Probable Space Group Research Articles

Rubicline, a new feldspar from San Piero in Campo, Elba, Italy

The rubidium analogue of microcline, rubicline, (Rb,K)AlSi3O8, ideally RbAlSi3O8, was found in a pollucite-bearing rare-element pegmatite at San Piero in Campo, Elba, Italy. Rubicline is the first mineral with rubidium as an essential constituent. It occurs as abundant but small (#50 mm) rounded grains in 1‐2 cm wide veins of rubidian microcline (6 albite, muscovite, quartz, and apatite) that crosscut pollucite. Rubicline is brittle, transparent, and colorless. Refractive indices are slightly higher than those of the host microcline. The birefringence is low (1st order gray interference colors), and crystals are apparently untwinned. In thin and polished sections, cleavage passes through both the host microcline and grains of rubicline; by analogy with microcline, the cleavage is {001} perfect and {010} good. Determination of additional physical properties is hindered by an average grain size of 20 mm, heterogeneous composition, and structural coherency of rubicline with the enveloping microcline. Rubicline is triclinic, probable space group P , with a 5 8.81(3), b 5 13.01(3) c 5 7.18(4) A ˚, a5 90.3(1), b5 1 115.7(3), g5 88.2(1)8, V 5 741 A ˚ 3 ,Z 5 4, and axial ratios a:b:c of 0.677:1:0.577 (calculated from electron-diffraction data). Chemical analysis by electron microprobe gave 58.68 SiO2, 16.48 Al2O3, 6.23 K2O, 17.47 Rb2O, 0.92 Cs2O, 0.12 Fe2O3, sum 99.90 wt% and the formula (Rb0.574K0.407Cs0.020)S1.001(Al0.993Fe0.005)Si3.001O8. Rubicline is, in many cases, structurally coherent with the host microcline; it formed by exsolution from a (K,Na,Rb)-enriched precursor, followed possibly by fluid-induced modification.

Cassia spectabilis DC seed galactomannan: Structural, crystallographical and rheological studies

The seeds of Cassia spectabilis DC (family: Leguminoseae), an Indian fast growing spreading tree, contain about 40% of endosperm and possess the characteristics of becoming a potential source of commercial gum. The purified galactomannan shows Mw 1.1×10 6, intrinsic viscosity [ η] 615 mL/g with k′=1.706×10 −1, and a mannose to galactose ratio of 2.65. The hydrolysis of the fully methylated polysaccharide reveals clearly the expected structure of legume galactomannans. The orthorhombic lattice constants of the hydrated gums are as follows: a=9.12 Å, b=25.63 Å and c=10.28 Å. The results of X-ray fiber studies show that the b dimension of the unit cell is very sensitive to relative humidity (RH), galactose substitution and orientation of the films. The probable space group symmetry of the unit cell is P2 12 12. Rheological studies of the galactomannan have shown that the transition from semi-dilute to dilute regime occurs at a critical concentration C c *=2.75. The slope of the log–log plot of specific viscosity versus C at zero shear rate is 5.87 in the more concentrated regime. The viscoelastic and critical shear rate behavior indicate the characteristics of a coil polymer. The large dependence of the viscosity on the coil overlap parameter is probably due to polymer–polymer interactions and peculiarity of the galactose distribution along the chain. Above 20 g/L concentration, the rheological behavior of the gum is like the one of a weak-gel.

