Nikmelnikovite, Ca12Fe2+Fe3+3Al3(SiO4)6(OH)20, a new mineral from Kovdor massif (Kola peninsula, Russia)

Abstract

Nikmelnikovite, Ca12Fe2+Fe3+3Al3(SiO4)6(OH)20, is a new mineral from the Kovdor massif, Kola peninsula, Russian Federation. It is the first trigonal member of the garnet supergroup. The mineral is named in honor of Academician Nikolay Nikolaevich Melnikov (1938-2018), an outstanding Soviet and Russian mining engineer, long-time (1981-2015) director of the Mining Institute of the Kola Science Center, Russian Academy of Sciences.

Trigonal variation in the garnet supergroup: the crystal structure of nikmelnikovite, Ca12Fe2+Fe3+3Al3(SiO4)6(OH)20, from Kovdor massif, Kola Peninsula, Russia

“…The crystal-chemical formula of nikmelnikovite refined from the crystal structure can be written as Ca 12 (Al 2.79 Fe 3+ 0.21 )(Fe 2+ 0.89 Al 0.11 )(Fe 3+ 1.89 Al 1.11 )(□ 0.882 Mn 0.118 )(SiO 4 ) 6.708 (OH) 17.64 or Ca 12 Al 4.01 Fe 3+ 2.10 Fe 2+ 0.89 Mn 0.118 (SiO 4 ) 6.708 (OH) 17.64 . This formula is in general good agreement with the empirical formula determined by the electron microprobe analysis reported by Krivovichev et al (2019) (see Introduction).…”

“…This latter formulation agrees well with the empirical and idealised formulae of nikmelnikovite (see above; Krivovichev et al , 2019). Note that both equations (2) and (3) are not charge-balanced.…”

“…which results in the essential anion deficiency compared to the "ideal" cubic garnets. For one formula unit, the substitution scheme can be re-written as 6(SiO 4 ) 4- 6(O 3 H 3 ) 3-+ 2(OH) -= 20(OH) -( 3), This latter formulation agrees well with the empirical and idealized formulae of nikmelnikovite (see above; Krivovichev et al 2019). Note that both equations ( 2) and ( 3) are not charge-balanced.…”

“…The chemical composition of the mineral corresponds to the empirical formula Ca 11.81 (Fe 2+ 0.74 Mn 0.12 Mg 0.16 ) Σ1.02 (Al 3.95 Fe 3+ 1.98 ) Σ5.93 [Si 6.14 O 24 ](OH) 20 ⋅1.18H 2 O (Krivovichev et al , 2019); its idealised formula can be written as Ca 12 Fe 2+ Al 4 Fe 3+ 2 [SiO 4 ] 6 (OH) 20 .…”

“…Such a phase was indeed found as thin crusts covering andradite crystals at Kovdor. Its detailed study led to its recognition as a new mineral species, nikmelnikovite, Ca 12 Fe 2+ Fe 3+ 3 Al 3 (SiO 4 ) 6 (OH) 20 , which, in contrast to all known garnet-supergroup minerals, has a trigonal symmetry (Krivovichev et al , 2019). In the present contribution, we provide the results of its crystal-structure determination and describe its relationship to other minerals of the garnet supergroup.…”