Lath crystal

P-form long chains giving crystals that cleave into laths parallel to the chain Figure 3.9 Schematic representation of the two forms of oxalic acid, (-C02H)2. 

Polycrystalline, roughly spherical morphology consisting of lath, fibrous or lamellar crystals emanating from a common centre. 

The basic morphologies of lepidocrocite are lath-like or tabular. No example of twinning has been reported. Macrocrystalline lepidocrocite in the form of tabular crystals has 010 as the predominant form (Fig. 4.13). Other massive varieties of lepidocrocite include micaceous and fibrous textures and aggregated scales. 

Synthetic crystals of lepidocrocite are platy or lath-like, elongated in the a-direction and terminate in 101 faces. The predominant face is 010 and crystals often lie on this face. Lepidocrocite is commonly formed by oxidation of Fe systems. The crystal... 

Fig. 4.14 Synthetic lepidocrocite produced by oxidation of a FeCl2 solution, a) Monodomainic, lath-shaped crystals, produced by oxidation with 100 ml air min " at 50°C and pH 7.5 shadowed with 5 nm chromium at 45° (Courtesy R.Ciova-noli). b) Multidomainic crystals obtained at pH 7-7.5 and room temperature (see Schwertmann Taylor, 1972a). c) Crystal aggregates produced in the presence of urotropin (courtesy R. Ciova-noli). d) Very small crystals showing (010) lattice fringes of 1 nm (Schwertmann. Taylor, 1979, with permission), e) Cubic crystals formed after ageing multidomainic crystals shown in (b) in M KOH containing 3.32 10 M Si at 80°C for 1749 h (Schwertmann Taylor, 1972, with per-...

Fig. 4.20 Synthetic hematites grown from ferrihydrite at temperatures <100°C (Schwertmann, unpubl.) a) Hexagonal plates grown at pH 7 and RT acicular crystals are goethite b) Laths grown at pH 11 and 80 °C in the presence of 2.5 10 M citrate (see Schwertmann et al., 1968). The fine granular material is unreacted ferrihydrite c) Framboids grown at pH 6 and 70°C in the presence of 2 10 M oxalate (see Fischer, ...

Fig. 16.9 Electron micrographs of soil lepidocro-cite. a) Large multidomainic lath-like crystal viewed perpendicularto [001] with laminar pores from a re-doximorphic soil, Natal, South Africa, b) Poorly crystalline grassy lepidocrocite crystals mixed with tiny ferrihydrite particles and pseudo-hexagonal kaolinite platelets. Origin as before (a. b courtesy P. Self), c) Small lepidocrocite crystal from a hydromorphic soil (with ferrihydrite) viewed perpendicularto [001] and showing (020) lattice fringes (see also Schwert-mann. Taylor, 1989,with permission).

Biogenic hpidocrodte was first discovered by Lowenstam (1967) in the radula teeth of a chiton. The crystals are lath-shaped and several tenths of xm long with terminal 101 faces (Webb et al., 1989) (see Fig. 4.14a). The lepidocrocite is often associated with magnetite and ferrihydrite suggesting an Fe" precursor (see Chap. 13). 

Precipitation of green rust from an FeCl2 solution at pH 7.5 with NH4HCO3 followed by oxidation with air at 50 °C at pH > 6.5 for ca. 8 h. This method gives well developed lath-like crystals (Giovanoli and Briitsch, 1974). 

Particle morphology refers to the external features or form of a powder s primary particles. This includes descriptions of shape, including aspect ratio, or crystal habit (plate, needle, lath, equant, etc.). Particles are not always observed as discrete entities. Rather, they are often associated with other particles, sometimes loosely held... 

Filter the solution through a folded filter and set it aside in a quiet place to crystallize. The crystals are glassy in appearance and should not be too large or grown together. Drain them on a Witte plate and evaporate the mother liquor for a second crop. They may be obtained as needles as lath-shaped crystals or in a compact, many-sided form. The last named is the most desirable form, and conditions should be varied to avoid supersaturation (Exercise 6) until such crystals are obtained. Meanwhile, prepare some effloresced sodium sulfate by gently warming about 100 g. of the crude crystals until they fall into white powder. Place this powder in the bottom of an empty desiccator and... 

These crystals cleave along two planes parallel to the axis of these strings, breaking up into long laths. The O—H 0 distances for both forms are about 2.65 A. 

Naturally occurring dextran is basically amorphous. However, single crystals with lath-like shape can be grown in a mixture of water/polyethylene glycol at temperatures ranging from 120 to 200 °C [47], Combined electron and X-ray diffraction studies indicate that the unit cell contains two antiparallel dextran chains of two glucopyranosyl residues each [48],... 

As-synthesized Na-ZSM-5, which possesses an isostructure with TPA-ZSM-5, exhibits a hexagonal-lath-shaped morphology. Liquid phase Sil /A Oo ratio is important in controlling the formation of Na-ZSM-5 and mordenite, and the silica source influences the liquid phase composition. High yield of Na-ZSM-5 is obtained only when a small-sized silica sol is employed. A linear relationship between Na/Al starting atomic ratio and the nucleation rate indicates that charge neutralization of SBU is required for zeolite crystallization. 

Hexagonal-lath-shaped crystals (Na-ZSM-5) are always obtained from the organic-free system, whereas sphere-like or crossed-discus (twinned) crystals (TPA-ZSM-5) are easily formed in the system containing TPABr (Figure 3). From XRD measurements of the lathshaped Na-ZSM-5, certain peak intensities were found to be sensitive to the manner of mounting the specimen onto an X-ray sample holder. 

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