Incorporation of Fe³⁺ in phase-X, A_2–xM₂Si₂O₇H_x, a potential high-pressure K-rich hydrous silicate in the mantle

Abstract

Phase-X, a potential sink for K in the mantle, is a synthetic hydrous K-rich silicate formed by the breakdown of K-amphibole at high pressure. It has the general formula A2–xM2Si2O7Hx where A = K, Na, Ca, □ (vacancy), and M = Mg, Al, or Cr. No other isomorphic substitutions, either for the A or the M site, were reported for such a synthetic compound. Here we report the crystal structure and chemical composition of a crystal of phase-X containinglarg e amounts of trivalent Fe. This crystal was synthesized in the model system garnet lherzolite–K2CO3 at lower pressure (P = 7 GPa) and higher temperature (T = 1450–1650°C) with respect to the stability range reported in the literature (i.e. P = 9–17 GPa and T = 1150–1400°C). Quantitative analysis led to the following formula: . The lattice parameters (hexagonal setting) are: a = 5.005(1), c = 13.148(2) Å, V = 285.23(9) Å3, and Z = 2. The structure was refined in space group P63cm to R = 5.06% using199 independent reflections and consists of MO6 octahedra layers stacked alongthe c axis and linked together by Si2O7 groups. The Si2O7 groups form pillars in the layer that contains A atoms in the cavities between the pillars. The Raman spectrum in the OH-stretching region indicates the existence of two different OH environments. However, the position of H could not be determined. The substitution of Fe3+ for Mg shortens octahedral bond distances. In addition, the entry of Fe3+ in M induces geometrical changes to the adjacent A site. The crystal-chemical characteristics are compared with published data on synthetic Fe-free phase-X.

Crystal structure of anhydrous phase X, K1.93(Mg2.02Cr0.02)Si2.00O7

“…X -ray diffraction analyses of these phase X's with various compositions (Yang et al, 2001, Mancini et al, 2001, 2002 revealed that both hydrous (x > 0.0) and anhydrous (x = 0.0) phase X's and the hydrous sodic phase X have identical structures with space group P6 3 cm, whereas anhydrous sodic phase X possesses a different structure with space group P3 ￣ 1m. Moreover, Fe (Bindi et al, 2007).…”

Equation of state of Fe3+-bearing phase-X

“…1) is a sheet di-silicate characterized by two tetrahedra sharing a vertex along the c-axis completely extended at room condition (Bindi et al 2007). This configuration is very stable and incompressible, as shown in other structures (e.g.…”

“…Phase-X has a hexagonal structure (Fig. 1), space group P6 3 cm, that consists of di-octahedral sheets (MO 6 octahedra) stacked along the c-axis and linked together by Si 2 O 7 groups which form pillars between the layers where A atoms are contained (Yang et al 2001;Mancini et al 2002;Bindi et al 2007; Matsuzaki et al 2010). In the Fe-free system, phase-X is stable over 1,150-1,400°C and 9-17 GPa (Luth 1997;Inoue et al 1998;Konzett and Fei 2000).…”

“…Despite the long drying before the experiment (15 h), the starting mixtures contained a small uncontrolled amount of water, which resulted in local crystallization of hydrous phase-X. A more detailed explanation is given in Bindi et al (2007). The sample was loaded in a membrane-type DAC equipped with 300 lm diamond culets and a pre-indented rhenium gasket with a hole of 160 lm.…”

“…substitution on the potassium end member through the data collected by high-pressure synchrotron X-ray powder diffraction. Bindi et al (2007), were used for the high-pressure synchrotron X-ray powder diffraction experiments at ID27 beamline (ESRF, Grenoble). The sample was synthesized with the high-pressure toroidal ''anvil-with-hole'' apparatus at 7 GPa and 1,650°C in the model peridotite system with 70 wt% of chemical K 2 CO 3 .…”