Ashcroftine, ca. K ₁₀ Na ₁₀ (Y,Ca) ₂₄ (OH) ₄ (CO ₃ ) ₁₆ (Si ₅₆ O ₁₄₀ ).16H ₂ O, a structure with enormous polyanions Available to Purchase

The elephantine crystal structure of ashcroftine, end member ca. K₁₀Na₁₀(Y,Ca)_24-(OH)₄(CO₃)₁₆(Si₅₆O_l40)⋅16H₂O has been independently investigated (Moore, Sen Gupta, Schlemper = MSS; Merlino = M) in two laboratories. Type material from Narsarsuk, Greenland, afforded crystals for both studies. The ashcroftine average structure is tetragonal holosymmetric, space group I4/m 2/m 2/m, a = 23.994(6), c = 17.512(5) (MSS) and a = 24.039(6), c = 17.538(8) (M), Z = 2.R = 0.058 for 1774 independent F₀ values (MSS) and 0.060 for 2276 F₀ values (M).

The structure is based on at least 39 atoms in the asymmetric unit. Of these, 27 are ordered and make up the [K₈Na₂Y₂₄(0H)₄(CO₃)₁₆(Si₄₈O₁₂₈)⋅ 10H₂O]^18- fraction. The underlying polyanion is a giant [Si₄₈O₁₂₈] ball whose connections define a polyhedron, hence a balosilicate. It has point symmetry (4/m 2/m 2/m), a near subgroup of (4/m 3 2/m) found for the related truncated cuboctahedron, an Archimedean solid. Breaks in the ball admit an extensive circumjacent disordered region and together define the $[Si56O140]∞1$ tubular inosilicate with channels and bulges, oriented parallel to [001]. All terminal oxygens point outside, away from the central channel. The ordered carbonate fraction defines, among other things, ordered [Na(CO₃)₄] clusters. The entire ordered region is called the curd.

The large balls are encrusted with a border region, or limbus, with Y-O, Na-O, and K-O bonds. The disordered region of 12 atoms in the asymmetric unit, or the whey, consists of partly occupied T(D) = (Si,B), Si; Na(D), K(D), and ɸ(ɸ = O^2-, OH^-, H₂O) sites.

It is proposed that the [Si₄₈] core constituted the template for ashcroftine’s growth and that other related monsters may occur in a similar paragenesis.