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The role of fluids in the formation of agates
Chemie der Erde 72 (2012) 283–286
Contents lists available at SciVerse ScienceDirect
Chemie der Erde journal homepage: www.elsevier.de/chemer
Short communication
The role of fluids in the formation of agates Jens Götze a,∗ , Werner Schrön b , Robert Möckel a , Klaus Heide c a
TU Bergakademie Freiberg, Institute for Mineralogy, Brennhausgasse 14, 09596 Freiberg, Germany Kernbergstrasse 52a, 07749 Jena, Germany c Institute for Geosciences, Friedrich-Schiller-University Jena, Burgweg 11, 07749 Jena, Germany b
a r t i c l e
i n f o
Article history: Received 11 July 2012 Accepted 18 July 2012 Keywords: Agate Silica Quartz Alteration Chemical transport reactions
a b s t r a c t Agates (banded, coloured chalcedony) are most frequently formed in SiO2 -rich (rhyolite, rhyodacite) and SiO2 -poor (andesites, basalts) volcanic rocks. The silica necessary for agate formation was discussed mainly by the alteration of the host rocks and is predominantly transported by diffusion as Si(OH)4 . The alteration processes are reflected in the occurrence of typical paragenetic minerals such as clay minerals, zeolites and/or iron oxides as well as high concentrations of Al, Fe, Ca, K and Na in the agates. However, the occurrence of calcite and fluorite and unexpected high concentrations of e.g. Ge, U and B especially in agates of acidic volcanics indicate that other fluids can play a role in the alteration of volcanic rocks and the mobilization and transport of SiO2 and other chemical compounds. Chemical transport reactions (CTR) of gases and liquids could be realized by stable fluorine (and chlorine) compounds such as SiF4 , BF3 , GeF4 and UO2 F2 . Such CTR could explain Si (Ge, B and U) transport better than exclusive transport by solution of SiO2 in water. The HF-release during the heat treatment and the enrichment of Ge, B and U in agates are positive indications of interaction of volatile fluids (HF). © 2012 Elsevier GmbH. All rights reserved.
1. Introduction Agates belong to the most fascinating mineral objects in nature because of their wide spectrum of colours and spectacular forms. Agates are in general banded chalcedony with the chemical formula SiO2 , but in detail they may represent a mixture of certain SiO2 polymorphs and morphological quartz varieties (e.g., Heaney, 1993; Graetsch, 1994; Moxon and Rios, 2004; Götze, 2011). Moreover, agates often contain considerable amounts of mineral and fluid inclusions, which may form spectacular internal structures or may be responsible for the different colouration of agates (Götze, 2011). Many authors (e.g., Walger, 1954; Harder, 1993; Holzhey, 1995; Pabian and Zarins, 1994; Möckel and Götze, 2007; Götze, 2011) assume that the formation of agates is associated with late- or post-volcanic alteration or weathering of volcanic host rocks (especially of volcanic glass). Evidence for the existence of alteration processes can be found in the chemical composition of the agates and the association with typical secondary minerals such as clay minerals, zeolites or iron oxides (Götze, 2011). Besides the accumulation of SiO2 , other chemical elements originating from the rock decomposition are enriched in agates such as Al, Fe, K, Na, Ca and
∗ Corresponding author. Tel.: +49 03731 392638; fax: +49 03731 393129. E-mail address: [email protected] (J. Götze). 0009-2819/$ – see front matter © 2012 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.chemer.2012.07.002
other trace elements (e.g., Götze et al., 2001; Möckel and Götze, 2007). Despite many new data from mineralogical and geochemical investigations during the last 20–30 years (e.g., Landmesser, 1984; Schrön and Blankenburg, 1985; Godovikov et al., 1987; Blankenburg, 1988; Moxon, 1991, 2009; Holzhey, 1993; Pabian and Zarins, 1994; Götze et al., 2001; Möckel and Götze, 2007; Götze, 2011), a lot of open questions and controversial discussions concerning the formation of agates exist. One of the main problems is the transport of the enormous amounts of Si and associated elements, which are necessary for the formation of agates.
2. Results and discussion Sufficient amounts of fluids (H2 O, but also CO2 or HF) have to be available both for the alteration processes and the transport of released silica. These fluids may include residual magmatic solutions but also meteoric water, which is often heated to temperatures of up to several 100 ◦ C during volcanic activities. There is some evidence from geochemical data that meteoric and magmatic fluids can be mixed (Götze et al., 2001). The discussion of the agate genesis has to be based not only on the system SiO2 –H2 O but also in a complex multi-component system. The main problem of the silica transport in solution is the extremely low solubility of SiO2 in water. The solubility depends on the temperature (100–140 mg/l at 20 ◦ C; 300–380 mg/l at 90 ◦ C
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Fig. 1. Distribution of selected trace elements in the host rock and different chalcedony and quartz layers within an agate from St. Egidien, Germany (size 7 cm × 5 cm); the data reveal the enrichment of Ge, B and U in the agate compared to the surrounding host rock (nd = not determined).
