Crystal chemistry of K-tourmalines from the Kumdy-Kol microdiamond deposit, Kokchetav Massif, Kazakhstan

Abstract

Selected crystals of natural K-bearing tourmalines, extracted from a quartzofeldspathic rock from the Kumdy-Kol microdiamond deposit (an ultrahigh-pressure region of Kokchetav Massif, northern Kazakhstan), were characterized using a scanning electron microscope, an electron microprobe and single-crystal X-ray diffraction to investigate the impact of K uptake on the tourmaline structure. All the studied crystals belong to the maruyamaite–oxy-dravite/dravite compositional field, with K contents ranging from 0.03 to 0.47 apfu (atoms per formula unit), and contain a minor fluor-uvite component that increases towards oxy-dravite and dravite. The compositional variability of our samples can be expressed as a sequence of substitutions ranging from maruyamaite to oxy-dravite, dravite and fluor-uvite (or vice versa). Specifically, the substitutions that lead from maruyamaite to oxy-dravite to dravite are (1) XK + AlTOT + O1O ↔ XNa + MgTOT + O1O and (2) XNa + MgTOT + O1O ↔ XNa + MgTOT + O1OH, respectively. Conversely, the substitutions that lead from oxy-dravite to dravite to fluor-uvite are (1) XNa + MgTOT + O1O ↔ XNa + MgTOT + O1OH and (2) XNa + MgTOT + O1OH ↔ XCa + MgTOT + O1F, respectively. By analysing the difference between the bond valence sum and mean formal charge at the X site, we show that an increase in the K content (K > 0.21 apfu) results in the compression of X–O bonds (overbonded cation). Conversely, lower K contents lead to the stretching of the bonds (underbonded cation). Compared to the K-dominant analogues with ZFeO6 povondraite-type framework, K-bearing tourmalines with a smaller ZAlO6 framework such as maruyamaite should only be stable at higher-pressure conditions, as pressure is necessary to squeeze the relatively large K cation into the tighter X cavity. In both cases, the essential condition for the formation of K-dominant tourmalines is the extremely high K activity in the crystallization fluid. The K-tourmaline from the Kokchetav Massif may have crystallized under high-pressure (HP) conditions, with an upper limit between 3.5–7 GPa, during retrograde metamorphism following the ultrahigh-pressure (UHP) metamorphic peak.