Complex sentence: The titanium minerals are mostly anatase and rutile and zirconium occurs in the mineral zircon in which due to the chemical similarity up to 5% of the zirconium is replaced by hafnium.
Split into simpler sentences: The titanium minerals are mostly anatase and rutile and zirconium occurs in the mineral zircon. Because of the chemical similarity, up to 5% of the zirconium in zircon is replaced by hafnium.