Symmetry and cation displacements in hollandites: structure refinements of hollandite, cryptomelane and priderite

Abstract

Precise single-crystal X-ray structure refinements of three hollandite-type minerals have allowed a detailed study of the hoUandite structure to be made. The minerals hollandite [(Bao.75Pbo.16Nao.loKo.04)(Mn,Fe, A1)s(O,OH)16], cryptomelane [(Ko.94Nao.25Sro.13-Bao.lo)(Mn,Fe,A1)s(O,OH)16], and priderite [(Ko.9o-Ba0.35) (Ti,Fe,Mg)sO16] were refined to residuals of R = 0.0165 (599 observations, 48 parameters), R = 0.0299 (623 observations, 53 parameters), and R = 0.0096 (316 observations, 29 parameters) respectively. The first two structures are monoclinic (I2/m) and priderite is tetragonal (I4/m). The symmetry of hollandite compounds depends on the ratio of the average ionic radius of the octahedral cations to that of the tunnel cations. Structures in which this ratio is >0.48 distort, reducing the tunnel volume, and thereby lowering the symmetry from tetragonal to monoclinic. The position occupied by a tunnel cation is determined primarily by the size of the cation. Relatively small cations, such as Ba 2+ in priderite and Pb 2+ in hollandite, displace from the special position, 2(a), to more stable sites that are at the sum of the ionic radii from the nearest O atoms. This study also indicates that the reduced form of Mn in hoUandite and cryptomelane is Mn3+; bond lengths calculated from the refinements suggest that Mn 3+ is more easily accommodated in the structures than the larger Mn 2+.

The mineralogical characterization of argentian cryptomelane from Xiangguang Mn–Ag deposit, North China

“…The compositional and structural properties of cryptomelane have been studied in considerable detail by (Bystörm and Bystörm, 1950;Post et al, 1982). It is a typical 2 × 2 tunnel structure manganese oxide, which is composed of edge-sharing double [MnO 6 ] octahedral chains (Post et al, 1982;Lu et al, 2003).…”

“…It is a typical 2 × 2 tunnel structure manganese oxide, which is composed of edge-sharing double [MnO 6 ] octahedral chains (Post et al, 1982;Lu et al, 2003). in the framework can be balanced by the insertion of large monovalent cation-K within the tunnel (Pasero, 2005).…”

Caesium substitution in the titanate hollandites Ba x Cs y (Ti3+ y+2x Ti4+ 8−2x-y )O16 from 5 to 400 K

“…3(a). When Cs is absent the (Ti--O) bond length is determined primarily by the Ti 3 +/Ti 4÷ ratio and the Ba ion in the tunnel has little influence (Post, von Dreele & Buseck, 1982). However, as the Cs level increases, the centres of the walls are compressed by the presence of these ions on either side of the wall and the wall thickness decreases at its centre.…”

An analysis of the structural characteristics of hollandite compounds

“…Ba, Cs) and B are smaller cations occupying octahedral sites such as A13+, Ti 3÷ and Ti 4+ (Kesson, 1983). They may be tetragonal [I4/m, a = 9.90 to 10.10/~ and c=2.9 to 3.0/~ (BystrSm & Bystr/Sm, 1950;Dryden & Wadsley, 1958;Sinclair, McLaughlin & Ringwood, 1980)] or monoclinic [I2/m, fl =90 to 91.5 ° (Cad6e & Verschoor, 1978;Post, Von Dreele & Buseck, 1982)]. The symmetry is usually monoclinic when the ratio of the cation radii RA/RB < 2.08 and tetragonal when this ratio >2.08 although this is by no means a general rule for all hollandites (Post et al, 1982).…”

“…They may be tetragonal [I4/m, a = 9.90 to 10.10/~ and c=2.9 to 3.0/~ (BystrSm & Bystr/Sm, 1950;Dryden & Wadsley, 1958;Sinclair, McLaughlin & Ringwood, 1980)] or monoclinic [I2/m, fl =90 to 91.5 ° (Cad6e & Verschoor, 1978;Post, Von Dreele & Buseck, 1982)]. The symmetry is usually monoclinic when the ratio of the cation radii RA/RB < 2.08 and tetragonal when this ratio >2.08 although this is by no means a general rule for all hollandites (Post et al, 1982). In their analysis of 0108-7681/86/030229-08501.50 radwaste substitution in SYNROC, Kesson (1983) and Kesson & White (1986) have shown that high concentrations of Cs can be immobilized in host hollandites containing Ti 3÷ on the B sites.…”