Palygorskite and Sepiolite Deposits in Continental Environments. Description, Genetic Patterns and Sedimentary Settings

Chapter 6

Palygorskite and Sepiolite Deposits in Continental Environments. Description, Genetic Patterns and Sedimentary Settings Emilio Gala´n* and Manuel Pozo{ *Departmento de Cristalografı´a Mineralogı´a y Quı´mica Agrı´cola, Facultad de Quı´mica, Universidad de Sevilla, Professor Garcı´a Gonza´lez 1. 41012 Seville, Spain { Departamento de Geologı´a y Geoquı´mica, Universidad Auto´noma de Madrid, Cantoblanco, 28049 Madrid, Spain

1. INTRODUCTION On the basis of their origin, two main types of clays are differentiated in the sedimentary environment: detrital and authigenic clays (Figure 1). Detrital clays are composed of inherited clay minerals as a result of exogenic processes including weathering, erosion, transport and clastic deposition. Authigenic clays are formed in situ through direct precipitation from solution (neoformation), reaction of amorphous gels, or by transformation of precursor minerals, mainly pyroclastics and detrital clays (Jones, 1986). The most important processes forming authigenic clays are neoformation and transformation. Neoformation is commonly the crystallization of a new mineral structure from simple or complex ions, in which there is no inheritance of a pre-existing mineral structure. However, transformation is the formation of a new mineral in which part or all of the pre-existing structure is inherited. In the sedimentary environment, transformation is mostly related to diagenesis, whereas neoformation can take place in both syngenetic (depositional) and diagenetic environments. Sepiolite and palygorskite are two excellent examples of authigenic clays. Palygorskite and sepiolite are relatively rare in nature; however, their origins have been studied frequently owing to the unique conditions required for their formation and stability, and the need to find new commercial deposits. According to Callen (1984), major deposits of these minerals were formed Developments in Clay Science, Vol. 3. DOI: 10.1016/B978-0-444-53607-5.00006-2 # 2011 Elsevier B.V. All rights reserved.

125

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SEDIMENTARY (depositional)

DIAGENETIC (post-depositional)

Evaporative

Intrasedimentary

Water

Clayey Sediment (detrital or chemical)

AUTHIGENIC CLAY

AUTHIGENIC CLAY

SEDIMENTARY NEOFORMATION

TRANSFORMATION

(direct precipitation from ionic or colloidal solutions)

(dissolution-precipitation from previous phases or epitaxial growth)

AUTHIGENIC CLAYS

DETRITAL CLAYS

SEDIMENTARY CLASTIC DEPOSITION (Inheritance)

Water

DETRITALCLAY

FIGURE 1 Sketch showing the origin of authigenic clays and their relationship with detrital clays in sedimentary and early diagenetic environments. Evaporative conditions commonly provoke the sedimentary neoformation (precipitation) of sepiolite. On the other hand, the early diagenesis of fine-grained sediments is often related to palygorskite formation by means of transformation of inherited Al-bearing clays. Under subaerial exposure Mg-clay paleosoils or reworked deposits can occur.

in three different environments: (1) in epicontinental and inland seas and lakes as chemical sediments or by reconstitution of former sedimentary clays; (2) in the open ocean in association with fore-arc basins and ocean ridges by hydrothermal alteration of basaltic glass, volcaniclastic sediments or existing clays; and (3) in calcareous soils by direct crystallization. With respect to their geotectonic setting, neoformed palygorskite and sepiolite are predominant in shallow shelf basins on passive margins or intraplate basins. Continental rift basins, continental “sag” basins or some perimarine basins, tend to contain significant deposits of these minerals, commonly formed in saline or hypersaline conditions (Figure 2; Merriman, 2005).

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Neoformed ± Inherited clays Inherited + Neoformed clays Neoformed clays

Continental ‘Sag’ Basin

Mid-ocean ridge and abyssal plain

Passive Margin

Continental Rift Basin

FIGURE 2 Neoformed clays at shallow shelf basins on passive margin, continental rift basins and continental “sag” basins (Merriman, 2005).

In soils of arid regions, palygorskite and sepiolite are common neoformed minerals (Singer, 1979), but traces of palygorskite in some arid soils may be inherited (Mackenzie et al., 1984; Shadfan and Dixon, 1984). The instability of these minerals in wet climates favours their preservation in dry or semi-arid climates. According to Singer (1984), palygorskite in soils is associated with one of the following conditions: (i) modern soils that, at present or in the past, were affected by rising groundwater of pH 7–8 and high salinity; (ii) in soils with distinct and sharp textural transitions because these minerals accumulate in the coarse fraction (this group includes many palaeosols); and (iii) in calcretes (caliches). In all these cases, palygorskite (and very rarely sepiolite, when Al is absent or immobilized) is precipitated by evaporation of the vadose water (Jones and Weir, 1983). Soils (duricrusts) can be cemented by palygorskite (palycretes; Rodas et al., 1994). Palygorskite is more readily formed in weathering environments than sepiolite, or else sepiolite is less stable under supergene conditions and, hence, is rarer. In the sedimentary environment, palygorskite of detrital origin can be found in oceans, having come from the continent, as in the case of palygorskite in the Atlantic close to Morocco, which was transported by SW winds from near-shore. Additionally, palygorskite may occur by recrystallization during diagenesis (e.g. Couture, 1977), or transformation of smectite in the marine environment (Lo´pez Galindo, 1986), sometimes in association with marine phosphorites (Chahi et al., 1999). However, these minerals can crystallize directly from solution (e.g. Jones and Gala´n, 1988; Weaver, 1984), either in lacustrine (e.g. Chahi et al., 1997; Gala´n and Castillo, 1984) or in perimarine (e.g. Singer, 1979; Velde, 1985; Weaver and Beck, 1977) environments. In this environment, sepiolite can also be of diagenetic origin through the transformation of magnesite at pH 10.5–11.5 in silica-rich lake waters (Ece, 1998).

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Jones and Gala´n (1988) summarized the occurrences of palygorskite and sepiolite and tabulated a summary of the favourable environmental conditions for palygorskite and sepiolite formation as compared to trioctahedral smectite (Table 1). As expected, with high values of Al, Mg and Si activity,

TABLE 1 Environmental Conditions of Formation of Magnesian Clays (Modified After Jones and Gala´n, 1988). Palygorskite

Sepiolite

MgSmectite

Moderate pH < 8,5

þþþ

þþ



Intermediate pH ¼ 8–9.5

þþ

þþþ

þþ

High pH > 9.5





þþþ

Major constituent ratios

High MgþSi/Al

þþ

þþþ



High MgþFe/Si





þþþ

Sediment-water pCO2

High





þþþ

Low

þþþ

þþþ



Alkali salinity

High





þþþ

Intermediate

þþ

þþþ

þþ

Moderate

þþþ

þþ



Siliciclastic or arkosic matrix

þþþ

þþ



Carbonate or mafic matrix

þþ

þþþ



Groundwater input dominant



þþþ

þþ

Surface runoff dominant

?



þþþ

Hypersaline





þþþ

Lagoon or tidal

þþþ

þþ



Deep sea

þþ



þþ

Chemistry pH and alkalinity

Environment Pedogenic calcrete or alluvial

Closed basin lacustrine

Marine

þþþ, favoured; þþ, less favoured; , not favoured.

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palygorskite is favoured over sepiolite. However, besides the presence of Al-rich clay phases (e.g. smectites), temporary variations in chemistry related to changes in environmental conditions such as evaporation, rainfall, freshwater flow, etc. affect the formation of palygorskite and sepiolite. Palygorskite originates in many cases from the transformation of smectite via a dissolution–precipitation process (Chen et al., 2004; Gala´n and Ferrero, 1982; Jones and Gala´n, 1988; Lo´pez-Galindo et al., 1996; Sa´nchez and Gala´n, 1995; Sua´rez et al., 1994). In some shallow restricted basins with evaporitic conditions, fibrous clay minerals and Mg-rich smectites (saponite, stevensite and kerolite) form as authigenic minerals, usually together with sulphate and carbonate minerals (Pozo and Casas, 1999). Sepiolite has also been formed in Japan from low-temperature hydrothermal solutions (Imai and Otsuka, 1984) and by the hydrothermal alteration of ¨ nlu¨, 1993). Vein-type sepiolite formed by hydrothermal volcanics (I˙rkec¸ and U processes is abundant in ophiolite complexes in Turkey (Yeniyol, 1986). The stability of palygorskite and sepiolite is mainly a function of pH. The transformation of palygorskite to smectite has been suggested by Golden and Dixon (1990), Merkl (1989), Golden et al. (1985), and Gu¨ven and Carney (1979). Golden and Dixon (1990) used transmission electron microscopy (TEM) data to show a close textural association of smectite and palygorskite in a series of experiments. Their work indicated that palygorskite readily converts to smectite above 100  C, although the reaction was sluggish at room temperature (22  C). They showed that at conditions near a pH of 12, the palygorskite to smectite transformation occurs over a period of several months. The transformation of palygorskite from the Meigs Member of the Hawthorne Formation, in southern Georgia, USA to smectite was analysed in detail using atomic force microscopy (AFM) and TEM techniques by Krekeler et al. (2005). AFM analysis indicated that palygorskite fibres in this horizon were commonly altered. Many AFM images of the altered fibres showed an oriented overgrowth on particles with platy morphology, which were interpreted as smectite. This transformation to smectite also accounts for the very low abundance of palygorskite in Mesozoic and older sediments. An implication of the transformation is that palygorskite deposits may have existed in abundance in the Mesozoic and perhaps even older sedimentary systems. However, Tertiary sedimentary rocks have different proportions of palygorskite. Orogenic activities, which resulted in the Tethys Sea being cut off during the late Cretaceous, gave rise to the development of shallow saline lakes during the Tertiary that were chemically favourable for the formation of fibrous clay minerals. In an evaporative environment, these conditions promoted the formation of gypsum and resulted in an increase in the Mg/Ca ratio that brought about the authigenic formation of a large amount of palygorskite, particularly in Neogene sediments. The positive correlation between the occurrence of palygorskite/sepiolite and gypsum and carbonates in sediments supports this hypothesis.

