COBISS 1.01
PALYGORSKITE IN CAVES AND KARSTS: A REVIEW
PALIGORSKIT V JAMAH IN KRASU: PREGLED
Pavel BOSAK1 2 & Nadja ZUPAN HAJNA2*
Abstract	UDC 551.3.051:549.6
Pavel Bosak & Nadja Zupan Hajna: Palygorskite in caves and karsts: a review
Palygorskite is fibrous mineral representing the transitional phase between chain silicates and layer silicates with modulated phyllosilicate structure. Although often found in carbonate environments, it forms quite uncommon constituent of cave fills. Palygorskite occurs in cave fills in two forms: (1) allogenic palygorskite which in arid and semiarid conditions can represents substantial constituent of cave fills, often associated with smectite, gypsum, calcite and halite; it is airborne or transported by surface run-off to caves from desert soils and paleosoils, calcretes, dolocretes and related deposits in cave surroundings. (2) Authigenic palygorskite occurs as in situ precipitate in cave fills from percolating water solutions and/or transformation of smectite and kaolinite in dry evaporative conditions and suitable geochemical composition of solutions. In carbonate hostrocks palygorskite fills fissures and faults and often it is found in cave walls. It occurs commonly as part of the "mountain leather" as a result of hydrothermal and/or weathering processes or represents a product of in situ chemical precipitation from percolating meteoric solutions with suitable pH a redox conditions and chemical composition. Key words: palygorskite, caves, karst.
Izvleček	UDK 551.3.051:549.6
Pavel Bosak & Nadja Zupan Hajna: Paligorskit v jamah in krasu: pregled
Paligorskit je vlaknat mineral, ki je prehodna fazo med inosili-kati in listastimi silikati z modulirano strukturo filosilikata. Čeprav je pogost v karbonatnih okoljih, je precej neobičajna sestavina jamskih sedimentov. Paligorskit se v jamah pojavlja v dveh oblikah: (1) kot alogeni paligorskit, ki je v sušnih in polsuhih razmerah lahko znaten sestavni del jamskih sedimentov, pogosto povezanih z montmorillonitom, sadro, kalcitom in halitom; v teh primerih gre za eolski nanos ali pa za transport v jame s površinskim transportom iz puščavskih tal in paleotal, kalkret, dolokret in podobnih sedimentov; (2) kot avtogeni paligorskit se pojavlja v jamah kot »in situ« oborina iz prenikajočih raztopin in/ali z obarjanjem med transformacijo montmorillonita in kaolinita v jamskih sedimentih v suhih razmerah izhlapevanja in primerno geokemično sestavo raztopin. V karbonatnih kamninah paligorskit zapolnjujejo razpoke in prelome in ga pogosto najdemo na jamskih stenah.
Običajno se pojavlja kot del "gorskega usnja", ki je posledica hi-drotermalnih in/ali procesov preperevanja. Lahko pa nastane tudi zaradi in situ kemičnega obarjanja iz prenikajočih meteornih raztopin z ustreznim pH, redoks potencialom in kemično sestavo.
Ključne besede: paligorskit, jame, kras.
1	Institute of Geology of the Czech Academy of Sciences, Rozvojova 269, 16500 Praha 6, Czech Republic, e-mail: bosak@gli.cas.cz
2	ZRC SAZU Karst Research Institute, Titov trg 2, 6230 Postojna, Slovenia, e-mail: zupan@zrc-sazu.si * Corresponding author
Received/Prejeto: 06.09.2017
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INTRODUCTION
Hill and Forti (1997) summarized palygorskite occurrence known at that time in caves. They stated that it represents quite uncommon mineral there. They listed several palygorskite-bearing sites described from Botswana (Gcwihaba Cave), Japan, Namibia (Leeurante-grot), New Zealand (Broken Hill, Oyster, and Luckie Strike), South Africa, United Arab Emirates (Kahf Wadi Gulam), United States (Carlsbad Cavern, Lechuguilla Cave, New Mexico; Pend Oreille mining district,
northwestern Washington), and Venezuela (Cueva Las Ursulas). They mentioned a general study of Broughton (1972) as a good source of information. After the summary of Hill and Forti (1997), palygorskite was newly described in some other caves. The location of caves in which palygorskite was reported are summarized on Figure 1.
