Berthierine-2H1 from Lovozero Alkaline Massif, Kola Peninsula, Russia: First Structure Model for Berthierine and Complexity-Stability Relations

Berthierine was found in a natrolite vein intersecting volcanogenic-sedimentary rocks on the slope of Mt. Quamdespakh in the upper reaches of the Suolwai River, Lovozero alkaline massif, Kola peninsula, Russian Arctic. The mineral occurs as well-formed translucent pyramidal crystals up to 250 μm in size. The chemical composition determined by electron microprobe analysis corresponds to the empirical formula VI(Fe2+1.99Al0.94Mg0.03Mn0.04)Σ3.00[IV(Si1.15Al0.85)Σ2.00O5] [(OH)3.92O0.08]Σ4.00; the idealized formula is VI(Fe2+2Al)[IV(SiAl)O5](OH)4. The crystal-structure determination (the first detailed crystal-structure characterization of berthierine) shows that the Lovozero mineral is hexagonal, P63cm (a = 5.3903(4), c = 14.0146(10) Å, V = 352.64(6) Å3, R1 = 0.053 for 338 unique observed reflections), and corresponds to the 2H1 polytype of serpentine-group minerals with 1:1 tetrahedral-octahedral layers. The unit cell contains two M3[T2O5](OH)4 layers (M = Fe2+,Al; T = Si,Al) stacked along the c axis. The calculations of information-based structural and topological complexity parameters indicate that berthierine is structurally and topologically simpler than its chlorite-group polymorph chamosite. Since berthierine usually crystallizes metastably in the stability field of chamosite, the complexity analysis is agreement with the Goldsmith rule that states that, in Ostwald sequences of crystallization, metastable phases are simpler and more disordered than their stable counterparts. This observation can be applied to a general case of the metastable formation of serpentine-group minerals prior to the crystallization of chlorites.