Superstructures and domain structures in natural and synthetic kalsilite

Transmission electron microscopy (TEM) investigation of natural kalsilite, (K,77Doo9Na,4b(Si209Alu9Feo12bOg, from San Venanzo, Italy, shows that it contains a pervasive domain structure related to a superstructure. The probable space group for the superstructure is P21. The relationship between the unit -cell parameters of the P6, subcell and those of the P21 supercell is a,upec ~ b,upec ~ v'3a,ub; c,upec = Cmb,and the a,upec axis is rotated 30° about the C axis with respect to the a,ubaxis. Nonperiodic distribution of elongated twin domains causes streaking of superstructure reflections normal to the c* axis. The average structure of this kalsilite sample has P6, symmetry. On the basis of previous structure refinements, this average structure has three 01 positions slightly displaced from the threefold axis, and each 01 position has a site occupancy of V3.High-temperature annealing studies indicated that there is a displacive phase transition from the P21 superstructure to the P63 substructure. The twin domains are related by threefold twin operations. The phase-transition temperature for kalsilite may range from 500 to 600°C and is dependent on the crystal composition (e.g., the proportions of Na, Ca, and vacancies). Kalsilite crystals prepared from nepheline by Na-K exchange are structurally complex. Domain structures in Na-bearing kalsilite crystals with more than 2.5 mol% NaSiAI04 are similar to those in the natural kalsilite sample but exhibit additional twin boundaries parallel to the c axis. Na-poor and Na-free kalsilite crystals are composed of (0001) domains with P63 and P31c symmetries, respectively, and merohedral twins between the neighboring P63 or P31c domains. The merohedral twins in the synthetic crystals formed during transformation of the nepheline structure to the kalsilite structure. The intergrown domains with P31c symmetry formed during the phase transition on cooling of the Na-poor kalsilite. There is no simple symmetry hierarchy among the known structures in the proposed phase diagram.

Single crystals of V amylose complexed with glycerol

Lamellar single crystals of amylose V glycerol were grown at 100°C by evaporating water from solutions of amylose in aqueous glycerol. The crystals which were square, with lateral dimensions of several micrometers, gave sharp electron diffraction patterns presenting an orthorhombic symmetry with a probable space group P2 12 12 1 and unit cell parameters: a = 1.93 ± 0.01 nm, b = 1.86 ± 0.01 nm and c (fiber axis) = 0.83 ± 0.03 nm. The amylose V glycerol crystal structure which is isomorphous to that of VDMSO consists of an antiparallel pair of left-handed six-fold amylose helices centered on the two-fold screw axes of the cell and probably separated by glycerol molecules. This packing mode is confirmed by de-solvation experiments where the V glycerol amylose crystals, annealed in ethanol, could be converted into V glycerol amylose without losing their external appearance. The V glycerol amylose crystals could be seeded by cellulose microfibrils to yield a shish-kebab structure where the amylose crystalline lamellae grew perpendicular to the microfibril directions.

Ferroelastic domain walls in the incommensurate phase of γ-KCoP04

Ferroelastic domains in γ-KCoP04 were studied by polarized light microscopy and by X-ray methods. These domains appear upon cooling as a result of a phase transition at about 449°C from an orthorhombic normal (N) phase (with probable space group Pnma) to a incommensurate phase (IC) which is monoclinic and has a average structure isotypic with CsLiS04 with space group P21/c. The modulation of the monoclinic structure decreases and the cell parameters change when the crystals are ground to a grain size of less than 7 μm and heated and cooled several times through the phase transition. A Rietveld refinement of the structure of γ-KCoP04 was achieved on a powder pattern. The cell parameters are: a = 5.3052(3), b = 8.5571(4), c = 10.3926(6), β = 120.833(2)° in [Å] and Z = 4.

Synthesis and Structural Systematics of Nitrogen Base Adducts of Group 2 Salts. V. Some Complexes With 3-Azapentane-1,5-diamine

