World of Dinosaurs/Mineralogy/Carbonates

The two most common carbonate minerals are calcite and aragonite.

Calcite is made of oxygen, carbon, and calcium.

Here is a 3D model of shells made of organic carbonates. Here is a 3D model of inorganic calcite.

Attributes

• Calcite can be clear or opaque.
• It can be bright white or a range of dusty warm colors (pink, orange, etc.)

It forms as trapezoid prisms - like cubes, but with a distinct lean.

• You can find big chunks of calcite in the desert mountains around Utah.
• You can spot cracks in rocks that are filled with calcite mineral pretty much all over the place in Utah.

• Calcite is easy to scratch and dissolve, and will break if hit with a hammer.
  • Calcite is made and destroyed by ionic bonding action!
    • One ingredient is a carbonate ion.
      • It's a covalent-bonded meatball of 1 carbon and 3 oxygens
      • Together, this meatball is sharing TWO extra electrons that they stole from some hapless hydrogen atoms.
      • The extra electrons give the whole meatball a negative charge, so we call it an ion (a charged particle).
    • The other ingredient is a calcite ion.
      • This is just one atom of calcium,
      • but it's missing TWO electrons because some other atom or molecule or mineral tore them off.
      • By keeping its protons and neutrons in the nucleus,
        • the calcium is still acting like calcium,
        • but the protons now outmatch the electrons.
        • So the calcium now has a POSITIVE charge of +2,
        • and we call it an ion.
    • Due to the equal but opposite charges the carbonate ion and calcite ion attract each other to form calcite.

• Broken calcite will usually keep making little trapezoid prisms!
  • This is because there are lines of weakness formed by ionic bonds.
  • The weak planes break, leaving crisp sides to the little baby trapezoids.
  • Calcite chunks can get sloppy edges.
    • Chemical weathering peels ions off the edges and corners most easily.
    • If you put acid on calcite, it will fizz.
      • This is why you can clean your sink or humidifier with vinegar to remove crusty build-up from tap water.
      • The fizzing happens because the calcium ions get stripped off, the carbonate ions interact with water, and carbon dioxide goes zipping into gas phase as bubbles.

Calcite can dissolve completely, but the ions can also travel in water and rebuild the mineral some place else!

Where to find Calcite and how it forms

Calcite forms naturally without help from life in sedimentary environments.

• Ions of carbonate and of calcite can travel in water, then join and form calcite wherever they reach a high enough concentration.
  • Examples include:
    • Caves
    • In between grains of sand in a sandstone
    • cracks in other rocks

However, life is usually around when calcite is formed.

Sometimes life is definitely the thing making the calcite.

• Most "seashells" are made of carbonate minerals, calcite and/or aragonite.
• Clams build shells out of the carbonate and calcium ions they harvest from seawater.
• Land snails build shells from the carbonate and calcium ions they harvest from water, soil, air, and their food.
• Hermit crabs STEAL shells from dead snails, but the crab exoskeleton is crunchy because it, too, harvests carbonate and calcium ions from seawater!
• Lots of microscopic drifting sea plankton make tiny carbonate frisbees that build up on the sea floor.

Other times life is sort of around and it can be difficult to tell what is happening

• Calcite forms layers, piles, spheres, or goo in watery places.
• These places are usually teaming with bacteria, cyanobacteria, and various gross stuff.
• It's often hard to tell whether the microbes are making the mineral form, or they're just enjoying the show.
  • Examples:
    • The Great Salt Lake has big crunchy domes, and sand made of tiny calcite spheres called "ooids".
    • Mono Lake exposes big spooky tufa towers made of low-density calcite from when the lake used to be deeper.
    • Any chunks of coral reef or broken shell sand might have carbonate goo forming in the cracks.

Aragonite is probably ALWAYS made by life when it forms on the Earth's surface. It will only form without life if the chemistry and physics are different, like in metamorphic rocks.

Importance in sedimentary rocks

Sandy beaches in Hawai'i and Florida include lots of carbonate, usually bits of broken sea shells and coral.

Carbonate sand grains can form by:

• animals make the mineral for their shell;
• shells get broken by waves;
• shell pieces get more broken as they shift along the beach.

LIMESTONE is what we call rocks mostly made of carbonate, whether they were sandy or gooey originally.

Limestone can metamorphose into MARBLE.