Feldspars albite-orthoclase

Kaolin - Kaolinite 4, 5, 6, Dickite 16. 27 Mica - Biotite, Phologopite, Muscovite Illite - Illite 36, Illite-Bearing Shale Mixed-Layer Clays - Metabentonite 37, 42 Montmorillonite - 21. 22A, 22B, 24, 25, 26. 31 Feldspars - Albite, Anorthite, Orthoclase Chlorite - Chlorite... 

Some of the most common igneous minerals are quartz, potassium feldspars (orthoclase and microcline), plagioclase feldspars (albite, anorthite), micas (muscovite, biotite), olivine, and members of the amphibole and pyroxene groups. Many other minerals are found in igneous rocks, most of which occur in only very small amounts. These are called accessory minerals, the most common of these are corundum, pyrite, magnetite, zircon, rutile, apatite, and members of the garnet group. 

The carbonates are mainly calcite, dolomite, or siderite. The occurrence of calcite is frequently bimodal. Some calcite occurs as inherent ash, while other calcite appears as thin layers in cleats and fissures. Iron can be present in small quantities as hematite, ankorite, and in some of the clay minerals such as illite. In addition to the more common minerals, silica is present sometimes as sand particles or quartz. The alkalies are sometimes found as chlorides or as sulfates but probably most often as feldspars, typically orthoclase and albite. In the case of lignites, unlike bituminous and subbituminous, sodium is not present as a mineral but is probably distributed throughout the lignite as the sodium salt of a hydroxyl group or a carboxylic acid group in humic acid. Calcium, like sodium, is bound organically to humic acid. Therefore, it too is uniformly distributed in the sample [10]. 

The composition of feldspar in a particular rock as calculated from the major element chemistry of the rock using a specific set of rules - the (CIPW) normative calculation. The feldspar content is expressed in terms of the weight % proportion of the Na-, K- and Ca-feldspars (Albite (Ab), Orthoclase (Or) and Anorthite (An)) Basalt from an arc environment which typically has about 20 wt% Al203, rather than the ca. 15 wt% found in oceanic basalts... 

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. 

By far the most important rock minerals are feldspars, which constitute about 60% of the igneous rock. They are essentially made of three fundamental types of minerals albite NaAlSi30g, orthoclase KAlSi30g, and anorthite CaAl SiO. Their structures are continuous three-dimensional network of [SiO ] and [AlO ] tetrahedrons, interspersed by positively charged sodium (Na(I)), potassium (K(I)), or calcium (Ca(II)). Albite and anorthite mix in arbitrary proportions such mixed minerals are known as plagioclase feldspars. Albite and orthoclase also make mixtures they are alkali feldspars. 

Figure 19. Variation in dimension of the X-fold cation site in alkali-feldspar for 2+ cations (), obtained by fitting the experimental data of Icenhower and London (1996) for Dca, >sr and Dsa at 0.2 GPa and 650-750°C. In performing the fits was set at 91 GPa for all runs. Error bars are 1 s.d. The positive slope is consistent with measured changes in metal-oxygen bond length from albite to orthoclase (cf Fig. 6). The solid line shows the best-fit linear regression given in Equation (35).

Table 5.62 Chemical analyses of some natural feldspars (Deer et al., 1983). (1) orthoclase (Mogok, Burma) (2) Adularia (St. Gottard, Switzerland) (3) anorthoclase inclusion in augite (Euganean Hills NE Italy) (4) sanidine from a nepheline-leucitite (5) albite from a pegmatite (6) anorthite from a calc-silicatic rock. ...

The feldspars are widely distributed and comprise almost two-thirds of all igneous rocks. Orthoclase and albite (NaAlSisOg) are feldspars in which one-fourth of the silicon atoms are replaced by aluminum and anorthite (CaAl2Si20g) and has one-half of the silicon atoms replaced by aluminum. Because the ionic radius of Na+ (0.095 nm) and Ca" " (0.1 nm) are the same, solid solutions are often formed between albite and anorthite. Good stones of albite and orthoclase are known as moonstones. 

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. 

The feldspar minerals are aluminosilicates of alkali and alkali-earth cations and are chemically classified by the mole percentages of three end-members orthoclase (Or KAlSi308) albite (Ab NaAlSi308) anorthite (An CaAl2Si208). The three constitute a ternary system with extensive solid solutions along the Or-Ab and Ab-An axes. The nomenclature on the basis of the mole percentages... 

The most common feldspars are those that contain potassium, sodium, and calcium cations. They are called, respectively, orthoclase (KAlSi30g), albite (NaAlSigOg), and anorthite (CaAl2Si20g). The solid solubility of orthoclase in albite is limited, and its solubility in anorthite is almost negligible. Albite and anorthite, however, are completely miscible at high temperatures and show complete solid solution. Offer an explanation for these observations, based on the tabulated radii of the K, Na, and Ca ions from Appendix F. 

Feldspars. Like the silica minerals, feldspars are framework silicates with all tetrahedral corners shared. By replacing one in four of the Si atoms by Al, the anionic framework Si3A10g is obtained from Si02 If the framework charge is balanced by structural K, the formula of potassium feldspar, KSisAlOg, is obtained. This feldspar has two common structural polymorphs (different structures with the same chemical formula), orthoclase and microcline. If the charge is balanced by structural Na, the feldspar obtained is albite, NaSi3A10g. 

The chemical composition of minerals was determined in 29 polished carbon-coated thin sections using a Cameca Camebax BX50 microprobe equipped with three spectrometers and a back-scattered electron detector (BSE). Operating conditions were 20 kV acceleration voltage, 8 nA (for carbonates and clay minerals) to 12 nA (for feldspars) measured beam current, and a 1-10 pm beam diameter (depending on the extent of homogeneous areas). Standards and count times were wollastonite (Ca, 10 s), orthoclase (K, 5 s), albite (Na, Si, 5 and 10 s, respectively), corundum (Al, 20 s), MgO (Mg, 10 s), MnTiOj (Mn, 10 s) and hematite (Fe, 10 s). Precision of analysis was better than 0.1 mol%. 

Sodium and potassium condense initially as the feldspar minerals albite NaAlSiaOg and orthoclase KAlSiaOg. Some of the albite reacts to form halite... 

Plagioclase feldspar (Na-Ca aluminosilicate), other than albite, stains red potash feldspar (microcline or orthoclase) stains yellow, and quartz and glassy slag are unstained. Calcite takes on various shades of pink and dolomite may be deep red. The particles are normally examined in immersion mounts, utilizing liquids of known index of refraction and a cover glass. 

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