Because of palygorskite rarity in cave fills we present the review here.
Fig. 1: Location of caves with reported palygorskite: 1. - 2. Southern Africa region (1. Botswana: Gcwihaba Cave, 2. Namibia: Luurante Cave, Rössing Cave, Tinkas Cave), 3. Japan (Ootakine Cave, Tateishi Syonyudo Cave), 4. - 5. New Zealand (4. North Island: caves Broken Hill, Oyster, Luckie Strike, Waitomo; 5. South Island: Riwaka), 6. United Arab Emirates (Kahf Wadi Gulam; Wadi Haqil), 7. - 8. Saudi Arabia (7. Coast: Hibashi lava tube; 8. Desert: Sulb Plateau, B31, Friendly, Surprise and Murubbeh caves, Jabal Al Qarah Caves), 9. -11. United States (9. New Mexico Guadelupe: Carlsbad Cavern, Lechuguilla Cave; 10. South Dakota's Wind Cave; 11. Washington: Pend Oreille), 12. Canada (Kootenay Cave), 13. Venezuela (Cueva Las Ürsulas), 14. Spain (Mallorca Island).
GENESIS OF THE MINERAL
Palygorskite belongs to the palygorskite-sepiolite mineral group, which includes several varieties enriched in some elements (e.g., Al, Mn, Fe, Na, K, Ni; Melka & St'astny 2014). It represents the transitional phase between chain silicates and layer silicates with chain bond of Si-O tetrahedrons, usually Mn-rich, dioctahedral with monoclinic and orthorhombic structural modifications. Although the mineral has fibrous character, it can be assumed as silicate with 2:1 layer modulated phyllosilicate structure (Guggenheim & Krekeler 2011; Melka & St'astny 2014; Fig. 2). Relationship between palygorskite and biological (biomineralization) activity was also proposed (see Cuevas et al. 2011). The synonymous mineral name - at-
tapulgite - often appearing in older studies, is still commonly used in industry (Guggenheim & Krekeler 2011, p. 7).
Palygorskite is commonly found in carbonate environments (see summary and references in Post & Crawford 2007; Singer & Galan 2011; Kaplan et al. 2013; Lacinska et al. 2014) and as alteration products of ore veins or serpentinites or at fissures of different rock types (Melka & St'astny 2014). The Al- and Si-bearing carbonate-alkaline environment is conductive to palygorskite neomorphism (e.g., Jones & Galan 1988; Bloodworth & Prior 1993; Hobbs et al. 2002). A general genetic model for palygorskite/sepiolite origin is based on the interac-
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(-b = 18,0 A->
O H20	O Hydroxyl	• Mg or AI
© (OH)2	O Oxygen	° Silicium
Fig. 2: Palygorskite structure (M. St'astny, original).
tion between Si- and Mg-bearing solutions (e.g., Jones & Conko 2011), in an environment with high pH and high salinity, and on the activity of Al in solution or the presence of reactive Al-bearing phases in the case of palygorskite (Ferrell 2011). Palygorskite is therefore common neomorphic (pedogenic) mineral in arid soils and pa-leosoils (Jenkins 1976; Elprince et al. 1979; Singer 1979; Mackenzie et al. 1984; Mashhady et al. 1980; Shadfan & Mashhady 1985; Shadfan et al. 1985; Aiban 2006, 2007; Gunatilaka 1989; El-Sayed et al. 1991; Aqrawi 1993; Ho-jati & Khademi 2011; Yalçin & Bozkaya 2011). Therefore, it is commonly associated with dolomite and gypsum,
in regions with less than 300 mm mean annual rainfall (Paquet & Millot 1973). Palygorskite directly crystallizes in duricrusts (calcareous soils, calcretes, caliches, dolocretes; Callen 1984, Maizels 1988; Karaka§ & Kadir 1998; El-Sayed 2001; Kadir & Eren 2008; Macklin et al. 2012; Kadir et al. 2010, 2014; Kaplan et al. 2013) often within fluvial/alluvial sequences and in ancient sabkhas enclosing ophiolitic rock fragments (Clarke & Walker 1977; Maizels 1988; El-Sayed 2001; Macklin et al. 2012; Lacinska et al. 2014). Most of palygorskite/sepiolite deposits are thus detected in depositional settings with evaporative regime in arid/semiarid climatic conditions (fluvial to lacustrine systems, including playas, alluvial fans, brackish to hypersaline environments, including sabkhas; for review see Galán & Pozo 2011). Palygorskite is transformed into smectite in wetter conditions (e.g., Paquet & Millot 1973; Heine 1988; Heine & Volkel 2010).