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The geochemistry of the post-Tethys Sea environment, which was significantly affected by climatic conditions and orogenic events during the Tertiary, controlled the formation of palygorskite and sepiolite. The present-day arid to hyper-arid environments prevailing in many areas have caused the preservation of these minerals (Singer, 1984). The deep-sea environment of the Tethys Ocean (geological formations older than Cretaceous) does not appear to have been suitable for the formation of palygorskite and sepiolite. The possible environments for palygorskite and sepiolite formation range from soils to marine and lacustrine deposits, hydrothermal veins in serpentinite and dolostone and the weathering of volcanic rocks. The primary question addressed by Singer (1979)—that is, whether these minerals owe their origin to transformation of precursor clay minerals or to precipitation directly from solution—continues to be the most controversial aspect in concerning the origin of these minerals. Nevertheless, their formation from a dissolution–precipitation mechanism that incorporates components (primarily sesquioxides) of pre-existing detrital material seems to be inescapable. These phases can therefore contain significant geochemical information regarding the precursor and the formation environment (Millot, 1970). Studies of sepiolite and palygorskite occurrences and origin in continental deposits are well supported by abundant literature. However, studies about their genetic relationships with other Mg-clays including saponite, stevensite, kerolite and associated mixed layers are relatively scarce, although fibrous and non-fibrous Mg-clays have been recognized, in the same deposit, suggesting a genetic relationship. Within sedimentary deposits, sepiolite and palygorskite occur commonly in beds, but locally also intrasedimentary (e.g. minerals that have precipitated within the sediment from the pore water after evaporation or because of transformation during diagenetic process) or forming clay clasts levels, the late as a result of reworking of desiccated and bioturbated beds (Bellanca et al., 1992; Trauth, 1977). The main aim of this contribution is to model the pathways leading to sepiolite–palygorskite (and other Mg-clays) formation based on worldwide continental occurrences. Also to establish the sedimentological models and related lithological associations, summarizing and updating the available experimental and geological data. For this purpose, the main characteristics of the most important sepiolite and palygorskite deposits of continental origin will be reviewed.

2. GENETIC CONDITIONS From a sedimentological perspective, the best conditions for the formation of fibrous clay minerals seem to occur in lacustrine evaporative environments where pre-existing sediments become exposed and diagenetic and soil

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processes alter the materials as described in a number of reviews (Calvo et al., 1999; Gala´n and Castillo, 1984; Jones, 1986; Jones and Gala´n, 1988; Singer, 1979). Palygorskite and sepiolite are common in lacustrine environments in arid or semi-arid climatic conditions when the interstitial and overlying fluids are alkaline solutions with high Si and Mg activity. The high Mg and Si may be the result of groundwater concentration near the surface, or of cyclical flooding and subsequent concentrations of the solution by evaporation. The fibrous clay materials are associated with carbonates (dolomite in particular), evaporites (gypsum, halite, thenardite) and chert. They are also often associated with other Mg-rich clays such as saponite, kerolite and stevensite. Depending on the regional geology, the source of Si, Al and Mg may be volcanic tuff (McLean et al., 1972; Starkey and Blackmon, 1984) or weathering of igneous and sedimentary rocks (Gala´n and Castillo, 1984; Hay and Stoessel, 1984). Direct precipitation and the transformation of precursor phases (by dissolution–precipitation) have been proposed as formation mechanisms in this setting (Table 2).

2.1. Experimental and Natural Evidence of Sepiolite Formation Experimental synthesis of sepiolite has been carried out under various conditions and the results generally agree (Couture, 1977; La Iglesia, 1978; Siffert and Wey, 1962; Wollast et al., 1968). Wollast et al. (1968) were able to produce sepiolite by evaporating seawater while controlling the pH and silica content. Sepiolite precipitates at pH > 8.2, and the activity of Mg2þ, OH and SiO2 is governed by the equilibrium constant k ¼ (a16Hþ)/(a8Mg2þ þ a12SiO2(aq)) ¼ 10 75.12. In other words, sepiolite can be formed at high aMg2þ/a2Hþ ratios when the SiO2 activity is

TABLE 2 General Criteria for Discriminating Between Neoformation and Transformation. Neoformation

Transformation

Low content in trace element (F is an exception) especially REE and TTE

Moderate to high content in trace elements (depending of the transformed previous phases)

Commonly detrital minerals are absent or in low content

Detrital minerals are common both in clay and higher grain-size fractions

Generally precursor minerals absent

Precursor minerals present

Clean textures in thin section

Often dirt textures in thin section

Locally lamination

Lamination absent or inherited

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low, while sepiolite is formed at low aMg2þ/a2Hþ ratios when SiO2 is high. They also observed that aluminium must not be present in solution or in reactive phases, a requisite later stressed by other authors (Birsoy, 2002; Jones, 1983; Starkey and Blackmon, 1979; Webster and Jones, 1994). Experiments performed by Siffert and Wey (1962) by evaporating solutions with fixed chemical ratios and varying pH were very productive. Sepiolite begins to precipitate at a pH of 8.5, although if pH is held at 9, smectite and talc can precipitate. In other words, an increase in Mg2þ is observed in precipitated solid phases when pH is increased, even leading to the formation of brucite at a pH of 10. Similar tests were carried out by La Iglesia (1978) where sepiolite was synthesized by homogeneous precipitation, with the observation that “crystallinity”, particle size and stability are greater when pH is increased, depending on the silica activity. The stability diagram produced by Jones (1986) shows that sepiolite, stevensite and kerolite equilibria depend on salinity (Na), pH and Mg concentration (Figure 3). High salinity favours stevensite formation, while sepiolite and

20

18

Talc (Kerolite)

ed Mix

ers lay

16

log a Mg2+/a 2H+

14 Sepiolite

Stevensite

12

10 Solution

8

6

4 −2

0

2

4

6

8

10

log a Na+/a H+ FIGURE 3 Stability of hydrous magnesium silicates as estimated from thermodynamic data and natural occurrences. Labels indicate fields of supersaturation of the appropriate mineral phase (adapted from Jones, 1986). The diagram explains the formation of kerolite at higher Mg/Si than sepiolite so as the predominance of stevensite at higher salinities.

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kerolite are formed in less saline conditions, with high Si/Mg ratios favouring sepiolite formation. Sepiolite stability is limited to solutions with log aMg/ a2H between about 12.5 and 16.1 and log aNa/aH ratios between  2 and 1. According to Jones and Gala´n (1988), the formation of sepiolite is controlled by several physico-chemical parameters including pH (alkalinity), pCO2 and salinity. A pH between 8 and 9.5, moderate salinity and low pCO2 seems to be the most favourable. More recently, Birsoy (2002) produced numerous stability diagrams for palygorskite and sepiolite and their associations with other minerals at 25  C in a system containing seven components (Figure 4; MgO–CaO– Al2O3–SiO2–H2O–CO2–HCl). She pointed out that when a solution has insignificant or very low aluminium activity (log [aAl3þ/(aHþ)3] < 7.5), direct precipitation of sepiolite occurs, diminishing as the activity ratio increases. Direct sepiolite precipitation was enhanced in solutions that were very rich in silica (log[aH4SiO4]   4.75), while in silica-poor solutions, sepiolite requires a higher pH for its formation than that required by palygorskite. In nature, sepiolite may precipitate from a water mass that is evaporating, but it may also precipitate from interstitial fluids in sediments subject to evaporation, or it may even be formed from magnesian-silicate substrates or pre-existing minerals during diagenesis. Leguey et al. (2010) recently proposed a relationship between dolomite biomineralization and sepiolite formation. They suggested a general process for dolomite dissolution as source of Mg favouring the fibrous clay neoformation.

A

B 21

21

chlorite

20

20

log [aMg2+/(aH+)2]

18

sepiolite

17 brucite talc

16 15

solution

magnesite

14

palygorskite dolomite

13 12 11 10 9

sepiolite

19

chrysotile

18 log [aMg2+/(aH+)2]

19

chlorite palygorskite

17 brucite 16 15

talc magnesite

14 13

solution dolomite

12 11

amorphous Mg-mont silica −8 −7 −6 −5 −4 −3 −2 −1 0 1 2 log [aH SiO ] 4

4



10 9

Mg-mont −8 −7 −6 −5 −4 −3 −2 −1 0 1 2 log [aH SiO ] 4

4

FIGURE 4 Representative 25 C and 1 bar phase diagrams of a seven component system (MgO– CaO–Al2O3–SiO2–H2O–CO2–HCl) with log aH2O ¼ 1, log aCO2 ¼ 3.5 and log[aCa2þ/ (aHþ)2] ¼ 13.06. Figures A and B show the stability fields of sepiolite and palygorskite for log [aAl3þ/(aHþ)3] ¼ 4.5 and log[aAl3þ/(aHþ)3] ¼ 9.2, respectively (adapted from Birsoy, 2002).

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Indeed, the most widely accepted interpretation for sepiolite formation is that of direct precipitation from a solution containing dissolved ionic species (silica and magnesium; Jones and Gala´n, 1988; Trauth, 1977). Sepiolite may also form from colloidal silica in a Mg2þ-rich environment, giving rise to a hydrated magnesium silicate precursor being deposited on a mineral support that would form sepiolite in periods of desiccation (Leguey et al., 1985; Pozo et al., 1990; Williams et al.; 1985). The role of aqueous CO2 in the precipitation of magnesium silicates under evaporative conditions has been reported by Deocampo (2005). Model calculations show that pCO2 is an important control on pH, thus affecting Mg-clay mineral stability. Geochemical analysis applied to water and sediments of African lakes indicates that a minimal evaporative concentration is required for Mg-silicate supersaturation, and a strong correlation (R2 ¼ 0.7, p < 0.001) is found between pCO2 and its solubility, independent of brine evolution. Obviously, initial supersaturation of water with respect to Mg-clays generally requires elevated pH, and subsequent pH suppression due to biotic or abiotic CO2 can prevent mineral precipitation (Jones, 1986). The influence of salinity as an inhibitor for sepiolite formation was described by Darragi and Tardy (1987). Aragonite and stevensite formed in alkaline salt lakes but not sepiolite. When pH and ion activity in solution are controlled by carbonate precipitation (calcite, dolomite), only limited evaporation is necessary for sepiolite to be formed from groundwater springs (Khoury et al., 1982). A d18O isotopic investigation of Mg-rich clays showed that the highest ratios are observed when stevensite is predominant, and the lowest are reached when kerolite predominates (Hay et al., 1995). The conclusion reached was that high salinity favours stevensite and low salinity favours kerolite. Values obtained for sepiolite (meerschaum type) are midway between the two. Sepiolite genesis may also be associated with the dissolution of Mg-rich clay minerals such as stevensite, saponite and kerolite. In this case, variations in salinity may give rise to ideal thermochemical and kinetic conditions for the formation of the fibrous mineral by means of a dissolution–precipitation transformation process (Chahi et al., 1997; Eberl et al., 1982; Khoury et al., 1982; Post and Janke, 1984; Pozo, 2000; Pozo and Casas, 1999). In laboratory dissolution experiments of sepiolite and kerolite (Stoessell, 1988), sepiolite is favoured in solutions in equilibrium with, or oversaturated with quartz, and that it is metastable with regard to kerolite at lower silica concentrations. In summary, according to the experiments and calculations cited, the physico-chemical conditions for sepiolite precipitation under normal conditions of temperature and pressure require relatively high alkalinity (low pCO2), with pH ranging between 8 and 9.5, medium salinity (brackish) and sufficient silica (log[aH4SiO4]   4.75), and magnesium activity. It is also essential

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that aluminium should be absent in solution (log[aAl3þ/(aHþ)3] < 7.5) or in reactive phases in the described conditions.

