Serpentine crystal structure

Analcite (NajOAljOj SiO I O), a cubic crystal structure, is formed at high temperatures. It is similar to acmite and also invariably is found beneath sludges of hydroxyapatite or serpentine or under porous deposits of iron oxides. 

Serpentine (3Mg02Si02 2H20) is a common silica-containing constituent of boiler waterside deposits. It typically appears as a flocculated sludge it has a monoclinic crystal structure. 

The crystal structures of all the minerals in the serpentine group contain the same basic building blocks. The basic unit is composed of a silicate sheet of composition (Si205) ", in which three of the O atoms in each tetrahedron are shared with adjacent tetrahedra (Fig. 2.2A), and a nonsilicate sheet of... 

Brindley, G.W., 1961b. Kaolin, serpentine and kindred minerals. In G. Brown (Editor), The X-Ray Identification and Crystal Structures of Clay Minerals. London Mineral. Soc., London, pp. 51-131. 

All terpenoid indole alkaloids are derived from tryptophan and the iridoid terpene secologanin (Fig. 2b). Tryptophan decarboxylase, a pyridoxal-dependent enzyme, converts tryptophan to tryptamine (62, 63). The enzyme strictosidine synthase catalyzes a stereoselective Pictet-Spengler condensation between tryptamine and secologanin to yield strictosidine. Strictosidine synthase (64) has been cloned from the plants C. roseus (65), Rauwolfla serpentine (66), and, recently, Ophiorrhiza pumila (67). A crystal structure of strictosidine synthase from R. serpentina has been reported (68, 69), and the substrate specificity of the enzyme can be modulated (70). 

The crystal structures of two heterobimetallic isopropoxides containing lead and zirconium (148), Pb2Zr4(O-/ -Pr)20 (Fig. 32) and Pb4Zr2(0-j-Pr))6 (Fig. 33), were determined by X-ray diffraction methods. These molecules adopt rare serpentine structures. 

Plesiopjc seres. The crystal structures of the members of these series are based on modules which have common features but may contain additional peculiar details. The family of serpentine-like structures (lizardite, chrysotile, antigorite, carlosturanite) is an example reported by Makovicky (1997). The members of this plesiotypic series are based on variously curled, reversed and/or interrupted TO (serpentine) layers. 

The serpentine group of minerals, which include chrysotile asbestos, are almost identical in composition. The chemical composition of unit cell is Mg6(OH)8Si40io. Chrysotiles have layered or sheeted crystal structure containing a silica sheet of (Si20s) in which silica tetrahedra point one way (Streib 1978). A layer of brucite, Mg(OH)2, joins the silica tetrahedra on one side of the sheet structure. Two out of every three —OH are replaced by oxygen atoms. X-ray and electron microscope studies indicate... 

Among the techniques used to characterize silica-supported Ni phases, FTIR spectroscopy is shown to be well adapted to identify ill-crystallized phases generated during the preparation by the competitive cationic exchange method. FTIR spectroscopy permits to discriminate a phyllosilicate of talc-like or serpentine-like structure from a hydroxide-like phase. Samples submitted to hydrothermal treatments have also been characterized by other techniques such as EXAFS and DRS spectroscopies. The pH and the specific surface area strongly influence the nature of the deposited phase, since they control the solubility and the rate of dissolution of silica. The results are discussed in terms of the respective amounts of soluble Si(OH>4 monomers and NP+ complexes at the interface. The relevant parameter as the Ni/Si ratio at the solid-liquid interface is assumed to control the routes to Ni-Si (Ni-Ni) copolyinerization (polymerization) reactions leading to supported Ni phyllosilicates (Ni hydroxide). 

Phosphate chemistry and silicate chemistry are much alike, but they are also very dissimilar. It was the expressed purpose of this work to prepare crystals of phosphates with properties similar to silicates referred to as asbestos, by substituting phosphorus atoms for silicon atoms in molecular structures like those found in serpentine minerals. It was hoped that many useful properties of silicates could be gained while retaining the long safety record of condensed phosphates. 

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