Pyrophyllite types

An additional structural variant for clay minerals is the chlorite-type structure. Chlorites are similar to the pyrophyllite-type structures with two tetrahedral sheets and an octahedral sheet making up each layer. Instead of alkali or alkaline earth interlayer cations, chlorites contain a brucite (Al-Mg hydroxide) layer between successive pyrophyllite-type layers [18]. 

The Y, C and B sub-types roughly correspond to types 1, 2 and 3 as defined by Urabe (1974a), who classified Kuroko deposits based on hydrothermal alteration and ore mineral assemblages type 1, kaolinite-pyrophyllite-diaspore-type type 2, sericite-chlorite-type type 3, sericite—chlorite-carbonate-type. Hydrothermal alterations in the Kuroko mine area are described in section 1.3.2. 

Pyrophyllite and diaspore alterations were reported from several Kuroko deposits, although they are not common (Urabe, 1974a). This type of hydrothermal alteration is thought to have occurred at a later stage than the hydrothermal alterations associated with Kuroko mineralization (sericite, chlorite, and zeolites) (Utada, personal communication, 1995). 

Shikazono, N. (1985e) K-Ar ages for the Yatani Pb-Zn-Au-Ag vein-type deposits and Otoge kaolin-pyrophyllite deposits, Yamagata Prefecture, northeastern part of Japan. Mining Geology, 35, 205-209 (in Japanese with English abst.). 

The number and exact composition of the sheets is used to classify the phyllosilicates. The most important classification for our purposes is the distinction between 1 1 and 2 1-type minerals (Figure 2.1). In 1 1 minerals such as kaolinite, the basal oxygens of the tetrahedral sheet are free to interact with octahedral OH groups forming hydrogen bonds. In contrast, 2 1 minerals such as pyrophyllite or talc contain two tetrahedral sheets sandwiched around an octahedral sheet. These minerals have only basal oxygens exposed on the faces of the tetrahedral sheets and are linked by weak van der Waals forces. The weaker interaction of one 2 1 layer with a second 2 1 layer results in interlayer spaces which, depending on the particular mineral, may be available for contaminant intercalation. 

High-pressure treatments were carried out in two units a hot isostatic press (HIP) and a multiple anvil-type high-pressure generator. The powder sample for HIP treatment were pelletized and sealed in Ag tubes in vacuo, and was then sintered at 0.15 GPa and 823 K for 2 h in N2. High pressures up to 5 GPa were applied to the powder sample in the cubic/ octahedral anvil apparatus according to a technique described elsewhere.34 Briefly, the raw powder sample was packed into a BN capsule and placed at a center of the inner octahedral anvils with pyrophyllite pieces as... 

Type II (TOT) silicates include pyrophyllite, AlSi2Os(OH) (Section 10.3.25) and talc, Mg3Si40io(OH)2 (Section 10.3.24). Muscovite, KAl2(AlSi3)Oio(OH)2 (Section 10.3.27), and phlogopite, KMg3(Si3Al)Oio(F, OH)2 (Section 10.3.28) are Type II minerals with K+ ions in interlayer sites. 

The three-sheet or 2 1 layer lattice silicates consist of two silica tetrahedral sheets between which is an octahedral sheet. These three sheets form a layer approximately 10 A thick. The oxygens at the tips of the tetrahedra point towards the center octahedral sheet and substitute for two-thirds of the octahedrally coordinated hydroxyls. The 2 1 clay minerals include the mica and smectite groups which are by far the most abundant of the clay minerals. The pure end members of this type are talc, a hydrous magnesium silicate pyrophyllite, a hydrous aluminum silicate and minnesotaite, a hydrous iron silicate. 

The pressure inside the heated chamber may also vary as a result of the local density changes produced by thermal expansion or phase changes resulting from the heating. For example NaCl may expand, melt, and thereby increase the local pressure, while pyrophyllite, a layer-lattice-type aluminum silicate, may transform into a denser assembly of coesite and kyanite, thereby reducing the local pressure. It follows that experimental results in high-pressure, high-temperature work must be interpreted with care. 

The derivation of clays from talcs and micas provides a direct way to understand the structures of the clays. The infinite-sheet mica pyrophyllite, Al2(Si40io)(OH)2, serves as an example. If one of six AI ions in the pyrophyllite structure is replaced by one Mg ion and one Na ion (which together carry the same charge), a type of clay called montmorillonite, MgNaAl5(Si40io)3(OH)g, results. This clay readily absorbs water, which infiltrates between the infinite sheets and hydrates the Mg and Na ions there, causing the montmorillonite to swell (Fig. 22.5). 

Layers of type (d) only. The two extremes are talc, Mg3(OH)2Si40io. and pyrophyllite, Al2(OH)2Si40io (Fig. 23.17). As in the kaolins the layers are electrically neutral, and there are only feeble attractive forces between neighbouring layers. These minerals are therefore soft and cleave very easily, and talc, for example, finds applications as a lubricant (french chalk). 

Charged layers of type (d) interleaved with ions. Replacement of one-quarter of the Si in talc and pyrophyllite layers gives negatively-charged layers which are interleaved with K ions in the micas phlogopite and muscovite (Fig. 23.17) ... 

Charged layers of type (d) interleaved with hydrated ions. In addition to the micas, which are anhydrous, there are some very important minerals, sometimes called hydrated micas , which are built of layers with smaller charges per unit area than in the micas, interleaved with layers of hydrated alkali or Mg ions. Such feebly charged layers can arise by replacing part of the AI2 in a pyrophyllite layer by Mg as in... 

Clay constitutes the most abundant and ubiquitous component of the main types of marine sediments deposited from outer shelf to deep sea environments. The clay minerals are conventionally comprised of the <2 pm fraction, are sheet- or fiber-shaped, and adsorb various proportions of water. This determines a high buoyancy and the ability for clay to be widely dispersed by marine currents, despite its propensity for forming aggregates and floes. Clay minerals in the marine environments are dominated by illite, smectite, and kaolinite, three families whose chemical composition and crystalline status are highly variable. The marine clay associations may include various amounts and types of other species, namely chlorite and random mixed layers, but also ver-miculite, palygorskite, sepiolite, talc, pyrophyllite, etc. The clay mineralogy of marine sediments is therefore very diverse according to depositional environments, from both qualitative and quantitative points of view. 

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