Examples from literature

• It contains a large proportion of mica, a small proportion of quartz and still less feldspar, but generally an abundance of iron pyrites in very minute crystals, which, on exposure, are decomposed. 
• It is the metallic base of mica, feldspar, slate, and clay. 
• It was made of black, glassy lava mixed with bits of feldspar. 
• The purest clay found in nature is porcelain clay, or kaolin, which results from the decomposition of a rock composed of feldspar and quartz, and it is almost always mixed with quartz. 
• What little feldspar there was is reduced to quartz and mica, and the quartz pebbles are drawn out into lenses.