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.

Attributes

• 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