More bromine is incorporated in sylvites and carnallites than in associated halites. In overlying halites the bromine content is apt to fluctuate or decrease as the overlying brine again diminishes its concentration. The Br/Cl ratio in the brine is about 35 times higher than in the halite at the onset of halite precipitation and then drops. During sylvite precipitation it is lowered from 34 to 30, during carnallite precipitation it diminishes further to about 28 times (Sonnenfeld, 1984). However, where carnallite [KCl.MgC1,.6H,O] is the primary potash mineral, the magnesium captures a disproportionate amount of bromine, leaving associated halites impoverished (Tret’ yakov, 1990). Many more lattice defects occur, therefore, in halites associated with primary sylvinites or secondary carnallites, compared to those associated with primary carnallites or secondary sylvinites. Solid solutions of chlorides with bromides have more lattice defects than pure chlorides (Wollam and Wallace, 1956) because of the great difference in anion sizes. Bromine in halite remains, therefore, more mobile than chlorine.
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Geology 94 (1995) 267-276
Bromine halite
leaching
as a proposed
cause of blue
Blue rock salt is absent beneath potash beds and adjacent to red sylvites that are derived from the leaching of carnallites (Chirvinskiy, 1943). This suggests that the occurrence of blue salt is not a function of the initial concentration of the halite precipitating brine and a rising bromine content of the precipitated halite. It appears to be due to the downward percolation of waters that have not reached potash saturation. Indeed, the blue halite often comes in distinct, often sharply defined clumps, mostly in the cm-to-dm range (Chirvinskiy, 19431, or in oviform shapes (Kirchheimer, 1976) in a clear matrix, suggesting some dissolution of coigns. The same conclusion can be reached from crystals that are blue only at the corners and in small amounts along the edges (Przibram, 1956). Altogether it is significant that in all instances blue patches are confined to sites where water seems to have percolated along bedding planes, induced recrystallization, leached or filled fractures. Waters percolating down to potash horizons saturate quickly for sodium and chlorine ions, but at the same time remain deficient in bromine. They tend thus to decompose the rarer NaBr, which occurs in enriched amounts only in the vicinity of potash horizons. Brines leaching the bromine, while being unable to mobilize the metallic sodium, are thus responsible for producing the naturally occurring blue color.
6. Conclusions Basically colorless halite is discolored by interstitial admixtures, or by colloids inserted into structural defects of the crystal lattice. Widely invoked radiation damage by 40K or X7Rb cannot be the cause of naturally occurring blue halite, because the blue rock salt occurrences have obtained no more radiation than adjacent undiscolored crystals. They both have grown in the same environment; moreover, discoloration is restricted to halites above sylvinites. Consequently, radioactivity, that readily discolors specimens in
274
P. Sonnenfeld /Sedimentary
the laboratory, is not the cause of discoloration in nature. The same objection can be raised against the lattice incorporation of trace elements being the cause, especially those of class IB (Cu, Ag, Au) or IIA (bivalent alkali earths) of the periodic table. The phenomenon appears to be concentrated along paths of circulating brines, saturated for NaCl, that leach, brecciate and recrystallize halites and preferentially dislodge bromine ions from crystal lattices, leaving metallic sodium behind.
References Andree, K., 1912. Uber ein blaues Steinsalz. Kali, 6(20): 497-501. Apollonov, V.N. and Koshchug, D.G., 1986. A study of halite and sylvite discolouration in potash deposits (in Russian). In: A.L. Yanshin and G.A. Merzlyakov (Editors), Fizikochimicheskie zakonomernosti osadkonakopleniya v solerodnykh basseynakh. Nauka, Moscow, pp. 44-52. Aprodov, V.A., 1945. Blue halite from the Solikamsk potash deposit (in Russian). Akad. Nauk SSSR, Dokl., 48(6): 346-348. Barrett, W.T. and Wallace, W.E., 1954. Studies of NaCI-KCI solid solutions I/II. J. Am. Chem. Sot., 76(l): 366-373. Bird, J.R., Rose, A. and Wilkins, R.W.T., 1981. Decoration of dislocations by proton irradiation of halite. Nucl. Instrum. Methods Phys. Res., 191(1-3): 19-22. Blank. F. and Smekal, A., 1930. Uber den Einfluss kleiner Verunreinigungen auf die Kohasionsgrenzen der Steinsalzkristalle. Naturwissenschaften, 18: 306-307. Blank, F. and Urbach, F., 1927. Colloidal gold in alkali halite crystals. Naturwissenschaften, 15: 700. Borchert, W., 1948. Uber die Verfiirbung von Steinsalz durch Rontgenstrahlen. Beitr. Mineral. Petrogr., 1: 203-212. Born, H.J., 1934. Der Bleigehalt der norddeutschen Salzlager und seine Beziehung zu radioaktiven Fragen. Chem. Erde, 9( 1): 66-87. Chirvinskiy, P.N., 1943. The blue rock salt of the Solikamsk district (in Russian). Zapisy Vserossijskogo Mineralogicheskogo Obshchestva (Proceedings of the All-Russian Mineralogical Society), 72(l): 51-55. Chirvinskiy, P.N., 1945. Blue rock salt as a possible indicator of the presence of rock salt deposits (in Russian). Zapisy Vserossijskogo Mineralogicheskogo Obshchestva (Proceedings of the All-Russian Mineralogical Society), 74(4): 313314. Cornu, F., 1907. Uber den farbenden Bestandteil des griinen Salzes von Hallstadt. Gsterr. Z. Berg- Hiittenwes., 55(47): 571-572. Cornu, F., 1908. Mineralogische und minerogenetische Beobachtungen. Neues Jahrb. Mineral. Geol. Palaontol., 1: 22258.
