Illite hydrothermal

Hydrothermal alteration at Pebble consists of a central, strong K-silicate assemblage with sparse magnetite, and peripheral sericitic alteration that overprints the deposit, with propylitic and illite assemblages present locally (Rebagliati Lang 2008). 

The size of the reactors is quite variable. In length, the biggest reactor has dimensions of 12 x 18 m and has a thickness of 20 to 50 cm (Fig. la). The core of the reactors consists of a 5 to 20 cm thick layer of uraninite embedded in clays (illite and chlorite). Clays around the reactors result from the hydrothermal alteration of the host sandstone during the fission reactions. This alteration occurred at a temperature close to 400 °C in the core. Temperature decreased drastically toward the vicinity with a thermal gradient of 100 °C/m (Pourcelot Gauthier-Lafaye 1999). The uranium content of the core ranges between 40 and 60%. Accessory minerals are mainly sulphides (pyrite and galena), hematite and phosphates (mainly hydroxyapatite). 

Probably the most passionately debated mineral (if one might use this adverb in a discussion of clays) found in argillaceous sediments, rivalling perhaps the enigmatic dolomite and dolomitization in the realm of sedimentary rocks, is the mineral or group name illite. Defined and redefined by its originator, R. E. Crim debated and further redefined, denied a proper existence and reprieved, this species has attracted the attention of clay mineralogists for the past two decades. It represents, in fact, the dominantly potassic, dioctahedral, aluminous, mica-like fraction of clay-size materials. Known as sericite or hydro-mica in studies of hydrothermal alterations, soil mica or illite in soils and illite... 

Studies of hydrothermal alteration products associated with ore mineralization in acidic rocks have established the general propensity for the original minerals to be replaced by illite, sericite or hydromica in the innermost zone near the source of hydrothermal fluids and by kaolinite or expandable minerals further from the vein or center of fluid emanation. The newly-formed "mica" can be 2M, 1M, or lMd in polymorph and range compositionally from muscovite to a low potassium, silicic species which can be assimilated in the term illite (Lowell and Guilbert, 1970 Schoen and White, 1966, 1965 Kelly and Kerr, 1957 Bonorino, 1959 Tomita, e al., 1969 Yoder and Eugster, 1955 Meyer and Hemley, 1959, among many authors). 

In hydrothermal alteration the transformation of muscovite to an aluminous illite has been noted (Kelley and Kerr, 1957). 

Normal aluminous illite common in pelitic sediments, altered volcanic ash beds, hydrothermal alterations of acidic rocks and weathering products of these rocks. 

In zones of hydrothermal alteration it is apparent that the formation of dioctahedral montmorillonites is limited by temperature. They almost never occur in the innermost zone of alteration, typically that of sericitization (hydro-mica or illite), but are the most frequent phase in the argillic-prophylitic zones which succeed one another outward from the zone where the hydrothermal fluid is introduced in the rock. Typically, the fully expandable mineral is preceded by a mixed layered phase (Schoen and White, 1965 Lowell and Guilbert, 1970 Fournier, 1965 Tomita, et al., 1969 Sudo, 1963 Meyer and Hemley, 1959 Bundy and Murray, 1959 Bonorino, 1959). However, temperature is possibly not the only control of expandable clay mineral occurrence, the composition of the solutions and the rock upon which they act might also be important. It is possible that high magnesium concentrations could form chlorite, for example, instead of expandable minerals. 

Clay minerals occur in all types of sediments and sedimentary rocks and are a common constituent of hydrothermal deposits. They are the most abundant minerals in sedimentary rocks perhaps comprising as much as 40% of the minerals in these rocks. Half or more of the clay minerals in the earth s crust are illites, followed, in order of relative abundance, by montmorillonite and mixed-layer illite-montmorillonite, chlorite and mixed-layer chlorite-montmorillonite, kaolinite and septachlorite, attapulgite and sepiolite. The clay minerals are fine-grained. They are built up of tetrahedrally (Si, Al, Fe3+) and octahedrally (Al, Fe3+, Fe2, Mg) coordinated cations organized to form either sheets or chains. All are hydrous. 

