斜長石の風化初期における微形態

概要

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The surface of fresh plagioclase usually shows smooth plane and has a weaving texture which suggests cryptic unmixing at vertical section (plate I-1). At the incipient stage of weathering, various features caused by chemical erosion appear on the plagioclase such as etch pits (plate II-5), conical hollows and square holes (plate I-4). Amorphous thin layer of which thickness is less than 0.5μm occurs successively or simultaneously on the surface (plate I-3). Occurrence of the thin layer suggests transitional state from frame work sillicate to clay minerals. The knobby substance, aggregate of fine and rounded spherules which is morphologically identified as spheroidal allophane occurs subsequently in the thin layer (plate II-7, 8). In the next stage of weathering, the following changes are observed; 1) Formation of imogolite (plate III-9) and gibbsite (plate III-10). 2) Formation of halloysite (plate III-11, 12). Imogolite occurs first as tiny bumps from the edges of the amorphous thin layer. Coexistence of gibbsite and imogolite is common in the same grain of weathered plagioclase. Gibbsite crystal shows short prismatic or tabular form (plate III-10 right upper). Observation of glassy matrix in the volcanic ash indicates that weathering should advance more easily in glassy material than in coexisting plagioclase (plate IV)
地学団体研究会の論文
1978-03-25