Palygorskite precipitation in joints in carbonate host-rocks is often associated with in situ neomorphism directly from percolation of subsurface meteoric water charged with soluble constituents sourced from the breakdown (weathering, dissolution) of minerals from e.g., soil profiles overlying carbonate host-rocks (Soong 1992; Hansen 2008). Hansen (2008) expected that both palygorskite formation and persistence require alkaline pH, high Mg and Si, and low Al activities. Some palygor-skite occurrences are connected with late stages of hydrothermal ore-forming processes (e.g., Dings & White-breat 1965) and alterations of Mg-rich host-rocks (like serpentinites, gabbroic rocks, carbonate rocks, clay sediments; Melka & St'astny 2014).
PALYGORSKITE IN KARST AND CAVES
PALYGORSKITE IN KARST Prevailing number of occurrences of palygorskite in caves is not related to speleogenesis itself, with cave-filling processes and/or transformation/diagenesis of cave fill. Palygorskite usually fills fractures in carbonate hostrocks (limestones, dolostones and their metamorphosed equivalents) and can thus be easily detected on cave wall where it can even hang out of fissures (e.g., Park & Cannon 1943) in a form of so-called mountain leather (e.g., Hunt 1960). We present here some of palygorskite occurrences in host-rocks which appear on cave walls but not related to speleogenesis and/or infilling processes, although we are sure, that the review cannot be exhausting.
Soong (1992) summarized palygorskite occurrences in New Zealand described before 1992; most of them are
associated with joints and faults in limestones. He also identified palygorskite at Canaan, Thomson Hill (direct neomorphism from meteoric waters that percolate through soils above marbles), and Riwaka (percolation of subsurface meteoric water through narrow passages and cavities in weathered dolomitic marbles) in northwest Nelson, South Island. Hansen (2008) studied the Latest Oligocene to earliest Miocene aged Otorohanga Limestone formation of the Te Kuiti Group. A distinctive leathery palygorskite occurs commonly as a chemical precipitate also in joints. Palygorskite is formed in situ, i.e., it is not washed into joint fill as allogenic material.
Palygorskite has been reported by Park and Cannon (1943), Halliday (1963), Dings and Whitebreat (1965), and Frost (1971) in the caves of the Pend Oreille mining
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district, northwestern Washington (USA), developed in Metaline metamorphosed limestone (Middle Cambrian). Palygorskite occurs as leathery deposits filling fractures and hangs from the roof and sides of the cave in dangling masses, and in many places it is wet and shiny. Park and Cannon (1943) and Dings and Whitebreat (1965) stated that palygorskite is commonly present in many of the caves where it fills fractures and hangs from the roofs and sides in dangling bodies resembling soiled and frayed rags or a wet and torn newspaper. Palygorskite is of hypogene origin formed at a late stage after most of the sulfides and coarse carbonates were developed.
In Venezuela, Urbani (1975a, b, 1996) described palygorskite from Cueva Las Úrsulas developed along a joint in quartz-mica-albite-schists as 1 mm thick leathery sheets and crusts on the walls and fractures. The sheets are light brown but also reddish due to iron-oxide staining. They are flexible and associated with minor amounts of calcite.
In the Yucatan Peninsula (Mexico) palygorskite is found in high contents in different localities and was used by the ancient Mayas to prepare especially the Maya blue pigment (a composite of organic and inorganic constituents, primarily indigo dyes derived from the leaves of añil Indigofera suffruticosa plants combined with palygorskite; Sanchez del Rio et al. 2011). Palygorskite can be a constituent of Tertiary lacustrine deposits (Sanchez del Rio et al. 2009).
