title: 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 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 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 Welch et al. (2012). The superstructure has been determined using a superspace approach as having the superspace group Cmcm(0,β,0)00s and t0= 1/16 with refined parameters a = 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 group Pbnm and unit-cell parameters a = 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 and Pbnm) has its own stability field, rather than being part of a continuous compositional series based upon a single structure type. As such, K1.5Mg2Si2O7H0.5should be considered as a potentially significant host of K in the Earth's mantle.
authors.affiliation: Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, England.
authors.forename: M D
authors.initials: MD
authors.lastname: Welch
authors.affiliation: Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, I-50121 Firenze, Italy.
authors.forename: L
authors.initials: L
authors.lastname: Bindi
authors.affiliation: Institute of Physics, ASCR, Cukrovarnicka 10, 162 00 Praha 6, Czech Republic.
authors.forename: V
authors.initials: V
authors.lastname: Petříček
authors.affiliation: Institute of Physics, ASCR, Cukrovarnicka 10, 162 00 Praha 6, Czech Republic.
authors.forename: J
authors.initials: J
authors.lastname: Plášil
keywords: Earth's mantle; high-pressure; hydrous silicate; superstructure