Crystallization of radiolaria amorphous silica in diagenesis

这项工作是在矿物学和结晶学的基础上阐明成岩作用。微化石放射虫的壳层由无定形二氧化硅组成，成岩作用时在阿尔法石英中结晶。结晶程度被认为与成岩作用密切相关，可以通过壳体中α-石英的结晶度来估计。从四国三巴川带侏罗纪的泥岩、板岩和硅质岩中提取了放射虫的特征，通过测量X-射线粉末花纹的线宽可以确定放射虫壳层的结晶度。以深海放射虫的无定形二氧化硅为标准。放射虫的每一种X射线衍射谱都表现为无取向的α-石英粉末花样。深海放射虫没有观察到无定形二氧化硅的衍射谱。线宽由高到低依次为灰岩、硅质岩和板岩。结果与成岩程度一致，并根据放射虫壳体中α-石英的颗粒大小估算了成岩程度。放射虫标本采自四国东部的沉积岩。用扫描电子显微镜观察颗粒大小。根据成岩作用的不同，颗粒尺寸分布在2-15微米之间。对放射虫贝壳的无定形二氧化硅进行了热处理。在60 0和70 0ﾟC条件下，α石英中的无定形二氧化硅结晶了1天。因此，放射虫壳层中的α石英的结晶度和粒度与成岩作用程度密切相关。

This work has been done to elucidate diagenesis based on mineralogy and crystallography. Shell of microfossil radiolaria consists of amorphous silica and is known to crystallize in alpha quartz in diagenesis. A degree of crystallization is considered to have a close relation with diagenesis and can be estimated by crystallinity of alpha quartz of the shell. Crystallinity of the shell of radiolaria canbe determined by measuring the line width of X-ray powder pattern.Specimens of radiolaria were obtained from mudstone, slate and chert of Jurassic period in Shikoku Sanbagawa belt. Amorphous silica of radiolaria in deep sea was used as standard. Every X-ray diffraction pattern of radiolaria shows unoriented powder pattern of alpha quartz. No diffraction pattern was observed from radiolaria in deep sea indicating amorphous silica. The line width becomes sharper in order of madstone, chert and slate. The results agree with the degree of diagenesis.The degree of diagenesis was also estimated from particle size of alpha quartz of radiolaria shell. Specimens of radiolaria were obtained from sedimentary rocks in Eastern part of Shikoku. Particle size was observed under Scanning Electron Microscope. The particle sizes are distributed in the range between 2 and 15 micron depending on diagenesis.Heat treatment of amorphous silica of radiolaria shell was carried out. Amorphous silica crystallized in alpha quartz at 600 and 700 ﾟC for 1 day.In conclusion crystallinity and particle size of alpha quartz of radiolaria shell have close relations with the degree of diagenesis.