Disordered silica with tridymite-like structure in the Twiggs clay
Disordered silica with tridymite-like structure in the Twiggs clay
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Twiggs 粘土中具有鳞石英结构的无序二氧化硅
DOI：
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发表时间：
2008
期刊：
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通讯作者：
R. Ferrell
中科院分区：
文献类型：
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作者：
Lori G. Eversull;R. Ferrell
摘要长期以来，人们一直认为乳白色二氧化硅是方石英和鳞石英结构域的无序共生体，这暗示了以鳞石英堆积为主的无序二氧化硅多晶型的存在。目前公认的无定形二氧化硅成岩转化模型也预测，早期形成的结构将主要由鳞石英堆叠。虽然方石英端元（opal-c）和中间结构（opal-ct）早已被认识，但鳞石英端元尚未被认识。这种端元的证据被发现在54 Twiggs粘土样品的X射线衍射（XRD）检查。样品的矿物学以蒙皂石和无序硅为主。硅相是生物成因的，其XRD特征与线加宽鳞石英一致，在4.107 nm附近具有宽的一次反射（21.6 °2θ），在约4.328 ° C处具有较低强度的其他宽峰（20.5 °2θ）和2.50 °（35.9 °2θ），在3.14 °（28.5 °2θ）和2.84 °（31.5 °2θ）处缺乏诊断方石英峰。目前，无序二氧化硅最广泛使用的分类是一个三重系统，它识别无定形相（蛋白石-a），方石英样端元（蛋白石-c），以及包含方石英和鳞石英域的中间结构（蛋白石-ct）（Jones和Segnit 1971）。这种分类不适用于在Twiggs粘土中观察到的主要鳞石英样结构。这些样品中的二氧化硅相更准确地描述为“无序鳞石英”。一个新的术语“蛋白石-t”被提出来描述这种端员相。
Abstract The existence of a disordered silica polymorph dominated by tridymite-like stacking is implied by the long-held interpretation of opaline silica as disordered intergrowths of cristobalite-like and tridymite-like domains. The currently accepted model for diagenetic transformation of amorphous silica also predicts that early formed structures will be dominated by tridymite stacking. Although the cristobalitic end-member (opal-c) and intermediate structures (opal-ct) have long been acknowledged, the tridymitic end-member has not yet been recognized. Evidence for such an end-member is found in 54 Twiggs clay samples examined by X-ray diffraction (XRD). The mineralogy of the samples is dominated by smectite and disordered silica. The silica phase is biogenic in origin and presents an XRD signature consistent with that of line-broadened tridymite, with a broad primary reflection near 4.107 Å (21.6 °2θ), additional broad peaks of lower intensity at approximately 4.328 Å (20.5 °2θ) and 2.50 Å (35.9 °2θ), and lacking diagnostic cristobalite peaks at 3.14 Å (28.5 °2θ) and 2.84 Å (31.5 °2θ). At present, the most widely used classification of disordered silica is a threefold system that recognizes an amorphous phase (opal-a), a cristobalite-like end-member (opal-c), and intermediate structures incorporating both cristobalite and tridymite domains (opal-ct) (Jones and Segnit 1971). This classification does not accommodate a dominantly tridymite-like structure as is observed in the Twiggs clay. The silica phase in these samples is more accurately described as “disordered tridymite.” A new term “opal-t” is proposed to describe this end-member phase.