Kenomicrolite, □2Ta2O4(OH)2□, the first vacancy-dominated pyrochlore-supergroup mineral from the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil

Kenomicrolite (IMA 2024-097), ideally □2Ta2[O4(OH)2]□, (where □ = vacancy) is a newly approved mineral species from the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. It is the first member of the pyrochlore supergroup to exhibit dominant vacancies at both A and Y sites and a Ta-dominated B site, making it a tantalum oxyhydroxide. It occurs as an accessory mineral within a microlite group assemblage, probably of secondary origin via weathering-induced leaching and hydration of fluorcalciomicrolite. Kenomicrolite appears as pale orange, transparent, isotropic octahedral crystals up to 200 μm in size, with a calculated density of 5.599 g·cm⁻3 and calculated refractive index of 1.880. Its empirical formula is A(□1.61Ba0.29Ce0.03U0.03Pb0.02Mn0.01Sr0.01)Σ2.00B(Ta1.74Nb0.11Sn0.08Si0.05Al0.01Ti0.01)Σ2.00 X[O4.65(OH)1.35]Σ6.00Y[□0.57(H2O)0.35F0.07K0.01]Σ1.00. The structure has been determined by single-crystal X-ray diffraction in the space group type, with a = 10.5911(6) Å. Spectroscopic analyses confirmed the presence of structural water primarily at the Y site. Ion-exchange experiments in Tl+-rich solutions showed minimal incorporation (∼0.19 Tl+ pfu), highlighting limited ion-exchange capacity due to vacancy-dominated tunnel sites. The structural stability and low hydration make kenomicrolite an important end-member for understanding vacancy-rich pyrochlore systems and their constraints on ion incorporation mechanisms.