Cyanite (Al2SiO5), quartz (SiO2), and leucite (KAlSi2O6) may be grouped together because they all contain?

The answer is silicon and oxygen. They may be grouped under the largest class of mineral known as silicates. They contain oxygen and silicon which are the two most abundant elements in the earth's crust and it is estimated that they comprise more than 90 percent of the earth's hard outer layer.