When aluminum replaces up to half of the silicon atoms in SiO₂, a mineral class called feldspars result. The feldspars are the most abundant rock-forming minerals, comprising about 50% of the minerals in Earth’s crust. Orthoclase is a feldspar in which Al replaces one-fourth of the Si atoms of SiO₂, and charge balance is completed by K⁺ ions. Determine the chemical formula for orthoclase.