Feldspars, 3-dimensional

Aluminosilicates. These silicates consist of frameworks of silica and alumina tetrahedra linked at all corners to form three-dimensional networks familiar examples are the common rock-forming minerals quartz and feldspar. Framework silicates generally form blocky crystals, more isotropic... 

Smith and coworkers recently proposed a specific and novel mineral-based solution to the problem of dilution and diffusion of prebiotic reactants. They have suggested [132-134] the uptake of organics within the micron-sized three-dimensional cross-linked network of pores found to exist within the top 50 xm, or so, of alumina-depleted, silica-rich weathered feldspar surfaces. These surfaces incorporate cavities typically about 0.5 pm in diameter along with cross inter-connections of about 0.2 pm. The nominal area of the weathered feldspar surface is apparently multiplied by a factor of about 130 arising from this network. The similarity of these pores to the catalytic sites in zeolite-type materials is pointedly mentioned. 

Feldspars are tectosilicates (multiplicity = 4, periodicity = 4, dimensionality = 3) the tetrahedral groups [TO4] share all their comers with neighboring [TO4] units, thus forming a rigid tridimensional network. In feldspars, 25 to 50% of the silicon atoms are replaced by AF. The basic stmcture of the network is made up of four-member rings of [TO4] groups, two-by-two in an upward- and downward-... 

Ultramarines are three-dimensional cage-like structures. They differ from feldspars and zeolites because of the large spaces within the structures that can contain cations and anions but not water, illustrating a natural buckeyball-like structure and cavity, and a diversity of environment between the internal and external cages. Ultramarines can act as ion-exchangers for both anions and cations. The blue color of ultramarines is due to the presence of the ion although a yellow ion S2 also exists in the same structure. 

As one might expect there is an approximate correlation between the solid state structure and the physical properties of a particular silicate. For instance, cement contains discrete 8104 units and is soft and crumbly asbestos minerals contain double chains of 8104 units and are characteristically fibrous mica contains infinite layers of 8104 units, the weak bonding between the layers is easily broken, and micas show cleavage parallel to the layers and granite contains feldspars that are based on three-dimensional 8104 frameworks and are very hard. 

If all the four corners of the SiO tetrahedra are shared, three-dimensional networks result. The different forms of silica (quartz, tridymite and cristobalite) discussed earlier in this section are typical examples. Feldspars are generated when part of the Si is... 

If we were to replace some of the Si4+ ions with Al3+, it would still be possible to have a three-dimensional Si—0—A1 network, but cations would be needed to counterbalance the now anionic structural framework. Thus, for each substituent Al3+, we must add, say, an Na+, a K+, or half a Ca2+ ion. The feldspars, which along with quartz and micas (see below) are typical constituents of granites, can be viewed in this way ... 

Three-dimensional or tecto-silicates Three-dimensional frameworks of [Si04] tetrahedra sharing all four oxygen atoms Feldspars, zeolites, ultramarines... 

A new application of thermoluminescence (TL) to the study of minerals is described. The intensity of TL light emission as a function of both temperature and wavelength defines a three- dimensional picture which is characteristic of a particular mineral or group of minerals. The apparatus is briefly described together with its application to the study of the physical structure of some alkali feldspars. Examples are given of a K-rich and Na-rich feldspar and a potential method is discussed for identifying the nature of the phases of samples of mixed K/Na composition. 

In the case of the crystalline silicates an approach which takes account of the partly covalent character of the Si-O bond is helpful. The [SiCL]4- tetrahedron is taken as a basic building unit, and in most of the silicates these tetrahedra are linked together in an ordered fashion to form strings as in diopside (MgCa(Si03)2), sheet structures as in clay minerals, or three-dimensional frameworks as in quartz and the feldspars. Within these frameworks isomorphic replacement of one cation type for another is extensive. For example, the replacement of Si4+ by Al3+ is common, with the necessary lattice charge balance being maintained either by the incorporation of interstitial cations such as Na+... 

SiO, 5.33 Three-dimensional structures as in quartz (with isomorphous replacement by A1 in feldspars and zeolites)... 

Micas, which are composed of infinite two-dimensional sheets of silicate tetrahedra, cleave, like graphite, into thin sheets because interatomic interactions are weaker between than within the sheets. On the other hand, minerals that crystallize with a three-dimensional network do not cleave so readily. Feldspars, such as orthoclase, KAlSiaOg, provide an example of this. The silicate tetrahedra are linked at all four corners and so give a strong three-dimensional network, like that of diamond. 

The most abundant and important of the aluminosilicate minerals in the earth s surface are the feldspars, which result from the substitution of aluminum for silicon in three-dimensional silicate networks such as quartz. The Al ions must be accompanied by other cations such as sodium, potassium, or calcium to maintain overall charge neutrality. Albite is a feldspar with the chemical formula NaAlSi308. 

By this hypothesis, Millot argued that aluminum tends to direct silica toward a two-dimensional (sheet) rather than three-dimensional (framework) structure under conditions usually prevalent in soil solutions (although this assumes that conditions are not strongly alkaline). Thus, feldspars are not known to neoform (by, say, the reverse of reaction 6.26) except perhaps under geological conditions favorable to metamorphosis. Yet thermodynamically less stable minerals such as zeolites do form under the alkaline conditions that are presumably favorable to the formation of feldspar. Zeolites are framework silicates with an arrangement of tetrahedra unlike that of feldspar, and are considered to be metastable with respect to feldspar. We find, then, that in weathering processes, the final mineral products may not be, in the thermodynamic sense, the most stable products. 

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