可可西里盐湖表层沉积物中粘土矿物的环境信息

Environmental information of clay minerals in salt lake surface sediments of Hol Xil area

通过对可可西里盐湖表层沉积物中粘土矿物的研究,以期获得近年来湖区的某些环境信息。研究结果表明：沉积物中的粘土矿物主要为伊利石和绿泥石,非粘土矿物主要为石英、方解石和长石以及少量白云石、文石、闪石和石膏等。样品中〈2μm的粘土矿物以伊利石和绿泥石为主,局部样品含少量有序伊/蒙混层。根据样品分布特点和粘土矿物的组合类型,将样品划分为3个区域：1区和3区的矿物组合为伊利石、绿泥石和有序伊/蒙混层;2区为伊利石和绿泥石,无伊/蒙混层矿物。其中1区和3区样品的结晶度（IC）范围为0.41~0.59,均值为0.51;2区样品的结晶度范围为0.25~0.40,均值为0.34,明显低于1区和3区,说明2区样品中的伊利石具有更高的结晶度。样品的伊利石化学指数（CI）均小于0.5,表明该伊利石为富Fe-Mg伊利石,主要是物理风化作用的结果。伊利石和绿泥石作为主要粘土矿物反映了近年来整个湖区的环境以干冷为主,局部区域（1区和3区）出现的少量有序I/S混层矿物则指示在短期内曾经历过短暂的干湿交替环境。此外,在酸性介质条件下,少量伊利石发生弱水解作用导致晶格中的K＋淋失,并转化为伊/蒙混层矿物,同时也降低了伊利石的结晶度。

Clay minerals of surface sediments in the salt lakes of Hol Xil area were studied in this paper with the purpose of obtaining some recent environmental information of the lakes.The results show that clay minerals are illite and chlorite in surface sedimentary samples,whereas non-clay minerals mainly include quartz,calcite and feldspar together with small amounts of dolomite,aragonite,amphibole and gypsum.Clay minerals with particle sizes smaller than 2 μm in samples are illite（65%～82%） and chlorite（13%～35%）,and small amounts of ordered Illite/Smectite（I/S） mixed layer（4%～15%） also exists in local areas.According to the distribution of samples and the combination characteristics of clay minerals,the samples can be divided into 3 districts： mineral combinations of samples in No.1 district and No.3 district are illite,chlorite and ordered I/S mixed layer;however,samples in No.2 district are composed of the combination of illite and chlorite,with no I/S mixed layer.Illite crystallinity in samples was obtained by measuring full width half maximum（FWHM） of 10 ？ diffraction peak according to Kübler Index.At the same time,standard curve of illite crystallinity was obtained by international standard illite samples under the condition of D/Max-RC equipment,and the linear equation is y=1.920 4 x-0.154 7 with related coefficient R2=0.982 8.On the basis of the linear equation for illite standard curve,the corrected illite crystallinity Kübler Index（IC） in samples was obtained,and the chemical index of illite in samples was also calculated.The crystallinity values for samples in No.1 and No.3 district are in the range of 0.41～0.59 with the average value of 0.51.However,illite in samples of No.2 district has better crystallinity because its crystallinity values are in the range of 0.25～0.40 with the average value of 0.34,obviously smaller than the values of No.1 and No.3 district.In addition,chemical indexes are all smaller than 0.5,showing that illite belongs to Fe-Mg illite resulting from physical weathering.In conclusion,illite and chlorite,as major clay minerals in samples,suggest that the environmental conditions of the studied lake area are arid and cold,whereas small amounts of ordered I/S mixed layers in local districts,i.e.,No.1 and No.3 districts,indicate that these districts have undergone the environment of alternate transient arid and humid in recent years.Furthermore,under acidic medium conditions,small amounts of illite experienced weak hydrolysis which led to the loss of potassium ions from illite crystal lattice,and then transformed to ordered I/S mixed layer,accompanied by the decrease of the crystallinity of illite.