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  • Author: M. Czank x
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Abstract

The structure of Li2Mg2[Si4O11] was determined by single crystal X-ray diffraction. It is triclinic, space group P[unk], with a = 8.645(1) Å, b = 7.401(1) Å, c = 6.884(1) Å, α = 104.71(1)°, β = 101.08(1)°, γ = 99.41(1)°, V = 407.5(6) Å3 and Z = 2. Least -squares refinements based on 1462 observed reflections with intensities I ≥ 3σ(I) resulted in R = 0.054 and Rw = 0.036.

The main structural elements are chains of corner-linked [SiO4] tetrahedra and ribbons consisting of edge-linked [MgO6] octahedra and [LiOn] polyhedra. The [SiO4] tetrahedral chain can be described as a loop-branched dreier single chain. This structure type has not been reported for other silicates. Tetrahedral chains and octahedral ribbons are arranged in layers of tetrahedra and octahedra, respectively, which are alternately stacked parallel to (110) and are linked by shared oxygen atoms. This structural arrangement, known from the structures of pyroxenes, pyroxenoids and amphiboles, is compared in detail with that of the closely related pyroxenoid-like dreier single chain silicates. The topology of the tetrahedral-octahedral linkage which occurs at the apical oxygen atoms of the chains is similar to that in serandite, while the linkage with the basal oxygen atoms of the chains is similar to that in wollastonite. Thus, Li2Mg2[Si4O11] represents a new structural member of the family of the pyroxenoid-like single chain silicates.

Abstract

Coarse-grained sapphirines in a high-grade metamorphic basic rock from the Eastern Ghats, India, contain in their cores fine, coherent exsolution lamellae of aluminous orthopyroxene. The interface is formed by the (010) plane of both structures and, in addition, (001)opx and ([unk]03)sapphirine are also subparallel. Therefore, both the chain direction and that of the octahedral strips are the same in both structures. In places, the orthopyroxene lamellae widen and become incoherent. The exsolution process started at high temperatures close to 900°C due to an increase of oxygen fugacity.

The new mineral biehlite was found in the Tsumeb mine, Namibia and has the composition Sb1.79As0.21MoO6. It crystallizes monoclinic, space group C2/c with lattice paramters a=18.076(5)Å, b= 5.920(5)Å, c=5.083(5)Å, β= 96.97(1)°. Refinement led to a final R(F) value of 0.065 for 679 symmetrically independent reflections.

[MoO6]6- octahedra share common edges and form zigzag chains which run parallel to the c direction. The octahedra are flanked by a [Sb2O2]2+ group on each side and infinite ribbons of composition [Sb4Mo2O12] are obtained. Neighbouring ribbons are only held together by weak Sb-O bonds in the directions of a and b.

The [MoO6]6- octahedra are strongly distorted with Mo-O distances ranging from 1.700(5) to 2.248(6) Å and O-Mo-O angles between 72.1(2)° and 105,3(4)°. The common O-O edge (2.492(8)Å) is shortened with respect to the remaining edges of the octahedra (2.704(8)Å-2.899(8)Å).

The Sb-cation forms three short bonds with oxygen at distances slightly smaller than 2Å. Six further oxygen atoms are located at distances ranging from 2.910(6) to 3.470(6)Å. Calculated distance for the lone-pair is 1.18 Å. Part of the Sb3+ is substituted by the smaller As3+ ion. The relationship of the new mineral to similar structures – especially to the mineral stibivanite and the synthetic compound Sb2MoO6 – are discussed.

Abstract

Diffraction patterns, bright-field images and dark-field images, mainly with so called c reflections (h + k + l = odd), were taken from anorthite (CaAl2Si2O8) crystals at different temperatures. From the results follows:

At temperatures somewhat above room temperature large antiphase domains exist with a displacement vector R = ½[111]. The anti-phase domains are associated with the whole structure. The size of the anti-phase domains does not change very much with increasing temperature although the anti-phase boundaries seem to be somewhat mobile. At the critical temperature Tc = 230°C the anti-phase domains disappear.

Above Tc dark-field images with c reflections show bright patches in a dark matrix. The bright patches are probably caused by the Ca atoms only which, in contrast to the body-centred (Si,Al)O2 framework, show a primitive ordered arrangement in some elementary cells. In the dark parts of the images the Ca atoms are either body centred or disordered.