Feldspars occurrence

The formation of epidote, K-feldspar, prehnite, wairakite and calcite in the geothermal area is considered to be due to the loss of CO2 gas and rapid precipitation from the solution supersaturated with respect to quartz (Browne, 1978). The widespread occurrence of these minerals in the Seigoshi district seems to be consistent with the above-mentioned consideration, namely that these minerals usually occur as veinlets rather than the replacements of original minerals and filling amygdule. In particular, many veinlets of epidote, prehnite and wairakite are found near the Au-Ag-quartz veins. 

The major occurrence of anatase and ilmenite, found in weathered carbonatite bodies, are found in Brazil. Occurrences of rutile and ilmenite in carbonatite-feldspar rocks are found in Mexico and Chile, and in recent years have been subject to extensive investigations. 

Alkali feldspars (K.NajAlSijOg) form a continuous solid solution series at high temperatures, but tend to exsolve upon cooling into a potassium-rich phase and a sodium-rich phase, resulting in perthitic intergrowths (2). Sodic plagioclases and potassium-rich feldspars tend to be studied together, in part because of similarities in structure, composition, occurrence, and weatherability (e.g., 1[). 

Occurrence. It is the commonest metallic element in the earth s crust. Aluminium occurs in many silicates such as micas and feldspars, hydroxo oxide (bauxite), cryolite (Na3AlF6). 

Occurrence. Silicon is the second most abundant element in the earth s crust, after oxygen (about 28% by weight). It occurs mainly in oxides (quartz, agate, opal, etc.) and a great variety of silicate minerals (feldspar, clay, mica, olivine, etc.). 

Pyroxenes (as feldspars) occupy a position that is chemically central in the realm of rock compositions (Robinson, 1982). They are therefore found ubiquitously and virtually in any kind of paragenesis developing in the P-T-Xconditions typical of earth s mantle and crust. Table 5.31 hsts the main occurrences. 

The P-T stability curve of muscovite intersects the incipient melting curve of granite in hydrous conditions at about 2.3 kbar total pressure and T = 650 °C. Thus, muscovite may crystallize as a primary phase from granitic melts above these P and T conditions (interstitial poikihtic crystals) or may form by reaction with pristine solid phases at lower P and T (muscovite as dispersed phase within feldspars, for instance). In this second type of occurrence, the following two equilibria are of particular importance ... 

Table 5.61 summarizes synthetically the observed occurrences of various trace elements in feldspars, based mainly on the indications of Smith (1983). 

Sr 10-100 ppm 100-5000 ppm In alkaline feldspars of pegmatites. In other occurrences. Marked correlation with Na in comagmatic series (Steele and Smith, 1982). 

Natural occurrences in hydrothermal areas show that the replacement of analcite + quartz by albite probably takes place near 150-180°C (Coombs, et al., 1959) at several hundred meters depth. The observed upper limit of analcite appears to be 100-125°C in deeply buried rocks in Japan 5Km depth). In other rocks for which no temperature data are available analcite can be found to coexist with sodium feldspar (High and Picard, 1965 Iijima and Utada, 1966 Iijima and Hay, 1968 Otalora, 196A Callegari and Jobstribitzer, 1964 Gulbrandsen and Cressman, 1960). Several authors have indicated that analcite replaces other zeolites in buried sequences of rocks (Moiola, 1970 Sheppard, 1970 Iijima and Hay, 1968 Iijima, 1970 Gude and Sheppard, 1967) but this is certainly not the rule since analcite is frequently associated with other zeolites as a primary mineral in soils sediments and sedimentary rocks (Hay, 1966). 

Due to the conflicting experimental results, we will rely upon natural occurrence and we will place an 80°C limit on the initial zeolite paragenesis where solid solution is maximum. An intermediate stage exists up to temperatures near 100°C at high water pressure where analcite, potassium feldspar and alkali zeolite can coexist as can analcite and albite. Analcite persists up to 180°C where the upper limit of the alkali zeolite facies is reached. 

These associations are noted by Hay (1966) as being found in sequences of sedimentary rocks or altered pyroclastics buried to depths greater than 3,000 meters and generally less than 10,000 meters. However, the limits are actually vague and the identifications imprecise. The relatively frequent occurrence and persistence of albite or potassium feldspar and alkali zeolite in such rocks leads one to believe that they can coexist stably in nature. This could be, however, a misleading conclusion based upon too few observations. The elimination of the silicic, alkali zeolites and the persistence of montmorillonite is known to exist in series of deeply buried rocks (Ii-jima, 1970 Moiola, 1970 Iijima and Hay, 1968). 