Geology 94 (1995) 267-276 Cornu, F., 1910. Zur Frage der Fiirbung des blauen Steinsalzes. Zentralbl. Mineral., 324-330. Dexter, D.L., 1951. Absorption spectra of pure and colored alkali halite crystals. Phys. Rev.. 83: 435-438. Dexter, D.L., 1954. X-ray coloration of alkali halides. Phys. Res., 93: 985-992. Doelter, C., 1910. Blaues Steinsalz. Tschermak’s Mineral. Petrogr. Mitt., 28: 559-560. Doelter, C., 1920. Kolloidpigmente in Mineralien. Kolloid-Z., 26: 23-27. Doelter, C., 1925. Die Farbung der Minerale durch Strahlung. Tschermak’s Mineral. Petrogr. Mitt., 38: 4566463. Doelter, C., 1929. Blaues Steinsalz. Neues Jahrb. Mineral., Monatsh., 12: 304-310. Emons, H.H., 1967. Untersuchungen an Salzsystemen in gemischten Liisungsmitteln. 3rd Int. Kali-Symp., Proc. VEB Deutscher Verlag fiir Grundstoffindustrie, Leipzig, I: 257-268. Friend, J.N. and Allchin, J.P., 1940a. Blue rock salt. Nature, 145(3668): 2666267. Friend, J.N. and Allchin, J.P., 1940b. Colloidal gold, a coloring principle in minerals. Mineral. Mag., 25(170): 584-596. Frisch, R., 1927. Der Einfluss langsamer Kathodenstrahlen auf Steinsalz. Akad. Wiss. Wien, Naturwiss. KI., Sitzungsber., [2a], 136: 57-64. Gawel, A., 1947. Geological conditions for the origin of blue salt, amethyst and violet fluorite tin Polish). Pol. Tow. Geol., Rocz.. 17: 39-60. Gmelin, L., 1964. Handbuch der anorganischen Chemie, Vol. 21. Natrium, 8th ed. Chemie G.m.b.H., Weinheim. Gmelin, L., 1973. Handbuch der anorganischen Chemie, 21. Natrium, Suppl. Vol. 6. Chemie G.m.b.H., Weinheim. Guthrie, F.C., 1929. Blue rock salt. Nature, 123: 130. Haberfeld, M., 1933. Uber das Auftreten und Verschwinden von Farbung in Steinsalzkristallen unter Druck. Akad. Wiss. Wien, Sitzungsber., [2a], 142: 135-154. Hacskaylo, M. and Otterson, D., 1953. On the presence of free sodium in sodium chloride crystals, containing color centers and color center precursors. J. Chem. Phys., 21: 5522553. Hacskaylo, M., Otterson, D. and Schwed, P., 1953. On the presence of free chlorine in sodium chloride crystals, containing color centers and color center precursors. J. Chem. Phys., 21: 1434-1435. Hartwig, G., 1954. Zur Kontrolle von Kohlendioxid in den Werra- und Fulda-Kalisalzlagerstatten. Kali Steinsalz, l(5): 3-26. Hite, R.J., 1982. Progress report on the potash deposits of the Khorat Plateau. Thailand. U.S. Geol. Sun/., Open File Rep. No. 82-1096, 70 pp. Hoffmann, J., 1934. Uber Mineralfarbungen. Z. Angew. Allgem. Chem., 219(2): 197-202. Holdoway, K.A., 1974. Behavior of fluid inclusions in salt during heating and irradiation. In: A.H. Coogan (Editor), 4th Symp. Salt. N. Ohio Geol. Sot., Cleveland, Ohio, 1: 303-312. Howard. CL. and Kerr, P.F., 1960. Blue halite. Science, 132(334): 1886-1887.
P. Sonnenfeld /Sedimentary Ivanov, A.A., 1953. Fundamentals of Geology, Methods of Exploration, Prospecting and Evaluation of Mineral Salt Deposits (in Russian). Gosgeoltekhizdat, Moscow, 204 pp. Kahanowicz, M., 1932. ober die seitliche Strahlung und die Natur der firbenden Substanz im natiirlichen blauen Steinsalz. Z. Phys., 76: 283-292. Kennard, T.G., Howell, D.H. and Yaeckel, M.P., 1931. Spectrographic examination of colorless and blue halite. Am. Mineral., 22: 65-67. Kirchheimer, F., 1976. uber Steinsalz und sein Vorkommen im Neckar- und Oberrheingebiet. Bundesanst. Geowiss., Geol. Jahrb., [D], 18: 1-139. Kirchheimer, F., 1978. Das blaue Steinsalz. Kali Steinsalz, 17(J): 271-287. Liermann, H. and Rexer, E., 1932. ober das Wesen des blauen Steinsalzes. Naturwissenschaften, 20: 561. Lopez-Rubio, F.B. and de la Rubia-Pacheco, J., 1950. Some physico-chemical considerations on the constitution of native alkali metal salts. Ion, 10: 586-588, 592. Ludewig, R. and Reuther, F., 1923. Untersuchung der durch Radiumstrahlen hervorgebrachten Farbinderung von Kristallen mit Hilfe des Ostwaldschen Farbmessungsverfahrens. Z. Phys., 18: 183-198; 26: 45-53 (1924). Lupinovich, Yu.1. and Kislik, V.Z., 1965. On ripple marks in rock salt of the second potassium horizon of Starobinsk deposits (in Russian). Litol. Polezn. Iskop., 2: 174-176. Maisch, W.G. and Drickamer, H.G., 1958. Effect of pressure on color centers in alkali halite crystals. J. Phys. Chem. Solids, 5(4): 328-336. Manecki, A. and Pawlikowski, M., 1975. Mineralogical and petrographic study of selected rock salt sequences from the Wieliczka salt deposit (in Polish). Pol. Akad. Nauk, Oddzial Krakowie, Kom. Nauk Mineral., Pr. Mineral., 40: 41-64. Miller, A.D. and Fisher, E.I., 1974. Dissolution of gold in ferric chloride solutions and its deposition on pyrite (laboratory model) (in Russian). Geokhimiya, 3: 411-417. Murata, K.J. and Smith. R.L., 1946. Manganese and lead as coactivators of red fluorescence in halite. Am. Mineral., 31(11-12): 527-538. Pieszczek, E., 1905. ober die Natur des blauen Steinsalzes. Pharm. Ztg., 50: 929-931; 51: 700-703. Popov, V.S., 1975. Geochemistry of gold in the Upper Jurassic evaporite formation of southern Central Asia. Akad. Nauk SSSR, Dokl., 224(4): 929-932; Acad. Sci. USSR, Dokl., Earth Sci. Sect. (Engl. Transl. Am. Geol. Inst.), 224(1-6): 204-207. Prinz, W., 1908. Studien an farblosem und blauem Steinsalz. Sot. Beige GCol. PalContol. Hydrol., Bull., 22: 63-82. Przibram, K., 1926. Eine Erklgrung der Farbiinderung in Salzen. Akad. Wiss. Wien, Math.-Naturwiss. Kl., Sitzungsber., [2a], 135: 213-216. Przibram, K., 1927a. ober die Farbiinderung von Steinsalz unter Druck. Akad. Wiss. Wien, Math.-Naturwiss. Kl., Sitzungsber., [2a], 136: 43-56, 345-346. Przibram, K., 1927b. Das RItsel des blauen Steinsalzes. Kali, 21(17): 253-255.
