Polygonal Serpentine and Chrysotile in the Kurosegawa Belt, Kyushu, Japan

AbstractThe basic crystallographic properties of polygonal serpentine (PS) have been revealed in previous studies, while their variation, occurrence, and formation conditions are poorly understood. This study defines four categories of chrysotile and PS: clinochrysotile (chrysotile 2Mc1), orthochrysotile (chrysotile 2Orc1), clino-type PS, and ortho-type PS. When chrysotile and PS coexist or are indistinguishable, we will call them fibrous serpentine (FS), regardless of their ratios. By researching the occurrence, distribution, microtexture, crystal structure, and chemical composition of PS and chrysotile based on these categories, we reveal the features of each type and discuss their relationships and formation processes. The chrysotile and PS of the Kurosegawa belt in Kyushu are enriched in ortho-type FS (O-FS) compared with other localities. O-FS-enriched samples (group-O) show a linear, wide vein occurrence and splintery texture or irregular veins with space-filling textures, while clino-type FS (C-FS)-enriched samples occur as thin veins in serpentinites or as massive-foliated aggregates. Polygonal serpentine is more frequently seen in group-O compared with group-C. In our observation, the crystal structure of chrysotile and PS is continuous, making it difficult to define a clear border between them. Ortho-type PS is frequently seen, whereas clino-type PS is absent in most localities. Orthochrysotile tends to grow into 15-sectored ortho-type PS, whereas clinochrysotile grows into 30-sectored clino-type PS. These features of the crystal structure and coexistence and proportion of the fibrous serpentines in each group were mostly common in all studied localities in the Kurosegawa belt in Kyushu, indicating similar formation conditions within group-O or -C. Based on the textures and occurrences, group-O formed by crystallization from the fluid in the fracture made by brittle deformation, while group-C formed in the last stage of alteration mainly by replacement by alteration. The serpentines in the Kurosegawa belt in Kyushu seem to have followed a similar process, at least at the late stage of serpentinization where FS forms and the difference in the ratio of C-FS to O-FS may have been derived from the difference in the timing of formation during the emplacement process. A formation model for chrysotile and PS is proposed based on these observations. In this model, C-FS and O-FS form under different conditions, such as differences in pressure. This model still needs verification by further observations at different localities and by synthetic experiments.