– Krauskopf, 1956), the pH (the solubility strongly increases at pH >9) and the silica phases which are in equilibrium with the solution. Therefore, it is unlikely that the transport and accumulation of large amounts of SiO2 , which are necessary for the formation of the agates, is realized via free solution transport. Up to 0.5 m3 water would have to flow in and out a cavity to form an agate of ca. 100 g assuming a mean concentration of 250 mg/l SiO2 in the solution. Another argument against a free solution transport model is the fact that adjacent agates should display the same structure or at least the same degree of filling, due to the “external” rhythm of filling. Many examples worldwide show, that this is only rarely the case (Götze, 2011). The lack of sufficient free liquid paths in most rocks is another fact that favours transport by diffusion against free liquid solution transport. Because of the large size (1–100 nm) and the very low diffusion velocity of colloidal SiO2 particles, the diffusion of monomeric silicic acid (H4 SiO4 ) was discussed as the main process for the transport of silica (Landmesser, 1984). Si(OH)4 (or oligomer) is able to diffuse in solution based on the concentration gradient to the cavities without loss of mass or water. This hypothesis is confirmed by recent investigations, which show the release of polymeric and monomeric silicic acid during weathering of certain silicate minerals. Monomeric silicic acid dominates over a broad range from pH 1 to 9, whereas polymeric forms with more than 10 Si-atoms are stable only above pH 10 (Dietzel, 2000). Because of a dominating pH below 9 of most weathering solutions a transport of dissolved silica (monomeric silicic acid), which penetrates the volcanic rocks, is assumed. The appearance of certain paragenetic minerals (e.g., calcite, fluorite) as well as the concentration of specific elements (e.g., B, Ge, U) in agates indicates that volatile fluids can play a role in the alteration of volcanic rocks and the mobilization and transport of SiO2 and other chemical compounds. For instance, CO2 is an essential fluid phase for the formation of calcite (especially in the core of the agate geodes depending on CO2 fugacity). The high concentrations of B, Ge and U are remarkable, in particular because of the fact that the contents of these elements could be higher in macrocrystalline quartz compared to chalcedony (Fig. 1). Moreover, the concentrations of these elements in quartz/chalcedony (e.g., Ge 10.4 ppm, U 17.1 ppm, B 33 ppm – see Fig. 1) can exceed the Clark concentrations (Ge = 1.4 ppm, U = 2.5 ppm, B = 12 ppm). In the case of Ge this can probably be explained by the similar valence and ionic radii of Si4+ and Ge4+ , which causes the common transport and incorporation into the quartz structure (Walenzak, 1969; Götze et al., 2004). The observed high concentrations of U are surprising. In the literature, U contents of up to 1200 ppm are published for
chalcedony in agates from acidic volcanic rocks (Konstatinov, 1968). The mobility of uranium during the alteration of volcanic rocks was investigated by Zielinski (1979), who observed a parallel accumulation of Si and U. Because of the crystal-chemical properties of U ions, a substitution of Si in the quartz lattice is not possible. Therefore, the incorporation may occur as mineral and/or fluid inclusions or adsorptive as uranyl complex. The geochemical data indicate that a transport of chemical compounds in aqueous fluids cannot be the only process in agate formation. Transport of gases and liquids can also be realized by stable fluorine (and chlorine) compounds (Schrön et al., 1988; Schrön, 1989). Concentrations of F of 25 up to 360 ppm (Blankenburg and Schrön, 1982) and Cl up to 154 ppm (Holzhey, 1995) have been measured in agates of different occurrences. The transport in form of SiF4 , BF3 , GeF4 or UO2 F2 complexes could explain the high concentrations of B, Ge and U in agates (Götze et al., 2001). The concentrations of these trace elements in agates are sometimes higher than in the surrounding host rocks. Assuming dominating processes of convection and diffusion of volatiles, the complexes of SiF4 , BF3 , GeF4 and UO2 F2 are moving by thermodynamically directed transport from cold to hot areas (Schrön et al., 1988; Schrön, 1989). If fluorine is released from the cooling magma, F-compounds will be transported to the hot rock areas (cavities). A sufficient supply of Ca can then support the formation of paragenetic fluorite (CaF2 ), which can often be observed in agates from acidic volcanic rocks (Holzhey, 2008). In silicified wood from volcanic environments such domains with fluoritization can also occur, which indicate similar processes (Matysová et al., 2010). These facts indicate that the system SiO2 –H2 O cannot completely explain the formation of agate by transport of SiO2 in water. The introduction of a volatile SiF4 -mobilization on the basis of the equilibrium reaction (1), with s = solid, g = gaseous, Me = metal, and X = fluorine and/or chlorine: MeX, g + H2 O, g ↔ Me-oxide, s + HX, g
(1)
e.g. SiF4 , g + 2H2 O, g ↔ SiO2 , s + 4HF, g
(1a)
is a good alternative to explain the agate genesis, because the transport effect in the gaseous phases is very efficient. As a result of a simple thermodynamic trend analysis on the basis of solid–gasequilibria, Si preferentially reacts with the available HF to form SiF4 in the hydrothermal and also in lower temperature ranges. We propose that during the cooling of magma, HX,g is released and available for reaction. The other elements enriched in agate such as B, Ge and U also preferentially react to form BF3 , GeF4 , as well as GeCl4 and UO2 F2 . The transport of these MeX-gas phases is
J. Götze et al. / Chemie der Erde 72 (2012) 283–286
285
Fig. 2. Saturation pressure psMex of metal halides (MeX) at different temperatures T.
