How do grain coats prevent formation of quartz overgrowths?

Abstract

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.