Chert and its sodium-silicate precursors in sodium-carbonate lakes of East Africa
Chert and its sodium-silicate precursors in sodium-carbonate lakes of East Africa
复制标题
东非碳酸钠湖中的燧石及其硅酸钠前体
DOI：
10.1007/bf00380740
复制
发表时间：
1968
影响因子：
3.5
通讯作者：
R. L. Hay
中科院分区：
文献类型：
--
作者：
R. L. Hay
燧石由两种钠硅酸盐矿物形成，麦羟硅钠石（NaSi 7，O 13（OH）3·3 H2O）和水羟硅钠石（NaSi11O20.5（OH）4·3 H2O），在肯尼亚和坦桑尼亚的马加迪湖和纳特龙湖的最上层更新世沉积物中。燧石由精细的结晶石英组成，其特征是形成不规则形状的结核，表面有网状图案的白色涂层。在马加迪湖、纳特龙湖和奥杜威峡谷附近的中更新世和下更新世湖相沉积中，也广泛存在类似的结核。虽然麦羟硅钠石和水羟硅钠石在下更新世和中更新世沉积物中不存在，但这些层中的硅质岩可能是由钠硅酸盐前体形成的。所有含硅质岩的沉积物都沉积在富含溶解的碳酸钠-碳酸氢钠的盐碱湖泊中。麦羟硅石（和硅质岩）可能形成薄而广泛的沉积物，也可能形成局部的块体，这些块体可能是横切的。薄，广泛的麦羟硅钠石层已沉淀的富硅盐水与淡水在化学分层湖混合;局部群众可能已形成的盐水与淡水进入湖底或边缘的相互作用。麦羟硅钠石和水羟硅钠石在与碳酸钠盐水接触时，可能在20，000年或更短的时间内被相对新鲜的水沥滤而变成燧石。硅质沸石斜发沸石和毛沸石在与麦羟硅钠石和燧石共生的粗面凝灰岩中占优势;硅质较少的钙十字石在无燧石序列的粗面凝灰岩中占优势。
Chert has formed from two sodium-silicate minerals, magadiite (NaSi7,O13(OH)3·3H2O) and kenyaite (NaSi11O20.5(OH)4·3H2O), in uppermost Pleistocene deposits of lakes Magadi and Natron in Kenya and Tanzania. The chert consists of finely crystalline quartz and characteristically forms nodules of irregular shape with white coatings having reticulate surface patterns. Similar nodules are widespread in lower and middle Pleistocene lacustrine deposits in the vicinity of Lake Magadi, Lake Natron, and Olduvai Gorge. Although magadiite and kenyaite are absent in the lower and middle Pleistocene deposits, the chert in these beds probably formed from a sodium-silicate precursor. All of the chert-bearing sediments were deposited in saline, alkaline lakes rich in dissolved sodium carbonate-bicarbonate.Magadiite (and chert) may form either thin, widespread deposits or localized masses which may be cross-cutting. Thin, widespread layers of magadiite have been precipitated by mixing of silica-rich brine with fresh water in a chemically stratified lake; localized masses may have been formed by interaction of brine with fresher water entering the floor or margin of the lake. Magadiite and kenyaite can alter to chert in contact with sodium-carbonate brine and possibly by leaching with relatively fresh water over a period of 20,000 years or less.The siliceous zeolites clinoptilolite and erionite predominate in trachyte tuffs associated with magadiite and chert; less-siliceous phillipsite predominates in trachyte tuffs of chert-free sequences.