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Przibram, K., 1929. cber natiirliches blaues Steinsalz. Akad. Wiss. Wien, Math.-Naturwiss. Kl., Sitzungsber., [2a], 138: 353-362, 483-495, 781-797; 141: 567-569 (1932); 143: 489-498 (1934). Przibram, K., 1931. Flrbung und Lumineszenz durch Becquerel-Strahlung. Z. Phys., 68(3): 403-422. Przibram, K., 1932. Umkristallisierung und Fiirbung. Akad. Wiss. Wien, Math.-Naturwiss. Kl., Sitzungsber., [2a], 141: 639-643. Przibram, K., 1936a. Das RZtsel des blauen Steinsalzes II. Kali, Verw. Salze, Erdiil, 30(J): 61-63. Przibram, K., 1936b. The yellow rock salt from Hall in Tirol. Nature, 137: 107-108. Przibram, K., 1947. Geniigen die bekannten Strahlungsquellen urn die Farbe von natiirlichem blauem Steinsalz und von Lhnlichen Mineralverfirbungen zu erklHren? Acta Phys. Austriaca, 1: 131-136. Przibram, K., 1950. ober das blaue Steinsalz. Tschermak’s Mineral. Petrogr. Mitt., [3], 2(l): 124-129. Przibram, K., 1951. cber die Rolle eines optimalen Stijrungsgrades und der Diffusion in der Verteilung gewisser Farben in Mineralen. 6sterr. Akad. Wiss., Math.Nat. Kl., Anz., 88: l-6. Przibram, K., 1953. Verfirbung und Lumineszenz. Beitrage zur Mineralphysik. Springer-Verlag, Vienna, 275 pp. Przibram, K., 1956. Irradiation Colours and Luminescence. A Contribution to Mineral Physics. Pergamon Press, London, 332 pp. (revised English translation of the preceding one). Przibram, K. and Belar, M., 1924. cber die Verflrbung durch Becquerel-Strahlung und die Frage des blauen Steinsalzes. Akad. Wiss. Wien, Math.-Naturwiss. Kl., Sitzungsber., [2a], 132: 261-277. Pustyl’nikov, A.M., 1975. Origin of blue color in halite in Cambrian salt deposits of the Siberian Platform. Lithol. Mineral. Resour. (Engl. Transl.), lO(3): 388-389. Reimer, T.O. and Utter, T., 1979. Kapillarit: Eine ungew6hnliche natiirliche Erscheinungsform des Steinsalzes. Neues Jahrb. Mineral., Monatsh., 3: 93-100. Repshe, I.K., 1936. Gold colloids in sylvite crystals (in Russian). Akad. Nauk SSSR, Acta Physicochim., 5: 173-188. Richter, A., 1962. Die Rotfirbung in den Salzen der deutschen Zechsteinlagerstltten I. Chem. Erde, 22(6): 508-546. Richter, A., 1964. Die Rotfirbung in den Salzen der deutschen Zechsteinlagerstitten II. Chem. Erde, 23(3): 179-203. Rose, H., 1863. Neue Reihe van Metalloxyden. Poggendorfer Ann. Phys., 120, 16 pp. Savostyanova, M., 1930. ober die kolloidale Natur der firbenden Substanz im verfiirbten Steinsalz. Z. Phys., 64: 262-278. Schauberger, O., 1935. Ein Vorkommen von lichtempfindlichem gelben Steinsalz im Haller Salzbergwerk. BergHiittenmann. Jahrb., 83: 115-l 17. Schultzky, J., 1926. uber den Ursprung der Blaufirbung natiirlichen Steinsalzes. PhD-Dissert., Univ. Halle. Seitz, F., 1946. Color centers in alkali halide crystals. Mod. Phys. Rev., 18: 384-408.
