ABSTRACTMelcherite (IMA2015-018), ideally Ba₂Na₂Mg[Nb₆O₁₉]·6H₂O, occurs as a vug mineral in the carbonatite of the Jacupiranga mine, Cajati county, São Paulo state, Brazil, associated with dolomite, calcite, magnetite, pyrrhotite, tochilinite, ‘pyrochlore’ and fluorapatite. This is also the type locality for zirkelite, quintinite, menezesite and pauloabibite. The mineral forms irregular, tabular crystals up to 200 µm in maximum dimension. Melcherite is transparent and displays a vitreous lustre; it is beige with a white streak. It is non-fluorescent. The mineral displays perfect cleavage on {001}. Chemical composition varies from Ba₂Na₂Mg[Nb₆O₁₉].6H₂O to (BaK)(NaCa)Mg[Nb₆O₁₉].6H₂O. Empirical formulae for the first and the second compositions are: (Ba_1.75K_0.19)_Σ1.94(Na_1.80Ca_0.19)_Σ1.99(Mg_0.96Mn_0.02Al_0.02)_Σ1.00Nb_6.02O_19.00·6H₂O and (Ba_0.99K_1.00)_Σ1.99(Na_1.02Ca_0.96)_Σ1.98(Mg_0.95Mn_0.05)_Σ1.00Nb_6.02O_19.00·6H₂O, respectively. Data for a single crystal with the second composition are: trigonal, R$\bar 3$, a = 9.0117(6) Å, c = 23.3986(16) Å, V = 1645.64(19) ų and Z = 3. Calculated density for this formula is 3.733 g/cm³, and the calculated mean refractive index is 1.924. Melcherite is a hexaniobate that has structural layers parallel to the xy plane that stack along the c axis with simultaneous 1/3 [110] displacement so as to produce an R lattice. The melcherite structure is built by layers of [(Ba,K)(O,H₂O)₉] polyhedra and the [Nb₆O₁₉]⁸⁻ super-octahedron (Lindqvist anion) interconnected by [(Na,Ca)O₆] polyhedra. Cations of Mg²⁺ are bonded to six water molecules each and are not associated with Lindqvist oxygen ions. The mineral is named in honour of Geraldo Conrado Melcher (1924–2011), a pioneer in Jacupiranga carbonatite studies.