Note 13 — Chapter I · THE NATURE OF THE MINERALS AND THEIR MODE OF OCCURRENCE

The phrase ‘rare earth minerals’ will be used whenever it is desired to indicate collectively those minerals of which the yttria and ceria earths form an important constituent, as contrasted to those in which only traces of these oxides occur. Such minerals may often contain titanium, zirconium, or thorium, and, for convenience, the term may be taken to include the commoner zirconium and thorium minerals, but not the commoner titanium minerals.