Unique Interlayer Research Articles

Pseudojohannite from Jachymov, Musonoi, and La Creusaz: A new member of the zippeite-group

Pseudojohannite is a hydrated copper(II) uranyl sulfate described from Jachymov, Northern Bohemia, Czech Republic (type locality). Pseudojohannite also occurs at the Musonoi quarry near Kolwezi, Shaba, Congo, and the La Creusaz prospect, Western Swiss Alps. At all three localities, pseudojohannite formed through the interaction of acid sulfate mine drainage waters with uraninite (Jachymov and La Creusaz) or uranyl silicates (Musonoi). Pseudojohannite forms moss green, non UV-fluorescent aggregates consisting of irregularly shaped crystals measuring up to 25 μm in length and displaying an excellent cleavage parallel to (101). d meas is 4.31 g/cm 3 , d calc 4.38 g/cm 3 , and the refractive indices are n min = 1.725 and n max = 1.740. A high-resolution synchrotron powder diffraction pattern on the material from Musonoi shows that pseudojohannite is triclinic ( P 1 or P 1), with a = 10.027(1) A, b = 10.822(1) A, c = 13.396(1) A, α = 87.97(1)°, β = 109.20(1)°, γ = 90.89(1)°, V = 1371.9(5) A 3 . The location of the uranium and sulfur atoms in the cell was obtained by direct methods using 1807 reflections extracted from the powder diffractogram. Pseudojohannite contains zippeite-type layers oriented parallel to (101). The empirical chemical formula calculated for a total of 70 O atoms is Cu 6.52 U 7.85 S 4.02 O 70 H 55.74 , leading to the simplified chemical formula Cu 6.5 [(UO 2 ) 4 O 4 (SO 4 ) 2 ] 2 (OH) 5 ·25H 2 O. The distance of 9.16 A between the uranyl-sulfate sheets in pseudojohannite shows that neighboring layers do not share O atoms with the same CuΦ 6 [Φ = (O,OH)] distorted octahedrons, such as in magnesium-zippeite. Rather, it is expected that CuΦ 6 forms a layer bound to the zippeite-type layers by hydrogen bonding, as in marecottite, or one apex of the CuΦ 6 polyhedron only is shared with a zippeite-type layer, as in synthetic SZIPPMg. The higher number of cations in the interlayer of pseudojohannite (Cu:S = 1.6:1) compared to marecottite (3:4) and SZIPPMg (1:1) indicates that pseudojohannite has a unique interlayer topology. Ab-initio powder structure solution techniques can be used to obtain important structural information on complex micro-crystalline minerals such as those found in the weathering environment. Pseudojohannite represents a new member of the zippeite group of minerals, and further illustrates the structural complexity of zippeite-group minerals containing divalent cations, which have diverse arrangements in the interlayer. Peudojohannite and other divalent zippeites are common, easily overlooked minerals in acid drainage environments around uranium deposits and wastes.

STRUCTURE TOPOLOGY AND HYDROGEN BONDING IN MARTHOZITE, Cu2+[(UO2)3(SeO3)2O2](H2O)8, A COMPARISON WITH GUILLEMINITE, Ba[(UO2)3(SeO3)2O2](H2O)3

The crystal structure of marthozite, Cu 2+ [(UO 2 ) 3 (SeO 3 ) 2 O 2 ] (H 2 O) 8 , a 6.9879(4), b 16.4537(10), c 17.2229(10) A, V 1980.2(3) A 3 , Pbn 2 1 , Z = 4, D calc = 4.37 g/cm 3 , has been solved by direct methods and refined to an R index of 5.7% for 4364 observed (| F o | > 4σ F ) reflections collected with a four-circle diffractometer fitted with Mo K α X-radiation and a CCD detector. There are three unique U sites, each occupied by U 6+ with two short U –O uranyl bonds (1.76–1.82 A) and coordination numbers of [8], [7] and [7], respectively. There are two unique Se sites, each occupied by Se 4+ and coordinated by three O atoms ( Se –O ≈ 1.70 A), forming a triangular pyramid with Se at the apex, indicative of stereoactive lone-pair behavior in Se 4+ . There is one Cu site, occupied by Cu 2+ in octahedral coordination by four (H 2 O) groups ( Cu –H 2 O ≈ 2.0 A) and two O atoms ( Cu –O ≈ 2.4 A). The structural unit is a sheet of composition [(UO 2 ) 3 (SeO 3 ) 2 O 2 ], comprised of chains of edge-sharing (Uϕ n ) polyhedra extending along [100] that are cross-linked in the [001] direction by (SeO 3 ) groups; this sheet is topologically identical to the structural unit in guilleminite, Ba [(UO 2 ) 3 (SeO 3 ) 2 O 2 ] (H 2 O) 3 . Adjacent sheets are linked through interstitial Cu 2+ cations via Cu 2+ –O apical bonds and via H bonds that involve both (H 2 O) groups bonded to Cu 2+ and interstitial (H 2 O) groups not bonded to any cation. The principal differences between marthozite and guilleminite involve the interlayer species. In guilleminite, the Ba atom is centrally positioned with respect to the chain of (UO n ) polyhedra, whereas in marthozite, the Cu 2+ atom is located off the pseudo-mirror plane of the chain of (UO n ) polyhedra. This differential positioning of the interstitial cations results in an intersheet separation in marthozite that is nearly 1 A greater than that in guilleminite, despite the fact that Ba (in guilleminite) is much larger than Cu 2+ (in marthozite). The H-bond arrangement in marthozite is very different from that in guilleminite. In marthozite, there are eight unique interlayer (H 2 O) groups; four of these (H 2 O) groups bond directly to Cu 2+ and four are held in the structure solely by H bonds. In guilleminite, there are two unique interlayer (H 2 O) groups, both of which bond to the interlayer Ba atoms.

Structural Factors Controlling Stacking Sequences in Dioctahedral Micas

AbstractDeviations in the symmetry of mica single layers from the ideal arrangement with all atoms in hexagonal arrays are caused by the structural factors such as compositional variation (substitutions), order-disorder, and distortions. The derivative crystal structure theory as applied to mica polymorphism, implies that the ideal symmetry of the mica single layer (C2/m) can be altered to one of its standard subgroups by these structural factors. The “derivative structure” of the single layer may allow a unique interlayer and a specific intralayer configuration and result in a regular stacking sequence.