Sepiolite and palygorskite are reported by Imai and Otsuka (1984) from the Ogano and Hanezuru mines in the southern part of the Ashio Mts. (Kuzuu District 80 km north of Tokyo, Japan) as the fill of faults in Paleozoic and Mesozoic limestones and dolostones. Other occurrences of palygorskite related to fill of fractures, fault and other tectonic structures and found in caves are reported from some caves in former Czechoslovakia and Czech Republic by Cílek (1984; plates in mud, fallen from fissure fillings in marbles), Morávek (1998; from fissures, in places protruding to small cavities, i.e., macroporos-ity in the Vitošov limestone Quarry in Moravia), Drbal (2007) and Krejča (2008) from the Chynovská Cave in Proterozic marbles in the association with boxwork selectively corroded from fissure fillings in marbles. Crusts and layers of fibrous aggregates in fissures in dolomites of the Bleiberg Pb-Zn deposit (Austria) was mentioned by Velebil (2005) and by Jeršek et al. (2006) from nearby Mežica Pb-Zn deposit (Slovenia). Rečnik (2013) described palygorskite as a few centimeters large cloth-like assemblies in fissures of silica-rich intraclastic dolosparite in the Idrija mercury mine (Slovenia). Lauritzen (2006) mentioned palygorskite from Western Spitsbergen in G18 Dobbeltgrotta (Paleozoic marbles) in small veins. Cílek (2012, pers. com.) discovered probably hydrother-
mal palygorskite in tectonically crushed zone with calcite at the Dolny vrch Plateau (Slovak Karst, Slovakia).
PALYGORSKITE IN CAVES In the USA, palygorskite was identified in South Dakota's Wind Cave by Bern (2004). Authigenic palygorskite is abundant in clay fill of caves of the Guadalupe Mountains, New Mexico (e.g., Davies 1964a, b; DuChene 1986; Hill 1987; Cunningham et al. 1995; Polyak & Guven 1996, 2000; Polyak 1998). Hill (1987) described waxy, colorful (blue, blue-green, pure-white and lavender) clay, and less waxy, soapy-feeling, colorful (gray-green, pink, brick-red and brown) clay fills sponge-work pockets in the limestone or underlies clastic sediment. The waxy clay usually occurs as veins, pods, or stringers within the soapy-feeling clay, with a sharp color differentiation displayed by the two clay types. The clay deposits have dried, compacted, and cracked so that they are now sloughing out of the spongework and are piling up as talus debris on the cave floor. Clays are composed of 10 A variety of halloysite (former endellite), smectite and palygorskite. Palygorskite was converted to hydrated halloysite and alunite in green clays (Polyak and Guven 1996). Palygor-skite is associated with smectite, illite, and kaolinite in the <2-^m fraction of these deposits according to Polyak & Guven (2000): the TEM micrographs show fibers of palygorskite radiating from oval-shaped smectite aggregates; the SEM images show palygorskite fibers disseminated in the clay-rich matrix of the laminated silt, and sometimes concentrated along quartz grain surfaces. They concluded that palygorskite is not a clay constituent of the carbonate bedrock, therefore it formed in a carbonate-alkaline environment produced by drip waters that saturated the silt and clay deposits, or by detrital grains of calcite and dolomite occurring in the laminated silt. The 40Ar/39Ar dating of alunite from these sediments ranged from 11.3 Ma for caves located at the higher elevations to 3.9 Ma for the Carlsbad Cavern at lower elevation (Polyak et al. 1998).
In Canada, Horne (2005, p. 60) reported that "the first known find, in a Canadian cave, of the mineral at-tapulgite is in a Kootenay Cave". Bates et al. (2008) related chlorite and palygorskite in siltstones from the Middle to Upper Triassic paleocavity fill at the sub-Watrous unconformity to (semi-)arid environment either through transformation of smectite or by neoformation in Al-rich soils and/or shallow water.
