This is a silicate mineral, often modified and sometimes part of the movable particles in a formation. Its most common form is potassium.

• A. Quartz
• B. Muscovite
• C. Calcite
• D. Feldspar

Answer: (D)

Feldspar represents a major group of silicate minerals that makes up a large portion of the crust. Within feldspars, the potassium-rich endmember is common, known as potassium feldspar (orthoclase/microcline). This mineral is often described as being modified because it readily alters during chemical weathering into clay minerals, a process that changes its structure and texture in rocks. In sedimentary formations, feldspar grains can be part of movable particles like sand until they weather or are transported away. Quartz is another silicate but does not have potassium as a dominant component, calcite is carbonate, and muscovite is a potassium-bearing mica with a different structure, so the clue about the most common form being potassium specifically points to feldspar.