Question: Why are mineral bands in metamorphic rocks light and dark colored? Need Assignment Help? Dark minerals rich in silica and magnesium separate from light minerals rich in iron and aluminum. Dark minerals rich in iron and magnesium separate from light minerals rich in silica and aluminum. Light minerals rich in iron and magnesium separate from dark minerals rich in silica and aluminum. Light minerals rich in iron and silica separate from dark minerals rich in magnesium and aluminum