The Greek word for egg is òoion. Hence something that is egg-shaped – such as an egg, or an egg-shaped sand grain – can be described as an ooid; pronounce it as oh-oid. All (except one) of the ooids above are sand grains from the Bahamas where sweeping and undulating tapestries of sand banks are composed of such grains:
Being somewhat lazy (and slightly busy), I’ll simply quote the good description of ooids from Wikipedia:
Ooids are small (< 2 mm in diameter), spheroidal, "coated" (layered) sedimentary grains, usually composed of calcium carbonate, but sometimes made up of iron- or phosphate-based minerals. Ooids usually form on the sea floor, most commonly in shallow tropical seas (around the Bahamas, for example, or in the Persian Gulf). After being buried under additional sediment, these ooid grains can be cemented together to form a sedimentary rock called an oolite. Oolites usually consist of calcium carbonate meaning they belong to the limestone rock family. Pisoids are similar to ooids, but are larger than 2 mm in diameter, often considerably larger, as with the pisoids in the hot springs at Carlsbad (Karlovy Vary) in the Czech Republic.
An ooid forms as a series of concentric layers around a nucleus. The layers contain crystals arranged radially, tangentially or randomly. The nucleus can be a shell fragment, quartz grain or any other small fragment. Most modern ooids are aragonite (a polymorph of calcium carbonate); some are composed of high-magnesium calcite, and some are bimineralic (layers of calcite and aragonite). Ancient ooids can be calcitic, either originally precipitated as calcite (as in calcite seas), or formed by alteration (neomorphic replacement) of aragonitic ooids (or the aragonite layers in originally bimineralic ooids). Moldic ooids (or molds later filled in by calcite cement) occur in both young and ancient rocks, indicating the removal of a soluble polymorph (usually aragonite).
The article includes some fine images that include ancient examples of ooids in an oolite from the balmy tropical and carbonate-rich seas that covered Utah around 170 million years ago. A close examination of the surface of the rock reveals that it is made up almost entirely of little sand-sized balls, and make a slice of this rock thin enough for light to pass through and scrutinise it down a microscope and the exquisite structure of these grains is revealed:
Ooids with radial crystals (such as the aragonitic ooids in the Great Salt Lake, Utah, USA) grow by ions extending the lattices of the radial crystals. The mode of growth of ooids with tangential (usually minute needle-like) crystals is less clear. They may be accumulated in a "snowball" fashion from tiny crystals in the sediment or water, or they may crystallize in place on the ooid surface. A hypothesis of growth by accretion (like a snowball) from the polymineralic sediment of fine aragonite, high-magnesium calcite (HMC) and low-magnesium calcite (LMC), must explain how only aragonite needles are added to the ooid cortex. Both in tangential and in radial ooids, the cortex is composed of many very fine increments of growth. Some modern (and ancient) ooids partially or totally lack clear layering and have a micritic (very fine grained) texture. Examination of such micritic ooids by scanning electron microscopy often shows evidence of microbial borings later filled by fine cement.
But enough of aragonitic ooids and micritic textures. There’s a bit of romance in the word, a rolling-off-the-tongue satisfaction to it. Mark Twain certainly encountered and enjoyed the word, and wrote, in Life on the Mississippi:
In the space of one hundred and seventy-six years the Lower Mississippi has shortened itself two hundred and forty-two miles. That is an average of a trifle over one mile and a third per year. Therefore, any calm person, who is not blind or idiotic, can see that in the Old Oolitic Silurian Period, just a million years ago next November, the Lower Mississippi River was upwards of one million three hundred thousand miles long, and stuck out over the Gulf of Mexico like a fishing-rod. And by the same token any person can see that seven hundred and forty-two years from now the Lower Mississippi will be only a mile and three-quarters long, and Cairo and New Orleans will have joined their streets together, and be plodding comfortably along under a single mayor and a mutual board of aldermen. There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
Now, as was often the case with his geological facts, Twain got things slightly wrong (the age of the “Old Oolitic Silurian Period” is around 400 million years) but no matter – ooids have taken us from the Bahamas to the nature of science via Utah – happy Easter!
[Bahamas image thanks to NASA; images of the Carmel oolite and Bahamas ooids from Mark A. Wilson, College of Wooster, via Wikimedia Commons]