Chiral edge-shared octahedral chains in liskeardite, [(Al,Fe)₃₂(AsO₄)₁₈(OH)₄₂(H₂O)₂₂]·52H₂O, an open framework mineral with a pharmacoalumite-related structure

Abstract

The type specimen of liskeardite, (Al, Fe)3AsO4(OH)6·5H2O, from the Marke Valley Mine, Liskeard District, Cornwall, has been reinvestigated. The revised composition from electron microprobe analyses and structure refinement is [Al29.2Fe2.8(AsO4)18(OH)42(H2O)22]·52H2O. The crystal structure was determined using synchrotron data collected on a 2 μm diameter fibre at 100 K. Liskeardite has monoclinic symmetry, space group I2, with the unit-cell parameters a = 24.576(5), b = 7.754(2) Å, c = 24.641(5) Å, and β = 90.19(1)º. The structure was refined to R = 0.059 for 9769 reflections with I > 3σ(I). It is of an open framework type in which intersecting polyhedral slabs parallel to (101) and (10) form 17.4 Å × 17.4 Å channels along [010], with water molecules occupying the channels. Small amounts (<1 wt.%) of Na, K and Cu are probably adsorbed at the channel walls The framework comprises columns of pharmacoalumite-type, intergrown with chiral chains of six cis edge-shared octahedra. It can be described in terms of cubic close packing, with vacancies at both the anion and cation sites. The compositional and structural relationships between liskeardite and pharmacoalumite are discussed and a possible mechanism for liskeardite formation is presented.

Ultra-flexible framework breathing in response to dehydration in liskeardite, [(Al,Fe)16(AsO4)9(OH)21(H2O)11]·26H2O, a natural open-framework compound

“…It is in the form of small white to pale green fibrous clumps of very thin needles. We have previously reported electron microprobe analyses and a crystal structure determination of liskeardite from the type locality [20]. A quantitative XRD phase analysis gave 2 wt% of pharmacoalumite as the main impurity.…”

“…Although the mineral liskeardite was discovered over 130 years ago [19], the structure was only determined recently [20]. It has monoclinic symmetry, space group I2, but is strongly pseudo-tetragonal, with aEcE24.7 Å, bE7.8 Å, and βE901.…”

“…Structural parameters for monoclinic liskeardite were taken from the single crystal refinement of liskeardite [20]. Due to the data limitations and the very large structure (62 independent nonhydrogen atoms) the structural parameters were fixed at the single crystal values for refinement of the heated samples.…”

Galeaclolusite, [Al6(AsO4)3(OH)9(H2O)4]⋅8H2O, a new bulachite-related mineral from Cap Garonne, France

“…The layers contain [001] spiral chains of edge-shared octahedra, decorated with corner connected AsO 4 tetrahedra, that are the same as in the mineral liskeardite (Grey et al, 2013).…”

M3+(H2AsO4)(H2As2O7) (M3+= Al, Ga) and In2(H2AsO4)2(H2As2O7)2: a new layer structure type and a new framework structure type containing the rare H2As2O72−group

“…Further water-containing Fe 3+ arsenates are Fe(H 2 AsO 4 ) 3 -(H 2 O) 5 (mineral kaatialaite), which also exhibits a diprotonated arsenate group (Boudjada & Guitel, 1981), the mineral hydroniumpharmacosiderite, (H 3 O)Fe 4 3+ (AsO 4 ) 3 (OH) 4 Á4H 2 O (Mills et al, 2010), FeAsO 4 Á0.75H 2 O (Jakeman et al, 1991), and the mineral parascorodite, FeAsO 4 Á2H 2 O (Perchiazzi et al, 2004). Hydrous Al arsenates include the minerals mansfieldite (already mentioned above), bulachite, Al 2 AsO 4 -(OH) 3 Á3H 2 O (Frau & Da Pelo, 2001; structure unknown), (Grey et al, 2016;Grey, Kampf et al, 2015), as well as liskeardite, (Al,Fe) 32 (AsO 4 ) 18 (OH) 42 -(H 2 O) 22 Á52H 2 O (Grey et al, 2013). A structurally characterized dehydration product of liskeardite has the formula [Al 16 -(AsO 4 ) 9 (OH) 21 (H 2 O) 11 ]Á8H 2 O (Grey, Brand et al, 2015).…”