第四纪研究  2016, Vol.36 Issue (4): 917-925   PDF    
柴达木盆地西部SG-1钻孔中伊蒙混层结构特征及环境意义
王春虹①,② , 李明慧 , 方小敏②,③ , 刘迎新 , 颜茂都②,③     
(①. 中国地质大学(北京)珠宝学院, 北京 100083;
②. 中国科学院青藏高原研究所, 北京 100101;
③. 中国科学院青藏高原地球科学卓越创新中心, 北京 100101)
摘要: 本文运用X射线衍射(XRD)的方法分析了柴达木盆地察汗斯拉图地区SG-1岩芯(38°24'35.30"N,92°30'32.70"E;长938.5m)的粘土矿物组成,重点讨论伊蒙混层结构的影响因素及其环境意义。样品取自非盐层中,取样间隔为2m。伊蒙混层矿物中的伊利石晶层含量为85%~90%,为高度有序的伊蒙混层矿物。伊利石晶层间隔层数(n)有高类型(H)和低类型(L)。主要影响因素可能是盐度和温度,盐度高的时候钾离子浓度也高,并且离子间的交换作用强,促进蒙脱石向伊利石的转化。因此,盐湖环境下H型的出现概率普遍高于咸水湖环境。盐湖环境下(2.2~2.0Ma和1.2~0.1Ma)伊利石晶层的间隔层数高,以H型为主,咸水湖环境下(2.8~2.2Ma和2.0~1.2Ma)以L型为主。钻孔中1.2~0.9Ma时期为盐湖环境,但H型出现概率却最低(4.94 %),这与全球性的极冷气候有关。与粘土矿物种类相比,矿物结构的变化具有更微观的环境意义。
主题词柴达木盆地     伊蒙混层     结构     XRD    
中图分类号     P588.22;P575.5;P532                    文献标识码    A

1 引言

伊蒙混层(简称I/S,I和S分别是伊利石和蒙脱石的缩写)主要分布于土壤、火山沉积物、热液脉岩、沉积岩和现代海洋湖泊沉积物中[1]。它是伊利石和蒙脱石两个端元组分之间的矿物,是由伊利石晶层和蒙脱石晶层沿C轴或者(001)方向组成的层状硅酸盐矿物[13],根据单元晶层在C轴上堆积的规律性可以分为无序(R=0)和有序(R > 0)伊蒙混层矿物两类:1)完全无序的I/S混层矿物(简称R0I/S),其中伊利石的含量小于60%,在地层中不稳定。此类伊蒙混层中伊利石和蒙脱石经常聚生在一起。2)规律有序I/S混层矿物。当伊利石晶层含量在60%~85%之间时,为R1I/S(IS型);当伊利石晶层含量大于85%时,为R3I/S(ISⅡ型),当伊利石晶层含量达到85%时,IS型向ISⅡ型转化[13]

伊蒙混层矿物的晶体结构和化学成分受风化作用及母岩性质的影响,在沉积作用和埋藏过程中经常发生变化[4],如伊蒙混层矿物的演化顺序为:离散的蒙脱石、完全无序的I/S、趋于IS两层有序的I/S、趋于ISⅡ四层有序的I/S,最后为离散伊利石[5]。成岩环境下,伊利石晶层含量可以代表不同的成岩阶段:早成岩阶段(伊利石晶层含量小于40%)、中成岩阶段(伊利石晶层为40%~60%)、高成岩阶段(伊利石晶层为65%~80%)和晚成岩阶段(伊利石晶层大于85%)[6]

目前,国内外主要通过粘土矿物含量、粘土矿物组合以及伊利石结晶度等来探讨古气候变化。如季俊峰等[7]和贾伟丽等[8]分析了黄土和古土壤剖面中伊利石结晶度的影响因素及环境意义;周世文等[9]认为高岭石/蒙脱石比值反映了华南珠江流域季风降雨的变化,比值增加代表流域降水增加;程峰等[10]分析了百色盆地红土剖面中的粘土矿物特征和伊利石结晶度,并恢复了中更新世以来盆地的气候演化序列;张磊等[11]利用粘土矿物含量讨论了内蒙古二连盆地的第四纪气候变化;钱鹏等[12]利用乌伦古湖中粘土矿物的类型和组合特征,探究了该地区16.2ka BP以来的气候环境演化;吴敏等[13]利用海洋表层沉积物中粘土矿物成分、含量和组合特征,讨论了海南岛地区的气候特征;Robert[14]利用粘土矿物组合特征分析了加利福尼亚南部海湾盆地的物质来源和气候变化,并利用蒙脱石相对丰度重构了区域降水模式;Huyghe等[15]利用粘土矿物组成推断新近纪喜马拉雅前陆盆地中的绿泥石和伊利石为碎屑来源,而高岭石和蒙脱石为自生;Fagel等[16]重建了贝加尔湖粘土矿物的环境意义,认为伊蒙混层比也具有气候指示意义。

