Zeolite Mineralogy and Cappadocian Erionite
A. Umran Doğan
Ankara University, Department of Geological Engineering, Ankara
The World Health Organization (WHO) recently classified erionite, a zeolite group mineral, as Group I carcinogens along with the six asbestos minerals (chrysotile, amosite, crocidolite, anthophyllite, tremolite, and actinolite). The classification is based upon evidence in humans, specific diseases from occupational exposures, and health effects noted in animal and cell experiments. In the tuff formations, a controversy mineral group called zeolites developed. In general, zeolite type minerals have excellent physical and chemical properties and they used in the industry widely. However, a fibrous form of zeolite, called erionite, has been proved to be the most toxic mineral for the humans, and erionite occurrences observed in the region.
Geology and Mineralogy of the Region
Previous studies about the general geology of the area include Sassano (1964), Beekman (1966), Pasquare (1968), Batum (1978), Kayakıran (1979), Mumpton (1979), Forster (1982), Aydın (1984), Atabey et al. (1987), Ercan et al. (1987), Atabey et al. (1988), Schumacher and Keller (1990), Schumacher et al. (1990), Le Pennec et al (1994), and Temel and Gündoğdu (1996). After endemic mesothelioma in the Cappadocia region is reported by Barış (1975, 1977), Ataman (1978) surveyed Karain village and found clinoptilolite, chabazite, mordenite, and erionite type of zeolites. Previous studies about mineralogy of the area include Elmes (1977), Pooley (1978, 1979), Sebastien et al. (1981a, b), Rohl et al. (1982), Suzuki (1982), Davis (1990), Bish and Chipera (1991).
The eruptions of two volcanoes on the Anatolian plateau, Erciyes (3917 m) and Hasandağ (3268 m) caused the region to be covered with a thick stratum of lava, volcanic ashes and a dense tuff layer formed on the surface. Natural factors such as rain and winds have created extraordinary shapes, deep valleys, and natural sculptures of fairy chimneys in the tuff formations.
Zeolites in sedimentary rocks are probably formed from volcanic ash or other pyroclastic material by reaction (dissolution - reprecipitation mechanism) of the amorphous aluminosilicate glass with pervading pore waters. Others originate by alteration of pre-existing feldspars, feldispatoids, biogenic silica or poorly crystalline clay minerals. The factors controlling zeolite type minerals include pressure, temperature, reaction time, activities of dissolved species, pH, silica, alumina, and alkali cations. In the Cappadocian area, tuffs accumulated in low areas through both direct airfall contributions and reworking of more widespread ash mantles. Single tuff deposits consist of successive accumulations of ash from more than one eruption event. Following deposition, tuffs have undergone a series of geochemical changes involving an early dissolution of glass surfaces and precipitation of grain coating smectite, followed by erionite growth in pore spaces. A chemical environment of increasing alkalinity is suggested to explain the observed mineralogical changes.
The word zeolite was come from Greek meaning "boiling stone" because of the loss of water when it is heated. Zeolite type minerals (Stilbite) were first discovered by Cronsted, a Swedish mineralogist, in 1756. A zeolite mineral formally defined as a crystalline substance with a structure characterized by a framework of linked tetrahedra, each consisting of four O atoms surrounding a cation. This framework contains open cavities in the form of channels and cages. These are usually occupied by H2O molecules and extra-framework cations that are commonly exchangeable. The channels are large enough to allow the passage of quest species. In the hydrated phases, dehydration occurs at temperatures mostly below about 400°C and is largely reversible. The framework may be interrupted by (OH, F) groups; these occupy a tetrahedron apex that is not shared with adjacent tetrahedra. The tetrahedral arrangement forms lattice structures with relatively large cavities connected by channels. These cavities contain water, and monovalent and divalent cations that balance the charge resulting from a trivalent aluminum ion replacing a quadrivalent silicon ion in the tetrahedra. The cations in the cavities can be exchanged with other cations including sodium, aluminum, potassium, calcium, copper, zinc, lead, silver, rubidium, cesium, and ammonium. The zeolite group of minerals include about 40 naturally occurring minerals (including natrolite, thomsonite, mesolite, phillipsite, harmotome, gismondine, chabazite, erionite, gmelinite, heulandite, clinoptilolite, stilbite, mordenite, and laumontite) and more than 60 synthetic analogies.