characterized by high and increasing stability in the case of temperature decrease (Schrön et al., 1988; Schrön, 1989). The stability of MeXg-type compounds also prevents back reaction of equilibrium reaction (1).
In case of chemical transport reactions (CTR) described by Schäfer (1962), circulation of the gases can accompany equilibrium reactions. In case of dominance of diffusion processes and of equilibrium reaction (1), the compounds SiF4 , BF3 and GeF4 are
Fig. 3. (a) Gas release profiles of an agate from Hohenstein-Ernstthal, Germany. Identification of fluorine release from the sharp peak in the partial pressure graph of m/z 19 (=[F/18 OH]-release) and m/z 20 (=[HF/H2 18 O]-release) e.g. at 819 ◦ C (see Heide et al., 2008). (b) Identification of the HF-release of the agate from Hohenstein-Ernstthal, Germany by the different slope of data in a m/z 19–m/z 17 graph. The slope of water release represents the natural 18 O/16 O-isotope ratio.
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transported from cold to hot, for example, directly into water filled cavities in the still warm rock. In addition, SiF4 , BF3 , GeF4 and UO2 F2 are characterized by very high saturation pressures psMex down to low temperatures (Fig. 2). These thermodynamic considerations result in stable transport of only Si, B, Ge and U as SiF4 , BF3 , GeF4 and UO2 F2 at low temperatures. The transition from gas to transport in aqueous solution described in Schrön et al. (1988) is also possible, e.g. in the case of formation of hydrothermal vein agate. The sharp HF-release from agates as shown in Fig. 3 could be an indication for the volatile transport of SiF4 and the formation of HF in interaction with H2 O (Eq. (1a)). We do not precisely understand what happens between MeX mobilization, transport and MeO (SiO2 ) deposition, but CTR with SiF4 , BF3 , GeF4 and UO2 F2 could explain Si (Ge, B and U) transport better than exclusive transport by solution of SiO2 in water. The simultaneous enrichment of Ge, B, and U in agates is a good argument for CTR in connection with agate genesis. Also the F concentrations in agates are evidence for SiF4 transport. At low temperatures below 200 ◦ C, the equilibrium reactions are very slow therefore, much time is necessary for agate formation. All the thermodynamic considerations are compelling arguments that there is no doubt that equilibrium reactions discussed here actually are important in nature. However, the actual reactions postulated here have left few if any traces of their existence in nature. References Blankenburg, H.-J., 1988. Achat. VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 203 pp. Blankenburg, H.-J., Schrön, W., 1982. Zum Spurenelementchemismus der Vulkanitachate. Chem. Erde 41, 121–135. Dietzel, A., 2000. Dissolution of silicates and the stability of polysilicic acid. Geochim. Cosmochim. Acta 64, 3275–3281. Godovikov, A.A., Ripinen, O.I., Motorin, S.G., 1987. Agaty. Nedra, Moskva, p. 368S. Götze, J., 2011. Agate – fascination between legend and science. In: Zenz, J. (Ed.), Agates III. Bode-Verlag, pp. 19–133. Götze, J., Tichomirowa, M., Fuchs, H., Pilot, J., Sharp, Z., 2001. Geochemistry of agates: a trace element and stable isotope study. Chem. Geol. 175, 523–541. Götze, J., Plötze, M., Graupner, T., Hallbauer, D.K., Bray, C., 2004. Trace element incorporation into quartz: a combined study by ICP-MS, electron spin resonance, cathodoluminescence, capillary ion analysis and gas chromatography. Geochim. Cosmochim. Acta 68, 3741–3759.