A consideration of natural occurrence and chemical composition of alkali zeolites allows a certain refinement of the zeolite facies concept previously proposed. The key factor is the grouping of the alkali zeolites into a continuous solid solution series. Other possible coexisting phases of similar composition are sodium and potassium feldspar, natrolite and analcite. The extent of solid solution decreases with temperature, possibly also with pressure. This effect allows the sequential series zeolite-K feldspar, zeolite-analcite-K feldspar, analcite-K feldspar-albite and eventually two feldspars to the exclusion of analcite, the alkali zeolite with the highest stability limits. 

Yund, R.A. (1984) in Feldspars and Feldspathoids Structure, Properties and Occurrences (Ed. W.L. Brown), D. Reidel Publ. Comp., Dordrecht... 

The occurrence of minerals which show CL is highly dependent on the type of meteorite. Possibly the most common phase which occurs is feldspar. Because this mineral accepts very little Fe into the structure, quenching is not a problem however, because the feldspar structure is quite open, the Na- and K-rich feldspars are easily damaged by electron beams. In contrast anorthite, the Ca rich variety, is quite stable. Pyroxene and olivine are common phases in meteorites but because they both usually contain iron, most do not luminesce. Only in the primitive meteorites do nearly pure enstatite and forsterite occur and both show brilliant CL. Other minerals are rare but include ... 

That the atomic weight of uranium lead is extremely variable has already been shown. In order to interpret this variability its sources must be studied both geologically and mineralogically. On the geologic side of the question the uranium ore can be divided in to three principal classes, which are sharply distinct. The definitely crystallized varieties of uraninite occur in coarse pegmatites, associated with feldspar, quartz, mica, beryl, and other minor accessories. The massive pitchblende is found in metalliferous veins, together with sulphide ores of copper, lead, iron, zinc, and so forth. As for camotite, that is a secondary mineral, found commonly as an incrustation on sandstone, and often, also upon fossil wood. There may be other modes of occurrence, but these are the most distinctive. 

Gandais, M., Willaime, C. (1984). Mechanical properties of feldspars. In Feldspars and Felspathoids Structures, Properties and Occurrences, edited by W. L. Brown, pp. 207-46. NATO ASI Series. Dordrecht D. Reidel. 

Feldspars constitute a significant component of igneous rocks and their occurrence in nature is thus very widespread. However, they are not found in pure form potassium feldspar always contains sodium, and sodium feldspar usually contains calcium feldspar. In addition to this, feldspars contain accessory minerals, mainly quartz, mica and other silicates. 

The Ca content of soils varies widely, ordinarily ranging from about 0.07 to 3.60%. Calcium is contained in a number of soil minerals including dolomite, calcite, Ca feldspars, apatite, amphiboles and many others. Coarse-tex-tured soils in humid regions, particularly those formed from rocks low in Ca minerals, are generally low in Ca. In spite of this, Ca deficiencies in crops do not appear to be of widespread occurrence, although a number of other factors affect nutrition in low base status soils. Fertilizers are not generally manufactured specifically to provide Ca as a plant nutrient, since this element is economically supplied by periodic applications of agricultural lime, as discussed subsequently. 

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]. 

Meteoric and surface waters, percolating through the thick layers of thin volcanic material, become always more basic and saline, as they penetrate in depth. This results in a vertical zonation with formation of clays near the surface and then, going downwards, zeolites, from open types (clinoptilolite, chabazite, phillipsite) to narrow-pore (analcimc), and finally alkali-feldspars. Generally, only one or two zeolite species are formed in open systems. A typical example of this type of occurrence is the John Day Formation, Oregon, characterised by fresh glass and clinoptilolite zones, plus sporadic K-feldspar [35]. 

This system is represented by a closed basin, made of impermeable rocks and filled in the past by a saline alkaline lake. Water in this case could not permeate downwards but only evaporate, so the deposit develops horizontally, instead of vertically as in the previous occurrence. Here pH and salinity in the fluids tend to increase, giving rise to brines, c.g., basic, alkali-rich solutions. Concentric zones of authigenic minerals are so formed, from an outer and upper ring of little altered glass and clay minerals, to zeolites, analcime and a finally alkali-feldspars. A good example for this type of occurrence is Lake Tecopa, California, where the zeolitic ring is constituted by phillipsitc, clinoptilolite and erionite, followed by the central feldspar zone [36]. 

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