Rock Digestion Using an Ammonium Sulfate and Ammonium Bifluoride Mixture for Element Analysis

The new sample preparation method of rock samples for elemental analysis using emission spectral analysis (ICP-AES) and inductively coupled plasma mass spectral analysis (ICP-MS) is proposed. International standard samples of rock composition GM (granite), JA-2 (andesite), BHVO-1 (basalt), and JR-1 (rhyolite) were the material for the study. The major elements detection in terms of oxides TiO₂, Al₂O₃, MnO, CaO, MgO, Fe₂O₃, Na₂O, K₂O, and P₂O₅ was carried out by inductively coupled plasma atomic emission spectrometry (ICP-AES). The ICP-MS method was used to determine the trace element composition of the studied samples: Li, Be, Sc, V Cr, Co, Ni, Cu, Zn, Ga, As, Rb, Sr, Y, Zr, Nb, Mo, Cd, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Pb, Th, U. The interaction of rocks with a bifluoride and ammonium sulfate mixture under heating up to 350°C for 2 h was applied for complete decomposition of minerals. It was found that at the process all components of the investigated samples are completely dissolved in 13% nitric acid. The elemental analysis for trace and major components of the solutions has shown good agreement with the certified characteristics, which indicates the completeness of the transition of the rock components into solution. The interaction mechanism of rock minerals with ammonium bifluoride and ammonium sulfate was investigated. It has been established that the combination of fluorination and sulfation stages allows for the effective decomposition of silicate rocks. This is achieved by breaking Si–O bonds with NH₄HF₂ with the formation of simple and complex fluorides and converting insoluble fluorides of the raw material elements into more soluble sulfates with (NH₄)₂SO₄. It was shown that in the sample preparation process silicon being the main rocks component is sublimated in the form of (NH₄)₂SiF₆ as a result of which a significant mass reduction of dissolved salts was achieved. The advantage of the proposed method, for example, compared to acid decomposition, is its rapidity and completeness of opening due to the reduction in the number of stages in the process of decomposition of rocks.