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

The high-pressure silicate K1.5Mg2Si2O7H0.5, synthesized and characterized by Welch et 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 andK2Mg2Si2O7 end-members, but it is not a solid solution. Single-crystal X-ray diffraction data for one of the original K1.5Mg2Si2O7H0.5 crystals 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 Welch et al. (2012). The superstructure has been determined using a superspace approach as having the superspace group Cmcm(0,,0)00s and t0 = 1/16 with refined parameters a = 8.7623 (10), b = 5.0703 (7), c = 13.2505 (11) A ° , V = 588.69 (12) A ° 3. This structure corresponds to one with the conventional space group Pbnm and unit-cell parameters a = 8.7623 (10), b = 20.281 (3), c = 13.2505 (11) A ° , V= 2354.7 (5) A ° 3 and 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 KMg2Si2O7HK2Mg2Si2O7 join and suggests that there is very limited solid solution, and so it can be expected that each of the three structures (P63cm, P331m and Pbnm) has its own stability field, rather than being part of a continuous compositional series based upon a single structure type. As such, K1.5Mg2Si2O7H0.5 should be considered as a potentially significant host of K in the Earth’s mantle.