Rocks containing more than 65% SiO₂ are

Rocks containing more than 65% SiO₂ are acidic.
Rocks with more than 65% of silica are called Felsic or acidic, those with between 55 and 65 percent silica are intermediate, and those with between 45 - 55% silica are mafic (Basic) and those with less than 45% are ultramafic.
Acidic magma solidifies in the crust commonly as granite and erupts as rhyolite lavas. Within an intrusion, crystals grow large within the magma chamber.