Erionite from Durkee, Oregon was defined by A. S. Eakle in 1898. The mineral occurred as white woolly fibers associated with opal in cavities in rhyolite tuff. Eakle proposed the name erionite from the Greek word "wool" because of its woolly aspect (Gottardi and Galli, 1985). Deffeyes (1959) improved the crystallographic data of erionite and described erionite from northern Jersey Valley, Sonoma Range Quadrangle, Nevada; Shoshone Range and valley of Reese River, Nevada; Pine Valley, Nevada; east of Sand Draw, Wyoming, and White River formation, South Dakota. Harada et al. (1967) has demonstrated erionite as an independent mineral.
The morphology of erionite is hexagonal prisms terminated with the basal pinacoid. Erionite usually occurs as thin fibers, often forming a compact felt, sometimes with delicate woolly appearance. The occurrence of intergrowth with offretite is common because both minerals have similar structures. Sometimes a single erionite crystal contains some stacking faults of the offretite as shown by TEM technique (Kokotailo et al. 1972). Macro intergrowths have been described (Rinaldi, 1976). Fibrous erionite-offretite intergrowths often built up a coating over levyne lamellae (Gottardi and Galli, 1985).
Erionite crystallize in hexagonal system and generally composed of
K2 (Na Ca0.5)8 (Al10 Si26 O72) .30H2O. Recently, three types of erionite is described as erionite-Na, erionite-K, and erionite-Ca (Coombs et al., 1997). If all reliable chemical analyses of erionite and offretites available in the literature are plotted in a discriminatory diagram based on the above chemical parameters, it is evident that none among the proposed criteria satisfactorily defines appropriate compositional fields apt to describe the literature information (Passaglia et al., 1998). Chemical analyses are considered to be reliable if
(Si+Al)=36, on the basis of 72 atoms; and balance error
E<10%where E% (Passaglia, 1970) is
100 x [(Al+Fe)ob - Alth]/Alth
Alth = Na + K + 2 x (Ca + Mg + Sr + Ba).
Methods of Study
In this study, samples (both bedrock and wall rock) were obtained from eight villages (Tuzköy, Sarıhıdır, Karain, Karacaören, Boyalı, Çökek, Yeşilöz, and Karlık). Initial screening of the rock samples was performed by optical microscopy and powder X-ray diffraction. Both bedrock and wallrock samples and where available tissue samples are examined using scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) and transmission electron microscope (TEM) equipped with energy dispersive spectroscopy and selected area diffraction (SAD). Enriched samples are also analyzed by ICP-MS. Then from the chemical analyses results crystal structures of erionites have been calculated and their balance errors have been computed.
In the study area, the samples are composed of silica minerals including quartz, cristobalite, and volcanic glass; feldspar minerals including K-feldspar and plagioclase; clay minerals including smectite, kaolinite, and illite; zeolite group minerals including clinoptilolite, chabazite, erionite, mordenite, and phillipsite; carbonate minerals including calcite and dolomite; gypsum, and halite. Substantial amount of unaltered, thin, irregular needles of volcanic glass shards have been observed. These shards have gas escape bubbles on their surfaces visible only with high-resolution electron microscopy. Erionites collected from the region have been characterized quantitatively and the reliability of results have been checked by the balance error computation.
Erionite is the only zeolite whose evidence of carcinogeneity has been evaluated and it is classified as a human carcinogen (IARC, 1987). In Turkey, erionite contaminated villages in Cappadoccia region provide a natural laboratory to study the health effect of these carcinogenic minerals.
In the USA, there are deposits of fibrous zeolites specifically at the western portion of the country. There are homes made of zeolite (erionite) in Oregon and weight stations made of the same materials in Nevada. Very large amounts of zeolites were also used in pozzolanic cements such as those used in the construction of the Los Angeles aqueduct in California. Recently, a few cases of zeolite-related pulmonary diseases have been reported in the USA. Therefore, the possibility of increased exposure to zeolites in the western states anticipated and potential carcinogenic dangers must be evaluated.
Interest in airborne minerals effecting human health has grown exponentially in the past decades. The study is now evolving from a "descriptive science" to a "process oriented" way. The respiratory organ interacts with the internal world with particles and gases, making the lungs an excellent organ for researchers who wish to develop an understanding of the interaction between host and the environment.
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