粘土矿物之间的相互转化是由温度、压力、介质、原始粘土矿物组成及时间等因素控制的复杂过程[17];伊蒙混层的形成只考虑温度是不够的,还需要考虑压力、pH值、水合阳离子特别是K的浓度[18]。一般认为,在转化过程中温度是主要控制因素,当温度因素不明显时,孔隙流体的化学成分尤其是K含量则成为主要因素[19]。随着埋藏深度的增加,温度升高,蒙脱石脱水,Na和Ca2+含量逐渐降低,四面体中的Si4+被八面体中的Al3+替代,引起层面负电荷增加,K进入晶层以平衡电荷[3, 19, 20],因此,K增加可以指示伊蒙混层中伊利石晶层的增加[21]。蒙脱石伊利石化的过程中Al3+、K不断增加,而Si、Fe、Na、Ca等离子不断减少[22],从而导致了伊蒙混层结构的变化。K的潜入,层间水的不断排开都是随机的,因此,这种混层必然是从无序开始的,逐渐演化到有序[23]

盐湖环境下的粘土矿物主要为碎屑成因,是由外力搬运进入湖盆[24];粘土矿物长期处于高矿化度卤水环境中,成分和结构不可避免地受到影响,即盐湖环境中的粘土矿物可能有自生成因[24]。因此,伊蒙混层矿物的结构是否具有环境指示意义值得去探讨。

柴达木盆地湖泊沉积物中含有大量粘土矿物,目前的研究主要是利用粘土矿物含量来恢复古气候环境,如张玉淑等[25]、赵东升和贺鹏[26]通过粘土矿物组合特征,分别讨论了晚更新世和第三系以来该盆地干旱气候的演化过程。2008年,我们在柴达木盆地西部的察汗斯拉图次级盆地获得1个近千米的钻孔SG-1,终孔深度938.56m;最底部938.5m处的古地磁年代为2.8Ma,顶部0m处年代为0.1Ma[27],钻孔位置如图 1[28]。目前,钻孔SG-1沉积物的各种环境指标均得到了高分辨率的分析测试,如逐个样品的沉积相分析[29]、粒度[29]、矿物测试[30, 31]以及地球化学元素分析[3235],等等,结果表明,2.8Ma以来伴随着气候的干湿变化,湖泊从深湖、半深湖逐渐演化为干盐湖。SG-1钻孔的高密度、高分辨率多种环境指标的研究分析是目前柴达木盆地其他钻孔所不具备的优势,但粘土矿物结构变化如何?是否具有与其他指标类似的环境意义?本文在已有研究基础上,利用X射线衍射(XRD)方法分析SG-1钻孔中伊蒙混层的结构变化、影响因素及环境意义。

图 1 柴达木盆地区域地理及钻孔位置图,引自杨一博[28] Fig. 1 The geographic map showing drilling location in Qaidam Basin, after Yang[28]
2 地质地理概况

柴达木盆地地处青海省西北部,盆地西高东低,西宽东窄,四周被昆仑山脉、祁连山脉、阿尔金山脉环抱,呈半封闭状态(图 1)。平均海拔2600~3000m,内部地貌呈同心带状分布,自中心至边缘分别分布着湖积淤泥盐土平原、湖积-冲积粉砂粘土质平原、冲积-洪积粉砂质平原、洪积砾石扇形地(戈壁);盆地内湖泊水质大多咸化,呈弱碱性,pH为7~8[28]

该地区气候为典型的高原大陆性气候,盆地内气候干旱、降水稀少、蒸发强烈、日照强且风沙多;盆地内降水以夏季为主,冬季降水极少,年降水量在16.09~189.73mm,多集中在4~10月份,而盆地年蒸发量为1973.62~3183.04mm,远大于降水量[36];年平均温度低于5℃,多年平均气温为1.53~4.77℃[36],气温变化剧烈,年较差为25~30℃[28]。盆地内常年盛行西风,风力强盛,风力侵蚀作用强烈[28]