The term sericite is frequently used to describe fine-grained dioctahedral micas. This material is usually coarser than illites and often hydrothermal in origin. Sericites... 

A number of Al chlorites in which both octahedral sheets are dioctahedral have recently been described. Dioctahedral Al chlorites have been reported in bauxite deposits (Bardossy, 1959 Caillere, 1962). These chlorites appear to have been formed by the precipitation-fixation of Al hydroxide in the interlayer position of stripped illite or montmorillonite. A similar type of chlorite, along with dioctahedral chlorite-vermiculite, occurs in the arkosic sands and shales of the Pennsylvanian Minturn Formation of Colorado (Raup, 1966). Bailey and Tyler (1960) have described the occurrence of dioctahedral chlorite and mixed-layer chlorite-montmorillonite in the Lake Superior iron ores. Hydrothermal occurrences have been described by Sudo and Sato (1966). 

Heystek (1955) hydrothermal alteration of illite, South Africa analyst D. Sampson. 

Mixed-layer clays form by the alteration of pre-existing micas and illites, by hydrothermal action, by the alteration of volcanic glass, and by diagenetic alteration of montmorillonite. During continental weathering K is leached from micas and illites and mixed-layer clays are formed. When these clays are carried to the sea, they may adsorb K and revert partially or entirely to illite. If they remain in a continental environment where K may not be available, the expanded layers can persist until K... 

The Marblehead illite used in hydrothermal laboratory experiments by Aja et al (1991a, 1991b) has the structural formula... 

NH4-rich phyllosilicates with K > NH4 and with (001) spacing values intermediate between illite and tobelite are referred to as NBU-rich illite. They occur in hydrothermal environments (Sterne et al. 1982 Higashi 1982 Von Damm et al. 1985 Wilson et al. 1992 Bobos and Ghergari 1999) in black-shales (Sterne et al. 1984) in regionally metamorphosed carbonaceous pelites (Juster et al. 1987 Daniels et al. 1996 Liu et al. 

Ylagan RF, Altaner SP, Pozzuoli A (2000) Reaction mechanisms of smectite illitization associated with hydrothermal alteration from Ponza Island, Italy. Clays Clay Minerals 48 610-631 Zhoukhlistov AP, Zvyagin BB, Lazarenko EK, Pavlishin VI (1977) Refinement of the crystal stracture of ferrous seladonite. Sov Phys Ciystallogr 22 284-288... 

A zonal distribution of clay minerals occurs in the East Hachimantai thermal area, characterized by smectite - illite/smectite - illite/chlorite, with the latter tending to occur in the vicinity of hot upflow zones. The smectite is characteristically Ca-smectite. The occurrence of Ca-bearing zeolite minerals, such as laumontite and wairakite correspond to the presence of hot hydrothermal fluids near in the center of the geothermal resource. In contrast, Na-smectite and Na-zeolite (e.g. clinoptilolite - mordenite - analcime) in marine sediments and pyroclastic sequences tend to envelope the main thermal area. Inoue et al. (2001) and Hara et al. (2001) have described the style and distribution of alteration in the Hachimantai area. The Na-enriched alteration zones contain higher Na concentrations than... 

Chermak, J.A., Rimstidt, ID. (1990). Hydrothermal transformation rate of kaolinite to muscovite/illite. Geochimica et Cosmochimica Acta, 54, 2979-2990. 

Illite occurs as fibrous material filling up sandstone pores, with or without smectite. It often replaees kaolinite pseudomorphously. The process of sericitization produces illite from feldspar at lower part of weathering zones. Illitization also takes place in hydrothermal environments around ore deposits. 

Glauconite Sedimentary rocks, low hydrothermal in igneous rocks ( ) a) From illite syn-sedimentary or by diagenesis (dissolution-repredpitation) b) From colloidal solutions in pore spaces of marine sediments To illite by leaching Neoformation in sediments or synsedimentary... 

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