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Mica minerals! Biotite is on the left and muscovite is on the right.
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Lisa Gardiner/Windows to the Universe

Mica Minerals

Mica minerals make some rocks sparkle! They are often found in igneous rocks such as granite and metamorphic rocks such as schist. They sparkle because light is reflected on their flat surfaces, which are where the mineral breaks along its plane of cleavage. These minerals break so easily along their cleavage that some crystals have broken into many thin layers that look like the pages of a little book. Colonial Americans used the “pages” of large mica crystals as glass for windows.

The characteristics of the two most common types of mica are listed below. Notice how they are very similar in all ways except for their color? That is how you can tell them apart!

Biotite

  • Shape: Monoclinic . Forms flat plates.
  • Luster: Pearly, metallic
  • Color: Dark brown, dark green or black
  • Streak: White
  • Hardness: 2.5-3 on Mohs Hardness Scale
  • Cleavage: Yes, one plane of cleavage
  • Fracture: The mineral is rather flexible and so it doesn’t fracture very easily. In fact you can bend it very far before it breaks.

Muscovite

  • Shape: Monoclinic. Forms flat plates.
  • Luster: Pearly, metallic
  • Color: Colorless or lightly tinted
  • Streak: White
  • Hardness: 2-3 on Mohs Hardness Scale
  • Cleavage: Yes, one plane of cleavage
  • Fracture: This mineral is also flexible and doesn’t fracture very easily.
(...and learn what shape, luster, color, streak, hardness, cleavage and fracture are all about!)

Last modified January 24, 2008 by Lisa Gardiner.

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TES XXVI, 3 fall 2010 The Fall 2010 issue of The Earth Scientist, focuses on rocks and minerals, including articles on minerals and mining, the use of minerals in society, and rare earth minerals, and includes 3 posters!

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