Serpentines, sheet silicates

Rocks that contain talc, or the chemically and structurally similar minerals mentioned previously, are usually the result of alteration and recrystallization of rock formations that contained magnesian silicate minerals. Steati-zation or serpentinization are the terms given the processes that create layered (sheet) silicates from chain or other tetrahedral arrangements adopted by silicate minerals (see Fig. 2.1). The recrystallization process is expedited by temperature and pressure, and especially through the action of hydrothermal solutions. 

Deer, Howie and Zussman (19J give physical and optical constants for amphibole minerals in Vol. two, Rock Forming Minerals, (chain silicates), and for Serpentine minerals (chrysotile) in volume three (sheet silicates). X-ray diffraction data is obtainable from the Powder Diffraction file, Inorganic (20). This data can be used for a double check of asbestos standards. 

Subclass Phyllosilicates (sheet silicates) In this structure each Si04 tetrahedron is linked to three adjacent tetrahedra to form an infinite sheet of tetrahedra. Each tetrahedron in the sheet thus shares three (out of four) apical oxygens, having the basic structural unit Si205. Some Si of the tetrahedral site may be replaced by AF, and the charge is balanced by inclusion of additional cations. The micas, clay minerals, chlorite, talc and serpentine minerals are examples of phyllosilicates. 

Chlorite group Iron group Serpentine group Sheet silicates group ... 

Silicates Sheet silicates Clays Kaolinite — Serpentine Kaolinite... 

Commercial talc is composed primarily of the mineral talc, a sheet sihcate, but may contain related sheet silicates such as chlorite and serpentine, plus prismatic tremolite, anthophyllite, and carbonates such as magnesite, dolomite, and calcite. Talc particles are characteristically platy in morphology and are oleophihc/hydrophobic they are wetted by oil instead of water. Talc s reinforcing and pigmenting properties, together with good color, make it desirable as a functional filler in both aqueous and nonaqueous appHcations. For nonaqueous uses its naturally good matrix compatibility can be further enhanced by surface treatment. 

Fig. 2.1 Configurations of the tetrahedral units and chain, double chain, and sheet structures in the silicate and aluminosilicate minerals. (A) Two-dimensional representation of a single silicate tetrahedron. (A ) Two-dimensional representation of an extended silicate chain. (B) Three-dimensional representations of single tetra-hedra in two orientations. The apexes of the tetrahedra point above or below the plane of the paper. (B ) Three-dimensional representations of extended silicate chains showing different orientations of the tetrahedra in two of the many possible configurations. Single chain pyroxenes (C), wollastonite (D), rhodonite (E). Double chains amphiboles (F). Sheets as found in the serpentines, micas, and clays (G).

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... 

Most serpentines and other layered silicate minerals, such as micas and clays, are composed of tetrahedral and octahedral sheets that lie virtually flat. In chrysotile samples, however, the layers curl, rolling up like a carpet, to form concentric hollow cylinders (Fig. 2.4). The average diameter of a cylinder, which is a chrysotile fibril, is about 25 nanometers (25 nm = 0.025 mi-... 

The micas are characterized by extended silicate sheets rather than chains. Their structures resemble the serpentine mineral group in that they are dom-... 

The silicate sheet in kaolinite, for example, has an 0-0 repeat distance in the sheet of 0.893 nm, whereas the octahedral or gibbsite sheet repeat is smaller, about 0.862 nm. Mismatch of the 1 1 sheets induces curvature with the smaller dimension sheet on the interior. The octahedral gibbsite layer in clays is postulated to be situated on the inside of the curve. This relationship contrasts with the hypothesis for chrysotile, in which the tetrahedral silicate sheet is smaller and is postulated to be the interior unit in the scrolled serpentine mineral. 

The 1 1 clay-mineral type consists of one tetrahedral sheet and one octahedral sheet. These two sheets are approximately 7 A thick. This two-sheet type is divided into kaolinite (dioctahedral) and serpentine (trioctahedral) groups. The kaolinite minerals are all pure hydrous aluminum silicates. The different members are characterized by the manner of stacking of the basic 7 A layers (Brindley, 1961b). 

Artificial synthesis of smectites, especially those with Mg in the octahedral sheet, is relatively easy. A solution saturated with monomeric silica, to which an equal number of moles of MgCl has been added, is adjusted to pH 11 with NaOH. This yields a crystalline precipitate of trioctahedral smectite at room temperature. Magnesium silicate with the 1 1 layer structure (serpentine) results if the Si/Mg mole... 

Brindley and Gillery observed that the 00/ intensities from an Fe-rich daphnite specimen from Cornwall were not in accord with a true chlorite structure. One-dimensional Fourier syntheses indicated the tetrahedral Si and O peaks to be unexpectedly low and broad and to extend closer to the interlayer sheet than normal. A model was postulated in which approximately one-third of the tetrahedra is inverted to link with the interlayer sheet instead of with the silicate octahedral sheet. This has the effect of changing a chlorite 14 A unit into a 7 A layer at the point of inversion, so that the structure can be described as a mixed layer 7-14 A structure. The inversion can be accomplished physically by shifting a Si atom to the opposite side of its basal triad and completing tetrahedral coordination with an interlayer anion. The reverse of this process may be the mechanism of the hydrothermal transformation of 7 A aluminian serpentines to chlorites. 

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