颗粒涂层如何防止石英过度生长?

IF 2 4区 地球科学 Q1 GEOLOGY
O. Walderhaug, Kristin W. Porten
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引用次数: 1

摘要

通过扫描电子显微镜对挪威大陆架深埋砂岩中绿泥石、伊利石、碎屑粘土、微石英和菱铁矿的颗粒涂层进行检查,结果表明,颗粒涂层下方的石英表面覆盖着由平面晶面界定的微小石英生长物。这些非常小的自形石英生长物也出现在涂层的间隙中,在这些间隙中没有物理屏障阻碍它们继续生长到相邻的大孔中,但只有在涂层间隙大于约5μm的情况下才能观察到这种向外生长和扩展到晶间孔隙中。我们认为,小于一定尺寸的自形石英无法通过晶粒涂层生长并形成大的孔隙填充石英过度生长,这是微米尺寸晶体与较大晶体相比溶解度增加的结果。由于表面能效应,矿物中最小的晶体无法在不阻止同一矿物中较大晶体生长的条件下生长。这是一种普遍的热力学效应,对于具有大表面体积比的微小晶体来说变得很重要,并通过奥斯特瓦尔德-弗伦德里希方程定量表达。微观生长物可以首先在石英颗粒表面上形成的原因可能是初始的预自形生长阶段能够在比自形生长物略低的二氧化硅过饱和度下生长。因此,在大多数砂岩中普遍存在的低过饱和度下的持续生长可能取决于生长物在大于临界尺寸后达到自形形状。在小于约5μm的晶粒涂层间隙中成核的生长物在达到临界尺寸之前形成平面晶面,因为在这些较小的间隙中没有足够的空间形成较大的自形生长物。因此,在最小开口中成核的生长物不能生长穿过晶粒涂层并到达相邻的孔隙空间,尽管可以自由进入相邻的大孔。因此,颗粒涂层可以防止石英胶结,而不会在微观尺度上连续。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
How do grain coats prevent formation of quartz overgrowths?
Examination of grain coats of chlorite, illite, detrital clay, microquartz, and siderite in deeply buried sandstones from the Norwegian continental shelf by scanning electron microscopy shows that the quartz surfaces beneath the grain coats are covered by tiny quartz outgrowths bounded by planar crystal faces. These very small euhedral quartz outgrowths also occur in gaps in the coats where there is no physical barrier to impede their continued growth into adjacent macropores, but such outward growth and expansion into the intergranular pore space was observed only where the gaps in the coats are larger than around 5 μm. We suggest that the inability of euhedral quartz outgrowths smaller than a certain size to grow through grain coats and form large pore-filling quartz overgrowths is a consequence of the increased solubility of micron-sized crystals compared to larger crystals. Due to surface energy effects, the smallest crystals of a mineral are unable to grow at conditions that do not prevent growth of larger crystals of the same mineral. This is a general thermodynamic effect that becomes important for tiny crystals with a large surface-to-volume ratio and is expressed quantitatively by the Ostwald-Freundlich equation. The reason microscopic outgrowths can develop on the quartz grain surfaces in the first place is probably that the initial pre-euhedral growth stages are able to grow at slightly lower silica supersaturations than euhedral outgrowths. Continued growth at the low supersaturations prevalent in most sandstones may consequently depend upon the outgrowths reaching a euhedral shape after they are larger than a critical size. Outgrowths nucleated in gaps in the grain coats smaller than around 5 μm develop planar crystal faces before they have attained the critical size because there is not enough space for larger euhedral outgrowths to form in these smaller gaps. The outgrowths nucleated in the smallest openings are therefore unable to grow through the grain coats and reach the adjacent pore space despite free access to the adjacent macropores. Grain coats can therefore prevent quartz cementation without being continuous at the microscopic scale.
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来源期刊
CiteScore
3.80
自引率
5.00%
发文量
50
审稿时长
3 months
期刊介绍: The journal is broad and international in scope and welcomes contributions that further the fundamental understanding of sedimentary processes, the origin of sedimentary deposits, the workings of sedimentary systems, and the records of earth history contained within sedimentary rocks.
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