2.2. Natural and Experimental Evidence of Palygorskite Formation The models for palygorskite formation have been reviewed and discussed by different authors (Isphording, 1973; Jones and Gala´n, 1988; Paquet, 1983; Singer, 1979). Palygorskite is commonly formed in lacustrine–palustrine environments (Millot, 1970; Sa´ez et al., 2003; Trauth, 1977; Webster and Jones, 1994). However, unlike sepiolite, the largest number of references to the distribution and genesis of palygorskite is in association with palaeosols and carbonate crusts (Paquet, 1970; Singer, 1984; Sua´rez et al., 1993). There have been considerable differences of opinion regarding palygorskite formation, with two models being established. One considers palygorskite to have originated from aluminous precursors (Trauth, 1977; Weaver and Beck, 1977), while the other considers it to be the result of direct precipitation from solution (Singer, 1979, 1984; Singer and Norrish, 1974). According to Trauth (1977), the mineral precursor for palygorskite is an aluminous-magnesian smectite in basic environments rich in Mg2þ and silica. Weaver and Beck (1977) in turn proposed montmorillonite as the aluminous precursor. For these authors, palygorskite occurs in environments with sufficient silica and Mg2þ in solution as well as a pH range between 8 and 9. Under these conditions, Mg2þ would migrate to the octahedral layer in the smectite, producing a distortion that would force the inversion of the tetrahedra, producing palygorskite. The stability relationships of palygorskite–smectite reported by Weaver and Beck (1977) show that for a fixed concentration of aluminium (log[Al(OH4)-)] ¼  5.5), the stability of one phase or the other at 25  C depends on the activity of silica and Mg2þ, and pH. Velde (1985) explains that the reactions between silicates containing alumina and the elements in solution, particularly silica and Mg2þ, play a very important role in the genesis of palygorskite. Thus, if the solubility of an ionic species is low (as occurs with aluminium), and the activities of silica and Mg2þ are high, a rapid process of transformation by dissolution–precipitation is possible. In other words, the aqueous dissolution of the precursor phase (smectite) would be enhanced so that a new phase (palygorskite) could be generated in a short time. Numerous researchers interpret the formation of palygorskite from a smectite precursor, both in soils (Abtahi, 1977; Yaalon and Wieder, 1976) and in sediments (Couture, 1977; Decarreau et al., 1975; El Prince et al., 1979; Giresse, 1980; Jamoussi et al., 2003). More recently, experimental studies by Birsoy (2002) show that the system MgO–CaO–Al2O3–SiO2– H2O–CO2–HCl (Figure 4), at a temperature of 25  C with high silica activity

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(either in the form of quartz or in the form of amorphous silica), does not easily form palygorskite by direct precipitation from solution. Rather, it does so by the transformation of certain aluminous precursors such as dioctahedral smectite. She also observed that palygorskite formation is favoured when the solution has an increasing aluminium activity (log[aAl3þ/(aHþ)3] from 4.5 to 9.2). Different investigators have shown the existence of a precursor in palygorskite formation by means of TEM (Sua´rez et al., 1994; Tazaki et al., 1986, 1987). The precursor phase generally proposed is montmorillonite, but beidellite (Sautereau and Decarreau, 1973), mica (Gala´n and Ferrero, 1982), chlorite (Gala´n et al., 1975) and primary silicates (Paquet, 1983) have also been implicated. As previously described, a model of direct precipitation from solution has also been proposed for palygorskite genesis. The work by La Iglesia (1977) demonstrated that palygorskite can be obtained experimentally by homogeneous precipitation. Singer (1979, 1984) also supports the precipitation model, while ruling out the previous transformation model based on crystal-chemical criteria and on the absence of an intermediate structure. This author states that the stability of palygorskite in solution depends on pH and Mg2þ and silica activity. For fixed Al and Fe activities, palygorskite can even be stable at a pH of 6, if silica and Mg2þ activities are high. However, when Mg2þ is scarce, high pH (> 8.5) and high silica concentrations are required. This model has also been proposed by Vanden Heuvel (1966), Singer and Norrish (1974), Watts (1976, 1980), Callen (1977), Hutton and Dixon (1981) and Este´oule-Choux (1984), mostly in calcretes. In conditions of high alkalinity, direct palygorskite precipitation would be inhibited, as the aluminium would coordinate tetrahedrally instead in octahedra as required by the mineral (De Kimpe et al., 1961), unless the octahedrally coordinated aluminium was inherited directly from the smectite precursor. Current investigators suggest that the main mechanism for palygorskite formation is transformation by dissolution–precipitation, given the abundance of fine detrital materials supplying aluminium. But, they have not ruled out that local neoformation by direct precipitation also takes place under more restricted conditions. The phase diagrams prepared by Birsoy (2002) show that precipitation of palygorskite can take place but only for log[aAl3þ/ (aHþ)3] value around 5.5 and a log[aMg2þ/(aHþ)3] value between 11 and 13. In the review by Jones and Gala´n (1988), it was established that the most favourable conditions for palygorskite formation were produced at pH < 8.5 with high Mg2þ, silica and aluminium activity and moderate salinity. Controversy exists concerning the most favourable salinity for palygorskite formation since against moderate salinity, Webster and Jones (1994) have proposed ephemeral-playa conditions.

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137

3. MAIN GENETIC CHARACTERISTICS OF THE WORLDWIDE DEPOSITS AND OCCURRENCES OF SEPIOLITE AND PALYGORSKITE IN CONTINENTAL SEDIMENTARY ENVIRONMENTS The main characteristics of the worldwide sepiolite and palygorskite deposits of continental origin are summarized in Tables 3 and 4, respectively. Concerning sepiolite the most important deposits are in Spain and Turkey, thus special attention will be paid to sepiolite in these two countries. On the contrary, the most important palygorskite deposit (Georgia–Florida, USA) will be not described here because of its perimarine origin. In relation with the Guanshan (China), probably the present biggest deposit of palygorskite is only briefly cited because it is described in detail in Chapter 10. Therefore, only some smaller deposits but of particular genetical features will be cited. Some X-ray diffraction (XRD) patterns of sepiolite and palygorskite from Vica´lvaro (Spain), Eskisehir (Turkey), Guanshan (China) and Torrejo´n (Spain) are shown in Figure 5. Sepiolite samples show a high purity (only traces of calcite in Eskisehir sepiolite) and high ordering. Palygorskite from Torrejo´n exhibits higher ordering than Guanshan sample, both with traces of associated minerals. Some representative chemical analyses from 10 most deposits cited in this section are displayed in Table 5. Sepiolite samples from Eskisehir and Vicalvaro are the purest samples showing the lowest Al2O3 and K2O content. Palygorskite samples display variation of MgO and Al2O3 content, being noteworthy the higher Fe2O3 and K2O percentages in the sample from India.

3.1. Spain Sepiolite and palygorskite are widely distributed in continental deposits in Spain (Figure 6), with special concentrations in the Tagus and Duero basins. However, the most noteworthy deposits are those located in the Tagus Basin.

3.1.1. Tagus Basin 3.1.1.1. Geological Setting The Tagus Basin is an intracratonic basin that was formed as a result of differential tectonic strains within the Iberian microplate during the Alpine orogeny. The lithostratigraphy of the Miocene epoch of the Neogene period is divided into three major units: Lower, Middle and Upper, which are separated by regional disconformities (Alberdi et al., 1984; Junco and Calvo, 1983). The Altomira Range (Mesozoic) divides the basin into two sub-basins named the Madrid Basin and Loranca Basin, located in the west and east of the Tagus Basin, respectively. The geological map of Madrid Basin and a representative lithological section are shown in Figure 7.

TABLE 3 Summary of Worldwide Sepiolite Deposits. Deposit (Country, Age)

Fibrous Clay Minerals (Thickness)

Lithofacies Assemblage

Mineralogical Association

Environment

Origin

Vicalvaro-VallecasCaban˜as de la Sagra (Spain) Miocene

Sepiolite Lower Unit. 1–5 m Upper Unit. Up to 10 m Two main beds

Arkose, clayey arkose, Mg-clay Sepiolite,dolomite, calcite, chert

Sepiolite (> 95%) (saponite, stevensite, illite, calcite, dolomite, quartz, feldspar)

Lacustrine/alluvial

Depositional neoformation Diagenetic

Batallones (Spain) Miocene

Sepiolite Lower Unit. Up to 9 m Upper Unit. Up to 2 m Two main beds

Sepiolite, sepiolite– palygorskite clay, Mgsmectite clay, calcrete, chert

Sepiolite (palygorskite, saponite, illite, quartz, feldspar, calcite)

Palustrine

Depositional neoformation Diagenetic

Eskisehir (Turkey) Miocene

Sepiolite 0.5–5 m Beds and nodules

Calcareous and gypsiferous clay, sepiolite,dolomite, conglomerate, magnesite, tuff, gypsum

Sepiolite (up to 90%): (dolomite, quartz, illite, feldspar, VFR)

Lacustrine

Depositional neoformation Diagenetic

Eskisehir (Turkey) Miocene

Loulinghite 0.6–5 m

Lutite, tuff, bentonite, chert

Loulinghita (sepiolite, analcime, smectite, illite, calcite, feldspar, palygorskite, opal)

Lacustrinevolcanosedimentary

Diagenetic

Mara-Orera (Spain) Miocene

Sepiolite 0.5–0.6 m (up to 1 m) Inserts

Carbonate, dolomitic marl, sepiolitic marl, chert

Sepiolite: (palygorskite, Mg-smectite, calcite, dolomite, zeolites, opal)

Palustrine– lacustrine

Depositional neoformation

Amboseli (Kenya–Tanzania) Pleistocene

Sepiolite 1–3 m One bed and nodules (meerschaum)

Sepiolite, dolomite

Sepiolite (dolomite, kerolite-stevensite, calcite, feldspar)

Lacustrine (margin)

Depositional neoformation

Amargosa (USA) Pliocene–Pleistocene

Sepiolite: Up to 1.5 m Two beds

Silt, silty clay, sand

Sepiolite (saponite, illite, kerolitestevensite, dolomite, calcite)

Lacustrine (playa)

Depositional neoformation Diagenetic

TABLE 4 Summary of Worldwide Palygorskite Deposits. Deposit (Country, Age)

Fibrous Clay Minerals (Thickness)

Lithofacies Assemblage

Mineralogical Association

Environment

Origin

Bercimuel (Spain) Miocene

Palygorskite 1–1.5 m Two beds

Silt, clay, calcrete

Palygorskite (illite, kaolinite, quartz, smectite, mixed layers)

Alluvial

Diagenetic (dissolution– precipitation), Al-smectite

Torrejo´n el rubio (Spain) Paleogene

Palygorskite 0.5–4 m One bed

Marl. Palygorskitic clay,arkose, gravel

Palygorskite (illite, sepiolite, chlorite, dolomite, saponite, quartz, feldspar)

Lacustrine– palustrine (alteration profile)

Diagenetic (dissolution– precipitation), chlorite

Andhra Pradesh (India) (Paleogene)

Palygoskite 0.5–3 m

Limestone, chert, marl, sandstone

Palygorskite (?)

