Silicates phyllosilicates

Clays are layer silicates (phyllosilicates) of particle size less than about 4 pm, produced by the weathering of aluminosilicate rocks. Clay minerals fall roughly into two structural classes the kaolinite type, based on paired sheets of tetrahedral (SiC>44-) and octahedral [A10n(0H) g " or... 

Quicksilver Pyromorphite SILICATES (Phyllosilicates) Alabaster Tyuyamunite... 

Micas. If the double-chain amphibole structure diagrammed in Figure 2.8a is extended in two dimensions by the bonding of all three basal 0 atoms of each tetrahedron with Si atoms of other tetrahedra, a sheet silicate (phyllosilicate) is formed with the structure shown in Figure 2.9a. This polymer, extended infinitely in two dimensions, has the formula (Si40to) and is the basis of the mica structure (as well as the layer silicate clays, as will be discussed later in this chapter). 

Micas are layer silicates (phyllosilicates) whose structure is based either on a brucite-like trioctahedral sheet [Mg(OH)2 which in micas becomes Mg304(0H)2] or a gibbsite-like dioctahedral sheet [Al(OH)3 which in micas becomes Al204(0H)2]. This module is sandwiched between a pair of oppositely oriented tetrahedral sheets. The latter sheet consists of Si(Al)-tetrahedra which share three of their four oxygen apices to form a two-dimensional hexagonal net (Fig. 1). In micas, the association of these two types of sheet produces an M layer, which is often referred as the 2 1 or TOT layer. 

The T and O sheets occurring in the mica structure are present in all layer silicates (phyllosilicates) the entire M (TOT) mica layer is present in 2 1 layer silicates only. The description of layer silicates is often given by emphasizing different stacking of T and O sheets, even if an explicit discussion in terms of modular series is absent from the literature. 

It should be emphasized that the researcher must decide before starting the analytical procedure what size fraction and/or mineral fraction is to be analyzed. Clay sized material is defined as any sediment finerthan 2.0 /xm (or sometimes 3.9 /rm see Last, this volume) whereas clay mineral material is defined as generally crystalline layer-lattice silicates (phyllosilicates). Many non-clay minerals can be present within the clay sized fraction and, conversely, clay minerals can often be present in the coarser-than-clay sized material of the lacustrine sediment. There are different approaches for each of these components. The following summarizes a general procedure for identification and quantification ofclay minerals within the clay sized fraction. 

Silicates Phyllosilicate subclass Talc Mg3Si40jo(OH)2 PyrophyUite Al2Si40jQ(0H)2 Chlorite group minerals Muscovite KAl2(Si3Al)OjQ(OH,F)2... 

Naroe(s) talc, hydrous magnesium silicate, phyllosilicate CAS 14807-96-6... 

The clay minerals can now be discussed in terms of their relationship with the phyllosilicates (sheet silicates). It is important to keep clearly in mind here the difference between clay - the material which is dug out of the ground, and which may be a mixture of different clay minerals, together with various nonclay minerals (such as quartz, pyrite, etc), as well as unaltered rock fragments and incorporated organic material (Grim, 1968) - and the clay minerals themselves, which are crystalline compounds of specified stoichiometry and structure. At this stage, we are only considering the structure of the clay minerals. 

Study of hydrated kaolinites shows that water molecules adsorbed on a phyllosilicate surface occupy two different structural sites. One type of water, "hole" water, is keyed into the ditrigonal holes of the silicate layer, while the other type of water, "associated" water, is situated between and is hydrogen bonded to the hole water molecules. In contrast, hole water is hydrogen bonded to the silicate layer and is less mobile than associated water. At low temperatures, all water molecules form an ordered structure reminiscent of ice as the temperature increases, the associated water disorders progressively, culminating in a rapid change in heat capacity near 270 K. To the extent that the kao-linite surfaces resemble other silicate surfaces, hydrated kaolinites are useful models for water adsorbed on silicate minerals. 