Syntheses and room-temperature single-crystal X-ray structural characterizations are recorded for an array of adducts of H2N(CN2)2NH(CH2)2NH2, ' dien ', with salts of Group 2 elements, MX2.n( dien ).[Mg( dien )2] Cl2 (1) is monoclinic, P 21/c, a 11.326(6), b 10.928(5), c 12.636(6) Ǻ, β 92.74(4)°, Z = 4 f.u .; R was 0.047 for No 1726 independent 'observed' (I > 3σ(I)) diffractometer reflections. [Mg( dien )2] Br2 (2) is monoclinic, P 21/c, a 9.541(7), b 14.55(1), c 11.894(8) Ǻ, β 100.42(5)°, Z = 4 f.u ., R 0.054 for No 1778. [Mg( dien )2] I2 (3) is isomorphous , a 9.969(2), b 14.907(2), c 12.300(3) Ǻ, β 100.02(2)°, R 0.039 for No 3180. In all three complexes, the two tridentate ligands are disposed mer in a six-coordinate MgN6 environment, the trans Mg-N(central) distances being somewhat shorter (2.161 (4)-2.181(9)Ǻ) than Mg-N(distal) (2.215(8)-2.249(4)Ǻ). [Ba(dien)3] Cl2 (4a) is orthorhombic, Pnaa , a 19.309(2), b 13.545(2), c 8.872(2)Ǻ, Z = 4 f.u ., R 0.055 for No 1070; the methanol disolvate (4b) is monoclinic, P 21/c, a 16.807(8), b 8.764(5), c 19.493(4) Ǻ, β 102.39(3)°, Z = 4 f.u ., R 0.11 for No 2613. A family of derivative structures (5)-(8), [M( dien )3] X2, M, X = Sr , Br (5), Sr , I(6), Ba , Br (7), Ba , I(8), have cells of half the volume (a ≈ 13.8, b ≈ 9, c ≈ 9.8 Ǻ), probable space group P21212, R 0.050, 0.061, 0.034, 0.037 for No 549, 914, 1044, 1801 respectively, with the cations disposed about crystallographic 2 axes with one disordered ligand . [ Ba( dien )3](ClO4)2 (9) is rhombohedral , R3c, a 10.240(2), c 45.19(1) Ǻ (hexagonal setting), Z = 6 f.u ., R 0.054 for No 508. Like (4)-(8), the barium environment is nine-coordinate with the three tridentate ligands here� disposed about a 3 axis through the metal.

Polymorphism, structures and phase transformation of K3[SO4

The phase K3[SO4]F was found to be dimorphic, undergoing a reversible tetragonal (β) to cubic (α) phase transition at 585 °C. The room-temperature polymorph, β-K3[SO4]F, is isostructural with Ba3SiO5, as confirmed by Rietveld refinement of X-ray powder diffraction data: a= 7.2961(5), c= 10.854(1)A, R= 0.0689 and Rwp= 0.0907. The α form is cubic, a≈5.43 A, with probable space group Pmm. Partial Rietveld refinement suggested that it has an anti-perovskite structure. Lattice parameter measurements as a function of temperature showed no discontinuity at the α–β transition, which is therefore suggested to be a displacive second-order transition.

Structure, magnetic susceptibility and heat capacity of ScMnO 3

Structure refinement of the room temperature X-ray powder diffraction data of ScMnO 3 was carried out by Rietveld technique, based on the most probable space group P6 3 cm. The manganese and scandium atoms are surrounded by five and seven oxygen atoms respectively. Three manganese atoms make three kinds of triangles (two regular and an isosceles). A weak ferromagnetic behaviour with small hysteresis was observed below 130 K on the magnetization curve and ScMnO 3 becomes completely paramagnetic above it. The heat capacity of ScMnO 3 was measured from 77 to 800 K by two types of calorimeters (a.c. calorimeter and differential scanning calorimeter). At 129 K a λ-type anomaly was seen in the heat capacity curve which corresponds to the magnetic transition. The enthalpy change and entropy change that accompanied the magnetic transition were 178 J mol −1 and 1.40 J mol −1 K −1 respectively. The baseline of the heat capacity from 150 to 250 K yields a Debye temperature of 615 K.

