熱水性炭酸塩鉱物の相関係と希土類元素の循環過程

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Distribution and behavior of rare earth elements in geosphere is a good indicator of geochemical conditions and migration processes of rare earth elements. Rare earth fluorocarbonate minerals, bastnaesite (Ln (CO3) F) and parisite (CaLn2 (CO3) 3F2) occur in the hydrothermal alteration veins with calcite (CaCO3) in Toki granite, central Japan. Bastnaesite shows lamellae-like texture in the parisite matrix. These minerals should be precipitated with calcite from hydrothermal fluid containing rare earth elements, calcium and carbonate ions. Compositions of bastnaesite, parisite and synchysite (CaLn (CO3) 2F) can be expressed in the calcite-bastnaesite binary system. Phase relations of these rare earth minerals with calcite is fundamental background to understand migration and fixation conditions of rare earth elements under calcite-saturated hydrothermal system, which is one of the most common and active process to circulate elements in geosphere.In this study, hydrothermal synthesis expe … More riments are carried out on starting materials with parisite-(La) composition by cold-seal pressure vessels. Starting materials are prepared from La carbonate and CaF2 reagents and sealed in Ag capsules with distilled water. Experimental conditions are as follows : 350〜650 degree C, 30〜100 MPa and 66〜910 hours. Mineral phases in the run products are identified by powder X-ray diffraction and observed by scanning electron microscopy. Bastnaesite and calcite occur in the run products between 400 degree C and 600 degree C, 100MPa and between 350 degree C and 500 degree C, 30MPa. La carbonate and CaF2 reagents are decomposed by the experiments, and calcite crystals grow more than tens of micrometers in length. At 650 degree C, 100MPa and 600 degree C, 30MPa, bastnaesite is unstable, and La oxide, fluorite and calcite are found in the run products. Parisite is not detected in any run products. Bastnaesite should be more stable than parisite under the conditions of this study. Formation of perisite may be controlled by partial pressure of CO2. Less