The crystal structure of stichtite, re-examination of barbertonite, and the nature of polytypism in MgCr hydrotalcites

Abstract

The crystal structure of stichtite, re-examination of barbertonite and the nature of polytypism in MgCr hydrotalcites Mills, Stuart J.; Whitfield, Pamela S.; Wilson, Siobhan A.; Woodhouse, Joanne N.; Dipple, Gregory M.; Raudsepp, Mati; Francis, Carl A. American Mineralogist, Volume 96, pages 179-187, 2011 AB S T R A C TStichtite, ideally Mg 6 Cr 2 CO 3 (OH) 16 •4H 2 O, from Stichtite Hill, Tasmania, Australia, and barbertonite, also ideally Mg 6 Cr 2 CO 3 (OH) 16 •4H 2 O, from the Kaapsehoop asbestos mine, South Africa, have been studied by powder X-ray diffraction and their structures have been refined using the Rietveld method. Stichtite from Stichtite Hill crystallizes in the rhombohedral space group R3m, with unitcell parameters a = 3.09575(3) and c = 23.5069(6) Å, V = 195.099(6) Å 3 , with Z = 3/8. Barbertonite from the Kaapsehoop asbestos mine crystallizes in the hexagonal space group P6 3 /mmc. The co-type specimens of barbertonite were found to be intergrown mixtures consisting of barbertonite and stichtite. Unit-cell parameters of barbertonite from the co-type specimens were a = 3.09689(6), c = 15.6193(8) Å, and V = 129.731(8) Å 3 and a = 3.09646(6), c = 15.627(1) Å V = 129.76(1) Å 3 , and Z = ¼. Rietveld refinements of both stichtite and barbertonite show that they are polytypes rather than polymorphs and do not represent distinct mineral species. Several possible nomenclature systems are discussed for the naming of hydrotalcite minerals and groups. Raman band assignments are also presented for stichtite from Stichtite Hill.Stichtite and hydrotalcite minerals make up a large proportion of the ore at the Mount Keith nickel mine in Western Australia. Bulk powder diffraction shows the ore contains 6.1 wt% stichtite and 5.6 wt% iowaite. Hydrotalcite group minerals provide an important potential reservoir of CO 2 . At Mount Keith, the amount of CO 2 mined as stichtite could exceed 45 000 metric tons per year, while exchange of Cl for CO 3 could fix in excess of 40 000 metric tons CO 2 per year if end-member iowaite is reacted to form pyaroaurite.

Dritsite, Li2Al4(OH)12Cl2·3H2O, a New Gibbsite-Based Hydrotalcite Supergroup Mineral

“…Considering the observed low intensity of the band at 1047 cm −1 (Figure 3a), it was assigned to the combination mode (594 + 461 = 1055 cm −1 , which differs from the value of 1047 cm −1 by the magnitude of the anharmonic shift of 8 cm −1 ). The assignment of this band to carbonate groups seems unlikely due to the absence of the v 1 band of the CO 3 2− anion in the spectrum ( [38,39], and at 1062 cm −1 (stronger) and 1046 (weaker) cm −1 for quintinite-2T, Mg 4 Al 2 (OH) 12 CO 3 (H 2 O) 3 [40].…”

“…The arrangement of Cl − anions and H 2 O groups in the interlayer space of dritsite shows statistical disorder ( Figure 5). The general architecture of the interlayer arrangement of dritsite is principally different from that of carbonate hydrotalcite supergroup minerals, namely quintinite, hydrotalcite, pyroaurite, stichtite, takovite, and zaccagnaite [38,[43][44][45][46][47][48][49][50], as shown in Figure 5c,d. In accordance with the approach by Bookin and Drits [23], the interlayer space can be considered as consisting of trigonal prisms formed by H atoms occupying the H1 site of the metal-hydroxide layers (Figure 5e,f).…”

The importance of proper crystal-chemical and geometrical reasoning demonstrated using layered single and double hydroxides

“…166, hexagonal axes), although many other derivative structures have been published that involve various cations and interlayer anions (e.g. Bellotto et al, 1996;Constantino et al, 1998;de Roy et al, 2001;Radha et al, 2007a,b;Johnsen et al, 2010;Manohara et al, 2011;Mills et al, 2011). O1 is the O atom of the octahedral layer and O2 belongs to the interlayer anion or water molecule.…”

Contribution to the crystallography of hydrotalcites: the crystal structures of woodallite and takovite

“…Bish and Brindley (1977) reported the following rhombohedral unit cell parameters (no space group was noted) based on powder diffraction for Serbian takovite: a = 3.0250(1), c = 22.595(3) Å, V = 179.06(2) Å 3 and Western Australian takovite: a = 3.0280(4), c = 22.45(2) Å and V = 178.31(15) Å 3 . Due to the limited information on the structure of the two minerals and their polytypes, and as part of our recent related studies (Mills et al 2011) and the nomenclature of the supergroup (Mills et al 2012), we decided to tackle the structures of both woodallite and takovite. The crystal structure results are reported herein.…”