In New Zealand, Laird and Donald (1961) and especially Lowry (1964) described in detail palygorskite from in the Free Attic Passage, Broken Hill Cave, and other caves in the Te Kuiti District (Oyster Cave, Luckie Strike and a cave on the property of Mr. Axel Juno). Lowry (1964 , p. 917) connected the palygorskite with
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Fig. 3: Unroofed caves filled by palygorskite bearing sediments on the slopes of Wadi Haqil (Musan-dam Mountains; Ras Al-Khaimah Emirate). a. locations of cave entrances and unroofed caves; b. with sediments filled cave from where are samples No. 1 (bottom), 2 (middle), 3 (top); c. brown fill in Meander cave with location of sample No. 6.; d. unroofed cave on the slope of the hill with location of sample No. 5 (grey sediment mixed with scree) (Photo: N. Zupan Hajna).
former flooding of the cave by the water: "it was probably at that time that the joints (in the host-rock) became filled with palygorskite". The presence of palygorskite ((Mg, Al)2Si4O10(OH).4H2O) in Broken Hill caves was confirmed recently by Onac and Forti (2011). Soong (1992) identified palygorskite at Canaan, Thomson Hill, and Riwaka in northwest Nelson, South Island. It is suggested that the clay mineral neoforms directly from subsurface meteoric water that percolates through the soil profiles overlying marble at Canaan and Thomson Hill, and through narrow passages and cavities in weathered dolomitic marble at Riwaka. Hansen (2008) studied Oligocene-Miocene Otorohan-ga Limestone Formation (Te Kuiti Group). A distinctive leathery clay mineral - palygorskite - occurs commonly as an in situ chemical precipitate both in joints and caves as an infill. Caves act as natural sediment traps for siliceous materials, the fills being enriched in clay minerals, including palygorskite. Waitomo Dis-ctrict Council (2009) mentioned the presence of important speleothems (calcite pearls and palygorskite) in the Waitomo Headwaters (Cave) System giving any details. Hill and Forti (1997) mentioned more study of
Kermode (1969) and of anonymous author (1964) on palygorskite in caves of New Zealand.
Kashima (1987, 1993) mentioned palygorskite in Japan from the Ootakine Cave in limestones (Fukushima Province) giving unfortunately no further details. Hill and Forti (1997) mentioned more study of the Cave Research Group (1983) from the Tateishi Syonyudo Cave.
In United Arab Emirates, palygorskite was observed in the caves of Kahf Wadi Gulam inside of mud cracks on the cave floor at ca 200 m a. s. l. Borreguero and Jeannin (1990) interpreted that palygorskite was related to activity of brackish waters during an ancient stage in cave development when sea level was higher and the cave was subjected to an ingression of sea water. Zupan Hajna et al. (2013, 2016a, b) described palygor-skite-rich cave fill from small caves of ancient hypogene origin from the entrance of the Wadi Haqil (Mu-sandam Mountains; Ras Al-Khaimah Emirate). Caves are situated close to the surface and they are partly or completely unroofed (Fig. 3a, b, d). Various types of calcite and gypsum crystals and flowstones are present together with allochthonous clastic sediments (Fig. 3c). According to the XRD, samples (for location see Fig. 3b,
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Fig. 4: Scanning electron microscope images of aggregates of mutually in ter-grown palygorskite fibers (P) accompanied with grains of other clay minerals (M), minute calcite rhombohedra (C) and rare iron sulfide grains (S), Wadi Haqil, United Arab Emirates (Photo: R. Skala).
c) contain quartz, gypsum, smectite, kaolinite, calcite, and palygorskite (10 wt. % to 26 wt. %; Fig. 4), some of them Fe-dominant chlorite, illite, feldspars and goe-thite. Higher palygorskite content is typical for samples of grey color (samples Nos. 1; Fig. 5) while the lowest content was identified in brown sample (No. 6; Fig. 5).
Calcite dominates in most samples; smectite prevails in clay fraction (Skala et al. 2011; Fig. 5).
Palygorskite empirical formulae between
(Mgl.09Al0.89Fe0.02)(Si,99Al0.0l)°i0(OH)0.90^4.11H2O and
(Mg,36Al0,1Fe0.02)(Si,90Al0,0)O10(OH)0,4^4.29H2O
were calculated (Zupan Hajna et al. 2016c). Cave sediments represent palygorskite-bearing weathering products and desert soils re-deposited from the cave surroundings by slope processes, wind and/or surface runoff. Fine-grained quartz fraction is probably airborne. Gypsum and calcite are the precipitates (crusts and/or cements), although gypsum can also be re-deposited from omnipresent gypsum-cemented surface sediments. High kaolinite content and negligible feldspar content may indicate (1) high degree of weathering of original source-rocks, and/or (2) re-deposition of older weathering crusts, coming at least partly from ultrabasic (mafic) magmatites (Zupan Hajna et al. 2016c). The marine transgression-related model proposed by Borreguero and Jeannin (1990) and the in situ palygorskite neomor-phism within the cave fill due to brackish water is unlikely here with the respect to geomorphic evolution of the area. Palygorskite in situ precipitation in mud cracks can be rather related to smectite or kaolinite transformation in dry evaporative condition and activity of Mg- or Al-rich solutions (dripwater) in this broader region.
