モンモリロナイトの鉱物学的性質と変質

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Previous works on bentonite deposits in United States, and bentonite and acid clay deposits in Japan are reviewed briefly. The outlines of genesis of bentonite deposits are discussed : bentonite deposits formed by alteration of volcanic ash or tuff in situ are most widely distributed abroad and in Japan also. Hydrothermal alteration (including deuteric alteration) of igneous rock and material is next an important mode of origin. Bentonite and acid clay deposits of economic importance in Japan are discribed on the location, parent rock, main exchange cations, form of deposit, geologic age of formation, etc. Some deposits abroad are also described on main exchange cations and form of deposit. Montmorillonite (the main constituent mineral of bentonite) is discussed first on terminology : the term smectite can be used as a group name for all clay minerals with an expanding lattice except vermiculite, and montmorillonite is a specific mineral name of the smectite group. Dioctahedral smectite is the main constituent clay mineral in bentonites except quite a few number of bentonite deposits composed of trioctahedral montmorillonite. In Japan no deposits composed of trioctahedral smectite, are found. Variations in composition in montmorillonite-beidellite series of smectite are nearly between end member of montmorillonite and boundary of montmorillonite and beidellite. Methods for caluculating structure formulae for the dioctahedral smectite are proposed : 1) fixed anionic charges 22 in form of O_<10>(OH, F)] 2) fixed anionic charges, including charges of exchange cations (O.33) 3) fixed cationic number (6.00) Populations of tetrahedral and octahedral sheets of Cheto and Wyoming montmorillonites are compared among calculations made with the second method by Grim and Kulbicki, with the third method by Grim and Guven, the first method by Takeshi and Uno. Cheto montmorillonite has the composition near end member of montmorillonite, though Wyoming montmorillonite has the composition near the boundary of montmorillonite and beidellite. Tsukinuno and Odo montmorillonites have compositions near Wyoming, and Nakajo montmorillonite has the composition near end member of montmorillonite, nearly the same as that of Cheto. Weathering of dioctahedral smectite is discussed : (Mg, Ca) montmorillonite like Nakajo, having the composition near end member of montmorillonite, is easily altered to halloysite in the wet climate and mild temperature in the following order : (Mg, Ca) montmorillonite→(H, Mg, Ca) montmorillonite→(H, Mg, Ca) abnormal montmorillonite→mixed-layer of montmorillonite and halloysite. Odo montmorillonite is not easily transformed like Nakajo one, because its composition of montmorillonite layer is near the boundary of montmorillonite and beidellite, although its main exchange cations are Mg and Ca, which are largely replaced by hydrogen at the surface of the deposit. Finally differential thermal analysis and the phase transformation of montmorillonite-beidellite series are discussed briefly.
1980-09-25

モンモリロナイトの鉱物学的性質と変質

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