Kintoreite, PbFe3(PO4)2(OH,H2O)6, a new mineral of the jarosite-alunite family, and lusungite discredited

AbstractKintoreite is a new lead iron phosphate mineral in the alunite-jarosite family, from Broken Hill, New South Wales, Australia. It is the phosphate analogue of segnitite and the iron analogue of plumbogummite. Kintoreite occurs as clusters and coatings of cream to yellowish green rhombohedral crystals up to 2 mm high and with the principal form {112}. The mineral also forms waxy, yellowish green globular crusts and hemispheres on other phosphate minerals. These associated species include pyromorphite, libethenite, rockbridgeite/dufrenite, apatite and goethite. Kintoreite formed during oxidation of primary ore rich in galena, in the presence of solutions with high P/(As + S) ratios. The mineral is named for the locality, the Kintore opencut, in which it is most common. A mineral closely resembling kintoreite in composition has also been found at several mines in Germany. Type material is preserved in the Museum of Victoria and the South Australian Museum.Electron microprobe analysis showed a nearly complete spread of compositions across the P-dominant portion of the segnitite-kintoreite series. The selected type specimen has an empirical formula of Pb0.97(Fe2.95Zn0.13Cu0.02)Σ3.10[(PO4)1.30(AsO4)0.39(SO4)0.18(CO3)0.11]Σ1.98(OH)5.45·0.74H2O, calculated on the basis of 14 oxygens and with all Fe trivalent. The simplified formula is PbFe3(PO4)2(OH,H2O)6. Kintoreite crystals are translucent with a vitreous to adamantine lustre, with globules appearing waxy. The streak is pale yellowish green and the Mohs hardness is ∼ 4. Crystals show good cleavage on {001} and are brittle with a rough fracture. The calculated density is 4.34 g cm−3. Kintoreite crystals are uniaxial negative with RIs between 1.935 and 1.955 and show light yellowish green to medium yellow pleochroism.The strongest lines in the X-ray powder pattern are (dobs, Iobs, hkl) 3.07(100) 113; 5.96(90)101; 3.67(60)110; 2.538(50)024; 2.257(50)107; 1.979(50)303; 1.831(40)220. The X-ray data were indexed on a hexagonal unit cell by analogy with beudantite, giving a = 7.325(1) Å, c = 16.900(3) Å, V = 785.3(5) Å3 and Z = 3. The probable space group is Rm, by analogy with beudantite and other members of the alunite-jarosite family. Powder X-ray diffraction data for several intermediate members suggest that the segnitite-kintoreite series may not represent ideal solid solution.During the study of kintoreite, part of the type specimen of lusungite from Zaïre was obtained and shown to be goyazite. The IMA's Commission on New Minerals and Mineral Names has voted to discredit lusungite as a species, and has approved the renaming of the ‘lusungite’ group as the segnitite group. However, as relationships between crystal structure, order-disorder and solid solution in the Pb-rich minerals of the alunite-jarosite family are not well documented, the nomenclatural changes resulting from this study should be seen as interim only.

Amylose AL associée à un déficit acquis en facteur V

Lamellar single crystals of amylose V glycerol were grown at 100°C by evaporating water from solutions of amylose in aqueous glycerol. The crystals which were square, with lateral dimensions of several micrometers, gave sharp electron diffraction patterns presenting an orthorhombic symmetry with a probable space group P212121 and unit cell parameters: a = 1.93 ± 0.01 nm, b = 1.86 ± 0.01 nm and c (fiber axis) = 0.83 ± 0.03 nm. The amylose Vglycerol crystal structure which is isomorphous to that of VDMSO consists of an antiparallel pair of left-handed six-fold amylose helices centered on the two-fold screw axes of the cell and probably separated by glycerol molecules. This packing mode is confirmed by de-solvation experiments where the Vglycerol amylose crystals, annealed in ethanol, could be converted into Vglycerol amylose without losing their external appearance. The Vglycerol amylose crystals could be seeded by cellulose microfibrils to yield a shish-kebab structure where the amylose crystalline lamellae grew perpendicular to the microfibril directions.

