Structure of aerugite (Ni_8.5As₃O₁₆) and interrelated arsenate and germanate structural series

Abstract

Aerugite (Nis.5As3Ol6), Mr=979.8, trigonal, R3m, r = 9-8105 (7) of (Ni,Mg)i0Ge3Ol6 is revised to R3m (final R = 0.013). Aerugite is isomorphous with Mgs.sAs30~6 and has a close-packed layer structure ([001]hex stacking direction), with tetrahedral As 5÷ and octahedral As 5+ and Ni 2÷ cations. Charge balance of the structure is achieved by partial occupancy (5/6) of the Ni(1) position in layers of mixed As(1)O 6 and Ni(1)O 6 octahedra. Partial occupancy is associated with local positional disorder about Ni(1) sites but there is no indication of vacancy ordering. The aerugite structure is closely related to the (Ni,Mg)10Ge30~6 structure, and cation charge per single close-packed layer is conserved between them. The aerugite structure may be analyzed in terms of a regular intergrowth of olivine-type and rock-salt-type layers parallel to (011)rh. This suggests the possibility of new structural series of the type M3n+5.sAs2n+lOs(n+l ) (M= Ni, Co, Mg), analogous to that reported earlier for germanate compounds, with aerugite and xanthiosite (Ni3As2Os) corresponding to the end members n = 1 and n = oo, respectively, for the Ni series.

A New Structural Family:M10n−2Ge3n+1O16nwithM=(Co, Mg) and (Ni, Mg)

“…As illustrated in Fig. 5, this parent structure actually corresponds to the M Ge O phase which has been described previously in the cases of the (Ni, Mg) Ge O and Co Ge O compounds (4,5,10). This cubic close-packed structure is closely related to the spinel structure type, consisting of a regular intergrowth of triple +111, spinel layers and single +111, rock-salt layers (Fig.…”

“…5a). The insertion of periodic glide planes parallel to the (011) plane of its rhombohedral unit cell generates the M and M structures as follows: a glide operation on every fifth (011) plane (i.e., n"5) results in the (ch) layer sequence of the M structure, the unit cell of which contains two twin slabs of identical width (5,5) and is, therefore, orthorhombic (Fig. 5b); similarly, a glide operation on every third (011) plane (i.e., n"3) yields the (ch) layer sequence of the M structure with a (3, 3) orthorhombic unit cell (Fig.…”

“…Later, a reinvestigation of the nickel system led to the structure determination of a new germanate, (Ni, Mg) Ge O (3)(4)(5), and its description as a member of a new structural family formulated then as M L> Ge L> O L> (6). The simultaneous presence of tetrahedrally and octahedrally coordinated Ge atoms in these compounds led to the speculation that silicate analogs might form under high-pressure condi-To whom correspondence should be addressed.…”

Co10Ge3O16

“…1, the ColO6, Co306 and Ge204 polyhedra make up a spinel-like triple layer, while the Co206 and GelO6 octahedra make up a rock-salt type single layer. Exactly the same spinelrock salt intergrowth also occurs in (Ni,Mg)10Ge3016 (Fleet & Barbier, 1988), while related cation-deficient structures are formed by the arsenate mineral aerugite, Ni8.sAs3016 (Fleet & Barbier, 1989), its Mg analog (Bless & Kostiner, 1973) and its Co analog (formulated as C08As3016; Krishnamachari & Calvo, 1970).…”

Nonstoichiometric sodium pentaaluminum dodecaaluminogermanate (Na2.90Al5[Al6.16Ge5.84O29.87l): A sechser double-ring structure

“…Germanium and aluminum have very similar mean atomic radii in tetrahedral coordination (c.f., Fleet, 1991) and this promotes greater cation disorder in aluminogermanates compared with comparable aluminosilicates. There are several germanate structures with germanium in both tetrahedral and octahedral coordination (e.g., Fleet and Barbier, 1989;Fleet, 1990). The absence of octahedrally-coordinated germanium in SPDAG could be attributable to a dominant octahedral-site preference for aluminum when competing with germanium.…”