[SCG73-P05] Mineralogy of polygonal serpentine and chrysotile from Kyushu Kurosegawa Belt
Keywords:serpentine, polygonal serpentine, chrysotile, Kyushu Kurosegawa belt
About 300 samples were collected at 28 areas in Kyushu Kurosegawa belt and analysed by XRD (Inoo and Uehara, 2009). Representative samples were chosen considering the result of X-ray diffraction (XRD), and were analysed by electron microprobe analyser (EPMA) and transmission electron microscope (TEM) in Kyushu university ultramicroscopy research center. Two types of TEM samples were prepared; dispersed grains on Cu-grids and ion milled thin sections.
The ortho-PS commonly occur as liner splintery veins, few millimetres in width (Shimotake, Fukami) or irregular veins (Wakayama) in host serpentinites. Clinochrysotile rich specimen occur as thin veins in serpentinites, or as massive aggregates. The clinochrysotile rich samples are Al poor (<0.02 apfu), while ortho-type PS rich samples showed various Al contents (0.01-0.09 apfu). Al contents of the ortho-type PS differed in localities; Fukami (0.01 apfu), Shimotake (0.04-0.05 apfu) and Wakayama (0.09 apfu). The Fe was mainly divalent estimated from charge balance, and showed no clear difference between the two types (0.03-0.11 apfu). The basic properties of ortho-type PS (15 sectored, 200-300nm in general) were common in all samples, but microtexture differed by locality. The chrysotile and PS in Fukami, have smaller diameters with sparse texture, while they occur as incomplete fractures with larger diameter in dense texture in Wakayama. Shimotake was intermediate.
Serpentine veins with different occurrence in Shimotake was observed to estimate the factor that causes different forming species. In Shimotake many splintery serpentine veins occur between spathic serpentinite, pale to apple green, opaque, few millimeters in width, which are all ortho-type PS near endmember. However there is also a wider vein (up to 1 cm) composed of many scale like blocks, yellowish to apple green, opaque to transparent, which is much less common. They are mixture of orthochrysotile and clinochrysotile, and the proportion change through out the vein. The composition of the chrysotile split into two types, Al rich phase (Al 0.02-0.04, Fe 0.04-0.08 apfu) and Fe rich phase (Al 0.01-0.02, Fe 0.09-0.12 apfu). The Al-rich phase is abundant in orthochrysotile rich specimen and Fe rich phase in clinochrysotile. Also considering that the chemical composition of Al rich phase is close to ortho-PS, the Al content may be responsible for clino-type vs ortho-type, but not for PS vs chrysotile. Another interesting point is that ortho-PS are always homogeneous, although the composition slightly differs from vein to vein.