A new commensurate modulated structure in orthoclase

Transmission electron microscopy shows that there is an ordered modulated structure in orthoclase (Or84.6Ab13.1An2.3) (Or = orthoclase, Ab = albite, An = anorthite) of an augite monzonite from Wulian, Shandong Province, Northern China. The modulated orthoclase is composed of a series of triclinic (010) layer domains with C\overline 1 symmetry. Each domain has a thickness of 4d010 and the domains are periodically arranged along the b axis. The modulation period along the b axis is thus 104 A (= 8d010). The relationship between the extended unit-cell parameters of the modulated structure (supercell) and the triclinic subcell parameters is: asup \simeqasub; bsup = 8d010 \simeq 8bsub; Csup \simeq Csub; βsup \simeq βsub. The probable space group of the commensurately modulated orthoclase is Pm. The modulated structure probably forms during the phase transition from sanidine (C2/m symmetry) to microcline (C\overline 1 symmetry) through the segregation of Al atoms from T2 sites into T1(0) and T1(m) sites, respectively, in neighboring domains, which are in the albite-twin relationship. The ordering of Al atoms in the tetrahedral sites during the phase transition results in a sinusoidal deviation of the crystal structure from monoclinic symmetry. The Al–Si distribution near the domain boundary positions is likely to be relatively disordered.

Schwertmannite, a new iron oxyhydroxysulphate from Pyhäsalmi, Finland, and other localities

AbstractSchwertmannite is a new oxyhydroxysulphate of iron from the Pyhäisalmi sulphide mine, Province of Oulu, Finland. It occurs there, and elsewhere, as an ochreous precipitate from acid, sulphate-rich waters. Associated minerals at other localities may include jarosite, natrojarosite, goethite and ferrihydrite. Schwertmannite is a poorly crystalline, yellowish brown mineral with a fibrous morphology under the electron microscope. A high specific surface area in the range of 100 to 200 m2/g, rapid dissolution in cold, 5 M HCl or in ammonium oxalate at pH 3, and pronounced X-ray diffraction line broadening are consistent with its poorly crystalline character.Colour parameters for the type specimen as related to CIE illuminant C are L* = 53.85, a* = + 15.93, and b* = +47.96. Chemical analysis gives Fe2O3, 62.6; SO3, 12.7; CO2, 1.5; H2O−, 10.2; H2O+, 12.9; total 99.9 wt.%. These data yield an empirical unit cell formula of Fe16O16(OH)9.6(SO4)3.2·10H2O after exclusion of CO2 and H2O−. The most general simplified formula is Fe16O16(OH)y(SO4)z·nH2O, where 16 − y = 2z and 2.0 ⩽ z ⩽ 3.5. Schwertmannite has a structure akin to that of akaganéite (nominally β-FeOOH) with a doubled c dimension. Its X-ray powder diffraction pattern consists of eight broad peaks [dobs in (Iobs) (hkl)] 4.86(37)(200,111); 3.39(46)(310); 2.55(100)(212); 2.28(23)(302); 1.95(12)(412); 1.66(21)(522); 1.51(24)(004); and 1.46(18)(204,542), giving a = 10.66(4), c = 6.04(1) Å, and V = 686(6) Å3 for a primitive, tetragonal unit cell. The probable space group is P4/m. Upon heating, schwertmannite transforms to hematite with Fe2(SO4)3 occurring as an intermediate phase. Bidentate bridging complexes between Fe and SO4 are apparent in infrared spectra. Mössbauer data show the Fe in schwertmannite to be exclusively trivalent and in octahedral coordination; it has a Néel temperature of 75 ± 5 K and a saturation magnetic hyperfine field of about 45.6 T. Pronounced asymmetry of the Mössbauer spectra indicates different locations for Fe atoms relative to SO4 groups in the structure. The name is for Udo Schwertmann, professor of soil science at the Technical University of Munich.

