Abstract: In recent years much progress has been made in various branches of experimental petrology, such as anhydrous silicate system under atmospheric pressure, hydrothermal synthesis of minerals with or without controlled oxygen partial pressures, and very high pressure studies on silicates and oxides. In this connection it is interesting that high power explosives can afford extremely high pressure, by which “impact metamorphism” is produced on the surrounding rocks, resulting in the brecciation or melting of the rocks, transformation from lower density phases to higher density phases, or isotorpization of minerals.
Petrologic studies of the rocks collected or drilled from some of the alleged “meteorite craters” have shown the presence of such impact metamorphism and have provided the positive proof for the impact origin of these craterlike topography on the earth's surface. Especially in Canada and the United States much efforts have been made in this branch of the so-called “astrogeology, ” and more than a dozen of these impact craters have recently been found in the Canadian shield. Presence in the rocks of these craters of maskelynite, a glassy form of plagioclase due to extremely strong shock, is very diagnostic. The tektite problems have also been discussed in the light of these new experimental data in astrogeology, and stimulating controversy pro and con to their lunar origin has been made. It is expected that the rock specimens which may be collected from the surface of the moon in near future will throw much light on the origin of tektites.

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A silica sublimate mineral which includes strong halogen acids solution and has a cell-like structure was found at Satuma-Iwo-Zima volcano. It is formed around some fumaroles of relatively low temperatures (above 100°C). The gases emitted from those fumaroles are rich in fluorine compounds.
The freshly formed mineral apparently resembles alunogen. The liquid included in it is easily squeezed out by mechanical stimulus. The liquid content in the sublimate decreases gradually with time passed after its formation (about 97%→2%). Hyalite found in this volcano seems to be a final product of this series.
Specimens which represent various stages of aging were collected and examined by chemical analysis, X-ray diffraction method and differential thermal analysis. The solid part of the mineral is mainly composed of amorphous silica and contains sulfur besides it in most cases. The liquid part contains high concentrations of hydrochloric (max. 4.5N) and fluorosilicic (max. 2.2N) acids but low concentration of sulfuric acid. Metallic ions do not exist in any appreciable amount. Some specimens contain large amounts of hydrogen iodide, hydrogen bromide and fluoroboric acid in their liquid part. They turn red on leaving in air owing to the oxidation of iodide to iodine.
The formation mechanism of this silica sublimate is concluded to be as follows: At first, water vapor in the volcanic gas is condensed at the oriffice of the fumarole and then, silicon tetrafluoride in the gas is hydrolyzed by liquid water forming amorphous silcia and fluorosilcic acid: 3SiF₄+2H₂O=SiO₂+2H₂SiFe₆. Hydrogen chloride, bromide and iodide are concentrated by the successive fractional distillation process.

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Detailed petrological studies of the assemblages of metamorphic minerals stably found in bore-hole cores from some active geothermal areas in Japan as well as in New Zealand show that temperature and depth of the point at which zeolite-bearing assemblage in rocks was transformed into feldspar-bearing assemblage with increasing temperature are available for the indicators of water pressure conditions or of the ratio of H₂O pressure/total pressure prevailed in deeper part of these geothermal areas.

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