The recently defined arctite supergroup contains nine mineral members defined as hexagonal intercalated antiperovskites, most of which have been found in pyrometamorphic rocks of the Hatrurim Complex, Israel. Three members of this supergroup: nabimusaite, gazeevite and zadovite, were identified for the first time in altered carbonate–silicate xenoliths from the Caspar and Scherer quarries, Bellerberg volcano in Germany. Present work focuses on the chemical, structural and spectroscopic investigation of these minerals and their correlation with holotype counterparts. The apparent differences are mainly related to the chemical composition, types of substitution in the tetrahedral and antiperovskite layers within the crystal structure, and position of bands in the Raman spectra. In the Bellerberg volcano xenoliths, the crystallisation of nabimusaite and gazeevite is caused by high-temperature alteration of early mineral associations (clinker-like phases) and their reaction with melt or gas generated by volcanic activity. In turn, the formation of zadovite is related to the Ba-rich silicate melt that filled the intergranular space between the rock-forming minerals.

The new mineral nabimusaite, KCa12(SiO4)4(SO4)2O2F (R3m, a = 7.1905(4), c = 41.251(3) Å, V = 1847.1(2) Å3, Z = 3), has been discovered in larnite-ye' elimite nodules of pyrometamorphic rocks of the Hatrurim Complex. Nabimusaite is colourless, transparent with a white streak, has a vitreous lustre and does not show luminescence. It is brittle, but shows pronounced parting and imperfect cleavage along (001). Nabimusaite is uniaxial (–), ω = 1.644(2), ε = 1.640(2) (589 nm), nonpleochroic, Mohs' hardness is ∼5 and the calculated density is 3.119 g cm–3. The crystal structure has been solved and refined to R1 = 0.0416. Its artificial analogue is known. The nabimusaite structure may be derived from that of hatrurite, also known as the clinker phase 'alite' (C3S = Ca3SiO5), and is built up by an intercalation of three positively charged hatrurite-like modules of composition [Ca12(SiO4)4O2F]3+ with inserted modules of [K(SO4)2]3–. The hatrurite-like modules in nabimusaite are characterized by octahedrally coordinated anion sites and tetrahedrally coordinated cation sites. The structure is representative of the intercalated antiperovskite type. In contrast to its synthetic analogue, nabimusaite is P-bearing. The shortened bond T–O lengths for one tetrahedral site indicates P preference at the Si2 site, located at the border of the hatrurite-like modules. Significant variations of isomorphous substitutions in nabimusaite suggest the possibility of other isostructural minerals occurring in Nature. It also seems likely that nabimusaite could serve as a prototype for new advanced synthetic materials, given the discovery of two other new minerals in the Hatrurim Complex with related modular structures, placed in the nabimusaite group. These are zadovite and aradite, as described in a companion paper (Galuskin et al., 2015a).

The mineral assemblage and paragenesis of nabimusaite suggests that nabimusaite formed as a result of the reaction of potassium-enriched, sulfate-bearing melt with larnite and ellestadite. This contradicts the isochemical model that pyrometamorphic rocks of the Hatrurim Complex formed relatively fast in a practically dry system.