Lacustrine

Diagenetic (dissolution– precipitation), Illite Depositional neoformation

Lake Nerramyne (Australia) Garford Paleochannel (Australia) (Miocene)

Palygorskite 4–9 m Palygorskite Up to 2 m

Clay, dolomite

Palygorskite (?) Palygorskite (illite, smectite, dolomite)

Lacustrine Lacustrine

No data

Guanshan (China) (Miocene)

Palygorskite 3–6 m

Clay, basaltic ash

Palygorskite (smectite > quartz  sepiolite, mica, dolomite)

Lacustrine–fluvial (alteration profile)

Diagenetic (basaltic ash and basalt))

Grevena (Greece) (Pliocene– Plesitocene)

Palygorskite 10–18 m

Clay, sand

Palygorskite (?)

Lacustrine

Diagenetic (saponitic sands, ultramafic rock)

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ESKISEHR

S = Sepiolite C = Calcite

S

900

12.06 Å

S

S S

S

C S S

S S

S

S

100 0

S

S

400

S

S

S

S

S S

S S

VICALVARO

3600

S 12.01 Å

S

1600 S S

400

S

S

S

S

S

S

S

S

S

S S

S

S S S

S

S S

S

0 10

20

30 40 Position [⬚2theta]

GUANSHAN

P P

P P

100

Q P P

P

70

P P

Q

Q

TORREJON

P 900

60

Q P 10.47 Å

0

50

P 400

P = Palygorskite Q = Quartz D = Dolomite

Q

10.49 Å

P P

Q P

D

P P

P

P

P Q

P P

100

P

Q P

P

P

0 10

20

30 40 Position [⬚2Theta]

50

60

70

FIGURE 5 XRD patterns of sepiolite and palygorskite from representative deposits.

Sepiolite is commonly found in the Madrid Basin (Brell et al., 1985; Calvo et al., 1999; Gala´n and Castillo, 1984; Ordon˜ez et al., 1991). Although not as common as sepiolite, the presence of palygorskite in the Neogene lacustrine and alluvial sequences of the Madrid Basin has often been reported (Gala´n and Castillo, 1984; Garcı´a-Romero et al., 2004; Leguey et al., 1985; Pozo et al., 1985). The sedimentological analysis of the lacustrine deposits and their lateral relationship with alluvial facies that border the basin margin suggests that the lake systems underwent significant changes throughout the Miocene (Calvo et al., 1989).

TABLE 5 Representative Chemical Analysis of Sepiolite and Palygorskite from Selected Worldwide Deposits. Sepiolites

SiO2

Al2O3

Fe2O3

Vallecas (Gala´n and Castillo, 1984)

63.10

1.08

0.27

Amboseli (Hay and Stoessel, 1984)

53.17

1.76

0.99

Mara (Arauzo et al., 1989)

58.29

2.32

Batallones (Pozo et al., 2010b)

56.46

¸ oban, 1994) Eskisehir (Ece and C

FeO

H2O

CaO

Na2O

K2O

23.80

0.49

0.16

0.21

24.70

0.23

0.45

0.97

0.17

8.29

1.24

22.21

1.73

0.13

0.15

0.62

14.06

1.17

0.37

22.67

0.19

0.06

0.18

0.08

18.57

56.95

1.05

0.93

23.35

2.45

0.11

0.41

0.15

14.60

Andhra Pradesh (Siddiqui, 1984)

53.70

7.78

7.96

8.45

0.92

0.14

1.57

1.23

18.13

Guanshan (Zhou and Murray, chapter 10 of this book)

55.21

8.16

4.05

12.52

0.24

0.03

0.83

0.50

18.40

Bercimuel (Castillo, 1991)

60.00

12.40

4.80

7.80

1.68

0.90

0.90

Torrejo´n (Gala´n et al., 1975)

51.50

10.03

2.36

0.04

TiO2

H2Oþ

MgO

LOI

10.88 8.79

Palygorskites

0.52

12.28

11.93 14.43

7.36

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Developments in Palygorskite-Sepiolite Research

FIGURE 6 Tertiary basins in the Iberian Peninsula (Spain and Portugal).

The Mg-clay deposits (sepiolite, Mg-bentonites and kerolitic clays) are located in a transition zone between alluvial and marginal lacustrine facies within the Miocene Middle Unit (Ordon˜ez et al., 1991). According to Doval et al. (1985) and Garcı´a et al. (1990), the distribution of Mg-rich clays in the Madrid Basin follows a pattern in which sepiolite is present in the marginal zones closest to detrital inputs, while Mg-rich smectites are found in the central areas. This palaegeographical distribution of clay minerals is not in accordance with the Millot’s model (Millot, 1970), that is, the most magnesian clays do not occur at the centre of the basin, as Millot proposed, but mostly in marginal sites. The facies bearing Mg-clays was named “Magnesian Unit” by Pozo and Casas (1995) in reference to its anomalous Mg content. A relationship between

0

MADRID

50 100

5 VICALVARO

V

UU

200 m

Arkoses

Legend K 5

VICÁLVARO

QUATERNARY

SD

UPPER LIMESTONE ARGANDA

4

IU

LIMESTONES CLAYS AND CHERT GREENISH CLAYS AND SANDS

N VACDEMORO

LU

ST

B

GYPSUM

BATALLONES

RED CLAYS

MA

3

100 m

IU

RO

IC

ESQUARAS

2

EB

DUERO

RAR

ESQUIVIAS

HES

E

JAR A

Sp

NEOGENE

MAGNESIC UNIT

ARKOSES

drid TAJO

SIF

MAS

Ma

C

LU

ARANJUEZ

CABAÑAS DE LA SAGRA

1

C

C

M

MAGÁN

Deposit

0m

SP SD ST I K

sepiolite AI-smectite Mg-smectite illite kaolinite

ER

O TAJ

RIV

30 km

VICALVARO BATALLONES ESQUIVIAS CABAÑAS MAGÁN

Main clay occurrence Sepiolite Sepiolite Kerolite-stevensite Bentonite Bentonite

TOLEDO

FIGURE 7 Geological map and representative lithological section and mineralogy of Neogene units at the Madrid Basin (LU, Lower Unit; IU, Intermediate Unit; UU, Upper Unit). The location of Mg-clay deposits including sepiolite, Mg-bentonite and kerolite-stevensite mixed layers is also showed (modified after Brell et al., 1985).

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occurrences of Mg-clays, source areas and depositional systems has been suggested by Doval et al. (1985) and Calvo et al. (1989). The mineralogical assemblages are composed of both detrital (inherited) and authigenic minerals. Authigenic silicate minerals include sepiolite, Mg-smectite (stevensite, saponite), mixed-layer kerolite/Mg-smectite, quartz (nodules) and zeolites (clinoptilolite–heulandite). Calcite, dolomite and barite were also detected. The Magnesian Unit facies are interpreted as alluvial-related, dry mudflat or palustrine deposits formed in a saline-alkaline lake margin. In this environment, alternating expansion/contraction episodes of detrital input (sand intervals) and sub-aerial exposure (palaeosols) can be determined. Granitic and metamorphic rocks from the bordering Central System and Toledo Mountains are the major source of inherited minerals, colloids and silica-laden waters. Fluctuating brackish to saline lake waters are the principal contributors of Mg, probably recycled from Mesozoic evaporitic successions. According to Castillo (1991), the sepiolite deposits present in the Madrid Basin are in two main palaeogeographic situations, one associated with the distal facies of the alluvial fans (e.g. Vica´lvaro-Caban˜as de la Sagra; Figure 7) and another associated with predominantly palustrine conditions (e.g. Cerro de los Batallones). A third, highly subordinated type is associated with the central lacustrine facies where it occurs with carbonates and is not suitable for exploitation. Several authors have also described sepiolite palaeosols in the Madrid Basin, both in marginal areas influenced by arkosic deposits (Calvo et al., 1986; Leguey et al., 1989) and in shallower internal areas of the basin (Leguey et al., 1985; Martı´n de Vidales et al., 1988). The existence of sepiolite with deposits of other Mg-rich clays is very common in the Madrid Basin (Martı´n de Vidales et al., 1991). According to Pozo (2000), the diversity of these Mg-clay occurrences and their geochemistry can be interpreted as a result of genetic processes controlled by the existence of precursor clay phases (inherited and/or authigenic), water sources (runoff, groundwater, lake water) and their hydrochemistry (e.g. Esquivias Mg-clay deposit). The geochemical variability of Mg-rich clays in the Madrid Basin has been described by a number of different authors (Pozo et al., 1999a,b; Torres-Ruı´z et al., 1994). These authors observed generally significant geochemical differences between sepiolite and magnesium smectite with respect to palygorskite, justifying the differences by the type of mineral genesis. The different types of clays can be clearly separated by F-Mg and Li-Mg ratios (e.g. Vicalvaro and Maga´n Mg-clay deposits). More recently, Pozo et al. (2005, 2010a) studied the geochemistry of sepiolite from different environments in the Madrid Basin, including alluvial fan facies (Vica´lvaro, Caban˜as de la Sagra), a palustrine complex (Cerro de los Batallones) and mudflat environments (Esquivias, Maga´n). The results allow sepiolites to be differentiated by the abundance of trace elements, or on the

