Rare earth element analyses

The rare earth element (REE) contents of the samples were determined with an ELEMENT XR Inductively Coupled Plasma Mass Spectrometer (ICP‒MS) operated at an ambient temperature of 23.5 °C and relative humidity of 41%.

The analytical procedure consisted of weighing 25 mg (up to 0.01 mg) of the powdered sample (< 74 µm in size), putting it into a Teflon vessel insert, and then dissolving it with approximately 1 mL of HF (1.16 g/mL) and approximately 0.5 mL of nitric acid (1.42 g/mL). The insert was sealed, put into an oven and heated to 185 ± 5 °C for 24 h. The insert was removed from the oven, cooled and heated nearly to dryness on an electric hot plate. Nitric acid (0.5 mL, 1.42 g/mL) was added to the tank and allowed to evaporate until dry. This step was repeated after 5 mL of nitric acid (1 + 1) was added to the insert and sealed. The samples were placed into the oven and heated at 130 °C for 3 h. The insert was removed from the oven, and the solution was transferred to a 25 mL plastic bottle and diluted to 25 mL with deionized water; it was shaken and prepared for ICP‒MS measurements.