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Orthoclase feldspar
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Feldspar

Feldspar is the most common mineral in the Earth’s crust, so you are very likely to find it in the rocks you collect! It is found it all of the three rock types, but is most common in intrusive igneous rocks like granite where the crystals look white or pink.

There are several types of feldspar. The characteristics of the two most common types are listed below. These two common types of feldspar are difficult to tell apart besides their color. Color can be helpful, but beware because the same mineral can often have different colors. The sure way to tell these two apart is by looking at the crystal surfaces for thin parallel groves called striations. Plagioclase feldspar has striations but orthoclase feldspar does not.

Orthoclase

  • Shape: Monoclinic (Flat tabular or prism-shaped crystals)
  • Luster: Glassy or pearly
  • Color: Cream to pink
  • Streak: White
  • Hardness: 6 on Mohs Hardness Scale
  • Cleavage: Yes
  • Fracture: Conchoidal or brittle

Plagioclase

  • Shape: Triclinic (Single prism-shaped crystals are very rare. You are much more likely to find many crystals that have grown together in a mass.
  • Luster: Glassy or pearly
  • Color: White to gray
  • Streak: White
  • Hardness: 6-6.5 on Mohs Hardness Scale
  • Cleavage: Yes
  • Fracture: Conchoidal or brittle

Last modified April 25, 2003 by Lisa Gardiner.

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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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