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INTRODUCTION
The serpentine minerals are the main constituent minerals of serpentinites mainly distributing in plate subduction zone. They are 1: 1 type phyllosilicates with tetrahedral sheet and trioctahedral sheet, and their ideal chemical composition is Mg3Si2O5(OH)4. Serpentine minerals are classified into chrysotile (stacked with cylindrical layers), lizardite (stacked with flat layers including more aluminum ions), and antigorite (stacked with corrugated layer). Each mineral is some of the clay minerals causing landside disasters. In addition, chrysotile had been used as the asbestos mineral since it takes the fibrous form.
Serpentine minerals are usually analyzed by X-ray diffraction (XRD), but the diffraction patterns resemble each other. So it is difficult to determine quantity in detail. Furthermore, a different type of serpentine is reported nowadays, which is shaped like a multiangular column microscopically and fibrous like chrysotile in macroscopically. It is called polygonal serpentine, since the fiber section is divided into fan-shaped polygonal sectors. Polygonal serpentine resembles lizardite and chrysotile in diffraction pattern because each sector is stacked with flat layers.
Though there were many studies on geology of serpentinites, the species of serpentine was not clear in many serpentinites. In addition, polygonal serpentine may coexist with chrysotile or lizardite that has been reported so far.The purpose of this study is qualitative and quantitative analysis of serpentine species, and to research whether polygonal serpentine exist in serpentines.
SPECIMENS AND METHODS
The specimens were collected from outcrops in the Kurosegawa tectonic belt, central Kyushu, Japan. The distribution of serpentinites has been researched in this area (e. g., Ono et al., 1977; Sakai et al., 1993; Saito et al., 2005), but the detailed serpentine mineralogy is not clear.
The specimens were analyzed by XRD, field emission scanning electron microscope (FE-SEM: JSM-6701F of the center of advanced instrumental analysis Kyushu university), and high-resolution transmission electron microscope (HRTEM: JEM4000EX of Kyushu university HVEM laboratory).
RESULTS
The constituent minerals of all specimens were analyzed by XRD. Two or three serpentine species coexist in most serpentinites, especially chrysotile and lizardite. Serpentinites with strong luster have a high content of serpentine minerals and whitish flaky serpentinites tend to include more chrysotile in this region. Some specimens include little serpentine and much chlorite.
The microstructures of chrysotile-rich specimens were observed by FE-SEM and HRTEM. The assemblages of chrysotile fibers are divided into two types. One is the fibers oriented their axes and the other is random orientation. Most of chrysotile has about 50 nm in diameter. Some chrysotile fibers have fan-shaped sectors partially, therefore they seem to be intermediates chrysotile and polygonal serpentine.
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