SG-1钻孔(38°24'35.30″N,92°30'32.70″E)位于柴达木盆地西北部的察汗斯拉图次级盆地的一个干盐滩中心,湖区无表面卤水,海拔2900m[28]。该盆地是上新世以来区域性构造抬升形成的次级凹陷,处于碱山和鄂博梁两个背斜之间。该地区第三纪侵入岩发育,各种岩石均有出露,但以中酸性岩石为主,主要岩性为灰色片麻岩、碎屑岩、白云岩、石英岩、大理岩、碳酸盐岩、千枚岩、橄榄岩、蛇纹岩、辉橄岩、闪长岩和花岗岩等[28]

SG-1钻孔中粘土层和纯盐层交替出现,岩性柱及环境演化如图 2,纯盐层中不含粘土矿物,而粘土层中则含有石膏石盐等盐类矿物。从下至上岩性为:938.56~825.00m灰黑色泥岩,偶见泥质粉砂岩;825~723m主要为泥质粉砂岩和灰质泥岩;723~657m泥岩、粉砂岩与少量的层状岩盐互层;657~590m为粉砂岩、泥质粉砂岩和块状泥岩;590~523m主要为黑色的泥岩,偶尔可见粉砂岩;523~414m灰色、灰绿色块状泥质粉砂岩与泥岩互层;414~324m较厚的粉砂岩与较薄的层状岩盐互层;324~231m灰白色、灰黄色、灰色泥质碎屑层与较纯洁的白色石盐层交互回旋;231~0m含碎屑石盐层、泥质碎屑层、灰黑色淤泥层,石盐层与粉砂、砂、淤泥层交替出现[29]

图 2 SG-1钻孔的岩性柱图[29] Fig. 2 The stratigraphic column of core SG-1[29]
3 样品及方法

粘土矿物取自非盐层中,间隔约为2m,共490个样品。采用Stokes的静水沉降法提取< 2μm的粘土悬浮液,依据常规程序分别制作自然片(N)、乙二醇饱和片(EG)和高温片(T,550℃加热),采用K值法进行定量分析。分析依据是中华人民共和国石油天然气行业标准《沉积岩中粘土矿物和常见非粘土矿物X射线衍射分析方法》(2010年版)。前处理和测试工作均在北京北达燕园微构分析测试中心完成,测试仪器型号为Dmax 12kW(CuKα,0.15418nm,40kV,100mA,扫描速度:4°(2θ)/分;步宽:0.02°)。特征衍射图谱见图 3

图 3 样品自然片(N)、乙二醇饱和片(EG)和高温片(T)衍射图 Fig. 3 The diffraction pattern of clay minerals. N, natural condition; EG, ethylene-glycol saturation; H, 550℃heating

伊蒙混层间层比主要是通过峰面积法计算得到,公式如下:

(1)

其中,I/(I/S)表示伊利石和伊蒙混层间层矿物含量比;I10(EG)表示乙二醇饱和片图谱上10×10-1nm峰面积,单位mm2;I10(550)表示550℃高温片图谱上10×10-1 nm峰面积,单位mm2

伊蒙混层中伊利石晶层间隔层数计算公式为:

(2)

其中n为两个蒙脱石层之间的伊利石晶层数量,S为蒙脱石晶层的含量。

4 实验结果

钻孔SG-1的粘土矿物中主要是伊利石和伊蒙混层,绿泥石、高岭石和蒙脱石含量较低。伊蒙混层矿物的含量在15%~45%之间(图 4),而伊蒙混层中伊利石晶层含量为85%~90%,均为高度有序的R3I/S型伊蒙混层矿物。由于测试仪器的分辨率不同,XRD仪器确定的R3I/S在透射电镜下可能是R1I/S。有研究认为,R1I/S是唯一稳定的有序伊蒙混层类型[38, 39]

图 4 钻孔SG-1中伊蒙混层矿物百分含量、伊利石晶层含量及伊蒙晶层间隔层数的变化年代、气候条件和沉积环境阶段的划分依据Wang等[29] Fig. 4 Curves of mixed illite /smectite(%), illite layer percentage of mixed I /S, the ratio of illite layer and smectite layer of mixed I /S versus depth. The data of age, sedimentary environment were after Wang et al.[29]