Photo-oxidation reactions, 32 118 Photoreduction, metal oxides, 31 123 Phthalic acid, esterification, 17 340 Phthalocyanines EDA complexes of, 20 328-330 catalytic activity for hydrogen exchange reaction, 20 329,330 electronic configuration of, 20 330 organometallic complexes, 30 276-277 Phyllosilicates, see Layer lattice silicates, catalysts... 

Phyllosilicates (clay minerals) - layered silicates such as Al4Si4O10(OH)8 (kaolinite) and (Ca,Na)o.7(Al,Mg,Fe)4(Si,Al)802o(OH)4.nH20 (montmorillonite, also called smectite). 

Matrix minerals are complex mixtures of silicates (especially olivine and pyroxene), oxides, sulfides, metal, phyllosilicates, and carbonates. The bulk chemical composition of matrix is broadly chondritic, and richer in volatile elements than the other chondrite components. Some chondrules have rims of adhering matrix that appear to have been accreted onto them prior to final assembly of the meteorite. Small lumps of matrix also occur in many chondrites. Presolar grains, described in Chapter 5, occur in the matrix. 

Figure 7.2 Silicate anion structures (o) orthosilicate, (6) pyrosilicate, (c) three-silicate ring, (d) six-silicate ring, (e) pyroxene, (/) amphibole, and (g) phyllosilicate.

It should be possible, through a unification of chemical and mineral structure data and the results of experimental studies on silicate phase equilibria, to develop a general picture of clay mineralogy based upon the known chemical behavior of phyllosilicates under various physical conditions. The major elements for such a study are presently available in a rough outline. It is fact the purpose of this essay to summarize the available information and create a general outline of clay mineral petrology. It is hoped that such an attempt meets with some success and, more important in the long run, that such an attempt will interest others in similar exercises, especially those of precision and revision. 

No new definitions will be proposed here but a simple outline of the basic vocabulary is given which will be used to permit a discussion of the problems of physical chemistry of phyllosilicates and other silicate minerals found in clay mineral suites. 

Most likely, the chemical system remains closed, as far as the other components in the silicate phases are concerned, as diagenesis or low grade metamorphism becomes more evident. Although there may be transfer of calcium, it seems, from bulk chemical analysis, that there is no systematic increase in potassium nor decrease in sodium content of argillaceous sediments. The transfer of Na and K is between the two size fractions—clay and coarse fraction—or between phyllosilicates and tectosilicates. Albitization of argillaceous rocks should be a common phenomenon where mixed layered phases are predominant in clay assemblages and especially evident in the illite-chlorite zone. 

CLAYS. The terms chy or cloys commonly refer to cither rocks that are consolidated or unconsolidated sediments, nr a group of minerals having unique properties. Traditionally, clays (rocks) are distinctive in al least two properties that render them technologically useful plasticity and composition. Clays are predominantly composed of hydrous phyllosilicates. referred to as clay minerals. These are hydrous silicates of Al. Mg. K, anti He. and other less ahundanl elements. Clay minerals arc extremely fine crystals or particles, often colloidal in size and usually plate-like in shape. The nonclay mineral portion of clays (rocks) may consist of other minerals, portions of rocks, and organic compounds. 

VUId Silicates with layered anions (the so-called phyllosilicates) Figure 7.11 represents a way to link Si04 tetrahedrons in one layer. The repetitive unit in the structure of figure 7.11 is represented in between the dotted lines and has the formula Si404-. The anion described here for example occurs in the minerals ... 

Clay minerals belong to a structural family of minerals known variously as sheet silicates, layer silicates, or phyllosilicates. Sheet silicates contain Si04 tetrahedra that are polymerized in two... 

Phyllosilicate clays are built from tetrahedral sheets made of SiO silicate units and of octahedral sheets resulting from the condensation of octahedra — such as the aluminate AlO octahedron — centered on a divalent or on a trivalent cation. Layers can be constituted of one tetrahedral sheet associated with one octahedral sheet (1 1). Or they can be made of one octahedral sheet sandwiched in-between two tetrahedral sheets (2 1). There are other combinations, associating up to four sheets. 

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