Abstract

The crystal structure of kraisslite, orthorhombic (pseudo-hexagonal), a = 8.1821(1), b = 14.1946(3), c = 43.9103(8) Å, V = 5099.8(2) Å3, Z = 4 and dcalc = 4.083 g cm−3, has been solved by direct methods and refined in the space group C2221 to an R1 index of 1.68% for 7432 observed (|Fo| > 4σ|F|) reflections. Electron-microprobe analysis gave the following chemical composition: As2O5 10.86, As2O3 6.18, SiO2 13.39, Al2O3 0.25, Fe2O3 2.06, MnO 51.14, ZnO 7.39, MgO 2.13, CaO 0.05, H2Ocalc = 4.50, sum 97.95 wt.%; and empirical formula: Zn2.91(Mn23.07Mg1.69Ca0.03)Σ=24.79(Fe0.833+Al0.16)Σ=0.99(As3+O3)2[(Si0.71As0.305+)O4]10(OH)16 calculated on the basis of 62 anions with (OH) = 16 and As3+/(As3+ = As5+) taken from the refined crystal structure. The general formula, [4]Zn3(Mn,Mg)25(Fe3+,Al)(As3+O3)2[(Si,As5+)O4]10(OH)16, differs from those given previously.

There is one As3+ site with a <As–O> distance of 1.780 Å and a stereochemistry typical of a stereoactive lone-pair of electrons. There are five tetrahedrally coordinated T sites with <T–O> distances from 1.635 to 1.692 Å; the T(1) site is fully occupied by As5+, and the T(2)–T(5) sites are occupied by both Si and As5+. There are two tetrahedrally coordinated Zn sites with <T–O> distances of ~1.996 Å, both of which are occupied by dominant Zn and minor Mn2+. There are thirteen octahedrally coordinated M sites, twelve of which are occupied by dominant Mn2+ with lesser Mg and minor Zn; <M–O> distances are in the range 2.197–2.284 Å. The <M(13)–O> distance is 2.083 Å and its lower site scattering indicates occupancy by Fe3+, Mn2+, Mg and Al. The structure consists of five crystallographically distinct layers of polyhedra, labelled m = 0–4. Layer m = 0 consists of corner-sharing Zn and (Si,As5+) tetrahedra, and layers m = 1–4 each consist of trimers of Mn2+ octahedra linked by (Si,As5+) tetrahedra and intrasheet hydrogen bonds (m = 1, 3) or (Si,As5+) tetrahedra and (Fe3+,Al) octahedra (m = 2) or (As5+) tetrahedra and (As3+O3) triangular pyramids and intrasheet hydrogen bonds (m = 4). The layers stack along [001] with reversals of the sequence m = 1, 2, 3, 4 at z = 0, ¼, ½ and ¾. Kraisslite is a member of the mcgovernite family.

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