A TEM study on the crystallization of amorphous Fe 73Si 3B 24 alloys

The crystallization of Fe 73Si 3B 24 amorphous alloys has been studied by transmission electron microscopy (TEM). A metastable phase P has been found and determined by a double tilting method in TEM. This phase belongs to a primitive tetragonal lattice with a = 0.62 nm and c = 1.43 nm. Its probable space group is P4nc or P4/mnc. In the Fe 2B phase, we found threefold twins which are rotated 120° around the [ 1 1 0] axis. The crystallization process of this alloy is suggested to be Amorphous→Amorphous + α-Fe(Si) + Fe 3B + P → α-Fe(Si) + Fe 3B + Fe 2B → α-Fe(Si) + Fe 2B.

Solid-State Synthesis, Structure, and Vibrational Spectra of NaGdP 2O 7

NaGdP 2O 7 was synthesized by the solid-state reaction of Gd 2O 3 with Na 2CO 3 and 4(NH 4) 2HPO 4 at 650°C. The X-ray powder diffraction data of NaGdP 2O 7 was indexed in the orthorhombic system with the approximate unit cell dimensions of a = 12.44 , b = 15.00, and c = 15.87 Å and the space group is Pmm2. Analysis of the vibrations of the P 2O 4- 7 ion according to C 2v symmetry and approximate band assignments for IR spectra are also reported in this work. The P-O-P band was found to be nonlinear, and some coincidences in the infrared and Raman spectra suggest that the most probable space group is noncentrosymmetric.

Ca0.2Na0.9Fe2.9O5: A New Me4O5 Structure Type

Examination of ferrite reactor product has resulted in the detection of a previously unreported Me4O5 oxide structure type with substituted composition Ca0.11Na1.11(Fe2.84Mg0.04)Si0.01O5. Investigation of the system Ca2xNa1-xFe3-xO5, x = 0.05 to 0.3 has shown that the simplified composition of this phase is Ca0.2Na0.9Fe2.9O5 (Z = 4). Using the orthorhombic unit cell dimensions determined by indexing powder X-ray data, a = 10.28, 10.29(1), b = 12.52, 12.46(1), c = 3.01, 3.01(1) Å (for Ca0.11Na1.11 (Fe2.84Mg0.04)Si0.01O5 and Ca0.2Na0.9Fe2.9O5 respectively), and the most probable space group Pnnm, the crystal structure of this phase has been determined and refined from the powder data. Ca0.2Na0.9Fe2.9O5 may be described as having layers composed of Fe octahedra and tetrahedra on either side of (010) planes of Na trigonal prisms. The overall relationship to CaFe2O4, CaFe3O5, and the lillianite homologous series is discussed. The Ca0.2Na0.9Fe2.9O5 structure type is apparently stabilized by incorporation of the minor Ca into the NaFe3O5 composition.

Transmission electron microscopy study of KDP crystals

Abstract KDP crystals were examined by transmission electron microscopy (TEM). Transmission electron diffraction (TED) patterns and images of some crystal areas indicated the existence of the second orthorhombic phase in tetragonal KDP crystals at room temperature. The lattice parameters of this orthorhombic phase are almost the same as in the case of the low temperature ferroelectric phase, a-and b-axes are the diagonals in the tetragonal XY-plane. The most probable space group of the orthorhombic phase is Aba 2. Partially coherent precipitates of about 500-1200 Å in diameter and noncoherent particles of about 1300-4500 Å in size are observed. The concentration of semicoherent particles is about 1011cm−3 and volume fractions of noncoherent particles are in the range from 1,0-1,5% to 4,5-5,0% in different crystals. One can distinguish the preferential [110]t (or [110]t) direction in the KDP crystals along which the particles are distributed.

