Alkali feldspars crystallization

Feldspars are found in nature in the form of mixed crystals of the alkali feldspars orthoclase K[AlSi308], albite Na[AlSi308] and anorthite Ca[Al2Si208]. Unlimited formation of mixed crystals of the alkali feldspars is only possible at high temperatures. During cooling demixing occurs. Potassium feldspar and calcium feldspar are not miscible. 

Colorless belite, according to Lee (1983), can occur as very small inclusions in alite and contains very little impurity. Large-crystal, ringlike belite nests (from coarse alkali feldspar) are also colorless because of iron-oxide deficiency. These occurrences of belite, therefore, do not reflect the cooling rate. Consequently, Ono (1978) recommends color observation of roughly 20 pm belite crystals in order to judge the cooling rate. 

The feldspars are subdivided into two groups the alkali feldspars (including mi-crocline, orthoclase, and sanidine), in which potassium is dominant with a smaller proportion of sodium and negligible calcium and the plagioclase feldspars, which vary in composition in a series that ranges from pure sodium feldspar (albite) through to pure calcium feldspar (anorthite) with negligible potassium. Feldspars form colourless, white, or pink crystals with a hardness of 6 on the Mohs scale. 

Sa. 13.12.65) Rhyodacite, altered olive-gray vitiic-crystal tuff. Quartz = 30%, alkali feldspar = 10%, plagioclase = 15%. Hawkes-Porphyry Member, Hill Nunatak. 

The crystals are generally euhedral and unaltered. The dolerite also contains two and, in some cases, even three pyroxenes augite, pigeonite, and hypersthene. In addition, all specimens contain opaque minerals (magnetite and ilmenite). Quartz and alkali feldspar are prominent in the rocks of the diorite pegmatite and granophyre zones. 

Granite Igneous rock composed of coarse crystals (greater than about 2 millimeters) of plagioclase (calcium-sodium silicate mineral), alkali feldspar (potassium-sodium silicate mineral), and quartz (all silicate with no other plus ions), and a few dark silicate minerals. 

The feldspar minerals have similar physical properties and often occur as prismatic or tabular crystals in igneous rocks, or as more anhedral grains in metamorphic and sedimentary rocks. They are colourless when fresh but are more commonly white due to incipient alteration impurities or inclusions result in coloured varieties, with green-brown alkali feldspars found in some metamorphic rocks, and orthoclase commonly found as pink. The surfaces of feldspar crystals are often iridescent due to twinning on a microscopic scale, with labradorite characterised by blue surface iridescence. Feldspars readily alter under hydrothermal action or chemical weathering to form members of the clay minerals group (. v.). Sodium-rich feldspars commonly decompose to form montmorillonite, in the presence of limited water, or to kaolinite with excess water alkali feldspars typically form illite or kaolinite sub-group (qq.v.) clay minerals (Deer et al, 1992 Rutley, 1988). 

Non-lattice sites may play an important role in the incorporation of large foreign ions in crystal structures during coprecipitation Pingitore (Chapter 27) discusses the importance of these sites in the formation of coprecipitates of calcium carbonate containing Srz+ or Ba. White and Yee (Chapter 28) discuss the diffusion of alkali ions into defect structures in the surfaces of glasses and crystalline feldspars. 

Zone two can be defined by the absence of montmorillonite and by the tie-line mica-opal (Figure 37). Zone one, which contains montmorillonite shows the coexistence of feldspar and montmorillonite (Figure 37a). Trona and halite found in the sediments are considered to indicate higher alkalinity and alkali content of the pore fluids that effected the crystallization of the feldspar "facies" in zone two at the lake center. Here the evaporated fluids became more concentrated. 

Beyond the positions of these atoms, we also need to specify their occupation factors. Feldspars are widely reported to exhibit incongruent dissolution, whereby the alkali and aluminum atoms are dissolved preferentially. Thus, we allow for removal of individual species at the surface by varying the occupation of the near-surface potassium ions. This is an additional multiplicative factor in the expression for the structure factor (Eqn. 14) for each atom in the crystal, and varies between 0 and 1. 

In this chapter are described the phenomena of gas flow in glasses and in crystals. While the structure of alkali halides and similar more simple crystals is well known, it is not so with the large families of derivatives of silica. Nevertheless, considerable advance has been made towards an understanding of structural relationships in some at least of these families of compounds. Since many silicates are of importance to the present chapter, a few of the known structural relationships will be given for substances such as silica, silicate glasses, zeolites, mica, clays, feldspars and ultramarines, the first four of which are of great importance in discussing the permeability data. 

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