玛瑙的结构、水含量和成因机制

Texture, water content and formation mechanism of agate

玛瑙是一种隐晶质硅质岩石,主要由玉髓以及少量蛋白石、斜硅石和微晶石英等组成.其在世界各地分布广泛,质地坚硬细腻、色彩鲜艳多样、纹理交错、造型千姿百态,是一种平凡而美丽的宝石.本文结合国内外最新研究进展,对玛瑙矿物的结构、水含量和成因机制进行了综合评述.玛瑙以纹带构造为特征,其内部纹带花纹和化学组成呈韵律性变化,并在不同观察尺度表征出来.因此,玛瑙也被定义为条带状玉髓.然而,多数玛瑙实际上是由纤维状玉髓、同心环带状玉髓、水平条带状玉髓、微晶石英和自形石英晶体等组成,它们的相对含量与玛瑙的产地和形成过程密切相关.X射线衍射和电子背散射衍射(EBSD)测量数据揭示,玛瑙中的硅质矿物多发生定向生长,其中微晶石英a轴倾向于垂直韵律环带,c轴则近似平行韵律环带.傅立叶变换红外光谱(FTIR)测量表明,玛瑙含有少量分子水和羟基水,并且总水含量随着硅质矿物结晶度提高而降低.至今实验室仍然无法合成玛瑙,还不清楚玛瑙纹带构造的形成机制和过程.一种代表性假设认为,玛瑙中的纹带源于硅质热液沉淀作用或者硅胶原位结晶作用.但是,最近研究发现玛瑙纹带中共生矿物的结晶度、晶体取向和含水量具有系统差异,揭示其形成过程可能与成岩序列密切相关.将实验岩石学与定量显微结构观察以及多种矿物微区分析测试手段有机结合,深入研究玛瑙环带内部微量元素、水含量和晶体取向数据,获得其系统性变化特征及其内部联系,可以为玛瑙成因机制和形成过程研究提供重要数据支撑和关键性的约束条件.

Agate, universally popular with infinitely variable appearance and color, contains a variety of silica polymorphs, amorphous and nanocrystalline silica, chalcedony and quartz. It is characterized by distinctive banded texture, and usually displays special rhythms with associated repetitive textures and compositions at different scales. Very often agate is simply defined as ＂ banded chalcedony＂, and for most practical purposes this should be sufficient. However, microstructural and spectroscopic observations reveal that most agates, depending on their genesis, are mixed from fibrous chalcedony, wall-banded chalcedony, horizontally-banded chalcedony, microcrystalline quartz and euhedral crystalline quartz. X-ray diffraction and electron backscatter diffraction（ EBSD） analyses indicate that most nanocrystalline and microcrystalline quartz in agate tends to grow with a-axes perpendicular to the growth substrate, typical of length-fast chalcedony. Fourier transform infrared spectroscopy（ FTIR） analysis reveals some molecular water and silanol group water in agate, with lower total water content in crystalline silica than in amorphous silica. Although agate is abundant worldwide, controversy surrounds the genesis of its characteristic banded textures and the consensus has not been achieved. A representative hypothesis suggests that the bands in agate are either from precipitation from siliceous hydrothermal influxes or by in situ crystallization of a silica gel. However, most of the recent literature indicates a viable model for the growth of chalcedony in agate based on diagenetic cycles reflected in the degree of crystallinity, crystallographic orientation and water content. Quantitative information on microstructural observations and micro-scale analyses with petrological experimental results will improve our understanding of the origin and formation processes of agate.