Goethite content in iron ore fines
The 35 percent by mass carve-out that lets iron ore fines be carried under the Group C IRON ORE schedule, and the shipper declaration that it requires.
The IRON ORE FINES schedule carries an exception that moves a cargo out of Group A entirely. Iron ore fines where the total goethite content is 35 percent or more by mass may be carried in accordance with the individual schedule for IRON ORE, which is Group C, provided the master receives from the shipper a declaration of the goethite content determined according to internationally or nationally accepted standard procedures.
The basis is mineralogical: goethite is a hydrated iron oxide whose structure holds water differently from hematite and magnetite, and a goethitic ore drains rather than building pore pressure. The full article will cover the mineralogy, the determination methods, the form of the declaration, and the verification problem a master faces when the whole classification turns on a laboratory figure.