Vacancy pairing and superstructure in the high‐pressure silicate K 1.5 Mg 2 Si 2 O 7 H 0.5 : a new potential host for potassium in the deep Earth

The high‐pressure silicate K 1.5 Mg 2 Si 2 O 7 H 0.5 , 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 KMg 2 Si 2 O 7 H and K 2 Mg 2 Si 2 O 7 end‐members, but it is not a solid solution. Single‐crystal X‐ray diffraction data for one of the original K 1.5 Mg 2 Si 2 O 7 H 0.5 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,β,0)00 s and t 0 = 1/16 with refined parameters a = 8.7623 (10), b = 5.0703 (7), c = 13.2505 (11) Å, V = 588.69 (12) Å 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) Å, V = 2354.7 (5) Å 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 KMg 2 Si 2 O 7 H−K 2 Mg 2 Si 2 O 7 join and suggests that there is very limited solid solution, and so it can be expected that each of the three structures ( P 6 3 cm , and Pbnm ) has its own stability field, rather than being part of a continuous compositional series based upon a single structure type. As such, K 1.5 Mg 2 Si 2 O 7 H 0.5 should be considered as a potentially significant host of K in the Earth's mantle.