东海沉积物中海绿石的研究

关于东海现代沉积物中的海绿石,前人曾有过报道,但都限于一般性的描述。近年来我们对东海表层沉积物中的海绿石进行了较为系统的研究,工作范围为东经121°-129°,北纬26°30′-32°00′,样品233个。分别进行了镜下鉴定、透射电镜和扫描电镜观察,并以多晶X-射线衍射法,化学全分析、差热分析、红外吸收光谱和电子探针等方法进行较系统的矿物学研究。本文就东海表层沉积物中海绿石的矿物学特征,产状及分布进行阐述,并对其成因作了初步的探讨。

GLAUCONITE IN THE SEDIMENTS OF EAST CHINA SEA

Glauconites in 233 bottom samples from the shallow portion of East China Sea were studied.In this region, most of glauconites is in various forms of pellets, but some are found in cavities of foraminiferal tests, ostracod shells, molluscal shells and bryozan zooecial chambers. The color of the glauconites varied considerably, ranging from black to light yellow. Most grains are black to dark green, some are yellow-green, a few of them are light yellow. The surface pattern of the glauconite grains are either shallow sutures or network of cracks. They show an aggregate polarization, with an average index of refraction of 1.578-1.615 and a biaxial negative interference figure, with 2V about 10°. The specific gravity varies from 2.49 to 2.67. The X-ray analysis revealed that the glauconites have a micaceous structure between the dioctahedral and triocta-hedral types. They can be regarded as a disordered interstratification of nonexpanding and expanding layers. The amount of the expanding layers is about 21%. The chemical composition of the glauconites is Al2O3 4.95-5.01%, Fe2O3 19.49-23.82%, FeO 1.44-2.00% and K2O 2.46-4.48%. Their crystal forms and thermal characteristics are very similar to that of micaceous clay mineral.Glauconite is widely distributed in the sediments of East China Sea, but its content is generally very low. Only in one sample the, glauconite’s content amounts to 5 per cent of the total sediment. In most cases, only a few grains were found.Glauconite distributes chiefly in the middle part of East China Sea, while only a small amount of glauconite can be found in its eastern and western part. The middle part of the investigated area may be divided into two regions with latitude 30°N as the dividing line. The content of glauconite in the south region is greater than that in the north region. It is worthy to note that glauconite as fillings of organisms is mainly found in the southern portion of this area.There is an inverse correlation between the distribution of glauconite and that of authigenic pyrite.Our research shwos that the well-sorted coarse sediments (0.1-0.25mm and 0.25-0.5mm) with a bottom temperature of 17-18℃, a salinity of 34.1-34.4%, a pH of 8-9 and a depth of 100-160m are favorable to the formation of glauconite in the region of relict sediments where modern sedimentation appears to be very slow.The parent materials of glauconite as fillings of organisms are micaceous clay fraction with a small amount of mineral fragments such as quartz and mica. The shells are often filled with these materials. The decay of organics iwthin shells created probably a micro-environment favorable to glauconite formation. The parent materials of glauconite pellets are biotites.