A Few Words About Ooids and Oolites

OOIDS are a distinctive type of sand that usually form on the sea floor. After they have been buried under additional sediment, such sand grains can be fused into a solid mass of sedimentary rock known as an OOLITE. The unusual name reflects the fact that grains of oolitic sand are highly reminiscent of fish eggs or roe when examined closely. Most ooids consist of the chemical compound CaCO3 (calcium carbonate), so most oolites belong to the group of rocks known as LIMESTONES.

Most types of sand are created through the breakdown of pre-existing rocks and crystals. Sandstones, for example, consist largely of quartz, feldspar, and fragments of other minerals typically found in granites and the other types of bedrock that make up our continents. Such minerals are rich in the element silicon, so they are referred to as silicates, but even most limestones consist largely of fragments of pre-existing carbonate materials. Much of the material in limestone is derived from the break-up of the calcareous hard parts of organisms such as clam shells and coral sticks, and more is derived from the erosion of previously deposited muds. Ooids, however, are something different. 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. Whether physical and/or chemical in nature, it appears that microbes play an important role in the growth of ooids, and they tend to form where there are strong waves and/or currents because ooids need to keep on moving in order to keep growing.

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. Like oolites, most of the Indiana limestones consist of well-sorted sands, but a high percentage of the sand grains in the Indiana limestones consist of fossil fragments, often with a thin or superficial coating like ooids, but not with the thick concentric and/or radial coating typical of true ooids. And even though most ooids consist of calcium carbonate, ooids can also form from other chemical compounds under unusual circumstances. For example, ooids made entirely of the mineral halite (table salt) can form where waves roll sand grains in waters of unusually high salinity, such as along the shores of the Dead Sea in Israel. Ooids can even form out of more exotic minerals. At certain times during earth’s prehistory, oxygen has been less plentiful in parts of the ocean than it is today, and ooids consisting of iron-rich minerals such as hematite and chamosite sometimes formed at such times and places. Even kidney stones are essentially a type of ooids – most of them are also rich in calcium, and when you cut them open, they show the same type of growth structures as ooids formed on the seafloor!

Dr. Bruce Simonson
Geology Department
Oberlin College

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