南極Victoria LandのDry Valley調査報告 : 1. 南極Victoria LandのMiers Valleyに産するEvaporiteについて

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Reconnaissance of Lake Miers in the Dry Valley region, Victoria Land was made by a summer party supported by the U. S. National Science Foundation This report summarizes the description of occurrence and characteristics of the crystalline salt deposits found on and beneath the morains on the bottom of the Miers Valley, in this report these deposits are called "evapontes" although the mechanism of the formation of these deposits is still obscure. The mam characteristics of the climate in the Dry Valley are quite arid condition and comparatively high temperatures in summer and strong wind, which are considered to be ascribed chiefly to the low albedo of widely exposed rocks to the insolation in summer and the compressional effect (pseudofoehn phenomenon) of katabatic wind. These circumstances may have a close relation to the mechanism of formation of the evapontes. The Miers Valley was once covered with glaciers and by the retreat of the glaciers the U-shaped valley was formed, the floor of which being widely covered with moraines. The moraines are composed of undifferentiated material of the Koettlitz Glacier origin, which was grouped into four types by BLANK. The moraines are quite flat in part, suggesting a levelling action of water (lake water?). Patterned ground is commonly developed in the moraines Lake Miers is located at the bottom of the valley and ill-defined old strand lines of the lake can be observed The lake water is not saline probably because of the presence of an outlet stream. The evaporites can be grouped into three types by the mode of occurrence (1) platelike aggregates of crystals spread over the flat moraines (Sample Nos. 65M60, 65M80, 65M90, 65M1001, 65M1002, Photo 3), (2) cloddy crystal aggregates on the ridge of moraines, some of which lie directly on the core of ice (Sample Nos 65M30, 65M402, 65M401, Photo. 4) and, (3) horizontal layers interbedded with moraines, exposed on the flank of a gorge which was formed by downcutting of an outlet stream (Sample Nos 65M501, 65M502, 65M503, 65M504, 65M506, Photo 5) Samples of the deposits were analyzed mineralogically and chemically X-ray diffraction pattern and emission spectrogram were also taken. The result revealed the presence of the following four evaporites gypsum CaSO_4・2H_2O, calcite CaCO_3, mirabihte Na_2SO_4・10H_2O and thenardite Na_2SO_4 Some of the deposits were found to be the mixtures of gypsum + calcite or mirabihte + thenardite and some others contained more or less quartz, feldspar and amorphous minerals (Table 4) Chemical analysis of four samples (Table 5) also confirmed that the main components are gypsum, calcite, mirabihte and thenardite Trace impurities in these crystals were detected by spectrographic analysis as shown in Table 6, indicating the presence of strontium in calcium minerals and calcium fluoride and barium in calcite Crystalline mirabilite sample was found extraordinarily pure, showing no indication of impurities other than traces of aluminum, magnesium and calcium

南極Victoria LandのDry Valley調査報告 : 1. 南極Victoria LandのMiers Valleyに産するEvaporiteについて

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