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

NMR techniques are employed to determine the relative fractions of boron atoms in Na2OB2O3SiO2 glasses of high soda content. The data show that if enough Na2O is added, four-coordinated borons are destroyed and borons with one or two non-bridging oxygens are created, but that both the beginning point and the rate of these processes depend strongly on the amount of silica present. These findings are shown to be quantitatively inconsistent with structural models previously suggested in the literature. Utilizing the concept of proportionate atomic sharing of the additional Na2O, a new structural model is proposed for K </8 (K = mol% SiO2/mol% B2O3) which is consistent with all the data including previously reported data for glasses in the region of relatively low soda content. Using R = mol% Na2O/mol% B2O3, the new model states that for RMAX = 12+16K </ R </ RD112+14K, all the additional Na2O is employed in forming non-bridging oxygens on the silica tetrahedra; then for RD1 </ R </ RD3 = 2 + K, the fraction (K + 14K)/(2 + K) of the additional Na2O destroys reedmernerite groups and forms pyroborate units plus silica tetrahedra with two non-bridging oxygens per Si atom, while the fraction (2 − 14K)/(2 + K) of the additional Na2O destroys diborate groups and forms additional pyroborate units.

11B NMR studies and structural modeling of Na2O-B2O3-SiO2 glasses of high soda content

Synthetic and natural muscovites

Structures of Layer Silicates

Layered double hydroxide (LDH) compounds are characterized by structures in which layers with a brucite-like structure carry a net positive charge, usually due to the partial substitution of trivalent octahedrally coordinated cations for divalent cations, giving a general layer formula [(M 1–x 2+ M 3+ x )(OH)2] x+. This positive charge is balanced by anions which are intercalated between the layers. Intercalated molecular water typically provides hydrogen bonding between the brucite layers. In addition to synthetic compounds, some of which have significant industrial applications, more than 40 mineral species conform to this description. Hydrotalcite, Mg6Al2(OH)16[CO3]·4H2O, as the longest-known example, is the archetype of this supergroup of minerals. We review the history, chemistry, crystal structure, polytypic variation and status of all hydrotalcite-supergroup species reported to date. The dominant divalent cations, M 2+, that have been reported in hydrotalcite supergroup minerals are Mg, Ca, Mn, Fe, Ni, Cu and Zn; the dominant trivalent cations, M 3+, are Al, Mn, Fe, Co and Ni. The most common intercalated anions are (CO3)2–, (SO4)2– and Cl–; and OH–, S2– and [Sb(OH)6]– have also been reported. Some species contain intercalated cationic or neutral complexes such as [Na(H2O)6]+ or [MgSO4]0. We define eight groups within the supergroup on the basis of a combination of criteria. These are (1) the hydrotalcite group, with M 2+:M 3+ = 3:1 (layer spacing ∼7.8 A); (2) the quintinite group, with M 2+:M 3+ = 2:1 (layer spacing ∼7.8 A); (3) the fougerite group, with M 2+ = Fe2+, M 3+ = Fe3+ in a range of ratios, and with O2– replacing OH– in the brucite module to maintain charge balance (layer spacing ∼7.8 A); (4) the woodwardite group, with variable M 2+:M 3+ and interlayer [SO4]2 –, leading to an expanded layer spacing of ∼8.9 A; (5) the cualstibite group, with interlayer [Sb(OH)6]– and a layer spacing of ∼9.7 A; (6) the glaucocerinite group, with interlayer [SO4]2– as in the woodwardite group, and with additional interlayer H2O molecules that further expand the layer spacing to ∼11 A; (7) the wermlandite group, with a layer spacing of ∼11 A, in which cationic complexes occur with anions between the brucite-like layers; and (8) the hydrocalumite group, with M 2+ = Ca2+ and M 3+ = Al, which contains brucite-like layers in which the Ca:Al ratio is 2:1 and the large cation, Ca2+, is coordinated to a seventh ligand of 'interlayer' water. The principal mineral status changes are as follows. (1) The names manasseite, sjogrenite and barbertonite are discredited; these minerals are the 2H polytypes of hydrotalcite, pyroaurite and stichtite, respectively. Cyanophyllite is discredited as it is the 1M polytype of cualstibite. (2) The mineral formerly described as fougerite has been found to be an intimate intergrowth of two phases with distinct Fe2+:Fe3+ ratios. The phase with Fe2+:Fe3+ = 2:1 retains the name fougerite; that with Fe2+:Fe3+ = 1:2 is defined as the new species trebeurdenite. (3) The new minerals omsite (IMA2012-025), Ni2Fe3+(OH)6[Sb(OH)6], and mossbauerite (IMA2012-049), Fe3+6O4(OH)8[CO3]·3H2O, which are both in the hydrotalcite supergroup are included in the discussion. (4) Jamborite, carrboydite, zincaluminite, motukoreaite, natroglaucocerinite, brugnatellite and muskoxite are identified as questionable species which need further investigation in order to verify their structure and composition. (5) The ranges of compositions currently ascribed to motukoreaite and muskoxite may each represent more than one species. The same applies to the approved species hydrowoodwardite and hydrocalumite. (6) Several unnamed minerals have been reported which are likely to represent additional species within the supergroup. This report has been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association, voting proposal 12-B. We also propose a compact notation for identifying synthetic LDH phases, for use by chemists as a preferred alternative to the current widespread misuse of mineral names.

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Joseph M. Axelrod

Papers

Loughlinite, a new hydrous sodium magnesium silicate

Reedmergnerite, NaBSi3O8, the boron analogue of albite, from the Green River Formation, Utah

The uranium minerals from the Hillside mine, Yavapai County, Arizona

Calculi and Other Stones Found in Mammals

Determination of thorium by fluorescent x-ray spectrometry