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Seitz, F., 1951. Color centers in additively colored alkali halide crystals containing alkaline earth ions. Phys. Rev., 83: 134-140. Seitz, F., 1954. Color centers in alkali halide crystals. Mod. Phys. Rev., 26: 7-94. Serra, A., 1949. Coloring principles in coloured minerals. Ric. Sci., 18: 1059-1060. Chem. Abstr., 43: 6116. Shcherbina, V.N., 1958. The irregularities of the blue coloring of halite and its possible causes. Akad. Nauk SSSR, Dokl., 2: 257-261. Shlichta, P.J., 1968. Growth deformation and defect structure of salt crystals. In: R.B. Mattox et al. (Editors) Saline Deposits. Geol. Sot. Am., Spec. Pap., 88: 597-617. Siedentopf, H., 1905. Ultramikroskopische Untersuchungen iiber Steinsalzverfarbungen. Phys. Z., 6(24): 855-866. Siedentopf, H., 1908. Uber kiinstlichen Dichroismus van blauem Steinsalz. Phys. Z., 8(23): 850-851. Smekal, A., 1927. Uber weitere Untersuchungen an deformierten Steinsalzkristallen. Akad. Wiss. Wien, Math.Naturwiss. Kl., Anz., 64: 22-24. Sonnenfeld, P., 1984. Brines and Evaporites. Academic Press, Inc., Orlando, Fla., 613 pp. Sonnenfeld, P., 1992a. Chemical facies of marine evaporites. Geol. Carpathica, 43(l): 3-14. Sonnenfeld, P., 1992b. Genesis of marine evaporites-a summation. Geol. Carpathica, 43(5): 259-274. Spezia, G., 1909. Metallic sodium as the alleged cause of the natural blue colour of rock salt. Zentralbl. Mineral., Geol., Palaontol., pp. 398-404. Chem. Abstr., 3: 2106. Steinmetz, H., 1932. Die blaue Farbe des Steinsalzes als eine Begleiterscheinung der Funkenentladung in Salzkristallen. Neues Jahrb. Mineral. Geol., Beilageband, [A], 65: 119128. Strutt, R.J., 1909. Helium in saline minerals and its probable connection with potassium. R. Sot. London, Proc., [4], 81: 278-279. Sun, M.S., 1962. Oriented growth of cryptomelane in sylvite, Carlsbad, New Mexico. Am. Mineral., 47(1-2): 152-155. Tammann, G., 1931. Umkristallisierung. Z. Elektrochem., 37: 429-436. Truesdell, A.H. and Jones, B.F., 1969. Ion association in natural brines. Chem. Geol., 4(l): 51-62. Tret’yakov, Yu.A., 1990. Distribution patterns of blue halite
Geology 94 (1995) 267-276 in potash salts (in Russian). In: Usloviya Obrazovaniya Mestorozhdeniy Kaliynykh Solei. Akad. Nauk, Sib. Otd.. Novosibirsk, pp. 35-40. Ujueta, L.G., 1969. Salt in the eastern Cordillera of Colombia. Geol. Sot. Am. Bull., 80(111: 2317-2320. Vakhremeeva, V.A., 1964. On microinclusions of hydrogen sulfide in salt rocks of the Upper Kama deposit (in Russian). Vsesoy. Nauchno-Issled. Proektn. Inst. Galurgii, Tr.. 46: 151-155. Valentinov, S., 1916. Heliumgehalt im blauen Steinsalz. Kali, 6(l): l-3. Vinokurov, V.M., 1958. Blue rock salt from the Solikamsk district (in Russian). Zapisy Vsesoyuznogo Mineralogicheskogo Obshchestva (Proceedings of the All-Union Mineralogical Society), 87: 5044507. Wardlaw, N.C., 1968. Carnallite-sylvite relationships in the Middle Devonian Prairie Evaporite Formation, Saskatchewan, Canada. Geol. Sot. Am. Bull., 79(10): 1273-1294. Wieninger, L., 1950. Verfarbung gedruckter Steinsalzkristalle durch Bestrahlung mit alpha-Teilchen von Radium. Gsterr. Akad. Wiss., Math.-Naturwiss. Kl., Sitzungsber., [2a], 159: 381-393. Wieninger, L., 1951. Uber die Natur und Herkunft der violetten oder blauen Farbung natiirlicher Steinsalzkristalle. Gsterr. Akad. Wiss., Math.-Naturwiss. Kl., Sitzungsber., [2a], 160: 147-180. Wieninger, L. and Adler, N., 1950. Der Einfluss von Erwarmung auf das Absorptionsspektrum von Steinsalz, das verfarbt ist durch alpha-Teilchen von Radium. Gsterr. Akad. Wiss., Math.-Naturwiss. Kl.. Sitzungsber., [2a], 159: 395-406. Wollam, J.S. and Wallace, W.E., 1956. A comparison of pycnometric and X-ray densities for the sodium chloridesodium bromide-potassium bromide systems. J. Phys. Chem., 60: 1654-1656. Yamamoto, T., 1938. Growth of crystals, IX. Relation between concentration of cations added to solution and their amount in the crystal formed. Jpn. Inst. Phys. Chem. Res., Bull., 17: 1278-1291. Zekert, B., 1927. Die Fat-bung von Steinsalz und Kunzit durch Becquerelstrahlen. Akad. Wiss. Wien, Math.-Naturwiss, Kl., Sitzungsber.. [2a], 136: 337-355.
Sedimentary
Geology
94 (1995) 267-276
The color of rock salt-a
review
Peter Sonnenfeld University of Windsor, Department of Geology, 401 Sunset Avenue, Windsor, Ont. N9B 3P4, Canada Received
14 April 1994; revised version
accepted
25 July 1994
Abstract Pure rock salt is colorless, but natural occurrences are often discolored. Halite may be discolored by: (a) the inclusion of interstitial matter; (b) impurities; or (c) structural defects in the crystal lattice. Blue halite is absent beneath all potash seams and may occur some small distance above sylvite, but not above carnallite zones. It is frequently found in tectonically disturbed zones, in breached anticlines, fault zones, or fracture and crevice fillings. It is easy to induce a blue color in the laboratory by: (a) various methods of irradiation; (b) a soaking in sodium vapors; or (c) the emplacement of various impurities. None of these methods seems to have occurred in nature. Natural blue discoloration is concentrated in oviform clumps along paths of circulating brines. Descending brines quickly saturate with sodium and chlorine, leach, brecciate and recrystallize the halites and preferentially dislodge bromine ions from crystal lattices. Platelets of metallic sodium left behind in lattice defects cause a blue (or rarer yellow) discoloration.
1. Introduction Hitherto the color of halite [NaCl] and sylvite [KCI] has for the most part been studied only in the laboratory, on samples completely divorced from sampling sites and without reference to the stratigraphic or regional position of the sample within an evaporite basin (e.g., Przibram, 1926; Steinmetz, 1932; Wieninger, 1950; Dexter, 1951, 1954). In the early part of this century, the investigation of colored rock salt samples centered on detailed studies of their response to various types of artificial irradiation (e.g., Pieszczek, 1905; Siedentopf, 1908; Przibram and Belar, 1924; Doelter, 1925; Frisch, 1927; Zekert, 1927; Przibram, 1929, 1931, 1953, 1956; Borchert, 1948; 0037.0738/95/$09.50 0 1995 Elsevier SSDI 0037-0738(94)00093-X
Science
Dexter, 1951, 1954), or what traces of impurities they might contain (e.g., Siedentopf, 1905; Prinz, 1908; Valentinov, 1916; Doelter, 1920; Blank and 1927; Blank and Smekal, 1930; Urbach, Savostyanova, 1930; Born, 1934; Repshe, 1936; Friend and Allchin, 1940a,b; Serra, 1949; Wieninger, 1951; Miller and Fisher, 1974; Popov, 1975). Sonnenfeld (1984) summarized some of the data on *discolored salt.
