Bohseite, ideally Ca4Be4Si9O24(OH4, from the Piława Górna quarry, the Góry Sowie Block, SW Poland

Bohseite is an orthorhombic calcium beryllium aluminosilicate with variable Al content and an endmember formula Ca4Be4Si9O24(OH4), that was discovered in the Piława Górna quarry in the eastern part of the Góry Sowie Block, ∼50 km southwest of Wrocław, SW Poland. It occurs in a zoned anatectic pegmatite dyke in close association with microcline, Cs-rich beryl, phenakite, helvite, 'lepidolite', probably bertrandite and unidentified Be-containing mica as alteration products after a primary Be mineral, probably beryl. Bohseite forms fan-like or parallel aggregates (up to 0.7 cm) of white, platy crystals (up to 2 mm long) with characteristic striations. It is white with a white streak, is translucent and has a vitreous lustre; it does not fluoresce under ultraviolet light. The cleavage is perfect on {001} and fair on {010}, and neither parting nor twinning was observed. Bohseite is brittle with a splintery fracture and Mohs hardness is 5–6. The calculated density is 2.719 g cm–3. The indices of refraction are α= 1.579, β = 1.580,γ = 1.597, all ±0.002; 2Vobs = 24(3)°, 2Vcalc = 27°; the optic orientation is as follows: X ^ a = 16.1°, Y ^ b = 16.1°, Z // c Bohseite shows orthorhombic diffraction symmetry, space group Cmcm, a = 23.204(6), b = 4.9442(9), c = 19.418(6) Å, V = 2227.7(4) Å3, Z = 4. The crystal structure was refined to an R1 value of 2.17% based on single-crystal data, and the chemical composition was determined by electron-microprobe analysis. Bohseite is isostructural with bavenite. Bohseite was originally approved with an end-member composition of Ca4Be3AlSi9O25(OH)3, but subsequent discovery of compositions with Be > 3.0 apfu led to redefinition of its end-member composition, holotype sample and locality, as reported here. There is extensive solid solution in bavenite–bohseite according to the scheme O(2)OH– + T(4)Si4+ + T(3)Be2+ ↔ O(2)O2– + T(4)Al3++ T(3)Si4+, and a general formula for the bavenite–bohseite minerals may be written as Ca4BexSi9Al4–xO28–x(OH)x, where x ranges from 2–4 apfu: Ca4Be2Si9Al2O26(OH)2 (bavenite) to Ca4Be4Si9O24(OH)4 (bohseite).

Electric linear dichroism and birefringence of biological polyelectrolytes

The phenomenon of electro-optic orientation was discovered by John Kerr in 1875 and has been used extensively for determining the optical polarizability anisotropy of small molecules and for high-speed transmission of optical signals. Measurements on biopolymers have been made at least since 1950, but only in the last decade have these yielded definitive structural and physical information. In the course of this review, it should become obvious that among the reasons for this late development is the inherent difficulty of analysing optical data that depend simultaneously on intrinsic optical-structural properties of the molecules, and on their degree of orientation under the conditions of the experiment. The problem has been particularly difficult far biopolymers such as the nucleic acids, whose polarization in an electric field is dependent on their special polyelectrolyte properties. These unique electrostatic properties are an important feature in the interpretation of the experimental observations.