SG-1钻孔的伊蒙混层矿物中,蒙脱石晶层含量S只有两个值10%和15%。依据公式(2)计算,当S=10%时,n=9,即每隔9个伊利石晶层才有蒙脱石晶层的出现;S=15%时,n=5.67。由于公式(2)中n应该为整数,假设n=5,那么S=16.67%,假设n=6,那么S=14.28%。因此推测,S=15%时应该是n=5和n=6型的混合。SG-1钻孔中伊蒙混层主要是n=5和n=6的混合型,即伊利石晶层间隔层数低的类型(L型),而n=9型较少,即伊利石晶层间隔层数高的类型(H型)少,具体分布如图 4表 1。与2.8Ma以来该盆地的9个演化阶段对应,伊蒙混层矿物结构的变化详述如下:

表 1 不同沉积环境下L-型和H-型伊蒙混层的出现概率 Table 1 The probabilities of L-type and H-type of I /S in different environments

938.56~825.00m(2.8~2.5Ma),半深咸水湖,半湿润气候[29],伊蒙混层矿物含量为20%~40%,伊利石晶层含量以85%为主,L型伊蒙混层的出现概率为84%,H型的出现概率为16%;

825~723m(2.5~2.2Ma),浅咸水湖,半干旱气候[29],伊蒙混层的含量为30%~42%,伊利石晶层含量以85%为主,L型伊蒙混层出现概率为91.11%,H型出现的概率为8.89%;

723~657m(2.2~2.0Ma),盐湖,干旱气候[29],伊蒙混层的含量为22%~36%,伊利石晶层含量以85%为主,L型伊蒙混层出现概率为77.42%,H型伊蒙混层出现的概率为22.58%;

657~590m(2.0~1.8Ma),咸水湖,半干旱气候[29],伊蒙混层的含量为20%~50%,伊蒙混层含量逐渐增加,伊利石晶层含量以85%为主,L型伊蒙混层出现概率为90.00%,H型伊蒙混层出现的概率为10.00%;

590~523m(1.8~1.6Ma),咸水湖,较湿润气候[29],伊蒙混层的含量为26%~40%,伊利石晶层含量以85%为主,L型伊蒙混层出现概率为78.79%,H型伊蒙混层出现的概率为21.21%;

523~414m(1.6~1.2Ma),咸水湖,半干旱气候[29],伊蒙混层的含量为22%~44%,伊蒙混层含量逐渐增加,伊利石晶层含量以85%为主,L型伊蒙混层出现概率为85.71%,H型伊蒙混层出现的概率为14.29%;

414~324m(1.2~0.9Ma),盐湖,干旱气候[29],伊蒙混层的含量为5%~45%,伊蒙混层含量逐渐增加,到375m处伊蒙混层含量开始下降,伊利石晶层含量以85%为主,L型伊蒙混层出现概率为95.06%,H型伊蒙混层出现的概率为4.94%;

324~231m(0.9~0.6Ma),盐湖,非常干旱气候[29],伊蒙混层的含量为20%~44%,伊利石晶层含量以85%为主,L型伊蒙混层出现概率为85.71%,H型伊蒙混层出现的概率为14.29%;

231~0m(0.6~0.1Ma),干盐滩暂时性盐湖和盐泥坪交替沉积环境,极端干旱气候[29],伊蒙混层的含量为15%~50%,伊利石晶层含量为85%~90%,L型伊蒙混层出现概率为69.44%,H型伊蒙混层出现的概率为30.56%,伊蒙混层的晶层间隔层数明显增加。

以上表 1中盐湖环境下和咸水湖环境下的平均值数据均通过显著性检测,显著性检测(t-检测)结果:P=0.04512 < 0.05,因此,数据具有显著性,可以进行以下讨论。

5 讨论

尽管盐湖沉积物中粘土矿物主要是碎屑来源[24],但是,粘土矿物搬运到盆地后,由于气候环境的变化,湖水或孔隙水性质如温度、盐度、K含量等会发生变化,可能会造成粘土矿物之间的相互转化[40, 41]。一般认为,蒙脱石的伊利石转化分为两个步骤,首先形成具有无序结构的伊蒙混层,其活化能为69.7KJ/mol,第二步反应形成具有有序结构的伊蒙混层,活化能为37.4KJ/mol[19],其转化率可以用以下方程计算:

(3)

其中S为伊蒙混层中蒙脱石含量;t为时间(s);A为频率因子,为8.08×10-4/s;Ea为活化能,为28KJ/mol;R为气体常数,为1.978Cal/deg·mol;T为温度;K为流体中钾离子浓度。