Graetsch, H., 1994. Structural characteristics of opaline and microcrystalline silica minerals. Rev. Mineral. Geochem. 29, 209–232. Harder, H., 1993. Agates-formation as a multi component colloid chemical precipitation at low temperatures. N. Jb. Mineral. Mh. H1, 31–48. Heaney, P.J., 1993. A proposed mechanism for the growth of chalcedony. Contrib. Mineral. Petrol. 115, 66–74. Heide, K., Woermann, E., Ulmer, G., 2008. Volatiles in pillows of the Mid-OceanRidge-Basalts (MORB) and vitreous basaltic rims. Chem. Erde 68, 353–368. Holzhey, G., 1993. Vorkommen und Genese der Achate und Paragensemineralien in Rhyolithkugeln aus Rotliegendvulkaniten des Thüringer Waldes. Unpubl. Ph.D. TU Bergakademie Freiberg. Holzhey, G., 1995. Herkunft und Akkumulation des SiO2 in Rhyolithkugeln aus Rotliegendvulkaniten des Thüringer Waldes. Geowiss. Mitt. Thüringen 3, 31–59. Holzhey, G., 2008. Rhyolithkugeln und ihre Mineralisation im fluidalen “Nesselkopporphyr” bei Tambach-Dietharz/Thüringer Wald. Veröff. Naturhist. Museum Schleusingen 23, 29–38. Konstatinov, W.M., 1968. Uranium-bearing spherical formation in acidic volcanics. Izv. Akad. Nauk SSSR Ser. Geol. 7, 43–49 (in Russ.). Krauskopf, K.B., 1956. Dissolution and precipitation of silica at low temperatures. Geochim. Cosmochim. Acta 10, 1–26. Landmesser, M., 1984. Das Problem der Achatgenese. Mitt. Pollichia 72 (5), 5–137. Matysová, P., Rössler, R., Götze, J., Leichmann, J., Forbes, G., Taylor, E.L., Sakala, J., Grygar, T., 2010. Alluvial and volcanic pathways to silicified plant stems (Upper Carboniferous, Triassic) and their taphonomic and palaeoenvironmental meaning. Palaeogeogr. Palaeoclimatol. Palaeoecol. 292, 127–143. Möckel and Götze, 2007. Achate aus sächsischen Vulkaniten des Erzgebirgischen Beckens. Veröff. Museum Naturkunde Chemnitz 30, 25–60. Moxon, T., 1991. On the origin of agate with particular reference to fortification agate found in the Midland Valley, Scotland. Chem. Erde 51, 251–260. Moxon, T., 2009. Agates from Western Australia found in a 3,480 million-year-old host rock. Rocks Miner. 85, 66–73. Moxon, T., Rios, S., 2004. Moganite and water content as a function of age in agate: an XRD and thermogravimetric study. Eur. J. Mineral. 4, 693–706. Pabian, R.K., Zarins, A., 1994. Banded agates – origins and inclusions. Educational Circular No. 12. University of Bebraska, Lincoln, 32 pp. Schäfer, H., 1962. Chemische Transportreaktionen. Verlag Chemie, Weinheim. Schrön, W., 1989. Solid–gas-equilibria in geo- and cosmochemistry – I. Geochemistry. Eur. J. Mineral. 1, 739–763. Schrön, W., Blankenburg, H.J., 1985. Zur Geochemie der Gangachate. Z. Geol. Wiss. 13, 677–686. Schrön, W., Oppermann, H., Rösler, H.J., Brand, P., 1988. Fest-Gas-Reaktionen als Ursache geo- und kosmogeochemischer Mobilisierungs- und Anreicherungsprozesse. Chem. Erde 48, 35–54. Walenzak, Z., 1969. Geochemistry of minor elements dispersed in quartz (Ge, Al, Ga, Fe, Ti, Li and Be). Arch. Mineralog. 28, 189–335. Walger, E., 1954. Das Vorkommen von Uruguay-Achaten bei Flonheim in Rheinhessen, seine tektonische Auswertung und seine Bedeutung für die Frage nach der Achatbildung. Jahresber. Mitt. Oberrhein. Geol. Ver. 36, 20–31. Zielinski, R.A., 1979. Uranium mobility during interaction of rhyolitic obsidian, perlite and felsite with alkaline carbonate solution: T = 120 ◦ C, P = 210 kg/cm2 . Chem. Geol. 27, 47–63.