2. Types of rock salt discoloration Both halite and sylvite are essentially colorless in their pristine state. However, they can also be milky white, brownish, grayish, or bluish black,
B.V. All rights reserved
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P. Sonnenfeld / Sedimentary Geology 94 (1995) 267-276
smoky, pink, red, lavender, violet, purple, yellow, green, pale or dark blue (Table 1). Sylvinite zones, i.e., zones of intimate mixtures of halite and
sylvite, may contain smoky, pink, light or dark blue, lavender, violet, purple or greenish halite (Cornu, 1908; Aprodov, 1945).
Table 1 Common
2.1. Colors due to interstitial
colors
in rock salt (for references
see text)
Color
Impurity
milky white
entrained inclusions
pink
finely dispersed hematite in sylvinite zone minor Mn-admixture
needles
red
finely dispersed
needles
orange
traces of sylvite particles 150-180
yellow
minor traces of hematite minor amounts of KC1 light bitumina sulfur as pigment particles 130-150 nm exposure to some radioactivity
green
atacamite [Cu,CI(OH),] chloritic clays particles 110-120 nm trace of KRr + Au traces of sylvite
grayish black
clay admixtures
brownish
organic
black
air bubbles
or fluid
hematite
nm
matter
blue
lattice defects/colour centers admixture of KBr + Au in sylvinite zone particles 90-110 nm Pb, Cu, Ag, Au as pigments by adding metallic Na in Na-vapor Cl- removal by ionization rapid growth
violet
slow growth particles SO-90 nm + 0.5% KCI or RbCl exposure to radioactivity
lavender
in sylvinite zone
purple
by cathode rays in sylvinite zone
black to dark blue
Na-vapor + pressure large FezO, platelets by cathode rays
smoky
in sylvinite zone
admixtures
The inclusion of interstitial admixtures, such as clay particles, bitumina, or minute crystals of other minerals, changes the color of rock salt. The intensity of the color is then a function of the amount of admixture. Milky white halite obtains its color from innumerable tiny air bubbles or fluid inclusions that are entrained during the primary precipitation (Reimer and Utter, 1979). Gray to black rock salt is a common product of clay admixtures (Manecki and Pawlikowski, 19751, e.g., at Chanaral in the Atacama Desert of Chile (L. Darapsky in Przibram, 1956). A black discoloration can be caused by an oriented growth of black hairs of cryptomelane, as in Permian sylvites of New Mexico (Sun, 19621, by an admixture of large platelets of hematite (Gawel, 1947; Richter, 1962, 19641, or by an ample presence of organic matter (Ujueta, 1969). Naturally green rock salt is produced by the admixture of atacamite [Cu,CI(OH),] in Austria (Cornu, 1907), green chloritic clay in Transcarpathia, or by traces of sylvite that contain goldbearing potassium bromide (Blank and Urbach, 1927). Finely dispersed hematite needles produce a red or bright pink rock salt or sylvite (Gawel, 1947; Richter, 1962, 1964). However, a dark pink color can also result from a slight admixture of manganese to halite (Murata and Smith, 1946). N-hydrides, such as urea, complex with MgCl, (Emons, 1967) and render it almost infinitely soluble, thus hindering its coprecipitation in carnallite. In a stratified brine covered by inflow the N-hydrides are preserved and foster the precipitation of the slowly nucleating sylvite. Carnallite precipitates only where the dissolved N-hydrides, that are derived from the decomposition of proteins, are destroyed by sunlight in a brine that is in contact with the atmosphere. The destruction of these N-hydrides produces innumerable fluid inclusions of organic N, (often associated also
P. Sonnenfeld /Sedimentmy
with NH,) in carnallites (the so-called “popcorn salt”). When that happens, the pH of the brine is reduced and the bivalent Fe-compounds in solution (derived from the decomposition of chlorophyll and hemoglobin) are oxidized to goethite [FeO(OH)] and precipitate. Brown goethite soon turns into needles and platelets of red hematite [Fe,O,] in the intensely hygroscopic brine. Consequently, primary carnallite contains hematite and is red. Leaching produces at first a red sylvite and eventually even a red halite (Sonnenfeld, 1992a). However, halite veins also may turn red by leaching ions from adjacent clays. Later recrystallization displaces the hematite needles to crystal boundaries, leaving a colorless halite or sylvite with red crystal outlines. A yellow color is introduced by: (a) Minute traces of hematite (Lupinovich and Kislik, 1965). (b) A small admixture of sylvite, whereby 5.4 ppt KC1 or RbCl suffice (Aprodov, 1945); most tested halites contain half or less of that amount (Przibram, 1956). (c) A bituminous contamination. The inclusion of organic matter may ultimately lead to dark brown to black hues (Ujueta, 1969). (d) Exposure to radiation, insufficient to produce blue or violet discoloration (Przibram, 1936a, 1950). The yellow color is more unstable than the violet or the blue (Przibram, 1956) and may turn to blue under overburden pressure (Shcherbina, 1958). A yellow discoloration in salt occurs at Solikamsk (Aprodov, 1945) and at Hallstatt, Austria, where daylight bleaches it in minutes, but it remains stable in the dark to 160-180°C (Schauberger, 1935; Przibram 1936b). In contrast, a blue discoloration induced by cathode rays decreases after irradiation ceases (Borchert, 1948), even if the sample is kept in the dark (Doelter, 1925). (e) Clear rock salt subjected to both beta-rays and to pressure quickly turns yellow in response to the stress (Ludewig and Reuther, 1923; Przibram and Belar, 1924; Zekert, 1927; Przibram, 1927a; Wieninger, 1950). It turns yellow faster under application of one-sided pressure of at least 1 kPa and turns brown at pressures in excess of 3.9 MPa. Either way it changes to blue after exposure to daylight or some heat (Przibram,