从转化率方程中可以看出,温度和K浓度是控制蒙脱石伊利石化转化速率的主要因素[41]。温度升高、K浓度增加,蒙脱石的伊利石化转化速率增加。因此,伊蒙混层矿物结构的变化可能与盐度、温度有关。随着盐度的增加,水介质中K含量增加,蒙脱石脱水,八面体的Al3+不断替代四面体的Si4+,引起层间负电荷增加,从而使K不断进入层间,置换出Na和Ca2+,致使膨胀层减少,伊利石晶层数或晶层间隔层数也增加。盐湖环境下H型伊蒙混层出现概率普遍大于咸水湖环境下的值(表 1),说明盐度高有利于蒙脱石的伊利石化,即伊利石晶层的增加。但是,在盐度相似的环境中,如均为盐湖或咸水湖时,概率也有明显的不同(表 1),可能的原因是温度的影响。以H型伊蒙混层出现概率的变化为例,具体分析如下:

2.8~2.5Ma和1.8~1.6Ma期间古湖盆均为半深湖,淡水至半咸水湖环境[29],H型伊蒙混层的概率分别为16.00%和21.21%。柴达木盆地鸭湖剖面孢粉和盐类化学指示,2.6Ma以后耐旱植物花粉含量与盐度指标均呈现明显长期增加趋势,气候快速向更干旱方向发展[43],这与全球性气候背景一致。Wrenn等[44]研究表明,上新世中期暖期(3.30~3.15Ma)之后逐渐变冷,2.6Ma以后北半球开始冰期-间冰期旋回,而在1.83Ma以后出现更干旱的气候[43],这正好验证了1.8~1.6Ma间的盐类矿物种类和含量均明显高于2.8~2.5Ma的气候因素[30],因此,1.8~1.6Ma期间H型出现概率高的主要原因可能是盐度高。利用公式(3)可以判断,K含量高时,蒙脱石向伊利石转化的速率高。

2.5~2.2Ma、2.0~1.8Ma和1.6~1.2Ma均为浅咸水湖[29],H型伊蒙混层的出现概率分别为8.89%、10.00%和14.29%,呈递增趋势。2.6~2.1Ma期间的干旱气候在青藏高原东北部贵德盆地也有记录,其湖相沉积物由前期杂色和暗红色泥岩变为土黄色亚粘土(水成黄土),说明气候的干旱化使风尘大幅度增加[45]。干旱环境下,随着盐度增加,伊利石化速率增加[41],H型概率值增加。但是,与2.8~2.5Ma和1.8~1.6Ma的咸水湖阶段相比,以上这3个阶段的概率值比较低(表 1),主要原因可能是温度较低。因为2.5Ma左右发生了明显的降温事件,2.2Ma和1.6Ma发生了干旱气候事件,均受控于全球变冷和区域性气候干旱化的发展[29]。而2.8~2.5Ma和1.8~1.6Ma时期的古湖虽然也是半深的咸水湖,但气候可能比这3个阶段温暖湿润。降水多,湖水补给充足,才有可能形成半深湖。

2.2~2.0Ma和1.2~0.9Ma均为永久性或常年性盐湖[29],H型伊蒙混层的概率分别为22.58%和4.94%,后者也是整个钻孔中的最低值(表 1)。根据盐类矿物种类和含量推测,1.2~0.9Ma期间的盐度明显高于2.2~2.0Ma,但出现概率却是整个钻孔中最低的,可能与低温环境有关。1.2Ma开始了MPT事件(Mid-Pleistocene Climate Transition),即气候主导周期从4.1万转换为10万年周期,非常寒冷[46, 47],其他指标也显示,这个时期柴达木盆地也进入冰冻圈,气温急剧降低[29],西部其他地区如黄土高原中部也记录了这个时期的低温气候,认为该地区低温事件开始于1.26Ma[48]。低温下,离子活动性减弱,离子间的交换作用减弱,进入层间的K急剧降低,根据以上的转化率计算公式(3),低温环境下转化率会大大降低,致使伊利石晶层含量降低,H型伊蒙混层的出现概率降低。

0.9~0.6Ma时期古湖为浅盐湖,气候非常干旱[29],H型伊蒙混层出现的概率为14.29%。钻孔中的碳酸盐碳氧同位素记录了1.0Ma以来湖区的干旱化[49],并且1.0Ma以来钻孔中开始出现厚层石盐,指示更加干旱的气候条件[30],盐类矿物种类和含量说明盐度进一步增加,高于早期的任何阶段,但出现概率却低于同为盐湖环境的2.2~2.0Ma时期的22.58%,低温可能是主导因素。虽然MPT冷事件的影响逐渐降低,但黄土高原中部0.53Ma仍然是低温气候[48],Clark等[50]认为MPT事件的结束时间可能为约0.7Ma,气候条件有所好转但仍然存在冷事件[28, 51]。因此,研究区这个时期的温度也可能不高。根据转化率的计算公式(3)判断,低温下,离子活动性减弱,离子间的交换作用减弱,进入层间的K降低,从而导致H型伊蒙混层较低的出现概率。