Contents lists available at SciVerse ScienceDirect
Chemie der Erde journal homepage: www.elsevier.de/chemer
Short communication
The role of fluids in the formation of agates Jens Götze a,∗ , Werner Schrön b , Robert Möckel a , Klaus Heide c a
TU Bergakademie Freiberg, Institute for Mineralogy, Brennhausgasse 14, 09596 Freiberg, Germany Kernbergstrasse 52a, 07749 Jena, Germany c Institute for Geosciences, Friedrich-Schiller-University Jena, Burgweg 11, 07749 Jena, Germany b
a r t i c l e
i n f o
Article history: Received 11 July 2012 Accepted 18 July 2012 Keywords: Agate Silica Quartz Alteration Chemical transport reactions
a b s t r a c t Agates (banded, coloured chalcedony) are most frequently formed in SiO2 -rich (rhyolite, rhyodacite) and SiO2 -poor (andesites, basalts) volcanic rocks. The silica necessary for agate formation was discussed mainly by the alteration of the host rocks and is predominantly transported by diffusion as Si(OH)4 . The alteration processes are reflected in the occurrence of typical paragenetic minerals such as clay minerals, zeolites and/or iron oxides as well as high concentrations of Al, Fe, Ca, K and Na in the agates. However, the occurrence of calcite and fluorite and unexpected high concentrations of e.g. Ge, U and B especially in agates of acidic volcanics indicate that other fluids can play a role in the alteration of volcanic rocks and the mobilization and transport of SiO2 and other chemical compounds. Chemical transport reactions (CTR) of gases and liquids could be realized by stable fluorine (and chlorine) compounds such as SiF4 , BF3 , GeF4 and UO2 F2 . Such CTR could explain Si (Ge, B and U) transport better than exclusive transport by solution of SiO2 in water. The HF-release during the heat treatment and the enrichment of Ge, B and U in agates are positive indications of interaction of volatile fluids (HF). © 2012 Elsevier GmbH. All rights reserved.
1. Introduction Agates belong to the most fascinating mineral objects in nature because of their wide spectrum of colours and spectacular forms. Agates are in general banded chalcedony with the chemical formula SiO2 , but in detail they may represent a mixture of certain SiO2 polymorphs and morphological quartz varieties (e.g., Heaney, 1993; Graetsch, 1994; Moxon and Rios, 2004; Götze, 2011). Moreover, agates often contain considerable amounts of mineral and fluid inclusions, which may form spectacular internal structures or may be responsible for the different colouration of agates (Götze, 2011). Many authors (e.g., Walger, 1954; Harder, 1993; Holzhey, 1995; Pabian and Zarins, 1994; Möckel and Götze, 2007; Götze, 2011) assume that the formation of agates is associated with late- or post-volcanic alteration or weathering of volcanic host rocks (especially of volcanic glass). Evidence for the existence of alteration processes can be found in the chemical composition of the agates and the association with typical secondary minerals such as clay minerals, zeolites or iron oxides (Götze, 2011). Besides the accumulation of SiO2 , other chemical elements originating from the rock decomposition are enriched in agates such as Al, Fe, K, Na, Ca and
∗ Corresponding author. Tel.: +49 03731 392638; fax: +49 03731 393129. E-mail address: [email protected] (J. Götze). 0009-2819/$ – see front matter © 2012 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.chemer.2012.07.002
other trace elements (e.g., Götze et al., 2001; Möckel and Götze, 2007). Despite many new data from mineralogical and geochemical investigations during the last 20–30 years (e.g., Landmesser, 1984; Schrön and Blankenburg, 1985; Godovikov et al., 1987; Blankenburg, 1988; Moxon, 1991, 2009; Holzhey, 1993; Pabian and Zarins, 1994; Götze et al., 2001; Möckel and Götze, 2007; Götze, 2011), a lot of open questions and controversial discussions concerning the formation of agates exist. One of the main problems is the transport of the enormous amounts of Si and associated elements, which are necessary for the formation of agates.
2. Results and discussion Sufficient amounts of fluids (H2 O, but also CO2 or HF) have to be available both for the alteration processes and the transport of released silica. These fluids may include residual magmatic solutions but also meteoric water, which is often heated to temperatures of up to several 100 ◦ C during volcanic activities. There is some evidence from geochemical data that meteoric and magmatic fluids can be mixed (Götze et al., 2001). The discussion of the agate genesis has to be based not only on the system SiO2 –H2 O but also in a complex multi-component system. The main problem of the silica transport in solution is the extremely low solubility of SiO2 in water. The solubility depends on the temperature (100–140 mg/l at 20 ◦ C; 300–380 mg/l at 90 ◦ C
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J. Götze et al. / Chemie der Erde 72 (2012) 283–286
Fig. 1. Distribution of selected trace elements in the host rock and different chalcedony and quartz layers within an agate from St. Egidien, Germany (size 7 cm × 5 cm); the data reveal the enrichment of Ge, B and U in the agate compared to the surrounding host rock (nd = not determined).
