Merwinite-structured phases as a potential host of alkalis in the upper mantle

Two previously unknown Na- and K-rich phases were synthesized near the solidus of the model CMAS lherzolite interacted with the CaCO 3 + Na 2 CO 3 + KCl melt at 7 GPa. They coexist with forsterite, garnet and chloride–carbonate melt. Stoichiometry and unit-cell parameters measured by means of powder diffraction indicate that one of the phases corresponds to (K,Na) 2 Ca 4 Mg 2 Si 4 O 15 (with about 0.1 a.p.f.u. Al). Although single-crystal X-ray measurements of this phase did not allow the solution of the crystal structure, we suggest that the structure of this phase includes mixed SiO 4 and Si 2 O 7 units. Single-crystal diffraction experiments of the other alkali-rich phase with composition (Ca 2.06 Na 0.86 K 0.08 ) Σ=3.00 (Mg 0.53 Si 0.45 Al 0.03 ) Σ=1.01 Si 2.00 O 8 showed that it exhibits the merwinite structure, space group P 2 1 / a , with lattice parameters a = 12.987(2), b = 5.101(1), c = 9.130(2) Å, β = 92.36(1)°, V = 604.3(2) Å 3 , and Z = 4. The structure was refined to R 1 = 0.031 using 2619 independent reflections. In the structure, Na is hosted at the large Ca sites, whereas Si replaces Mg at the octahedral site and occurs in the usual tetrahedral coordination. Ordering-induced distortion provokes a change in coordination of the (Ca, Na) atoms with respect to pure merwinite. Merwinite phases with lower K + Na contents (0.08–0.18 a.p.f.u.) coexist with forsterite, clinopyroxene and immiscible carbonate–chloride and silicate melts at higher temperatures (up to 1510 °C) at 7 and 5.5 GPa. These phases (including alkali-rich ones at solidus) show a general formula [Ca 3−2 x (Na,K) 2 x ][Mg 1− x Si x ]Si 2 O 8 (with x up to 0.45), where the Na + K content negatively correlates with Ca and positively correlates with Si. The present experimental and crystal-chemical data prove that merwinite-structured phases may be efficient hosts for alkalis in the upper mantle. They are mineralogical indicators of either the interaction of mantle peridotites with alkaline carbonatitic liquids or high-pressure crystallization of silica-undersaturated CaO and alkali-rich melts.