Ooids, Pisoids, and Kidney Stones: Concentrate Concretely!

     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.

       (3) They cannot form in areas where a great deal of river runoff occurs as the ooids and pisoids need greatly concentrated waters full of CaCO3 to form. This brings us to the kidney stone tie in:

       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)

 

Petrographic Thin Sections, Deadlines, and Nature's Stained Glass

     Deciding on this week's Partial Ellipsis of the Sun topic was harder than usual. The genetically modified organisms (GMOs) we saw on the western side of Kaua'i, guarded by several people, trucks and barbed wire, was an option. I decided that topic needed more research than driving past miles of corn (in various developmental stages) where sugar cane and pineapples used to grow ten years ago and reading a few articles. I will also read Frankenstein's Cat: Cuddling up to Biotech's Brave New Beasts by Emily Anthes before addressing this topic. (Great suggestion, Jan).

     So...deadlines to deadliness :-), on this Tuesday I decided to write about petrographic thin sections because they are something I know and they are amazing beautiful.

      Here's a piece of olivine, a magnesium iron silicate (called peridot when of gem quality):

      

     Amazingly beautiful, right? How about cutting a nice, clean, thin slice of this and attaching it to a glass slide. And then, how about looking at this thin slice of rock (or thin section) under a petrographic microscope? Still not impressed?

      Well, how about under crossed Nicols filters (see below) under a petrographic microscope, in thin section?

 
      The  two polarizing filters on a petrographic microscope work similarly to polarizing sunglasses. Each filter allows light vibrating at one particular orientation to pass through. One of the two filters is fixed. The other can be moved in and out of the light path. That filter's  polarizing direction is perpendicular to the fixed filter. 

      If you put a thin section of a rock in between the two polarizing filters (referred to as looking at the section under "crossed Nicols") it creates the stunningly beautiful portraits we see above of olivine and below of  zircon: 

     The uncrossed filter photograph of zircon, a nesosilicate, is below:

      
       Here's one more crossed Nicols thin section of basalt, a fast-cooling, fine-grained volcanic rock from Thailand with two large spectacular olivine crystals in the center.

       And, for completeness, a sample of basalt:

           I will end this week's illustration-rich blog with a photo of one of my favorite thin sections of gabbro, coarser-grained (i.e. slower cooling), intrusive igneous rock found near mid-oceanic rift zones (under crossed Nicols):

         The brightly colored orthopyroxene crystals are surrounded by striped plagioclase feldspar. Slower cooling produces some amazing crystals.

          Stained glass: made by nature.

          Enjoy! Next stop, a field trip to Iceland to see the mid-Atlantic ocean ridge on land to see that basalt and gabbro up close!

Petrographically yours,

Word Woman (Scientific Steph)