– Krauskopf, 1956), the pH (the solubility strongly increases at pH >9) and the silica phases which are in equilibrium with the solution. Therefore, it is unlikely that the transport and accumulation of large amounts of SiO2 , which are necessary for the formation of the agates, is realized via free solution transport. Up to 0.5 m3 water would have to flow in and out a cavity to form an agate of ca. 100 g assuming a mean concentration of 250 mg/l SiO2 in the solution. Another argument against a free solution transport model is the fact that adjacent agates should display the same structure or at least the same degree of filling, due to the “external” rhythm of filling. Many examples worldwide show, that this is only rarely the case (Götze, 2011). The lack of sufficient free liquid paths in most rocks is another fact that favours transport by diffusion against free liquid solution transport. Because of the large size (1–100 nm) and the very low diffusion velocity of colloidal SiO2 particles, the diffusion of monomeric silicic acid (H4 SiO4 ) was discussed as the main process for the transport of silica (Landmesser, 1984). Si(OH)4 (or oligomer) is able to diffuse in solution based on the concentration gradient to the cavities without loss of mass or water. This hypothesis is confirmed by recent investigations, which show the release of polymeric and monomeric silicic acid during weathering of certain silicate minerals. Monomeric silicic acid dominates over a broad range from pH 1 to 9, whereas polymeric forms with more than 10 Si-atoms are stable only above pH 10 (Dietzel, 2000). Because of a dominating pH below 9 of most weathering solutions a transport of dissolved silica (monomeric silicic acid), which penetrates the volcanic rocks, is assumed. The appearance of certain paragenetic minerals (e.g., calcite, fluorite) as well as the concentration of specific elements (e.g., B, Ge, U) in agates indicates that volatile fluids can play a role in the alteration of volcanic rocks and the mobilization and transport of SiO2 and other chemical compounds. For instance, CO2 is an essential fluid phase for the formation of calcite (especially in the core of the agate geodes depending on CO2 fugacity). The high concentrations of B, Ge and U are remarkable, in particular because of the fact that the contents of these elements could be higher in macrocrystalline quartz compared to chalcedony (Fig. 1). Moreover, the concentrations of these elements in quartz/chalcedony (e.g., Ge 10.4 ppm, U 17.1 ppm, B 33 ppm – see Fig. 1) can exceed the Clark concentrations (Ge = 1.4 ppm, U = 2.5 ppm, B = 12 ppm). In the case of Ge this can probably be explained by the similar valence and ionic radii of Si4+ and Ge4+ , which causes the common transport and incorporation into the quartz structure (Walenzak, 1969; Götze et al., 2004). The observed high concentrations of U are surprising. In the literature, U contents of up to 1200 ppm are published for
chalcedony in agates from acidic volcanic rocks (Konstatinov, 1968). The mobility of uranium during the alteration of volcanic rocks was investigated by Zielinski (1979), who observed a parallel accumulation of Si and U. Because of the crystal-chemical properties of U ions, a substitution of Si in the quartz lattice is not possible. Therefore, the incorporation may occur as mineral and/or fluid inclusions or adsorptive as uranyl complex. The geochemical data indicate that a transport of chemical compounds in aqueous fluids cannot be the only process in agate formation. Transport of gases and liquids can also be realized by stable fluorine (and chlorine) compounds (Schrön et al., 1988; Schrön, 1989). Concentrations of F of 25 up to 360 ppm (Blankenburg and Schrön, 1982) and Cl up to 154 ppm (Holzhey, 1995) have been measured in agates of different occurrences. The transport in form of SiF4 , BF3 , GeF4 or UO2 F2 complexes could explain the high concentrations of B, Ge and U in agates (Götze et al., 2001). The concentrations of these trace elements in agates are sometimes higher than in the surrounding host rocks. Assuming dominating processes of convection and diffusion of volatiles, the complexes of SiF4 , BF3 , GeF4 and UO2 F2 are moving by thermodynamically directed transport from cold to hot areas (Schrön et al., 1988; Schrön, 1989). If fluorine is released from the cooling magma, F-compounds will be transported to the hot rock areas (cavities). A sufficient supply of Ca can then support the formation of paragenetic fluorite (CaF2 ), which can often be observed in agates from acidic volcanic rocks (Holzhey, 2008). In silicified wood from volcanic environments such domains with fluoritization can also occur, which indicate similar processes (Matysová et al., 2010). These facts indicate that the system SiO2 –H2 O cannot completely explain the formation of agate by transport of SiO2 in water. The introduction of a volatile SiF4 -mobilization on the basis of the equilibrium reaction (1), with s = solid, g = gaseous, Me = metal, and X = fluorine and/or chlorine: MeX, g + H2 O, g ↔ Me-oxide, s + HX, g
(1)
e.g. SiF4 , g + 2H2 O, g ↔ SiO2 , s + 4HF, g
(1a)
is a good alternative to explain the agate genesis, because the transport effect in the gaseous phases is very efficient. As a result of a simple thermodynamic trend analysis on the basis of solid–gasequilibria, Si preferentially reacts with the available HF to form SiF4 in the hydrothermal and also in lower temperature ranges. We propose that during the cooling of magma, HX,g is released and available for reaction. The other elements enriched in agate such as B, Ge and U also preferentially react to form BF3 , GeF4 , as well as GeCl4 and UO2 F2 . The transport of these MeX-gas phases is
J. Götze et al. / Chemie der Erde 72 (2012) 283–286
285
Fig. 2. Saturation pressure psMex of metal halides (MeX) at different temperatures T.
