Vacancy pairing and superstructure in the high-pressure silicate K1.5Mg2Si2O7H0.5: a new potential host for potassium in the deep Earth

Abstract

The high-pressure silicate K1.5Mg2Si2O7H0.5, synthesized and characterized by Welchet al.[(2012),Am. Mineral.97, 1849–1857], has been re-examined with the aim of determining the nature of the superstructure noted in their study. The composition corresponds to a 1:1 combination of KMg2Si2O7H and K2Mg2Si2O7end-members, but it is not a solid solution. Single-crystal X-ray diffraction data for one of the original K1.5Mg2Si2O7H0.5crystals synthesized at 16 GPa/1573 K, has been collected using a much longer exposure time in order to improve the intensity statistics of weak superlattice reflections identified by Welchet al.(2012). The superstructure has been determined using a superspace approach as having the superspace groupCmcm(0,β,0)00sandt0= 1/16 with refined parametersa= 8.7623 (10),b= 5.0703 (7),c= 13.2505 (11) Å,V= 588.69 (12) Å3. This structure corresponds to one with the conventional space groupPbnmand unit-cell parametersa= 8.7623 (10),b= 20.281 (3),c= 13.2505 (11) Å,V= 2354.7 (5) Å3and is based upon a super-sheet motif in which ordering involves rows of pairs of vacant interlayer K sites. This is the third topologically distinct structure type for the KMg2Si2O7H−K2Mg2Si2O7join and suggests that there is very limited solid solution, and so it can be expected that each of the three structures (P63cm, P\bar 3 1m andPbnm) has its own stability field, rather than being part of a continuous compositional series based upon a single structure type. As such, K1.5Mg2Si2O7H0.5should be considered as a potentially significant host of K in the Earth's mantle.