Chapter

6

Palygorskite and Sepiolite Deposits in Continental Environments

145

basis of whether they originated through precipitation from solution or under diagenetic conditions. In the case of diagenetic origin, the geochemistry of authigenic sepiolite would be related to the Mg-clay/carbonate support involved and the composition and pH of the groundwater input. The abundance of palygorskite in the Madrid Basin is much lower than sepiolite (Pozo et al., 1985). Until now, the most important occurrence of palygorskite is at Tabladillo (province of Guadalajara), in Miocene beds belonging to the so-called Loranca Basin, between the fluvial (detrital) and lacustrine (evaporitic) facies (Martı´n Pozas et al., 1981). The palygorskite beds are 0.4–2 m in thickness, intercalated between detrital, marly and gypsiferous facies. The palygorskite can reach a proportion of 70%, accompanied by sepiolite, mica, quartz, calcite, dolomite and opal. This has been interpreted as the result of transformation from precursor aluminous phases. 3.1.1.2. Vallecas–Vica´lvaro and Yunclillos Deposits These sepiolite deposits, the world’s most important, are associated with alluvial deposits in a band running in a NE–SW direction. The Vallecas-Vica´lvaro deposits, close to Madrid, are located at the northern, and the Caban˜as-Yunclillos deposits, in the province of Toledo, are at the south (Figure 7). The sepiolite occurs in two stratigraphic levels that are topped by arkosic filling materials originating in distal alluvial facies (Figure 8). In the exploitation of the Vallecas-Vica´lvaro area, two lacustrine units with sepiolite mineralization are distinguished (Figure 9). They form lenticular subhorizontal layers with thicknesses varying between 2 and 12 m (Castillo, 1991; Gala´n and Castillo, 1984). These units are separated by arkosic sands forming coarsening-upward sequences of alluvial origin with the absence of erosive bases or channels, and with a thickness varying between 10 and 50 m (Baltuille et al., 1996). The sepiolite-rich beds mostly consist of sepiolite (> 80 wt%) and smectites (15 wt%), with traces (< 5 wt%) of calcite, dolomite, quartz and feldspars. The first sepiolite-bearing unit (lower lacustrine sequence) is formed by a main bed of poorly ordered sepiolite (XRD; 1–5 m thick) with a lenticular morphology and with intercalations of dolomitic carbonates, Mg-smectite clays and silex on the margins. The unit ends locally in a laminated bed of marly smectite-rich clays. The second sepiolite-bearing unit (upper lacustrine sequence) has at its base a bed of laminated sepiolite that is rich in detrital minerals (quartz and feldspar) and reddish in colour. The sepiolite bed has a lenticular morphology and presents carbonate, smectite and silex intercalations at its margins. It becomes separated into two exploitable layers, reaching a thickness of 10 m, high purity (up to 95 wt%) and good XRD ordering (see Figure 5). Towards the top and laterally, it grades into smectite-rich clays, calcareous marls and marly limestone finishing in a laminated clay bed formed by

146

Developments in Palygorskite-Sepiolite Research

DISTAL ZONE INTERMITTENT LAKES (PLAYA – LAKE)

GRANITIC BED ROCK

PROXIMAL ZONE

PERENNIAL LACUSTRINE ZONE

B SEPIOLITE

A ALLUVIAL FAN

MUD FLAT

IV III II

I

SO.

10 m.

0 m.

E. Vertical

0

1

2 Km.

E. Horizontal

Sepiolite

Mg/AI Semectite

Arkose

Mg-Smectite, Sepiolite

Calcrete

Gypsum

Lutite/Sepiolitic arkose

Dolocrete

Chert nodule

Black clay

Silcrete

Zeolite

FIGURE 8 Block diagram showing the Madrid Basin sedimentary environments during Neogene and representative cross section of the Vicalvaro sepiolite deposit. The main sepiolite beds are labeled A and B. Roman numbers indicate boreholes position (adapted from Gala´n and Castillo, 1984 and Sa´nchez Rodrı´guez et al., 1995).

smectite and/or sepiolite. The stratigraphic succession ends with an interval of up to 20 m of arkose in fining-upward sequences, with thin inserts of clays and a basal bed of gravel. The primary mechanism for the formation of the sepiolite seems to be neoformation by precipitation. The differences in the mineralogical associations between the two established sepiolite-bearing beds have been related to the locally greater availability of magnesium and higher pH, which would explain the greater presence of Mg-rich smectites in the lower sepiolite-bearing unit (pH close to 9) and the better ordering of the fibrous mineral in the upper sepiolite-bearing unit (Gala´n and Castillo, 1984). A detailed study of the beds containing smectite phases in the Vica´lvaro sepiolite deposit (Cuevas et al., 2003)

Chapter

6

Palygorskite and Sepiolite Deposits in Continental Environments

147

YUNCLILLOS SECTION 0 m.

B

2 m.

Arkoses

SEPIOLITE–SMECTITE CLAY WITH CHERT SAND AND CLAYEY SAND SEPIOLITE ILLITE–SMECTITE CLAY AND SEPIOLITE

12 m.

A 15 m.

ILLITE–SMECTITE CLAY

20 m.

Arkoses

VICALVARO SECTION

SAND AND CLAYEY SAND CHERT AND SILICEOUS LIMESTONE SMECTITE–SEPIOLITE CLAY

20 m.

SEPIOLITE

B 25 m.

ILLITE–SMECTITE CLAY

Arkoses

DOLOMITE AND SMECTITE– SEPIOLITE CLAY GRAY LIMESTONE

33 m.

A

FIGURE 9 Representative lithological sections from the Yunclillos and Vicalvaro sepiolite deposits. The main sepiolite beds are labelled A and B (adapted from Gala´n and Castillo, 1984).

148

Developments in Palygorskite-Sepiolite Research

reviews a mixture of stevensite, saponite and a micaceous mineral. Sepiolite is interpreted as the result of a diagenetic alteration in the Mg-rich smectites. At Yunclillos-Caban˜as de la Sagra area (province of Toledo), a sepiolite deposit of truly great economic interest (Gala´n and Castillo, 1984; Pozo et al., 1999a,b) is associated with Mg-rich bentonites. In this deposit, two sepiolite beds can be defined (see Figure 9). The upper bed has smectites and chert nodule impurities, while the lower one is thicker (3 m) and of higher purity. The sepiolite occurs in a unit of fine-grained sands with chert and carbonates (Garcı´a et al., 1990). The sepiolite-bearing beds can also contain Mg-rich smectite (saponite) in varying proportions, although the sepiolite bed can reach several metres thick and with high purity. The sepiolite is considered to be authigenic. The general sedimentary environment for the formation of the sepiolite would have been related to flooding events over the fringe facies between distal alluvial fan deposits and those corresponding to lacustrine mudflats of a saline–alkaline lake (Figure 10). In this situation, the sepiolite also occurs in sediments of the alluvial fringe and associated with calcretes (Calvo et al., 1986). 3.1.1.3. Batallones Deposit Sepiolite associated with palustrine environments appears to be well represented in the Valdemoro-Esquivias sector. The sepiolite-bearing unit extends in a NE–SW direction for over 20 km. Only one mineralized bed is recognized where sepiolite is the major mineral; however, it can be accompanied towards the top of the unit by palygorskite. The Batallones sepiolite deposit is located near the villages of Torrejo´n de Velasco and Valdemoro (province of Madrid; Figure 7). Both mapping and sedimentological analysis allow three main lithological units to be distinguished (Pozo et al., 2004): (I) mudstones, dolomitic mudstones and Mg-bentonites; (II) sepiolite and opal-CT; and (III) limestones, marls and siliciclastic sediments (Figure 11). Mudstones from Unit I are interpreted as mudflat deposits associated with a Mg-rich lake margin. The sepiolites and opals of Unit II are interpreted to represent thick polyphasic sepiolite palaeosols developed in a similar alkaline lake margin environment undergoing periods of prolonged sub-aerial exposure and groundwater inputs. Deposited in disconformity, the Unit III comprises associated siliciclastic and carbonate deposits forming sequences that are interpreted to represent deposition in a freshwater palustrine to shallow lacustrine environment. The sepiolite unit in Batallones consists of three sedimentary sub-units forming sequences II.1, II.2 and II.3, each with distinct mineralogical characteristics. Sequence II.2 contains sepiolite of the highest purity and XRD ordering (Pozo et al., 2010b). A noteworthy fact of these sequences is the existence of at least three main textures: laminated, massive and brecciated-intraclastic.

Lake margin

Open lake

Alluvial Groundwater recharge

Pond

Mudflats

Evaporation Oscillation of lake water level

Carbonate facies

Clay mineralogy Carbonate mineralogy

Massive bedded Calcretes/ Nodular Nodular Stromatolites, Bedded limestones dolostones and dolostones, dolocretes limestones dolostones tepees gypsum molds Mg-smectites Polygorskite/ Sepiolite (stevensite, saponite) Illite sepiolite Kerolite/stevensite Calcite/ Calcite Dolomite Dolomite Calicite/dolomite dolomite

PALYGORSKITE

SEPIOLITE

Mg-SMECTITE

FIGURE 10 Idealized sketch of marginal to open lake environments (palustrine–lacustrine) in the Madrid Basin during Miocene. The whole clay minerals displayed are authigenic and commonly associated with carbonate facies (adapted from Calvo et al., 1999).

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Developments in Palygorskite-Sepiolite Research

UNIT III. DETRITAL FACIES AND CARBONATES

II.3

Spain Madrid

N

Sp-Pk

Sp

UNIT II. SEPIOLITE II.2 LEGEND

Sp

Limestones, chert,

U.3 mudstones, marls and sandy clays

U.2 Sepiolites and opal Green mudstones

U.1 and bentonites

II.1

Transition

Mudstones and gypsum

Sp-SmT

Lithological sections

200 m m 1

SmT

UNIT I. GREEN CLAYS, SILTS AND DOLOMITIC MARLS

1.LADERA 2.POSTE 3.BALLO 4.JF2 5.BAT8!9

0

FIGURE 11 Geological map and representative lithological section of the Batallones sepiolite deposit (modified after Pozo et al., 2004).

The appearance of these textures both in optical microscopy and in scanning electron microscopy is shown in Figure 12. Massive texture shows a chert-like groundmass locally with detrital grains and bioturbation features. Under the scanning electron microscope (SEM), sepiolite randomly oriented fibre bundles are displayed. Laminated texture exhibits thin lamination, sometimes containing intraclasts and/or detrital grains. Under the SEM, laminar microfabric with orientation of fibre aggregates is observed. Brecciated-intraclastic texture (Figure 12) shows several degree of brecciation sometimes forming intraclasts. Under the SEM, the intraclasts are commonly groundmass-supported indicating at least two sepiolite genetic stages. Pozo et al. (2009) suggest that the sepiolite was formed from the destabilization of Mg-rich smectite (saponite) by means of an intrasedimentary mechanism with the subsequent action of neoformation processes from solutions or Si–Mg gels. A reduction in the salinity and silica input from groundwater would have favoured these processes. In fact, an association between groundwater discharge and sepiolite formation was reported by Pozo et al. (2006) in the vicinity of the Batallones deposit. In contrast to sepiolite formation, the presence of palygorskite in sequence II.3 would have been the result of the transformation of inherited clays from the input of surface waters and detrital sediments in an environment favouring the formation of calcretes (Pozo et al., 2010c).

Chapter

6

Palygorskite and Sepiolite Deposits in Continental Environments

151

FIGURE 12 Thin section (crossed polars) and SEM images of main textures in the Batallones sepiolite deposit (Madrid Basin). Massive and brecciated-intraclastic textures are the most common often forming sequences. Laminated textures are more subordinated and have been observed mainly at the base of sepiolite unit.

