Abstract The Reflejos de Mar Li-pegmatite, located in northwestern Argentina, is part of the Villismán pegmatite group, Ancasti District, Pampean Pegmatite Province. Four garnet crystals from the outermost part of the pegmatite were analyzed by major and minor elements (SiO 2 , TiO 2 , Al 2 O 3 , Cr 2 O 3 , MgO, CaO, MnO, FeO) using electron microprobe. The pegmatite belongs to the rare-element class, spodumene type, LCT (Li-Cs-Ta) petrogenetic family. Based on their Mn and Fe contents, the analyzed garnet can be assigned to the spessartine-almandine serie. The cores and rims of the analyzed garnets show significant differences for the divalent components in VIII-coordination, especially, Mn and Fe. The average MnO and FeO contents in the cores of the grains is 34.6 and 8.29 wt%, respectively, while in the rims is 29.31 and 12.95 wt%, respectively. The Fe/Mn ratio at the core of the grains is 0.24 while at the rims it is 0.44. Cr 2 O 3 and TiO 2 contents are very low (˂0.17 wt%) and the values of SiO 2 and Al 2 O 3 are ~36 and ~21 wt%, respectively. The mean chemical and molecular formulas of the core can be expressed: [(Mn 2.40 Fe 0.57 Ca 0.02 Mg 0.02 ) 3.01 (Al 1.99 Cr 0.002 ) 2.00 (Si 2.99 Ti 0.01 ) 3.00 O 12 ]; {Sps 79.8 Alm 18.9 Grs 0.7 Prp 0.5 }; and the rim as: [(Mn 2.04 Fe 0.89 Ca 0.05 Mg 0.04 ) 3.02 (Al 2.00 Cr 0.002 ) 2.00 (Si 2.98 Ti 0.005 ) 3.00 O 12 ]; {Sps 67.6 Alm 29.5 Grs 1.6 Prp 1.3 }. The chemical composition of garnet from the Reflejos de Mar pegmatite is similar to other worldwide examples in similar rocks, especially LCT pegmatites, which are highly evolved and associated with Li mineralization. Therefore, its composition could be used as an additional tool in the exploration of Li-bearing pegmatites in the Pampean Pegmatite Province. The differences in Fe-Mn contents between core and rim of the crystals would be controlled by variations in composition of the pegmatitic melt and, in addition, by the simultaneous precipitation of other mineral phases, for example, schorl and Mn-Fe-bearing phosphates.