characterized by high and increasing stability in the case of temperature decrease (Schrön et al., 1988; Schrön, 1989). The stability of MeXg-type compounds also prevents back reaction of equilibrium reaction (1).
In case of chemical transport reactions (CTR) described by Schäfer (1962), circulation of the gases can accompany equilibrium reactions. In case of dominance of diffusion processes and of equilibrium reaction (1), the compounds SiF4 , BF3 and GeF4 are
Fig. 3. (a) Gas release profiles of an agate from Hohenstein-Ernstthal, Germany. Identification of fluorine release from the sharp peak in the partial pressure graph of m/z 19 (=[F/18 OH]-release) and m/z 20 (=[HF/H2 18 O]-release) e.g. at 819 ◦ C (see Heide et al., 2008). (b) Identification of the HF-release of the agate from Hohenstein-Ernstthal, Germany by the different slope of data in a m/z 19–m/z 17 graph. The slope of water release represents the natural 18 O/16 O-isotope ratio.
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transported from cold to hot, for example, directly into water filled cavities in the still warm rock. In addition, SiF4 , BF3 , GeF4 and UO2 F2 are characterized by very high saturation pressures psMex down to low temperatures (Fig. 2). These thermodynamic considerations result in stable transport of only Si, B, Ge and U as SiF4 , BF3 , GeF4 and UO2 F2 at low temperatures. The transition from gas to transport in aqueous solution described in Schrön et al. (1988) is also possible, e.g. in the case of formation of hydrothermal vein agate. The sharp HF-release from agates as shown in Fig. 3 could be an indication for the volatile transport of SiF4 and the formation of HF in interaction with H2 O (Eq. (1a)). We do not precisely understand what happens between MeX mobilization, transport and MeO (SiO2 ) deposition, but CTR with SiF4 , BF3 , GeF4 and UO2 F2 could explain Si (Ge, B and U) transport better than exclusive transport by solution of SiO2 in water. The simultaneous enrichment of Ge, B, and U in agates is a good argument for CTR in connection with agate genesis. Also the F concentrations in agates are evidence for SiF4 transport. At low temperatures below 200 ◦ C, the equilibrium reactions are very slow therefore, much time is necessary for agate formation. All the thermodynamic considerations are compelling arguments that there is no doubt that equilibrium reactions discussed here actually are important in nature. However, the actual reactions postulated here have left few if any traces of their existence in nature. References Blankenburg, H.-J., 1988. Achat. VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 203 pp. Blankenburg, H.-J., Schrön, W., 1982. Zum Spurenelementchemismus der Vulkanitachate. Chem. Erde 41, 121–135. Dietzel, A., 2000. Dissolution of silicates and the stability of polysilicic acid. Geochim. Cosmochim. Acta 64, 3275–3281. Godovikov, A.A., Ripinen, O.I., Motorin, S.G., 1987. Agaty. Nedra, Moskva, p. 368S. Götze, J., 2011. Agate – fascination between legend and science. In: Zenz, J. (Ed.), Agates III. Bode-Verlag, pp. 19–133. Götze, J., Tichomirowa, M., Fuchs, H., Pilot, J., Sharp, Z., 2001. Geochemistry of agates: a trace element and stable isotope study. Chem. Geol. 175, 523–541. Götze, J., Plötze, M., Graupner, T., Hallbauer, D.K., Bray, C., 2004. Trace element incorporation into quartz: a combined study by ICP-MS, electron spin resonance, cathodoluminescence, capillary ion analysis and gas chromatography. Geochim. Cosmochim. Acta 68, 3741–3759.