3.1.2. Other Spanish Deposits Sepiolite and palygorskite are also found in the Duero Basin and in small basins, particularly those of Torrejo´n el Rubio, Calatayud, As Pontes and Campo de Calatrava and locally in a brackish lacustrine environment of perimarine origin, in the Guadalquivir basin (Lebrija) (Figure 6). Most of the occurrences of palygorskite and sepiolite in the Duero Basin correspond to materials included in the Cuestas Facies composed of marls, dolostone, limestone, clays and gypsum (Armenteros et al., 1989; Pozo, 1987) in the central areas of the basin. The only significant accumulation is located on the south-eastern edge of the basin in the Bercimuel palygorskite

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Developments in Palygorskite-Sepiolite Research

deposit where the fibrous clay mineral is associated with fine-grained detrital alluvial fan deposits and carbonate crusts (Armenteros, 1986; Martı´n Pozas et al., 1983). Sepiolite and palygorskite content in the Cuesta Facies can reach 28 and 73 wt%, respectively, accompanied by illite, smectite and traces of kaolinite (Pozo and Carame´s, 1983). Their formation in the carbonate-rich lacustrine environments has been interpreted as the result of neoformation from silica and magnesium gels both by intrasedimentary and by evaporitic processes. The palygorskite formed from silico-aluminous precursor phases (Pozo et al., 1990). Vivar (2010) recently studied different outcrops and cores of this facies establishing sequences starting with inherited mineral associations (illite, smectite and kaolinite) and finishing with neoformed minerals (Mgsmectite, palygorskite and sepiolite). Sepiolite was mostly neoformed, while palygorskite and magnesium smectite would be the result of transformation processes. The Bercimuel palygorskite deposit is associated with Neogene detrital materials deposited by alluvial fan systems that change laterally to lacustrine facies. This palygorskite deposit has been studied by Sua´rez et al. (1989, 1993) and Sua´rez (1992). A horizontal clay unit with average palygorskite content of 60–70 wt% overlying a carbonated bed (crust) is commercially exploited in Bercimuel. The unit consists of two beds 1–1.5 m thick. The palygorskite is associated with quartz, illite and kaolinite as inherited minerals, and with smectites and interstratified minerals (smectite–illite) as transformation minerals. According to Sua´rez (1992), the origin of the palygorskite is associated with the weathering of phyllosilicates and the dissolution of the quartz present in the sediment and by the action of solutes dissolved in surface runoff resulting from the weathering of pre-Neogene silicate and carbonate rocks. The palygorskite deposit at Torrejo´n el Rubio (province of Ca´ceres, Spain) contains Paleogene palygorskite-rich marls overlying the basement made of Cambrian slates. This deposit is located in a small Tertiary fault basin limited by quartzite and granite outcrops to the west of the Tagus Basin (see Figure 6). According to Gala´n and Castillo (1984), the maximum thickness of the clay unit ranges between 6 and 50 m. The detrital-clay unit has two mineralized beds of which only the lower one (0.5–4 m thick), overlying the slate basement, is exploited (Figure 13). The palygorskite content can reach 70 wt% and is associated with illite and varying quantities of sepiolite, chlorite, dolomite, smectite (saponite), feldspars and quartz. The lower bed is the result of weathering of the slate basement in a lacustrine–palustrine environment. The fluvial–alluvial conditions would have originated with the exposure of slate areas that were previously weathered and exposed as the consequence of movement by the faults limiting the basin. Thus, the continuity between slates and palygorskite-rich clays observed at a number of points suggests the formation of palygorskite in association with weathering of the metamorphic

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N Sra.

R IV E

de

las

Co

rch

R

uela

s

JO TA

TORREJON EL RUBIO

S-274 S-365

ST-385 ST-301

340 319

ST-342

367

ST-199

ST-289 ST-266

351 322

389 356 347 ST-415

394

EXPLANATION − “RANA” AND/OR PEDIMENT − “RANA” DETRITAL-CLAYEY BED

0 5 10m

QUARZITE

0

1

2Km.

SLATE (BASEMENT)

Pa > I I >> Pa DETRITAL MINERALS ST-385 = LOG NUMBER MINE 319 = ELEVATION IN M

FIGURE 13 Geological map and correlation of cores from the Torrejo´n el Rubio Basin. It is noteworthy the thickness variability of the palygorskite-rich bed.

rock in an acidic environment. Weathering of chlorite contained in the slate would have provided the necessary input of components required for the formation of palygorskite (Gala´n and Castillo, 1984; Gala´n et al., 1975). Other authors (Fernandez Macarro and Blanco, 1990) suggest the palygorskite would have originated from smectite in palaeosols formed on a stable alluvial plain. The Mara sepiolite deposit located in the Calatayud Basin province of Zaragoza (see Figure 6) originated during the upper Miocene in a palustrine– lacustrine environment (Arauzo et al., 1989). The sepiolite bed consists of alternating clays, marls and carbonates, with thicknesses between 10 cm and 1 m, and average values of 50–60 cm. The mineralogical associations in the sepiolite beds are complex: detrital minerals (illite, interstratified illite-smectite, quartz, feldspars, dioctahedral smectite, chlorite and kaolinite); neoformed phyllosilicates (sepiolite, trioctahedral smectite, palygorskite); carbonates (calcite and/or dolomite); and, also occasionally, heulandite– clinoptilolite, opal-A and opal-CT (Mayayo et al., 1998). Both the sepiolite and the trioctahedral smectite formed by precipitation in the lacustrine basin. The palygorskite is post-depositional resulting from the transformation of aluminosilicate phases. The volcanic region of Campo de Calatrava is located in the south-central zone of the Iberian Peninsula (province of Ciudad Real) and belongs to the Cenozoic Manchegan Basin (see Figure 6). The presence of palygorskite in

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this basin has been reported by Pozo et al. (1986), Pozo and Martı´n de Vidales (1989), and Sa´nchez and Gala´n (1995). The genesis of the palygorskite is a result of dissolution–precipitation phenomena at the expense of the dioctahedral smectites and/or illite during retractive stages of the marginal lacustrine system. The origin of the Mg is interpreted as the result of hydrolytic alteration of chloritic slates from the basement, from which this element would have been released. The weathering of volcaniclastic materials, however, would have been responsible for releasing Mg, Ca and silica into the aqueous phase. The main characteristics of the Spanish sepiolite–palygorskite deposits are summarized in Tables 3 and 4.

3.2. Turkey 3.2.1. Geological Setting References to the presence of sepiolite and palygorskite in continental deposits in Turkey have grown notably in recent years. Although there is a common presence of both fibrous clay minerals, the sepiolite deposits acquire special relevance owing to their extensive distribution and their quality, with special reference to the Ezkisehir Basin deposits. Three sectors can be established with sepiolite deposits or containing sepiolite: Eskisehir–Konya, Denizli and Sivas (Figure 14). The Eskisehir–Konya sector is a band of lacustrine deposits running in a NW–SE direction in the west-central area of Turkey. The northern area contains the deposits of Eskisehir (Ece and C¸oban, 1994) and Yenidogan (Yeniyol, 1992), both in the Neogene Eskisehir Basin. Because of the economical interest of the first one, it will be described in Section 3.2.2.

FIGURE 14 Geographic map showing the main zones with sepiolite deposits in Turkey. 1. Eskisehir-Konya sector, 2. Denizli sector, 3. Sivas sector.

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Sepiolite and palygorskite have also been reported in the Neogene Konya Basin to the southeast, but the presence of sepiolite is less significant in the Konya Basin than in the other sector. Here, three types of sepiolite associated with carbonates have been identified (Karakaya et al., 2004). The first is a brown sepiolite, rich in organic material with common occurrences of bioclasts; another is poor in organic material with associated dolomite (50 wt%); and the third type with only 5–20 wt% sepiolite is present in light-coloured carbonates (dolomite, calcite). The presence of chart is common as nodules or lenses. The presence of palygorskite together with sepiolite has also been recognized in nearby areas (Asagi Pinarbasi Basin) in carbonate units (calcite and dolomite; Karakas¸ and Kadir, 1998). The Denizli sector is located southwest of Eskisehir–Konya with sepiolite having been discovered in the Neogene Serinhisar-Acypayam Basin. Akbulut and Kadir (2003) describe the presence of sepiolite, palygorskite and saponite in the Neogene lacustrine deposits of the basin. The deposits occupy the depression formed by a graben resulting from extensional stress. The sedimentary filling is mainly clayey material deposited in fluvio-lacustrine environments. The alkaline lake palustrine environments favoured the periodic development of sepiolite, palygorskite, saponite and dolomitic sepiolite. At some points, the sepiolite predominates but with intercalated saponite. At other points where there is evidence of volcanic activity, the saponite accompanies palygorskite. In all cases, the contact between the fibrous clay minerals is clear, which can be interpreted as rapid changes in the physicochemical conditions in the environment. The source of Fe and Al is inferred to be synsedimentary basaltic volcanism, while the SiO2 and Mg would have originated in the surrounding lithological units including ultrabasic rocks, detrital rocks and perhaps volcanic rocks. The conclusion reached is that the Mg-rich clays were formed either through the direct precipitation in the alkaline waters of the lake or in the pore water existing between the dolomite crystals. The Sivas sector is located to the east of the previously described sectors. The presence of sepiolite and palygorskite has been cited for the Neogene Kengal Subbasin that is a part of the Sivas Basin (Yalc¸ın et al., 2004). These authors describe the presence of sepiolite and palygorskite in lacustrine facies (20–60 m in thickness) consisting mainly of carbonates (calcite, dolomite and magnesite) with alternating layers of clays and chert. The palygorskite, sepiolite and dolomitic sepiolite are present in layers with varying thicknesses of 0.1–1 m. A detailed description of occurrences and deposits of sepiolite and palygorskite in Turkey is included in this book (see Chapter 7).

