Coarse-grained igneous rocks (such as granite, diorite, and gabbro) inherently contain feldspar as a primary essential mineral by definition, making this a matter of basic petrology and common geological knowledge that requires no citation.
The claim states that coarse-grained igneous rocks contain measurable amounts of feldspar. Feldspar is the most abundant group of rock-forming minerals in the Earth's crust and is a defining constituent of common phaneritic (coarse-grained) igneous rocks like granites, syenites, diorites, and monzonites. This is a matter of standard scientific definition and everyday petrological observation. Therefore, it is classified as common knowledge, and no citation is needed.