The trip to Iceland with a discussion of plate tectonics and mid-oceanic rifts has been postponed due to snow. This week's blog will migrate to some more tropical sedimentary features like these ooids (from the Greek for egg-like). I never wondered why Dr. Allen Curran of Smith College chose to study them in the Bahamas:
and pisoids (from the Greek word for pea-like).
I tried to play the word ooid in a recent Scrabble game; the Official Scrabble Dictionary says it is not a word. Oo id is! Pisoid is also not acceptable~~and it's such a fun word. . .
The main difference between the two sedimentary concretions is their size. Ooids are less than 2 millimeters in size and pisoids are 2 millimeters and greater in size. Ooids and pisoids are spheroidal, layered or coated grains, usually composed of calcium carbonate (CaCO3). Some pisoids and ooids contain iron (siderite for you Latin fans) or phosphates as well. They form as a series of concentric layers (see thin section below--crossed nicols or not--you be the judge!) around a nucleus of a crystal, shell fragment, or other small grain in shallow seas where the water is highly concentrated in calcium carbonate.
And if you use a scanning electron microscope (SEM) to view ooids you will see not only the concretions that mark the growth of ooids, but also the pitting and cracking of the grains caused by various microbes:
Three interesting things you should know about ooids and pisoids:
(1) They have formed under different conditions called calcite seas (where low-magnesium calcite is the primary CaCO3 precipitate and which occurred during my favorite period, the Jurassic, and the Ordovician period) or aragonite seas (where aragonite and high magnesium calcite is the primary CaCO3 precipitate during most of the rest of geologic time, including now). Thus, the calcite seas are found in the early Paleozoic time period when life was relatively new and during the middle of the Mesozoic (sometimes called the Age of Dinosaurs). Both of these time periods were time periods of rapid sea-floor spreading. (Ok, a wee bit of plate tectonics today).
(2) Ooids and pisoids generally require microbial action in their formation.
It had not occurred to me until today that the process of ooid and pisoid concretion is similar to kidney stone formation (It even says so in Wikiipedia :-)). Kidney stones tend to form in patients with concentrated urine (no extra fresh water running in).
Oh, a fourth item:
(4) Ooids and pisoids ought to be Scrabble-acceptable words.
And to bring this concentrated topic concretely back in focus, this view of pisoids shows the highlighted concreted layers that have been colorized to show the structure of the layers.
Looking forward to your crystallized, concentrated comments, diluted not at all by your new-found enthusiasm for the ooid and the pisoid :-)
Ooidally (but not pisoidally this evening),
Word Woman (Scientific Steph)