The chemical formula of feldspar is

$\begin {array} {1 1} (a)\;KAlSi_3O_8 & \quad (b)\;Na_3AIF_6 \\ (c)\;NaAIO_2 & \quad (d)\;K_2SO_4.Al_2(SO_4).4Al(OH)_3 \end {array}$

$(a)\;KAlSi_3O_8$