This study demonstrates that LIBS mapping and spatially resolved local analysis is an efficient and practical approach for the classification of mineral grains (quartz, feldspar, biotite, amphibole) and for prospecting of technologically relevant, low-Z elements (e.g. Be and Li) in granitoid rock samples. We tested three statistical approaches (classification tree (CT) based on indicator elements, linear discriminant analysis (LDA) and random forest (RF)) for the classification of the mineral grains and found that each of the three methods provides fairly similar, very good classification accuracies. RF and LDA provided better than 92% accuracy for all minerals, whereas CT showed a somewhat poorer (around 80%) accuracy for quartz in particular. Our results also demonstrate that using multiple analytical locations within each grain and resting the classification on the majority vote of these individual analysis gives more reliable discrimination (grain-based accuracy is better than location-based accuracy). We also demonstrated that LIBS elemental mapping can provide valuable information about the distribution of chemical elements among the minerals, especially if it is combined with matrix-matched calibration of emission intensity data. We illustrated this by the successful assessment of ng to μg amounts of Be and Li in the studied mineral grains. Our results suggest that mining for Be and Li in granitoid rocks should be aiming for biotite and amphibole grains.

Red mud is a by-product of alumina production containing lanthanides. Growth of green microalgae on red mud and the intracellular accumulation of lanthanides was tested. The best growing species was Desmodesmus quadricauda (2.71 cell number doublings/day), which accumulated lanthanides to the highest level (27.3 mg/kg/day), if compared with Chlamydomonas reinhardtii and Parachlorella kessleri (2.50, 2.37 cell number doublings and 24.5, 12.5 mg/kg per day, respectively). With increasing concentrations of red mud, the growth rate decreased (2.71, 2.62, 2.43 cell number doublings/day) due to increased shadowing of cells by undissolved red mud particles. The accumulated lanthanide content, however, increased in the most efficient alga Desmodesmus quadricauda within 2 days from zero in red-mud free culture to 12.4, 39.0, 54.5 mg/kg of dry mass at red mud concentrations of 0.03, 0.05 and 0.1%, respectively. Red mud alleviated the metal starvation caused by cultivation in incomplete nutrient medium without added microelements. Moreover, the proportion of lanthanides in algae grown in red mud were about 250, 138, 117% higher than in culture grown in complete nutrient medium at red mud concentrations of 0.03, 0.05, 0.1%. Thus, green algae are prospective vehicles for bio-mining or bio-leaching of lanthanides from red mud.