Phyllosilicates, and among them clay minerals, are of great interest not only for the scientific community but also for their potential applications in many novel and advanced areas. However, the correct application of these minerals requires a thorough knowledge of their crystal chemical properties. This chapter provides crystal chemical and structural details related to phyllosilicates and describes the fundamental features leading to their different behaviour in different natural or technical processes, as also detailed in other chapters of this book. Phyllosilicates, described in this chapter, are minerals of the (i) kaolin-serpentine group (e.g. kaolinite, dickite, nacrite, halloysite, hisingerite, lizardite, antigorite, chrysotile, amesite, carlosturanite, greenalite); (ii) talc and pyrophyllite group (e.g. pyrophyllite, ferripyrophyllite); (iii) mica group, with particular focus to illite; (iv) smectite group (e.g. montmorillonite, beidellite, nontronite, saponite, hectorite, sauconite); (v) vermiculite group; (vi) chlorite group; (vii) some 2:1 layer silicates involving a discontinuous octahedral sheet and a modulated tetrahedral sheet such as kalifersite, palygorskite and sepiolite; (viii) allophane and imogolite and (ix) mixed layer structures with particular focus on illite-smectite.

Structure and Mineralogy of Clay Minerals

Interstratified Clay Minerals

Structures of Layer Silicates

Publisher Summary This chapter describes structures and mineralogy of clay minerals. Phyllosilicates considered in this chapter ideally contain a continuous tetrahedral sheet. Each tetrahedron consists of a cation, T, coordinated to four oxygen atoms and linked to adjacent tetrahedra by sharing three corners (the basal oxygen atoms, Ob) to form an infinite two-dimensional hexagonal mesh pattern along the a, b crystallographic directions. The free corners (the tetrahedral apical oxygen atoms, Oa) of all tetrahedra point to the same side of the sheet and connect the tetrahedral and octahedral sheets to form a common plane with octahedral anionic position Ooct. Ooct anions lie near to the center of each tetrahedral 6-fold ring, but are not shared with tetrahedra. The 1:1 layer structure consists of the repetition of one tetrahedral and one octahedral sheet, while in the 2:1 layer structure one octahedral sheet is sandwiched between two tetrahedral sheets. In the 1:1 layer structure, the unit cell includes six octahedral sites (i.e., four cis and two trans-oriented octahedral) and four tetrahedral sites. Six octahedral sites and eight tetrahedral sites characterize the 2:1 layer unit cell. Structures with all the six octahedral sites occupied are known as “trioctahedral.” If only four of the six octahedra are occupied, the structure is referred to as “dioctahedral.” The structural formula is often reported based on the half unit-cell content—that is, it is based on three octahedral sites.

Chapter 2 Structures and Mineralogy of Clay Minerals

The crystallization kinetics of poly(lactic acid) / talc composites were determined over a range of 0 to 15 wt% of talc. Talc was found to change the crystallization kinetics. The presence of talc increases the crystallization rate and this increase is related to talc concentration and to crystallization temperature. In order to understand the effect of talc and PLA crystallinity on mechanical properties, dynamic mechanical thermal analyses were performed on poly(lactic acid) / talc composites before and after an annealing process. It was demonstrated that the presence of crystals improves thermo- mechanical properties but in order to achieve good results at high temperatures the reinforcing effect of a filler such as talc is necessary.

http://www.expresspolymlett.com/letolt.php?file=EPL-0002366&mi=c

Crystallization kinetics of poly(lactic acid)-talc composites

The crystal structure of a sample of talc from Harford County, Maryland, has been deter- mined by least squares refinement from X-ray diffraction photographs. A triclinic cell with a = 5.293, b = 9.179, c = 9.496A, a = 90'57 ~ = 98-91 ~ y = 90.03, space group cT is adopted. The layers of the structure have almost monoclinic symmetry but the nearly hexagonal rings of oxygen atoms on the surfaces of the layers, formed by the bases of the silica tetrahedra, are not held in register by inter- layer ions as they are in micas but are partly displaced so that the stack of layers forms a triclinic crystal. The hexagons of surface oxygens are distorted by a 3.4 ~ twist of the tetrahedra so that the b axis is 0.2 per cent shorter than in a structure with regular hexagons, and the twist brings the oxygen ions a little closer to the octahedral magnesium ions.

The Crystal Structure of Talc

Pyrophyllite gives a diffraction pattern consisting of sharp and diffuse reflections. The unit cell for all the reflections is monoclinic with a = 5.17 A, b = 8.92 A, c = 18.66 A, β = 99.8°. From the absences, the space group is either C2/c or Cc but there are additional absences of hkl reflections with k ≠ 3n and l even. The additional systematic absences show that the structure is partially disordered, the disordered state being based on a small subcell defined by the sharp reflections. This subcell is monoclinic with a′ = a = 5.17 A, b′ = b/3 = 2.97 A, c′ =c/2 = 9.33 A, β′ = β = 99.8° and belongs either to the space group Cm or C2/m. Co-ordinates have been found in the space group C2/m by Fourier and least-squares methods, which give R = 0.180 for h0l reflections and R = 0.148 for hkl reflections with h = 0, 1, 2. The Si—O tetrahedra are twisted 10°–10.5° from the “idear” arrangement, leading to a ditrigonal array of oxygens on the surfaces of the layers. The average Si—O bond length is 1.610 A and the average octahedral site—O distance is 2.025 A. The surfaces of the layers can come together in three ways; the O—O contacts for any pair of layers are either approximately parallel to (010) with contacts 2.890 A and 3.028 A alternately or approximately parallel to planes making angles of ±120° with (010) with contacts 3.009 A, 3.066 A, 3.009 A, 3.343 A sequentially.

J. H. Rayner

Papers

The Crystal Structure of Talc

Structure of Pyrophyllite