Hydrous mica

FIuorosihca.tes, Compared to the simple sUicates, these crystals have more complex chain and sheet stmctures. Examples from nature iaclude hydrous micas and amphiboles, including hornblende and nephrite jade. In glass-ceramics, fluorine replaces the hydroxyl ion fluorine is much easier to iacorporate ia glass and also makes the crystals more refractory. Eour commercial fluorosUicate glass-ceramic compositions and thek properties are Usted ia Table 2. 

Carr et al. (1953) hydrous mica from Yorkshire (Great Britain) fire clay. 

Analysis No. 1 represents the only truly tetrasilic celadonite. The average value for Si in the 17 analyses is 3.83. Except for Nos. 17-19 variations are between 3.73 and 4.00. Octahedral occupancy is close to 2 for all celadonites, with an average of 2.05. Interlayer cations often number substantially less than 1.0 and average 0.81 with a low value of 0.38 (No. 14). This, of course, is typical also for glauconites and hydrous micas. ... 

Carr, K., Grimshaw, R.W. and Roberts, A.L., 1953. Hydrous mica from Yorkshire Fireclay Mineral. Mag., 30 139-144. 

Heystek, H., 1955. Some hydrous micas in South African clays and shales. Proc. Natl. Conf. Clays Clay Miner. 3rd-Natl Acad. Sci Natl. Res. Counc., Pubi, 395 337-355. 

Illite (a hydrous mica) is a 2 1 clay mineral with isomorphous... 

Vermiculite, a 2 1 clay mineral, is also a hydrous mica, with isomorphous substitution in the tetrahedral sheet, resulting in a charge of between 1.2 and 1.9 mol negative charge per unit cell. In this case the... 

Cole, W.F., Moisture Expansion Relationships for a Fired Kaolinite-Hydrous Mica-... 

Secondary silicates form as clay minerals in soils after weathering of the primary silicates in igneous minerals. The secondary silicates include amorphous silica (opal) at high soluble silica concentrations and the very important aluminosilicate clay minerals kaolinite, smectite (montmorillonite), vermiculite, hydrous mica (il-lite), and others. Kaolinite tends to form at the low silicate concentrations of humid soils, whereas smectite forms at the higher silicate and Ca concentrations of arid and semiarid soils. The clay fraction of soils usually contains a mixture of these day minerals, plus considerable amorphous silicate material, such as allophane and imogolite, which may not be identifiable by x-ray diffraction. 

Micas are abundant in soils as primary minerals inherited from parent materials. Micas are not known to form to any significant extent in soils. Micas are precursors for other 2 1 layer silicates, notably venniculites. Micas are commonly present in soils as components of particles that have been partially transformed to expansible 2 1 minerals. As a result, mica is often interstratified with other minerals. Altered mica containing less K and more water than well-ordered mica is called hydrous mica (formerly illite). 

Compound Average of Igneous Rocks (%) Hydrous Mica (Scotland) (%) Montmoril- lonite (France) (%) Kaolinite (Virginia) (%) Allophane (Belgium) (%)... 

Aluminium, on the other hand, accumulates in the clay mineral fraction because it forms insoluble aluminosilicates and hydroxyoxides. The AI remains behind in the soil as other ions leach away. Iron also accumulates in soils but this is not apparent from Table 7.3 because the silicate clay minerals, with the exception of hydrous mica, are low in Fe. Iron precipitates in soils only as hydroxyoxides. Hydrous mica is altered parent material and is not reconstituted from the soil solution as are kaolinite, montmorillonite, and allophane. The <105° C water in Table 7.3 is, roughly speaking, adsorbed water the >105° C water is hydroxyl ions and water within crystal structures. 

Despite the unlikelihood of secondary mineral formation by ion substitution into or movement within an existing solid, some secondary 2 1 layer silicates apparently are formed by solid-phase changes of mica fragments inherited from the parent material. Hydrous mica, for example, is a product of chemical weathering as well as mechanical breakdown of mica. Hydrous mica, in turn, can be modified directly to vermiculite, montmorillonite, or chlorite. The process is not completely understood, but seemingly involves the outward diffusion of K+ from between the layer lattices and a subsequent or simultaneous reduction of charge within the layer lattice. 

Forty years have passed since the first petrogenetic application of the illite structural changes for characterizing diagenetic processes in sedimentary basins (Weaver 1960). Weaver s sharpness ratio as well as Kibler s (1964, 1968) empirical illite crystallinity index, have been easy-to-use X-ray powder diffraction (XRD) measures of the manifold, inter-related changes that the hydrous, mica-like phyllosilicates experience during increasing burial. 

Soils with less than 4% potassium saturation of the CEC are termed potassium frxers . Potassium is fixed especially in the clay mineral vermiculite, where it is entrapped in openings of the surface layers of oxygen atoms in adjacent aluminosilicate layers. In smectites and hydrous micas saturated with either Na-, Ca- or Mg-ions, the potassium ions displace these hydrated cations from the peripheral interlayer space, which leads to a collapse of this space and a decrease in CEC. 

There exists a group of minerals that is structurally similar to the micas, but contain less potash and more combined water than the latter. These materials are called illites, but have also been called hydrous micas or sericites. 

Another member of the illite group is hydrous mica, in which the principal interlayer cation is K. A smectite you might encounter is montmorillonite smectites can expand by incorporating water or organics between the structural layers. Vermiculite is derived from the Latin vermiculare, which means to breed worms, and describes what appears to happen when the material is heated rapidly. Otherwise it is very similar to phlogopite mica. As you would guess, most of these minerals have complex chemical compositions. 

These minerals are hydrated aluminosilicates which are characterized by a sheetlike structure and can be conveniently divided into three groups (1) the kaolinite group, (2) the mont-morillonite group, and (3) the potash clay (or hydrous mica) group (Table 7.7). In the kaolinite group, all have the same chemical composition and differ only in individual crystal structures. The montmorillonite group can be represented by means of ion substitutions in the general chemical formula. For example, in montmorillonite itself, approximately 16% of the aluminum... 

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