Crystal structure of potassium dimagnesium disilicate hydroxide, K1.3(Mg0.95Al0.03Cr0.02)2Si2O6.4(OH)0.6

Incorporation of Fe3+ in phase-X, A2–xM2Si2O7Hx, a potential high-pressure K-rich hydrous silicate in the mantle

“…2) comprises di-octahedral sheets of MO 6 octahedra stacked along the c axis and linked by layers of Si 2 O 7 groups and A cations, with the Si 2 O 7 groups lying above and below the vacant octahedron. The A atoms are located within the Si 2 O 7 layer in nine-fold coordination to form a distorted trigonal antiprism with an average bond distance of 2.787 Å , a value that is consistent with those observed in previous studies for Fe-free phase-X (Mancini et al, 2001(Mancini et al, , 2002Yang et al, 2001). As pointed out by Mancini et al (2001), the fact that the average AÀO distance is significantly shorter than the sum of the ionic radii (2.95 Å ) could be linked to a contraction due to partial occupancy of the A site.…”

“…The analysis of the systematic absences (hh0l: l = 2n; 000l: l = 2n), together with statistical tests on the distribution of |E| values, lead us to choose P6 3 cm or P6c2 as the acentric space groups. As hydrous phase-X has been solved in space group P6 3 cm (Mancini et al, 2001(Mancini et al, , 2002Yang et al, 2001), this space group was chosen initially for the refinement. The fullmatrix least-squares refinement on F 2 was done using the program SHELXL-97 (Sheldrick, 1997).…”

“…Inspection of the difference-Fourier map revealed that maximum positive and negative peaks were 0.86 and À0.73 e À /Å 3 , at 0.71 and 0.50 Å from the K atom, respectively. Mancini et al (2001) observed large residuals around the A position in the crystal structure of their phase-X, which led them to propose a split A site. In the present study the A site was located at the Wyckoff position 4b and no residuals were observed around this position.…”

“…According to Mancini et al (2002), the general formula of phase-X structures can be written as (K 1ÀxÀn ) 2 (Mg 1Àn [Al,Cr] n ) 2 Si 2 O 7 H 2x (with H 2 O contents ranging from 1.8 to 4.2 wt.%) with the general formula of A 2Àx M 2 Si 2 O 7 H x where A = K, Na, Ca, &, and M = Mg, Al and Cr 3+ . The crystal structure of endmember phase-X with composition K 2 Mg 2 Si 2 O 7 was determined independently by Yang et al (2001) and Mancini et al (2001Mancini et al ( , 2002 in the space group P6 3 cm. The crystal structure consists of di-octahedral sheets of MO 6 octahedra which alternate along the c axis with layers containing di-silicate groups.…”

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).…”