In Saudi Arabia, Forti et al. (2004) and Pint et al. (2005) described palygorskite from Ghar Al Hibashi lava tube as snow-white soft tufts of densely interlaced thin elongated vitreous fibres on a burned jaw of an animal. Pint and Pint (2005) and Forti et al. (2008) found rather common palygorskite in the desert caves in Saudi Arabia (As Sulb Plateau, B31, Friendly, Surprise and Murubbeh caves). It occurs as light milky white cotton tuffs consisting of elongated and banded fibres, sometimes as acicular
Fig. 5: Semi-quantitative analysis of fine-grained fractions from cave sediments from Wadi Haqil, Ras Al-Khaimah. Legend: 1. grey sediment, 2. laminated sediment, 3. red sediment, 4. yellow sediment, 5. grey sediment with gypsum crystals; 6. cemented brown sediment; Q - quartz, Pal - palygorskite, Sm -smectite, Ka - kaolinite, Gy - gypsum, KF - potassium feldspar, Ch - chlorite, IL - illite (from data in Skala et al. 2011).
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milky white crystals on walls of the small voids among the halite crystals. It is associated with gypsum. They expected that palygorskite developed inside natural caves. The Jabal Al Qarah Caves of the Hofuf Area contain wind-blow dust and speleothems (Hussain et al. 2006); palygorskite and Mg-rich smectite dominate among clay minerals. The origin of those clay minerals was related to the deposition in the ephemeral saline lake or saline flood plain. Hussain et al. (2006) reported common occurrence of palygorskite in soils in the studied region and the whole Arabian Peninsula (see also Jenkins 1976; Mackenzie et al. 1984).
In Southern Africa region, Martini (1993) summarized the occurrences of number of minerals in caves of Botswana and Namibia. In Botswana, Martini (1996) described palygorskite from the Gcwihaba Cave. In Namibia, Heine (1988) and Heine and Volkel (2010) mentioned high proportion of palygorskite (35 to 65 %) in
airborne cave fill in the Rössing Cave (late Quaternary) and in the Tinkas Cave (Little Ice Age). It is associated usually with smectite, illite, kaolinite and mixed-layer clay minerals. In the Rössing Cave, palygorskite is associated with at least three noticeable climatic fluctuations on a time-scale around 2 ka occurred during which more humid conditions were replaced by more arid (and more windy) ones and vice versa in a period between ca 34 and 27 ka. Martini (1993) reported whitish powdery palygorskite associated with minerals of smectite group, halite and gypsum from the Luurante Cave. It resulted from the transformation of residual kaolinic clay by Mg-rich solutions concentrated by evaporation.
Eolian origin of palygorskite (component of dust deposits) found in karst sediments is reported from Mallorca Island (Spain) by Fiol et al. (2005), nevertheless the mineral has not been detected in Late Pleistocene to Recent cave fill.
GENETIC MECHANISMS OF PALYGORSKITE IN CAVES AND KARSTS
Hill and Forti (1997) summarized the origin of palygorskite in caves from different sources. Palygorskite is related to different processes within caves: (i) to the weathering of the cave walls which contain quartz, plagioclase, muscovite, and calcite (Cueva Las Úrsulas; Urbani, 1975a, b; 1997); (ii) to the transformation of smectite to palygorskite under dry conditions and to 10 A variety of halloysite (former endellite) in acidic conditions, which can further transform into halloysite by drying (Davies 1964a, b; Hill 1987; Hill & Forti 1997); (iii) to the transformation of residual kaolinitic clay by magnesium-rich solutions concentrated by evaporation (Leeurantegrot; Irish et al. 1991; Leevante Cave; Martini 1993); (iv) to an ingression of sea water into the cave (Kahf Wadi Gulam; Borrequero & Jeannin 1990), or (v) to the hydrothermal (hypogenic) processes (the Pend Oreille; Frost 1971).
