World of Dinosaurs/Mineralogy/Mica

Mica is made of oxygen silicon and lots of tiny cations like calcium, potassium, etc.

Here is a 3D model of a dark mica. Here is a 3D model of a light mica.

Attributes

• Mica is usually a little bit transparent.

• It has a shine and texture like new plastic.
  • One chemical form makes the sheets almost black, with a little bit of a metallic, bronze look.
  • The other form makes the sheets a yellow color like an old glass bottle.

• It forms sheets when stacked together.
  • Big sheets are hard to find in nature, where they are usually seen in shiny flakes in rocks or sand.
  • Large sheets are easy to buy at stores because they are sold commercially.

• It is easy to break, scratch and dissolve.
  • Like quartz, it is made of silicon and quartz bound with strong covalent bonds.
  • The sheets themselves are only bound with weak ionic bonds.
  • This makes it easy for water to strip the ions out from in between the sheets.

• Mica can erode and weather into little flakes or dust as it travels with sediment and sand.
  • It is easy to see due to its glitter look.
  • It is easy to removed from a natural system by breaking and dissolving it completely.

Where to find Mica and how it's formed

Mica forms naturally without help from life in two main ways:

• It can form crystals from magma
  • Magma that cools slowly, under the surface of the earth, can allow mica sheets to form.
    • Examples include granite and diorite.
    • Mica is the glittery sections in these rocks.
• Mica also forms in metamorphic rocks.
  • If the parent rock has the right atoms, the heat and pressure of the Earth's crust can shift them into mica.
  • Examples include schist and gneiss (pronounced "nice").
    • These rocks are valuable as building materials due to the shimmery look tiny mica sheets give them.

Importance in sedimentary rocks

Sandy beaches in California, Oregon, and Washington include lots of quartz grains, and SOME of these beaches also have lots of tiny mica sheets.

These tiny mica sheets were formed by:

• Weathering the original rock where the mica formed, maybe an igneous granite or metamorphic gneiss.
• Transporting the pieces of mica a long distance, possibly via a river, where they then broke into smaller pieces.
• Further breaking the mica pieces as they shift along the beach.

Mica can be destroyed through enough weathering and erosion.

• "Mature" sand will have no mica, because it has been weathered and eroded so much.
• Sand with mica is "immature" because not all the mica has been destroyed.

Observing mica in sand or sedimentary rocks helps paleontologists determine:

• How long the sand had been moving;
• How far the sand travelled;
• Or how active the depositional environment was.
  • Mica flakes drift easily in water.
  • If there are mica flakes in sandstone the water must have been calm enough to let the mica settle.
  • A lack of mica might mean that the sediment is more mature.
  • A lack might also mean the water was not calm enough for it to settle.

Sand can make sandstone in the right conditions.

Mica is also very common in cosmetics, especially eye shadow, because:

• It is shimmery.
• It occurs naturally so it's cheap to gather and can be marked as "all natural".
• Naturally-occurring dark mica is a good source of rich, dark color.
• Naturally-occurring bright mica is easy to dye brighter colors.
• Mica doesn't react chemically with skin or soak into it.