Graetsch, H., 1994. Structural characteristics of opaline and microcrystalline silica minerals. Rev. Mineral. Geochem. 29, 209–232. Harder, H., 1993. Agates-formation as a multi component colloid chemical precipitation at low temperatures. N. Jb. Mineral. Mh. H1, 31–48. Heaney, P.J., 1993. A proposed mechanism for the growth of chalcedony. Contrib. Mineral. Petrol. 115, 66–74. Heide, K., Woermann, E., Ulmer, G., 2008. Volatiles in pillows of the Mid-OceanRidge-Basalts (MORB) and vitreous basaltic rims. Chem. Erde 68, 353–368. Holzhey, G., 1993. Vorkommen und Genese der Achate und Paragensemineralien in Rhyolithkugeln aus Rotliegendvulkaniten des Thüringer Waldes. Unpubl. Ph.D. TU Bergakademie Freiberg. Holzhey, G., 1995. Herkunft und Akkumulation des SiO2 in Rhyolithkugeln aus Rotliegendvulkaniten des Thüringer Waldes. Geowiss. Mitt. Thüringen 3, 31–59. Holzhey, G., 2008. Rhyolithkugeln und ihre Mineralisation im fluidalen “Nesselkopporphyr” bei Tambach-Dietharz/Thüringer Wald. Veröff. Naturhist. Museum Schleusingen 23, 29–38. Konstatinov, W.M., 1968. Uranium-bearing spherical formation in acidic volcanics. Izv. Akad. Nauk SSSR Ser. Geol. 7, 43–49 (in Russ.). Krauskopf, K.B., 1956. Dissolution and precipitation of silica at low temperatures. Geochim. Cosmochim. Acta 10, 1–26. Landmesser, M., 1984. Das Problem der Achatgenese. Mitt. Pollichia 72 (5), 5–137. Matysová, P., Rössler, R., Götze, J., Leichmann, J., Forbes, G., Taylor, E.L., Sakala, J., Grygar, T., 2010. Alluvial and volcanic pathways to silicified plant stems (Upper Carboniferous, Triassic) and their taphonomic and palaeoenvironmental meaning. Palaeogeogr. Palaeoclimatol. Palaeoecol. 292, 127–143. Möckel and Götze, 2007. Achate aus sächsischen Vulkaniten des Erzgebirgischen Beckens. Veröff. Museum Naturkunde Chemnitz 30, 25–60. Moxon, T., 1991. On the origin of agate with particular reference to fortification agate found in the Midland Valley, Scotland. Chem. Erde 51, 251–260. Moxon, T., 2009. Agates from Western Australia found in a 3,480 million-year-old host rock. Rocks Miner. 85, 66–73. Moxon, T., Rios, S., 2004. Moganite and water content as a function of age in agate: an XRD and thermogravimetric study. Eur. J. Mineral. 4, 693–706. Pabian, R.K., Zarins, A., 1994. Banded agates – origins and inclusions. Educational Circular No. 12. University of Bebraska, Lincoln, 32 pp. Schäfer, H., 1962. Chemische Transportreaktionen. Verlag Chemie, Weinheim. Schrön, W., 1989. Solid–gas-equilibria in geo- and cosmochemistry – I. Geochemistry. Eur. J. Mineral. 1, 739–763. Schrön, W., Blankenburg, H.J., 1985. Zur Geochemie der Gangachate. Z. Geol. Wiss. 13, 677–686. Schrön, W., Oppermann, H., Rösler, H.J., Brand, P., 1988. Fest-Gas-Reaktionen als Ursache geo- und kosmogeochemischer Mobilisierungs- und Anreicherungsprozesse. Chem. Erde 48, 35–54. Walenzak, Z., 1969. Geochemistry of minor elements dispersed in quartz (Ge, Al, Ga, Fe, Ti, Li and Be). Arch. Mineralog. 28, 189–335. Walger, E., 1954. Das Vorkommen von Uruguay-Achaten bei Flonheim in Rheinhessen, seine tektonische Auswertung und seine Bedeutung für die Frage nach der Achatbildung. Jahresber. Mitt. Oberrhein. Geol. Ver. 36, 20–31. Zielinski, R.A., 1979. Uranium mobility during interaction of rhyolitic obsidian, perlite and felsite with alkaline carbonate solution: T = 120 ◦ C, P = 210 kg/cm2 . Chem. Geol. 27, 47–63.