3.2.2. Eskisehir Deposit The presence of sepiolite in beds and as nodules in the Eskisehir Basin (Turkey) has been described by Ece and C ¸ oban (1994) (see Figure 14). The sedimentary record is complex and is represented by calcareous clays, clayey carbonates,

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dolomite, gypsiferous calcareous clays, siliceous tuffs, sepiolite, sepiolite-bearing dolomite and ultramafic conglomerates. The basement is an ophiolitic complex, and intercalations of weathered bentonitic tuffs are common occurrences in the stratigraphic succession. Four types of sepiolite are distinguished: one nodular and three bedded. The nodules appear in a bed (50–100 cm) of poorly sorted conglomerate from ultramafic rocks that overlie the ophiolitic basement. The sepiolite-bearing beds can be black (0.5–2 m); passing upward to brown (0.5–3.5 m); or white dolomitic sepiolite (4–5 m). The deposit is lacustrine and dates from the Miocene, filling a fault basin or graben of extensional origin (rifting). The existence of accumulations of stockwork magnesite very near the lacustrine deposits acted as the source for the formation of the sepiolite nodules (meerschaum), which would have formed as the result of diagenetic alteration of magnesite blocks under alkaline conditions. The sepiolite formed by direct precipitation in the saline–alkaline waters of the lake, with supersaturation of silica. The environment would have been alkaline–saline in arid to semi-arid climatic conditions with possible wet intervals owing to seasonal fluctuations. Yeniyol (1992) studied the geology, mineralogy and genesis of the Yenidogan sepiolite deposit, near Eskisehir (50 km to the SE) in the same basin. According to this author, this is the most important deposit in the Neogene Eskisehir Basin. The stratigraphic record shows a thickness of about 200 m in three lithological associations, of which the third contains the sepiolite deposit. The sepiolite occurs in two beds in the upper interval of the Pliocene sequence, which consists of an alternation of dolostone and dolomitic marls. The lower bed (3 m thick) is formed of sepiolite and dolomitic sepiolite. The upper bed has a thickness of up to 10 m and a broad lateral extension. It is formed by an alternation of sepiolite with sepiolite-rich layers and lenses. The sepiolite (up to 90%) is accompanied by quartz, feldspar, illite and grains of pumice. Dolomitic sepiolite is particularly abundant in the upper bed, but it does not exceed 50%. The formation of the sepiolite is associated with the shallow margins of an alkaline lake, ephemeral flooding events and wetlands (marshes), resulting from direct precipitation of the lake water. A diagenetic origin from solutions circulating through the intergranular porosity and along the desiccation cracks is also suggested. The primary source of Mg would have been the weathering of ultramafic rocks (serpentinite), common in the basement. Loughlinite (sodium-rich variety of sepiolite) is currently mined at Eskisehir. According to Yeniyol (1997), the loughlinite-bearing layers are present in Miocene deposits where layers can have thickness of 0.6–5 m, intercalated with lutites, volcanic tuffs, chert and bentonites. The loughlinite beds can be accompanied locally by analcime, smectite calcite and illite. Kadir et al. (2002) associate loughlinite in the same area to a lacustrine-volcano-sedimentary unit of highly variable sediment composition (clastic,

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157

clayey, evaporitic, dolomitic and siliceous sediments) and volcanic tuff deposits. In sharp contact with the sepiolite, the loughlinite contains dolomite, opalCT and analcime, especially in weathered tuffs and yellow or green lutites. At other times, the loughlinite is intercalated as lenses in the sepiolite. In this case, the associated materials are palygorskite, analcime, feldspar, opal-CT and calcite. According to Yeniyol (1997), loughlinite formed by the reaction of Naþ and Mg-rich waters with the vitreous components of pyroclastic deposits.

3.3. Other Deposits and Occurrences 3.3.1. Amargosa Desert Deposit (USA) Khoury et al. (1982) described the presence of Mg-rich clay deposits of economic interest in the Amargosa desert of southern Nevada. Sepiolite (up to 1.2 m in thickness) is associated with extensive kerolite/stevensite deposits and travertine accumulations of Plio-Pleistocene age (Eberl et al., 1982). The sepiolite is present in two main stratigraphic intervals and overlies a unit formed by sepiolite and Mg-rich smectites. In addition to trioctahedral smectite (kerolite/stevensite) and sepiolite, the minerals identified are dolomite, calcite and local occurrences of detrital mica. Based on textural evidence, Khoury et al. (1982) indicate that the sepiolite could have been formed not only as the result of direct precipitation but also from kerolite/stevensite dissolution. According to Hay et al. (1986), these deposits were deposited in the lacustrine environment of a playa-lake, with caliche, in areas influenced by groundwater infiltration. The review by Miles, Chapter 11, presents complementary information on the Amargosa deposits in Nevada, USA. 3.3.2. Amboseli Deposit (Kenya–Tanzania) According to Stoessel and Hay (1978), the Amboseli sepiolite deposit is located in the Pleistocene formation known as the Sinya Beds, characterized by the common occurrence of elongated mounds or domes. Lithologically, the deposit consists of massive white dolostone conformably overlain by green sepiolitic clays ending with a level of clays and silts with calcrete. Hay and Stoessel (1984) differentiate the sepiolite from the deposit into two varieties: waxy (green) sepiolite and meerschaum type (massive, light and porous) in cavities. Dolomite is brecciated in layers 1.5–5 m in thickness, and it is overlaid with 1.3 m of waxy green sepiolite accompanied by calcite, dolomite and potassic feldspar. Most of the meerschaum is located in the dolomitic breccia and this is the sepiolite of economic interest. The carbonate and sepiolite beds formed by deposition in marshland, or shallow lakes or ponds (palustrine). The aSiO2/aMg2þ ratio of the pore fluids would have been the main control on precipitation of sepiolite and kerolite (Hay et al., 1995).

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3.3.3. Guanshan Deposit (China) In China, there are more than 20 palygorskite deposits on the border of Jiangsu and Anhui provinces. The Guanshan palygorskite deposit in Anhui province seems to be the most important. This palygorskite is present in a unit varying in thickness between 18 and 54 m overlying the Miocene Huaguoshan Formation. The base of this unit consists of volcanic rocks (olivine basalt and basalt ash), transitioning to palygorskite-rich clays towards the top with a thickness varying between 3 and 6 m. The palygorskite content ranges from 55 wt% to more than 90 wt% accompanied by low quartz content (< 10 wt %) and traces of sepiolite, mica and dolomite (Liu and Cai, 1993). A model is proposed explaining the formation of the palygorskite as the result of the transformation of components of the volcaniclastic deposits in a lacustrine environment. Further information regarding the palygorskite occurrences in China is included in Chapter 10. 3.3.4. Ventzia Basin Deposit (Greece) In Greece, Kastritis et al. (2003) describe the existence of a deposit of palygorskite and palygorskite and saponite-rich clays in the Ventzia Basin. This basin is located in the vicinity of Grevena in the Greek region of Macedonia. The basin is 22 km in length, has a maximum width of 6 km and was formed at the end of the Pliocene and beginning of the Pleistocene. The palygorskite is present in beds with thicknesses varying between 10 and 18 m, with 60– 90% purity. The formation of the palygorskite would have been associated with the diagenetic transformation of saponite-rich sands resulting from the weathering of ultramafic rocks (Vourinos ophiolitic complex), which would justify the high iron content (up to 11% Fe2O3) of this palygorskite (Kastritis et al., 2005). 3.3.5. El Bur Deposit (Somalia) Singer et al. (1998) describe the abundant presence of sepiolite (meerschaum) in central Somalia (El Bur). The sepiolite is associated with limestone, dolostone, gypsiferous marls and evaporites (gypsum and anhydrite) facies. Mineralogically, the sepiolite shows a high grade of purity with traces of associated minerals: calcite, quartz and halite, and traces of illite in the clay fraction. According to these authors, the characteristics of the deposit are similar to those of Spanish and Turkish deposits. The exception is its age (Quaternary), which links them in time with the Amboseli (Hay et al., 1995) and Amargosa Desert (Hay et al., 1986) deposits. The environment is interpreted as an ephemeral lacustrine and evaporitic one, where groundwaters play an important role. Based on the absence of other phyllosilicates, they conclude the formation of the sepiolite to be due to chemical precipitation.

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4. FINAL REMARKS 4.1. Continental Sedimentary Environments for Sepiolite and Palygorskite Formation A schematic sedimentary model for palygorskite formation is given in Figure 15. The image shows a cross section where the transition from an alluvial fan environment to a palustrine–lacustrine one can be observed, taking into account the presence of a source area comprised igneous and metamorphic rocks and the interaction of three water types: groundwater, runoff and lacustrine water. The rocky substrate of the parent area is fundamental in order to justify inputs of Mg or Si (Figure 16; Table 6). The silica input can come from any magmatic rock, particularly those with a felsic composition, or from silica-rich metamorphic rocks and siliciclastic or biosiliceous (diatomite) sedimentary rocks. The Mg source can be any type of rock containing Mg-rich minerals, including magmatic rock (basalt, gabbro and peridotite), metamorphic rock (slate, serpentinite) or sedimentary rock (dolomite, magnesite and magnesian clays). The Si and Mg released in these zones can travel distances and accumulate in flooded lacustrine zones. As previously mentioned, given its low solubility except with very alkaline pH levels, aluminium

+ + +

+

+

Alluvial fan system

+

+

Lacustrine–palustrine

+ +

Runoff waters

+

Lake waters

Lacustrine deposits Fine-grained alluvial fan facies Medium-grained alluvial fan facies

Groundwaters

Coarse-grained alluvial fan facies Parent rock (igneous) Parent rock (metamorphic)

Calcretes

Palygorskite

Saponite

Al-smectite illite Palygorskite Weathered clays from the parent rock

FIGURE 15 Sketch of palygorskite deposit formation in continental sedimentary environments. The transformation of Al-bearing clays to palygorskite can take place close to the parent rock supplying Mg2þ, Si(OH)4 and Al-rich colloids but also related to marginal alluvial–palustrine–lacustrine environments.

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PALYGORSKITE FELSIC-INTERMEDIATE MAGMATIC ROCKS METAMORPHIC ROCKS Al-rich silicates

DIATOMITES Si-rich sediments

Mg2+

DOLOMITE MAGNESITE

Si(OH)4

Al from Al-clays

Mg-rich carbonates MAFIC-ULTRAMAFIC MAGMATIC ROCKS METAMORPHIC ROCKS Mg-CLAYS

SEPIOLITE Mg-rich silicates

FIGURE 16 Scheme showing some lithological sources delivering Mg2þ and Si(OH)4 in the sedimentary environment. The presence of inherited Al-rich clays favoured palygorskite over sepiolite formation.

TABLE 6 General Sources of Magnesium and Silica in the Sedimentary Environment. Si(OH)4 Sources

Mg2þ Sources

Runoff waters

Runoff and lake waters

Groundwaters

Groundwaters

Alteration/dissolution of detrital minerals (quartz, feldspars, micas)

Mineralizing fluids from calcite and gypsum formation (Mg/Ca increase)

Alteration of clay minerals (both detrital and authigenic)

Alteration/dissolution of Mg-rich minerals (silicates, carbonates)

Dissolution of biosiliceous facies (diatomites)

Dedolomitizacio´n

will tend to be less mobile and remain in the vicinity of its parent rock area, or it may be transported as sediment in aluminous clay particles or colloids. Phenomena occurring in the parent rock area can favour the formation of aluminous clay minerals and the release of Mg and Si, which would favour palygorskite formation in lacustrine conditions, mainly by transformation. Detrital aluminous clay minerals are abundant in distal alluvial fan facies. Palygorskite may be formed there as the result of the transformation of the inherited aluminous clays in the vicinity of a lacustrine environment, particularly during periods of exposure. This process favours calcite precipitation.

