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 host-rocks 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. Paligorskit v jamah in krasu: pregled Paligorskit je vlaknat mineral, ki je prehodna fazo med inosilikati in listastimi silikati z modulirano strukturo filosilikata. Ceprav je pogost v karbonatnih okoljih, je precej neobicajna sestavina jamskih sedimentov. Paligorskit se v jamah pojavlja v dveh oblikah: (1) kot alogeni paligorskit, ki je v susnih 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 povrsinskim transportom iz puscavskih tal in paleotal, kalkret, dolokret in podobnih sedimentov; (2) kot avtogeni paligorskit se pojavlja v jamah kot »in situ« oborina iz prenikajocih raztopin in/ali z obarjanjem med transformacijo montmorillonita in kaolinita v jamskih sedimentih v suhih razmerah izhlapevanja in primerno geokemicno sestavo raztopin. V karbonatnih kamninah paligorskit zapolnjujejo razpoke in prelome in ga pogosto najdemo na jamskih stenah. Obicajno se pojavlja kot del “gorskega usnja”, ki je posledica hidrotermalnih in/ali procesov preperevanja. Lahko pa nastane tudi zaradi in situ kemicnega obarjanja iz prenikajocih meteornih raztopin z ustreznim pH, redoks potencialom in kemicno sestavo. Kljucne besede: paligorskit, jame, kras.