We summarize, that palygorskite in cave fills is always connected with dry (arid, semiarid, evaporitic) conditions both of external and cave climates. Two forms
can be distinguished: (1) allogenic palygorskite can represent substantial constituent of cave fills in places. It is commonly associated with smectite, gypsum, calcite and halite. It is airborne or transported by surface run-off to caves from desertic soils and paleosoils, calcretes, dolo-cretes and related deposits in cave vicinity; and (2) authi-genic palygorskite occurs as in situ precipitates in cave fills from water solutions percolating through carbonate host-rocks and overlying soils or weathering profiles and/ or transformation of smectite and kaolinite in cave fills in evaporative conditions and with suitable geochemical composition of solutions (especially Mg-rich).
In carbonate host-rocks (limestones, dolostones and their metamorphosed equivalents) it fills fissures and faults and usually occurs in a form of "mountain leather". It is result of hydrothermal and/or weathering processes or represents a product of in situ chemical precipitation from percolating meteoric solutions with suitable pH a redox conditions and chemical composition.
CONCLUSIONS
Palygorskite is fibrous mineral representing the transitional phase between chain silicates and layer silicates with modulated 2:1 layer phyllosilicate structure. The carbonate-alkaline environment containing alumina and
silica is conductive to palygorskite neomorphism, which is based on the interaction between Si- and Mg-bearing solutions in an environment with high pH and high salinity, and on the activity of Al in solution or the presence
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of reactive Al-bearing phases (e.g., Jones & Conko 2011; Ferrell 2011). Therefore palygorskite represents common neomorphic (pedogenic) mineral in arid soils and pale-osoils often associated with dolomite and gypsum (calcareous soils, calcretes, caliches, dolocretes, gypcretes) or halite, in regions with low mean annual rainfall. Most of palygorskite/sepiolite deposits are thus detected in depo-sitional settings with evaporative regime in arid/semiarid climatic conditions (fluvial to lacustrine systems, including playas, alluvial fans, brackish to hypersaline environments, including sabkhas; for a review see Galán & Singer 2011).
Although often found in carbonate environments, palygorskite forms quite uncommon constituent of cave fills. Palygorskite occurs in cave fills in two forms: (1) as allogenic palygorskite in arid and semiarid conditions can represents substantial portion of cave fills, often as-
sociated with smectite, gypsum, calcite and halite; it is airborne or transported by surface run-off to caves from desertic soils and paleosoils, calcretes, dolocretes and related deposits, and (2) as authigenic palygorskite occurs as in situ precipitates in cave fills from percolating solutions and/or transformation of smectite and kaolinite in cave fills in dry evaporative conditions and with suitable geochemical composition of solutions.
In carbonate host-rocks (limestones, dolostones and their metamorphosed equivalents) it fills fissures and faults and is often found in cave walls. It occurs usually in a form of "mountain leather" as a result of hydrothermal and/or weathering processes or represents a product of in situ chemical precipitation from percolating meteoric solutions with suitable pH a redox conditions and chemical composition.
ACKNOWLEDGEMENTS
The research was supported by His Highness Saud bin Saqr Al Qasimi, Sheikh of the Ras Al-Khaimah Emirate. We are grateful to the Emirates Geographical Society and especially to Mrs. Asma Al-Faraj for arrangements of fieldworks in 2011 and 2016. Tadej Slabe, Franci Gabrovsek, Metka Petric, Martin Knez and Janez Mulec (ZRC SAZU Karst Research Institute) assisted during fieldwork in 2011. The research was carried out within the research program Karst research financed by
Slovenian Research Agency (research core funding No. P6-0119); Plan of the Institutional Financing of the Institute of Geology of the Czech Academy of Sciences No. RVO67985831; and UNESCO IGCP project No. 598. We acknowledge the help of Mr. Martin St'astny and Mr. Roman Skala with the preparation of figures; Mrs. Jana Rajlichova has re-drawn Fig. 2. Fruitful comments of reviewers are highly acknowledged.
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