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In areas nearest to a lake, more specifically in the zone containing intertidal mudflats where the water is more alkaline, the transformation of aluminous smectite into saponite would be favoured instead of forming palygorskite. The starting sedimentary model for sepiolite genesis is similar to that for palygorskite (Figure 17). Nevertheless, sepiolite in this case is formed in the water column or in the pore fluids in a lacustrine wetland, where inputs of silica-bearing groundwater would play a significant role. The type of magnesian clay formed will be conditioned by the pH, salinity and Si/Mg ratio. Consequently, in conditions of high salinity, pH equal to or higher than nine and relatively low Si/Mg ratios, the formation of magnesian smectite is favoured. If there is moderate salinity and the pH is between 8 and 8.5, the magnesian phase formed will depend on the Si/Mg ratio, with kerolite precipitated where this is low and sepiolite the result of a high ratio. In other words, environments that are very rich in Mg will favour the formation of kerolite or magnesian smectite depending on the pH and salinity of the environment, with transitions from one to the other by means of kerolite–smectite interstratified minerals. Sepiolite is precipitated with moderate salinity and slightly alkaline pH in a Si- and Mg-rich environment.

+ + +

+ + +

Alluvial fan system

+

+ + +

+

Lacustrine–palustrine

+ +

+

Runoff waters

+

Lake waters

Lacustrine deposits Fine-grained alluvial fan facies Medium-grained alluvial fan facies Coarse-grained alluvial fan facies

Groundwaters Basement (igneous and metamorphic rocks)

Neoformation

high salinity, pH ³ 9 Lower salinity, pH = 8–8,5 Si/Mg¯

Mg-smectite

Freshening

SYNGENETIC (evaporative neoformation)

Sepiolite

Si/Mg

Transformation Sepiolite (kerolite, Si/Mg¯) Mg-smectite, kerolite-stevensite

Groundwaters (SiO2)

DIAGENETIC (replacement and/or diagenetic neoformation)

FIGURE 17 Sketch of sepiolite deposit formation in continental sedimentary environments. Salinity, pH and Mg/Si ratio play an important role in the syngenetic formation of sepiolite, kerolite or stevensite. A drastic change in salinity–pH (freshening) during early diagenesis of Mgclays favours the intrasedimentary formation of sepiolite.

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During diagenesis, changes in the hydrochemistry of runoff, lacustrine and groundwater, can cause sepiolite formation by transformation at the expense of other magnesian clays. Likewise, it could precipitate directly from solution in pores. This can give rise to the formation of several generations of sepiolite. In exposed sedimentary environments, intrasedimentary sepiolite can form in fine-grained dolomitic facies (dolomicrites). On occasions, after the dolomitization process, sepiolite can be arranged by covering the dolomite crystals or by cementing the intercrystal porosity. Sepiolite can also be formed in the presence of silica as the result of dedolomitization processes with the development of calcite crusts and the release of Mg. Palygorskite can also be formed by these processes but only if there are particles of aluminous clays mixed with the dolomite. Representative field photographs of quarry faces with sepiolite beds and nodules deposited in different continental sedimentary environment are shown in Figure 18.

FIGURE 18 Representative field photographs of sepiolite deposits in different sedimentary environments from the Madrid Basin. Sepiolite is associated to other Mg-clays (saponite, kerolite–stevensite, stevensite) well bedded (1) or intrasedimentary (3, 4) in palustrine and mudflat facies, respectively. Associated to alluvial fan deposits sepiolite occurs inserted between detrital facies (2).

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163

4.2. Origin of Deposits with Sepiolite and Palygorskite: Lithological Associations In lacustrine environments, fibrous clay minerals can occur near to or overlying materials supplying the elements and particles required for their formation, or faraway from them through the involvement of a sedimentary transport system. Near parent rocks the development of palygorskite will be rather more favourable than for sepiolite. This is because there will be a high proportion of Al-rich particles in weathering profiles, which would favour its formation over that of sepiolite. This occurs in the deposits at Torrejo´n (Spain), on a slate substrate, or Guanshan (China) and Andhra Pradesh (India) over a mafic volcanic substrate. Obviously, if the bedrock lacks Al and is rich in Mg, the fibrous mineral that could be formed is sepiolite, provided that pH and salinity conditions and Si/Mg ratio are adequate as in the nodular sepiolite at Eskisehir. After transport, Al-clay particles accumulate in alluvial or lacustrine deposits where, in the presence of Mg, they can form palygorskite, with post-depositional calcretes (e.g. Bercimuel, Spain). Palygorskite was developed by diagenetic transformation. The arrival of Si and Mg in swamp environments will enable sepiolite formation to take place in different sub-environments within an alluvial–palustrine–lacustrine setting. With regard to distal alluvial deposits, sepiolite will form during ephemeral flooding events where inputs of silica-bearing groundwater play a significant role. The result can be thick accumulations (> 10 m) of sepiolite mainly produced through precipitation (neoformation). The most noteworthy example of this is the sepiolite deposit at Vica´lvaro (Spain). The importance of groundwater in the formation of nodular sepiolite has also been proposed for El Bur (Somalia) and Amboseli (Kenya–Tanzania). In zones near to a palustrine–lacustrine wetland with fluctuations in the lacustrine water mass, mudflat deposits may contain sepiolite beds alternating with detrital facies, carbonates and even evaporites (mainly gypsum). The sepiolite beds have moderate thicknesses (1–3 m), in cyclical sequences in correlation with the evolution of the lake. Precipitation in conditions of evaporation would be the cause for their formation under suitable physico-chemical conditions. Exceptionally, in extensional tectonic regimes, sepiolite could also be formed in deeper water where the presence of organic material would indicate anoxic conditions. The sepiolite deposits of the Eskisehir Basin (Turkey), Mara (Spain) and Amargosa (USA) are examples of the previously mentioned environments. At other times, sepiolite can be formed in palustrine conditions, that is, wetland zones subject to frequent flooding and frequent exposure episodes. In this case, an overlapping with edaphic processes is produced. Owing to amalgamation of the sepiolite beds, important thicknesses (up to 10 m) are achieved. The Batallones (Spain) sepiolite deposit would be representative of this type.

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As previously indicated, sepiolite and palygorskite can be formed between dolomite crystals during diagenesis. The presence of intrasedimentary sepiolite and/or palygorskite associated with dolomitic facies is common in the Tertiary basins of Spain and Turkey, associated in some cases with sepiolite deposits. On the basis of the above, six different lithological associations have been established that are representative of sepiolite–palygorskite deposits (Figure 19).

4.2.1. Bedrock: Fibrous-Clay Mineral Association (LA-1, LA-2, LA-3) The fibrous clay mineral is present in the vicinity of or overlying the parent rock area supplying the basic components. This is particularly interesting in the case

Sp Ke/stev

Sp

Magnesite

Dolomite Sp

LA-3

Gypsum

Detrital facies

Saponite

Sedimentary rocks

Pk

Detrital facies

LA-6

Chemical facies (Mg–clays) Slate/serpentinite

LA-2

Sp Limestone

Metamorphic rocks Detrital facies

Pk Mudstones

Pk

Sp

LA-4

Dolomite

Detrital facies Gypsum

Tuff/lava flow

LA-1

LA-5

Chemical facies (carbonate–gypsum)

Volcanic rocks

FIGURE 19 Idealized sketch of stratigraphic sections showing the different lithological associations (LA) established. LA-1 and LA-2 show the occurrence of palygorskite in palustrine–lacustrine conditions near or overlying the parent rock source of Si, Mg and Al. In the case of LA-3, the presence of sepiolite is the result of replacement of magnesite in a silica-rich medium. Relatively far away from parent rock supplying elements, the formation of sepiolite and palygorskite can be related to marginal lacustrine facies where detrital input from alluvial fan deposits should take place, playing groundwaters an important role. Lithological association LA-4 displays the presence of sepiolite and palygorskite associated to sandy and muddy facies, respectively. LA-5 and LA-6 show the occurrence of sepiolite as a consequence of chemical precipitation and diagenetic replacement of previously formed Mg-clays (saponite, kerolite-stevensite).

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of palygorskite, where situations can be differentiated depending on whether the bedrock is mafic magmatic rock (basalts, tuffs), as is the case of the Guanshan (China) and Andhra Pradesh (India) deposits, or metamorphic rock (slate), as occurs at Torrejo´n (Spain), or ultramafic rocks and saponitic sands, as at Grevena (Greece). In all of these cases, the bedrock and the palygorskite are in direct contact and can reach thicknesses of up to 6 m, although thicknesses of up to 18 m have been reported in Greece (possibly referring to the unit containing the palygorskite). With sepiolite, only the Eskisehir (Turkey) nodular sepiolite deposit would fall within this lithological association.

4.2.2. Detrital Facies: Fibrous-Clay Mineral Association (LA-4) The sepiolite layers occur between carbonate-poor detrital facies. This is the case of the Vica´lvaro (Spain) deposit. Palygorskite can also form deposits associated with the clayey detrital facies of alluvial deposits, as occurs at the Bercimuel (Spain) deposit. Finally, with regard to detrital mudflat facies, sepiolite can also form in carbonate-poor palustrine conditions but with abundant chart nodules and beds, as is the case of the Batallones (Spain) sepiolite deposit. 4.2.3. Carbonatic Chemical Facies: Fibrous-Clay Mineral Association (LA-5) Sepiolite predominates in these facies associations, intercalated between dolomite, dolomitic marl, limestone and gypsum and showing certain cyclicity. Typical examples of this are the Eskisehir (Turkey) and Mara (Spain) sepiolite deposits. 4.2.4. Clayey Chemical Facies: Fibrous-Clay Mineral Association (LA-6) Sepiolite is formed more locally as the result of diagenetic replacement of other magnesian clays, leading the sepiolite to form nodules or discontinuous beds in a lateral transition (towards the lake) to non-fibrous magnesian clays. This is the case of the sepiolite occurring at the Maga´n and Caban˜as (Spain) bentonite deposit and the Esquivias (Spain) kerolite–stevensite deposit. To summarize script, sepiolite and palygorskite are the result of the interaction between silica- and magnesium-bearing solutions in an environment with suitable physico-chemical conditions (mostly pH and salinity), and in the presence of aluminium in solution or reactive phases in the case of palygorskite. The physico-chemical conditions of the environment control the formation of the magnesian clay mineral, also determining its fibrous or laminar nature and, consequently, its composition. As seen throughout this chapter, these conditions occur in specific continental sedimentary environments where they give rise to sepiolite and palygorskite deposits of commercial interest. The lithological characteristics of the parent area supplying Si, Mg

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and aluminous phases, and the hydrochemistry of the participating water (runoff, groundwater and flooding events), play an important part in the formation of these fibrous clay minerals and in the complexity of their deposits.

ACKNOWLEDGEMENTS This work has been partially supported by the Project CGL-2008-05813-CO202 and by the Government of Andalusia through the Research Group Applied Mineralogy (RNM135). We would like to thank Prof. Ray E. Ferrell for his revision, discussion and comments of this chapter, which improved the manuscript.

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