Named for having perfect cleavage on one plane and good cleavage on a second plane at 90 degrees to the first, for 10 points, name this most common alkali or potassium feldspar that is often juxtaposed with plagioclase.
Its high-temperature polymorph can contain more sodium, giving a lower value of 2V that can distinguish this mineral from its polymorph sanidine. It can exhibit Manebach or Baveno twinning, and most commonly displays Carlsbad twinning, but its lack of lamellar twinning can distinguish it from its low- temperature polymorph, microcline. This common component of syenite and granite appears just above muscovite on Bowen's reaction series as the first mineral after the continuous branch, which represents the other major type of feldspar. Named for having perfect cleavage on one plane and good cleavage on a second plane at 90 degrees to the first, for 10 points, name this most common alkali or potassium feldspar that is often juxtaposed with plagioclase.
Its high-temperature polymorph can contain more sodium, giving a lower value of 2V that can distinguish this mineral from its polymorph sanidine.
{orthoclase}