Feldspars, 3-dimensional silicates

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... 

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. 

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. 

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

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+... 

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. 

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. 

One of the most well-known silicate minerals is quartz (Si02), which consists of a continuous three-dimensional network of silica and oxygen without any atomic substitutions. It is the second most abundant continental mineral, feldspars being most abundant. The network of covalent bonds (between silicon and oxygen) is responsible for the well-known hardness of quartz and its resistance to weathering. Although pure quartz is clear and without color, the presence of small amounts of impurities may result in the formation of gemstones such as amethyst. 

Framework silicates comprise, as the name suggests, an open, three-dimensional network of joined sihca tetrahedra. Quartz itself is a classic example of a framework sihcate. The feldspars and feldspathoids (including lazurite, qq.v.) also demonstrate this structure. 

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