天然ゼオライトの産状
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Natural zeolites form under a wide range of environments from cold deep-sea oceanic bottom sediments to hot hydrothermal alteration and from the Earth's surface to a great burial depth. Based on the states of water related to zeolite formation, the occurrence of zeolites is classified into several types : in order of increasing temperature, 1) deep-sea oceanic bottom. 2) weathering in arid to semi-arid regions, 3) saline, alkaline lakes, 4), percolation of meteoric water, 5) burial diagenesis. 6) hydrothermal alteration and contact metamorphism, and, 7) primary magmatic. Volcanic glass in ashes and vitric tuffs is by far most suitable for raw material of natural zeolites. Specific zeolite reaction series in different types of occurrence cause characteristic zonal distribution of unaltered glass zone, zeolite zone, and authigenic feldspar zone when appeared. In saline, alkaline lake deposits, for example, the horizontal zonal distribution is attributed to chemical gradient in pore water. In thick marine sequences, on the other hand, the vertical zonal distribution is attributed primarily to geothermal gradient. Rate of zeolitic reactions is a big geological problem to be solved, though it is difficult. The difficulty results largely in recognizing real reacting time in nature. The reacting time is able to be estimated either from the age of the youngest zeolitized tuff in alkali soil profiles and in saline, alkaline lake deposits, or from very slow rate of sedimentation of phillipsite-containing pelagic sediments. The rate of the analcime to albite transformation during burial diagenesis is estimated to be approximately 400,000 years ; this figure is known from the thickness of a transitional zone (analcime coexisting with albite) between Zone III (analcime) and Zone IV (albite) using the burial history diagram as shown in Fig. 4. Zeolites in any types of occurrence change ultimately to thermodynamically stable alkali feldspars or other non-zeolite minerals through intermediate zeolites, such as analcime and laumontite, due to the increase of temperature and pressure, to the increase of concentration of pore water, or to ageing. As a result, zeolite assemblage tends to be simpler in the later stage than in the early stage (Fig. 5). This probably reflects on the fact that the number of zeolite species, except for veins and vugs, decreases markedly with ascending geologic age of host rocks (Fig. 6).
- 日本粘土学会の論文
- 1986-06-05
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