0.6~0.1Ma古湖到干盐湖阶段[29],碳酸盐碳同位素值逐渐升高,0.6Ma以后快速升高[49],说明干旱化急剧增加,此时常年咸水湖被间歇性干盐湖取代[29],发生在约0.6Ma的干旱事件仍然具有普遍性[52, 53]。盐类矿物种类和含量说明这个时期的盐度是整个钻孔中最高的,H型伊蒙混层的概率为30.56%,也是整个钻孔中的最高值,由于冷气候事件影响较小[25],此阶段盐度的影响占据主导作用。

6 总结

柴达木盆地察汗斯拉图地区SG-1钻孔粘土矿物中的伊蒙混层矿物含量在15%~45%左右,伊蒙混层中伊利石晶层含量为85%~90%,蒙脱石伊利石化程度很高,为高度有序的伊蒙混层矿物。2.8Ma以来伊蒙混层矿物中的伊利石晶层间隔类型分布和变化趋势与湖泊的盐度和温度存在相关性。盐湖环境下(2.2~2.0Ma和1.2~0.1Ma)伊利石晶层间隔层数高,以H型为主,咸水湖环境(2.8~2.2Ma和2.0~1.2Ma)伊利石间隔层数低,以L型为主。盐湖环境下的间隔数普遍高于咸水湖环境下的值,其中1.2~0.9Ma时期为盐湖环境,但H型伊蒙混层最低的出现概率(4.94%)与全球性的极冷气候有关。

尽管盆地的粘土矿物为碎屑来源,但伊蒙混层矿物的结构变化与湖泊环境的关系有明显相关性,可以推断,粘土矿物埋藏后确实受到湖水盐度和温度的影响。因此,与矿物种类相比,粘土矿物结构具有更微观的环境意义。

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Structural characteristic of mixed-layer illite/smectite clay minerals of the SG 1 core in the western Qaidam Basin and its environmental significance
Wang Chunhong①,②, Li Minghui, Fang Xiaomin②,③, Liu Yingxin, Yan Maodu②,③     
(①. School of Gemmology, China University of Geosciences(Beijing), Beijing 100083;
②. Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101;
③. CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101)

Abstract

This paper presents the structural characteristics of illite/smectite mixed minerals (I/S) and their environmental significance of the core SG-1. The 938.56m-long core (38°24'35.30"N, 92°30'32.70"E) is located in the Chahansilatu subbasin, western Qaidam Basin. It was magnetostratigraphically dated to be ca.2.8Ma(938.56m)to ca.0.1Ma(0m). Lithofacies variation suggested that it evolved from a deep brackish lake into a playa since ca.2.8Ma. A total of 490 samples were sampled at about 2m intervals from non-salt layers in the core. < 2μm fractions of these samples were separated from the suspension using the Stokes' static setting method, and then made into flakes under natural, ethylene-glycol saturated and 550℃heated conditions for X-ray Diffraction(XRD)analyses. The results show that the illite-layer percentage of I/S ranges from 85 % to 90 %, and all of them were highly ordered. According to the number (n) of illite-layer within two smectite layers, the structure of I/S was divided into high(H)and low(L)types. The H-type(n=9)was dominant during the periods of 2.2~2.0Ma and 1.2~0.1Ma, while the L-type(n=5, 6)was dominant in the rest of the core. The main influencing factors for their distribution are possibly salinity and temperature of brines or porewater. The higher salinity of brines(or porewater), the richer K, as a result, the stronger ion exchange actions between clay minerals and brines(or pore water), and so as to the higher temperature. The two factors play an active role on the transformation from smectite into illite, thus high number of illite-layer of I/S. Therefore, the H-type was dominant in the saline lake environments during the periods of 2.2~2.0Ma and 1.2~0.1Ma, while the L-type was dominant in brackish lakes during the 2.8~2.2Ma and 2.0~1.2Ma. The H-type was the lowest(4.94 %)in saline setting during 1.2~0.9Ma. We believe that it was due to the global extremely cold climate during the period. Compared with whole clay minerals, the structural characteristics of I/S could have more microscopic environmental significance.
Key words: Qaidam Basin     illite/smectite mixed mineral     structure     XRD