Composition dependence of carrier concentration and conductivity in (Ba 1−xSr x)PbO 3−δ system

Measurements of electrical resistivity, Hall coefficient, lattice parameter for the system (Ba 1−xSr x)PbO 3−δ(0.0 ≤ x ≤1.0) were carried out. This system is orthorhombic and probable space groups in 0.0 ≤ x ≤ 0.5 and 0.5 ≤ x ≤ 1.0 are Imma and Pbnm, respectively. The resistivity increases with x and metallic behavior disappears at x = 0.5. The carrier concentration decreases with x and decreases rapidly in x ≥ 0.5. The mobility of electrons in this system shows a minimum at x = 0.5, being correlated with the randomness of A-site containing ions with different sizes. These electric properties are considered to be closely related with the tilting of PbO 6 octahedra, affecting the overlap between orbitals Pb 6s and O 2p which is the origin of the metallic conductivity of this system.

Molecular and crystal structure of the regenerated form of (1 → 3)-α- d-mannan

The crystal structure of the hydrated form of (1 → 3)-α- d-mannan, obtained by solid-state deacetylation of the partially O-acetylated mannan, was analyzed by combined X-ray diffraction and stereochemical-model refinement techniques. The structure crystallizes in a four-chain, monoclinic unit cell with parameters a = 11.33 Å, b = 18.36 Å, c (fiber repeat) = 8.25 Å, and γ = 101.75°, and the most probable space group is P2 1. In the most probable structure the chain-backbone conformation is a two-fold helix, but with all four O-6 rotational positions nonequivalent. The chains pack with antiparallel polarity and are connected by pairs of intermolecular hydrogen bonds that form an infinite, zig-zag sheet. There are 16 water molecules in the unit cell, generally embedded between the sheets in crystallographic positions, providing additional hydrogen bonding and establishing a three-dimensional hydrogen-bond network in the crystal structure. The reliability of the structure analysis is indicated by the X-ray residual R″ = 0.281, based on 98 hkl reflection intensities.

Molecular and crystal structure of konjac glucomannan in the mannan II polymorphic form

A probable crystal structure of konjac glucomannan (mannose:glucoe ratio = 1.6) is proposed based on X-ray data and constrained linked-atom least squares model refinement. The structure crystallizes in the mannan II polymorphic form, in an orthorhombic unit-cell with a = 9.01 Å, b = 16.73 Å, c (fiber axis) = 10.40 Å, and a probable space group I222. The backbone conformation of the chain is a two-fold helix stabilized by intramolecular O-3O-5′ hydrogen bonds, with the O-6 rotational position gt. The unit cell contains four chains with antiparallel packing polarity and eight water molecules which reside in crystallographic positions. Intermolecular hydrogen bonds occur exclusively between chains and water molecules, establishing a three-dimensional hydrogen-bond network in the crystal structure. The glucose residues replace mannoses in the structure in isomorphous fashion, although some disorder appears possible. A structure having alternating gg - gt O-6 rotational positions and conforming to space group P222 appears to describe the disorder regions of the crystal. The reliability of the structure analysis is indicated by the X-ray residuals R = 0.276 and R″ = 0.223.

Molecular and crystal structure of (1 → 3)-α- d-glucan triacetate

A crystal and molecular structure for GTA I, the low temperature polymorph of (1 → 3)-α- d-glucan triacetate, is proposed on the basis of X-ray diffraction analysis of well-oriented films, combined with stereochemical model refinement. The unit cell is monoclinic with parameters a = 30.17 A ̊ , b = 17.42 A ̊ , c (fibre axis) = 12.11 A ̊ , and β = 90° C. The probable space group is P2 1 with b axis unique. Six molecular chains pass through the unit cell with alternating polarity and with three independent chains comprising the asymmetric unit. The chain axes are located in a hexagonal packing arrangement. The chain backbone conformation is a left-handed, three-fold helix, but all nine O(6) acetyl groups of the asymmetric unit are in non-equivalent rotational positions. The most probable structure is indicated by X-ray residuals R = 0.261 and R ″ = 0.283, based on 62 reflection intensities (41 observed and 21 unobserved).