Baghdadite, a new calcium zirconium silicate mineral from Iraq

Baghdadite, a new calcium zirconium silicate mineral has been found in melilite skarn in contact with banded diorite, from the Qala-Dizeh region, NE Iraq. Electron microprobe analysis yielded: SiO2 = 29.26, ZrO2 = 27.00, TiO2 = 2.11, Fe2O3 = 0.11, Al2O3 = 0.03, MgO = 0.05, CaO = 41.44, Na2O = 0.02, sum = 100.02 wt. %. The mineral contains about 0.16% HfO2. This analysis calculates to Ca3.00(Zr0.89Ti0.11)(Si1.98Fe0.01)O9 which leads to the ideal formula Ca3Zr[O2|Si2O7]. X-ray single crystal study showed it to be monoclinic with space group P21/a. The unit cell dimensions are: a = 10.42(2), b = 10.16(2), c = 7.36(1) Å, β = 91.1°, Z = 4 and cell volume = 779.04 Å3. The seven strongest lines in the powder diffraction pattern are (d,I,hkl): 7.30 (45)(110), 3.23 (80)(130), 3.04 (75)(2̄02), 2.98 (85)(202), 2.88 (70)(320,212), 2.84 (100)(230), 1.702 (40)(522). It is colourless, lustre vitreous, no cleavage and VHN50 = 725–783 kg mm−2 with H ∼ 6. Calculated density = 3.48 g cm−3 which is very close to 3.46 measured density of a synthetic Ca3ZrSi2O9. It is optically biaxial, positive, 2 V ∼ 72°, dispersion indiscernible. The cathodoluminescence colour is dull grey with a greenish tint. Refractive indices: α= 1.652, β = 1.658, γ = 1.670. The crystal habit is stumpy prismatic and a contact twin with b as twinning axis is observed. Optic orientation: α = c, β//b, γ = a.

Structure and Fabric on the Burroughs Glacier, South-East Alaska

The Burroughs Glacier, south-east Alaska, is a slow-moving remnant (14×3km.) of a much more extensive glacier. It is now entirely below the firn line; ablation has revealed ice structures and fabric once 300 m. or more below the glacier surface.At the present glacier surface three kinds of ice are identified—foliated ice, coarse-grained border ice and very coarse-grained basal ice.Two systems of fine-grained foliation are present. Differential movement in the glacier has caused recrystallization along closely spaced planes. At the glacier surface this produces a steeply dipping longitudinal foliation. A gently dipping foliation, having a regional trough-like structure, may be associated with former stratification planes or with former spoon-shaped shear surfaces.The optic orientation of crystals in the coarser layers of the foliated ice shows three weak maxima, and in the finer layers a single weak maximum, corresponding to one of the coarse layer maxima, and normal to the gently dipping foliation plane. The other maxima in the coarse layers are orientated close to the poles of principal fracture planes.In the coarse ice the fabric shows a pattern with three maxima similar to that obtained in torsion shear experiments. In the glacier the pattern may be formed by shear near the glacier bottom or along gently dipping foliation planes. Grain-size increases towards the glacier terminus, especially in the stagnant ice zone.Structural evidence suggests that in the early stages of the Little Ice Age the ice flow was from west to east. Later it was to east and west from an ice crest in the upper Burroughs Glacier. Structures produced by present movement have been superimposed on older structures.

Structure and Fabric on the Burroughs Glacier, South-East Alaska

The Burroughs Glacier, south-east Alaska, is a slow-moving remnant (14×3km.) of a much more extensive glacier. It is now entirely below the firn line; ablation has revealed ice structures and fabric once 300 m. or more below the glacier surface.At the present glacier surface three kinds of ice are identified—foliated ice, coarse-grained border ice and very coarse-grained basal ice.Two systems of fine-grained foliation are present. Differential movement in the glacier has caused recrystallization along closely spaced planes. At the glacier surface this produces a steeply dipping longitudinal foliation. A gently dipping foliation, having a regional trough-like structure, may be associated with former stratification planes or with former spoon-shaped shear surfaces.The optic orientation of crystals in the coarser layers of the foliated ice shows three weak maxima, and in the finer layers a single weak maximum, corresponding to one of the coarse layer maxima, and normal to the gently dipping foliation plane. The other maxima in the coarse layers are orientated close to the poles of principal fracture planes.In the coarse ice the fabric shows a pattern with three maxima similar to that obtained in torsion shear experiments. In the glacier the pattern may be formed by shear near the glacier bottom or along gently dipping foliation planes. Grain-size increases towards the glacier terminus, especially in the stagnant ice zone.Structural evidence suggests that in the early stages of the Little Ice Age the ice flow was from west to east. Later it was to east and west from an ice crest in the upper Burroughs Glacier. Structures produced by present movement have been superimposed on older structures.