How is oolitic sand formed

Unlike these grains that are created through the abrasion and diminution of pre-existing materials, ooids actually form by growing larger and accreting material as they move around. They accomplish this either 1 by the physical attachment of fine-grained material as they roll around, much the way a ball of snow grows when you roll it around to make a snowman, and 2 by the chemical precipitation of material out of solution, much like salt crystallizes from water when you evaporate it.

Where ooids grow through physical accretion, they tend to have thin concentric layers analogous to tiny tree rings, except they are spherical instead of cylindrical. In contrast, ooids that grow by chemical precipitation are more likely to have radiating sprays of crystals arranged like the spokes in a bicycle wheel; but many ooids grow via a combination of these two processes and show both concentric and radial structures internally.

The limestones in Indiana that have been quarried for over a century now are often referred to as oolitic, but few of them are really good examples of this rock type. While some scientists suspect that ooid growth is driven by the chemical process of minerals precipitating out of the water, new research led by geologist Bob Burne of Australian National University adds evidence to a competing hypothesis that microbes are at the root of the process.

Ooids are eggshell white and spherical. The word ooid is derived from the Greek word for egg, and up close the oolitic sand looks like an endless sea of tiny eggs. Beneath the polished surface of each ooid are concentric layers of calcium carbonate that look strikingly similar to tree rings when viewed under a microscope.

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