Samples Containing Rare Earth Elements

Minerals

The most common minerals are silicates, phosphates, tantalates, and niobates that are summarized as follows:

Silicates - Gadonite (contains Fe, Be, Y and Gd thru Lu plus small amounts of Ce); Cerite (silicates of La, Ce, Pr, Nd, Sm and Eu); Allanite (contains Ca, Fe, Al, La, Ce, Pr, Nd, Sm and Eu).

Phosphates - Monazite (contains mainly phosphates of Ce and La, Pr, Nd, Sm and Eu plus small amounts of the other RE as well as Th).

Tantalates and Niobates - Fergusonite (contains Ta, Nb, Y, Gd thru Lu, Ce and Tb); Euxenite (contains Nb, Ti, Ce, U, Y, and Gd thru Lu); Samarskite (contains Nb, Ta, Y, Fe, Ca, U, and Tb).

Since the RE minerals contain such a wide assortment of other elements, a variety of sample preparation methods exist, including fusions with Na₂O₂, Na₂CO₃, Li₂CO₃ and Li₂B₄O₇, as well as digestions with strong mineral acids such as HF/Perchloric /nitric/hydrochloric acid combinations. Of these fluxes, the use of lithium tetraborate (Li₂B₄O₇) has proven to be a very useful way of opening out minerals associated with the REs.

A procedure reported by Knaack, C., Cornelius, S.B. and Hooper, P.R., GeoAnalytical Lab, Washington State University, December, 1994 involves fusion of the sample (2 grams) with an equal amount of the lithium tetraborate flux in a carbon crucible at 1000 °C in a muffle furnace for 30 minutes. The fuseate is then treated with HF, nitric, and then perchloric acids with a repetition of the perchloric acid treatment to dissolve fluorides. The Final solution is made to 60 mL and contains nitric acid, hydrogen peroxide and HF, where In, Re, and Ru are added as internal standards for an ICP-MS measurement. This method is reported to be applicable for the lanthanides (La thru Lu) together with Ba, Rb, Y, Nb, Cs, Hf, Ta, Pb, Th, Sr, and Zr.

Many analysts prefer acid digestions using mineral acids singularly or in combinations depending upon the mineral involved. For example 1:1 HNO₃ can be used to extract the RE from the sample leaving behind silica and other residues. The extraction is typically performed for 2-4 hours using heat. Both glass and Teflon vessels are commonly used. An acid digestion/extraction is particularly useful if silica is absent. The following procedure has been used at Inorganic Ventures for the preparation of silicate mineral samples for ICP-OES and ICP-MS measurement:

1. Weigh ~ 20 grams of sample into a 500 mL Erlenmeyer flask.
2. Add 100 mL of 1:1 HNO₃.
3. Place flask on a hot plate and bring to a gentle boil for 2 hours.
4. Cool, filter and transfer to a 200 mL volumetric flask and dilute to volume with DI water.

The accuracy of the extraction on specific minerals would need to be confirmed by analyzing the same sample (same aliquot that was extracted) using a complete digestion approach after extraction. This approach is considered better than comparison of independent samples prepared by the two methods based upon studies performed at Inorganic Ventures that suggest that sample homogeneity issues complicate extraction studies where recoveries between 60 and 100% have been found on samples thought to be the same. The measurement technique suggested is ICP-MS.

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