Geology 94 (1995) 267-276
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1927b; Wieninger and Adler, 1950). A gradual recrystallization over time of either halite or sylvite causes a loss of radiation-induced color, and can be arrested by applying a pressure of almost 1 GPa (Przibram, 1929, 1932). (f) Blue halite turns yellow when it is powdered and compressed again (Przibram, 1927b). 2.2. Blue colors due to crystal defect structures Halite crystals harbor a great variety of crystal lattice defects, which act as color centers. Shlichta (1968) and Gmelin (1973) have tabulated the halite crystal defect structures, that potentially lead to discoloration. Lattice spacings and Schottky defects reach a maximum near a 30% concentration of KC1 (Barrett and Wallace, 1954), causing an optimum number of structural disturbances and an optimum concentration of impurities (Przibram, 1951, 1953, 1956). A blue discoloration in halites or a violet one in sylvites is almost always due to the emplacement of various colloidal metals into defects in crystal lattices. This is true of gold, silver or copper or even bivalent alkali earth ions (Siedentopf, 1905; Serra, 1949; Seitz, 1951). Colloidal gold discolors both halite and sylvite (Blank and Urbach, 1927; Repshe, 1936; Friend and Allchin, 1940a, b). Bromine concentrations in the brine and bromide substitutions in the crystal lattice plays an important role, because bromine in brines catalyzes the mobilization of gold. Gold-bromide complexes are stable (Miller and Fisher, 1974; Popov, 1975). Depending on the amount of the gold-bearing KBr the rock turns green or blue (Blank and Urbach, 1927). Most of these metals do not occur naturally in sufficient quantities in halite lattices to cause any sizable discoloration. Halite crystals occasionally absorb light at wave lengths typical of sulfur ions and yield a blue discoloration. This feature indicates both an increase in the number of color centers and the presence of colloidal sulfur, which in some instances discharges upon heating as fine globules (Prinz, 1908; Doelter, 1920; Wieninger, 1951). Some samples of blue halite discharge organic liquids upon heating (Prinz, 1908) others evolve
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P. Sonnenfeld /Sedimentary
gas upon solution (Cornu, 1908), some cloud-over upon heating because hydrated cations have been built into the precipitating halite (Yamamoto, 1938; Truesdell and Jones, 1969). By far the most common cause of a blue discoloration of rock salt is the occurrence of a stoichiometric excess of metallic sodium in crystal lattices (Cornu, 1910; Seitz, 1946, 1954). Overall, a rapid rate of crystal growth fosters the occurrence of lattice errors and results in a deep blue color, slower growth in a clearer, more violet color (Przibram, 1929). The precise hue and the intensity of the color depend not only on the particle size, but also on the enrichment of submicroscopic sodium subchloride Na,Cl-crystals (Przibram, 1927b; Kahanowicz, 19321, or of colloidal sodium particles (Pustyl’nikov, 1975). A discoloration by the formation of the Na,Cl due to a beta-particle bombardment was suggested by Pieszczek (1905) and disputed by Siedentopf (1905). The existence of the subchloride, first postulated by Rose (1863) has since been questioned (Gmelin, 1964). In addition to colorless sodium chloride [NaCll and sodium subchloride [Na,Cl] there is also a rock salt of peppery taste, which could be an admixture in a ratio of 1: 18 of NaCl of a specific gravity 2.165 and Na-Cl = Cl-Na (or Na,Cl,) of a specific gravity of 4.328 (Lopez-Rubio and de la Rubia-Pacheco, 1950). The transparent variety has a specific gravity of 2.2779 with some bromine substituting for chlorine, the blue variety has a specific gravity of 2.262 with empty lattice spaces instead of bromine substitutions. 2.3. Color hues The hues are caused by different degrees of dispersion and depend on the diameter of the colloids: particles 80-90 nm in size discolor to a bluish violet hue; particles 90-110 nm in diameter to a blue hue; particles 110-120 nm in size to a greenish hue; particles 130-150 nm in diameter to a yellowish hue; and particles 150-180 nm in size to an orange hue (Doelter, 1910, 1920, 1925, 1929). Purple rock salt may contain dispersed small particles of metallic sodium, blue rock salt large ones (Savostyanova, 1930; Kahanowicz,
Geology 94 (1995) 267-27ti
1932; Liermann and Rexer, 1932; Przibram, 1936a, 1947; Holdoway, 1974; Tret’yakov, 1990). In Cambrian blue halites of Siberia Pustyl’nikov (1975) found colloidal particles S-100 nm in size arranged either in parallel bands and strings of nodular shapes or dispersed in patches around solution cavities.
3. Properties
of blue rock salt
Compared to ordinary rock salt, blue salt: (1) has a conchoidal fracture; (2) is harder (Pustyl’nikov, 1975); (3) dissolves more rapidly in water (Vinokurov, 1958); (4) may give an alkaline reaction when dissolved (Cornu, 1908; Guthrie, 19291, disputed by Spezia (1909); (5) has a lower index of refraction (Pustyl’nikov, 1975); (6) shows birefringence and pleochroism (Pustyl’nikov, 1975); pleochroic halos of irregular shape are unevenly distributed; (7) may have a higher pH (Vinokurov, 1958); (8) has a lesser magnetic susceptibility Winokurov, 1958); (9) requires an increase in the number of color centers per unit of volume (Siedentopf, 1905); (10) at times offers a distinct glow at 350°C (Guthrie, 19291.
4. Radiation
as a cause of discoloration?
