World of Dinosaurs/Mineralogy/Feldspars

Feldspar is made of oxygen, silica, and lots of little cations, like calcium, potassium, etc.

Here is a 3D model of a feldspar. This is another example of a slightly different feldspar.


  • Feldspar is opaque with a greasy luster.
  • One chemical form is a dusty pink.
  • The other form is a dirty white.

  • It forms as a trapezoid prisms - like cubes, but with a little bit of a lean.
    • You can find big chunks - the size of a golf ball or racketball - while hiking in the mountains.
    • Usually we see feldspar as the opaque bits of granite, as the opaque grains in sand, or as the dull and orange-y bits in sandstone.

  • Feldspar is hard to scratch, hard to dissolve, and will break if a hammer hits it.
    • It has a mix of ionic and covalent bonds.
    • Dissolving or breaking feldspar is easier than dissolving or breaking quartz, but harder than mica.

  • Broken feldspar will keep making smaller and smaller 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.

  • Feldspar chunks can get sloppy edges.
    • Chemical weathering can attack the edges of feldspar chunks.
    • It's easier to dissolve feldspar than quartz.
    • Tiny sand grains made of feldspar mineral might look a little bit more rounded off than their quartz neighbors.

Where to find Feldspar and how it's formed


Feldspar forms naturally without help from life in igneous rocks.

  • It can form in magma that cools slowly, deep within Earth's crust.
    • Examples include granite and diorite.
    • Feldspar crystals are the opaque sections that look like white chocolate or have a pink hue.

Importance in sedimentary rocks


Sandy beaches in California, Oregon, and Washington include lots of quartz grains, and often an equal number of feldspar chunks.

Feldspar forms by:

  • Weathering the original rock where the feldspar formed, possibly an igneous granite;
  • Transporting and breaking the feldspar in a river;
  • Further breaking the feldspar as they shift at a beach.

Feldspar CAN get destroyed by enough weathering and erosion.

  • Extremely "mature" sand will have no feldspar, because it's been weathering and eroding for so long.
  • You can tell how "mature" a sand, or the grains in a sandstone, are by:
    • The minerals present
    • How rounded-off the grains of feldspar and quartz look.
      • More angular chunks means fresher grains, less travel, etc.
      • More rounded-off means longer time or space traveled.

Sand can make SANDSTONE in the right conditions