Role of alteration minerals in geothermal exploration is discussed in reference with Matsukawa-type alteration found characteristically in Japan. In order to get exploration guids for the Matsukawa-type alteration, following three studies are desired from an engineering viewpoint.
(1) Mapping of the distribution of alteration minerals which is good indication to the permeability of geothermal fluids, with adequate data processing such as frequency analysis in which the cutting of short wave length is required.
(2) Study on the applicability of a reservoir model of White (1971) based on the idea that acid alteration solution is originated from saturated steam formed in deeper part. It should be inspected whether the alternative theory that, together with the saturated steam, magmatic emanation are also the sources supplying the alteration solution.
(3) Study on thermo-barometric condition of pyrophyllite-diaspore paragenesis characterizing the Matsukawa-type alteration in parallel with the comparison study to pyrophillite deposits of Cretaceous and Tertiary ages.

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Non-allophanic Andisols have mostly thick, very dark humus horizons, reflecting repetition of intermittent ash depositions and humus accumulation. It is impossible to differentiate these non-allophanic Andisols from allophanic ones according to morphology. Nonallophanic Andisols show absence or virtual absence of allophane and imogolite andabundance of 2: 1 layer silicates in the clay fractions. It is considered that formation of allophane and imogolite is inhibited by supply of a large quantity of organic matter from the vegetation under this weathering condition. Various hypotheses have been proposed concerning the sources of 2: 1 layer silicates.
Non-allophanic Andisols show many unique physical and chemical properties such as low bulk density, high water retention, a large amount of available water, remarkable accumulation of organic carbon, high CEC dominated by variable charge, high fluoride reactivity, high phosphate retention, etc. These properties are also common to allophanic Andisols. However, there are significant dissimilarities between non-allophanic Andisols and allophanic ones. The former have Al-humus complex as the main active Al, very strong soil acidity, high KCl-exchangeable Al, which is toxic to plant roots, critical pH (H₂O) of about 5, etc.

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