4.1. Arguments in favor of radiation as a cause of discoloration Because it is easy to create a blue discoloration by exposing halite to either metallic sodium vapor or strong irradiation with various energy sources, it has been generally accepted that radiation is the cause of discoloration, as naturally occurring rock salt has never been exposed to sodium vapors. Blue rock salt can be produced artificially: (a) in about 10% of all crystallization runs from a solution saturated for both KC1 and NaCl and containing 320 g/l of MgCl, (Kirchheimer, 1976); (b) by adding sodium vapor or a mother liquor containing vaporized sodium (Siedentopf, 1905); (c) by electric sparks (Steinmetz, 1932); the blue color decreases even in the dark after the irradia-
P. Sonnenfeld/SedimentaryGeology 94 (1995) 267-276 tion ceases; (d) by ultraviolet irradiation (Dexter, 1951); (e) by subtracting chlorine ions through ionization (Siedentopf, 1905; Hoffmann, 1934); (f) by irradiation with gamma-rays (Dexter, 1954); (g) by irradiation with beta-rays from a Co-60 or a cathode-ray source (Przibram, 1953, 1956). Cathode rays used for ionization produce a metallic coating of sodium on halite, the sodium being the sputtered cathode material (Siedentopf, 1908; Frisch, 1927). The hues always grade from blue-black closest to the radiation source to pale blue and purple farther from it, the darkening of the crystal being caused by ionic diffusion (Seitz, 1946). The discoloration by a beta-particle bombardment occurs by splitting off chlorine to leave a sodium-enriched sodium chloride complex (Hoffmann, 1934; Hacskaylo and Otterson, 1953) or by adding an electron to a sodium ion and neutralizing it (Przibram, 1929, 1931, 1953, 1956; Apollonov and Koshchug, 1986). X-ray radiation produced free chlorine in halites only in minutest quantities (Hacskaylo et al., 1953). Nevertheless, it is easier to separate chloride ions and leave a sodium enrichment, than to achieve the opposite. The formation of color centers by proton irradiation depends on the dosage, rate, temperature, exposure to light, degree of strain, and the distribution of any impurities (Bird et al., 1981). However, even if the color appears to be identical in the visible spectrum, there occur important differences in the ultraviolet part, if X- or cathode-rays are used, or the sample is heated in sodium vapors (Seitz, 1946, 1954). Sylvite turns violet if subjected to radiation, but the color is likewise not permanent at room temperature, and fades rapidly in sunlight (Ludewig and Reuther, 1923). Minute traces of violet sylvite suffice to discolor halite. Some samples of blue rock salt are enriched in helium Walentinov, 19161, but the helium content is still about four times smaller than that of sylvite and slightly smaller than that of carnallite or kieserite (Strutt, 1909). The presence of helium may thus be merely accidental. Together with helium, lead chloride incorporated into halites may have radium as progenitor and may, therefore, be a potential cause of some blue
271
discoloration (Born, 1934); however, it merely increases the elastic limit about threefold (Blank and Smekal, 1930). Much lead is precipitated with primary halite and gypsum, and is then mobilized very early during recrystallization (Murata and Smith, 1946; Sonnenfeld, 19841, leaving a very insignificant amount in ancient halites. 4.2. Arguments against radiation as cause of discoloration The evidence for radiation damage is not unequivocal. After an exhaustive evaluation of all the data pertaining to light-sensitive blue and purple halite, Kirchheimer (1976, 1978) cautioned that radioactivity may not be the exclusive explanation for the discoloration. Most blue rock salt obtained no more radiation than adjacent undiscolored crystals. If radiation were the prime cause, all halite in contact with sylvite, superjacent or subjacent to it, should over time have turned blue, as the radiation is not particularly focussed. The discoloration should he most intense in direct contact with the potash seams and not occur in discrete bodies some distance from them. Moreover, the blue discoloration occurs only above, but not below potash horizons (Chirvinskiy, 1943). Naturally occurring blue salt shows several significant differences in properties compared to rock salt turned blue by irradiation or by exposure to sodium vapor in the laboratory. Natural blue halite displays thermoluminescence, blue halite due to a soaking in sodium vapors does not (Gmelin, 1964). Higher temperatures or friction are required to bleach natural blue halite, but even more for halite colored by sodium vapor. It has been known since 1775 that grinding makes the blue color disappear, and since 1864 that heating to 275°C has the same effect (Kirchheimer, 1976). Preheating the sample to 150°C inhibits the discoloration (Przibram, 1926). However, a discoloration induced by heating the sample in sodium vapor is retained even beyond 4Oo”C, especially if the specimen has first been exposed to lateral compression (Spezia, 1909; Przibram, 1929; Ivanov, 1953). Gmelin (1964) places the loss of discol-
272
P. Sonnenfeld / Sedimentary Geology 94 (1995) 267-276
oration at 570°C. The sample eventually turns black, if it becomes deformed under pressure (Przibram, 1927a; Tammann, 1931). Colorless salt next to blue salt thereby discolors faster than other colorless salt (Ludewig and Reuther, 1923). This happens probably because such halite contains more lattice defects than a halite that precipitated at a greater vertical distance from a sylvinite deposit from less concentrated brines. However, the greater the degree of deformation, the more rapidly the color disappears (Haberfeld, 1933). Peaks in optical absorption centers of blue halites, discolored by radiation, are shifted compared to natural blue crystals (Kirchheimer, 1976). Illumination by electric light bleaches irradiated blue halite within a few hours, but fails to bleach the natural variety (Howard and Kerr, 1960). Most artificial radiation sources that are utilized to produce a color change, are several orders of magnitude stronger than the radiation naturally occurring in evaporite beds. The 40K radiation normally attenuates in a few millimeters of rock salt, since evaporites contain only 119 ppm of 40K in potash seams (Kirchheimer, 1976). Fluid or sylvite inclusions in blue halite are always surrounded by a halo of colorless halite approximating the thickness of any potential radiation influences. Przibram (1953, 19561, Shcherbina (1958) and Pustyl’nikov (1975) argued that the dosage of beta-radiation is adequate to create radiation damage in halites over time, but Kirchheimer (1976) countered that the halo of colorless halite around sylvite inclusions should present the deepest blue colors, if discoloration were introduced by radiation. 4.3. Blue salt in structural
settings
And&e (1912) and Kennard et al. (1931) took issue with the idea of coloring pigments, and ascribed the discoloration to purely structural causes. This is supported by a preferential occurrence of blue salt near faults, fractures, shear zones or contorted strata (Schultzky, 1926; Howard and Kerr, 1960; Gmelin, 1973), or in domal parts of positive structures near the sur-
face, where the halite beds were subject to deformation, recrystallization or brecciation. The blue discoloration of halite in fracture fillings is deemed to have occurred during or after the application of pressure, judging by the orientation of the blue color (Cornu, 1908; AndrCe, 1912). Blue halite is absent on deeper flanks (Pustyl’nikov, 1975). While linking blue halite to zones of recrystallization, Tret’yakov (1990) noticed the discoloration to be rare in anticlines, except where deformation had opened paths for percolating solutions. Atmospheric oxidation is then able to bleach plastically deformed crystals (Maisch and Drickamer, 1958). Plasticized portions of the rock salt show a lasting, more intense coloration due to an increase in sodium sites where the lattice was loosened (Smekal, 19271; the degree of disarrangement of the crystal lattice determines the hue, with the deepest one occurring at points of strongly disturbed crystal structure (Blank and Urbach, 1927). These are the very points where percolating brines find the easiest entry. Frequently, open channels with mobile intercrystalline fluids occur in the Zechstein evaporites of Germany (Hartwig, 1954). The occurrence of undiscolored fibrous halite inside blue halite crystals (Tret’yakov, 1990), normally a mark of fracture fillings, supports this concept. Fibrous or columnal halite occurs also at Hallein and Hallstatt in Austria (Przibram, 1956). 4.4. Association
of blue salt with sylvinite
While blue halite is often restricted to the contact zone between halite and sylvinite (Przibram, 1929) and thus to potash salt sites (Chirvinskiy, 1945), it is not always associated with bedded pure sylvite (Gawel, 1947; Pustyl’nikov, 1975). Sylvinites that are not affected by circulating waters preserve their content of fluid inclusions and develop only a weak blue discoloration (Tret’yakov, 1990). In Oligocene evaporites of the Upper Rhine valley, blue halite occurs together with swept-in variegated marls and clays, more than 15 cm above a sylvinite horizon (Kirchheimer, 1976).
P. Sonnenfeld /Sedimentary
Very coarse halites in the Middle Devonian Prairie Evaporite Formation of Saskatchewan, Canada, can be light blue and carry abundant air bubbles (Wardlaw, 1968). Sapphire-blue halite crystals, up to 50 mm long and with interctystalline void spaces up to 15 mm in diameter, occur in a near-surface position on top of Lower Cretaceous sylvites in Thailand and represent a dissolution front that has left a porosity of about 40% (Hite, 1982). A secondary sulfatization by percolating meteoric waters that affected all Permian evaporite deposits worldwide (Sonnenfeld, 1992a, b) fostered a blue discoloration in Upper Kama halites (Vakhremeeva, 1964) and in Werra halites (Zechstein Z-1) (Hartwig, 19541, both superjacent to red sylvites and KMg-sulfates. 5. Bromine in the crystal lattice The bromine content of halite, precipitated from increasingly concentrated brines, increases from the onset of halite precipitation very gradually until potash saturation. More bromine is incorporated in sylvites and carnallites than in associated halites. In overlying halites the bromine content is apt to fluctuate or decrease as the overlying brine again diminishes its concentration. The Br/Cl ratio in the brine is about 35 times higher than in the halite at the onset of halite precipitation and then drops. During sylvite precipitation it is lowered from 34 to 30, during carnallite precipitation it diminishes further to about 28 times (Sonnenfeld, 1984). However, where carnallite [KCl.MgC1,.6H,O] is the primary potash mineral, the magnesium captures a disproportionate amount of bromine, leaving associated halites impoverished (Tret’ yakov, 1990). Many more lattice defects occur, therefore, in halites associated with primary sylvinites or secondary carnallites, compared to those associated with primary carnallites or secondary sylvinites. Solid solutions of chlorides with bromides have more lattice defects than pure chlorides (Wollam and Wallace, 1956) because of the great difference in anion sizes. Bromine in halite remains, therefore, more mobile than chlorine.
273
Geology 94 (1995) 267-276
Bromine halite
leaching
as a proposed
cause of blue
Blue rock salt is absent beneath potash beds and adjacent to red sylvites that are derived from the leaching of carnallites (Chirvinskiy, 1943). This suggests that the occurrence of blue salt is not a function of the initial concentration of the halite precipitating brine and a rising bromine content of the precipitated halite. It appears to be due to the downward percolation of waters that have not reached potash saturation. Indeed, the blue halite often comes in distinct, often sharply defined clumps, mostly in the cm-to-dm range (Chirvinskiy, 19431, or in oviform shapes (Kirchheimer, 1976) in a clear matrix, suggesting some dissolution of coigns. The same conclusion can be reached from crystals that are blue only at the corners and in small amounts along the edges (Przibram, 1956). Altogether it is significant that in all instances blue patches are confined to sites where water seems to have percolated along bedding planes, induced recrystallization, leached or filled fractures. Waters percolating down to potash horizons saturate quickly for sodium and chlorine ions, but at the same time remain deficient in bromine. They tend thus to decompose the rarer NaBr, which occurs in enriched amounts only in the vicinity of potash horizons. Brines leaching the bromine, while being unable to mobilize the metallic sodium, are thus responsible for producing the naturally occurring blue color.
6. Conclusions Basically colorless halite is discolored by interstitial admixtures, or by colloids inserted into structural defects of the crystal lattice. Widely invoked radiation damage by 40K or X7Rb cannot be the cause of naturally occurring blue halite, because the blue rock salt occurrences have obtained no more radiation than adjacent undiscolored crystals. They both have grown in the same environment; moreover, discoloration is restricted to halites above sylvinites. Consequently, radioactivity, that readily discolors specimens in
274
P. Sonnenfeld /Sedimentary
the laboratory, is not the cause of discoloration in nature. The same objection can be raised against the lattice incorporation of trace elements being the cause, especially those of class IB (Cu, Ag, Au) or IIA (bivalent alkali earths) of the periodic table. The phenomenon appears to be concentrated along paths of circulating brines, saturated for NaCl, that leach, brecciate and recrystallize halites and preferentially dislodge bromine ions from crystal lattices, leaving metallic sodium behind.
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