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Sorption
Published in Igor Bello, Vacuum and Ultravacuum, 2017
Zeolites are mostly aluminosilicates. They are a blend of alumina and silica. Hence, they are opaque and have a white chalk-like appearance. Detailed description of zeolites and their structures can be found, for example, in the reference by Čejka et al.357 Zeolites can be found in nature and they can also be prepared synthetically. In nature, about 40 types of zeolites have been identified. One of the natural zeolites is stilbite composed of Na2Al2Si3O10–2H2O molecules. Zeolites are crystalline porous aluminosilicates, which confine water and loosely bonded sodium (Na+), potassium (K+), calcium (Ca+2), or magnesium (Mg2+) cations. These cations give rise to different pore sizes. Synthetic zeolites have an advantage over natural zeolites in purity and engineering of the pore sizes for specific use. More than 150 types of synthetic zeolites have been prepared so far. Synthetic zeolites, used in vacuum cryosorption pumps, are prepared in bead or pellet forms with a diameter of 1.5–4 mm and pores typically ranging from 3 to 10 Å (0.3 to 1.0 nm). The A and X types (the capital letter refers to the zeolite structures) are common commercial zeolite structures used in vacuum technology. For example, zeolite molecular sieves 4A is the sodium aluminosilicate (typical chemical formula Na2O/Al2O3/2SiO2/4.5H2O) with the A zeolite structure and a pore size of 4 Å in diameter. Molecular sieves 4A are used for drying and removing carbon monoxide. Substitution of sodium cations in the 4A structure by potassium cations gives molecular sieves 3A with pores 3 Å in diameter, while the substitution of two sodium cations by a single calcium cation yields zeolite molecular sieves with pore sizes of 5 Å in diameter. Due to the small pore size, the molecular sieve 3A zeolites have selective adsorption properties. Similarly, the size of pores in the X structure can be engineered by substitution of one type of alkali cation with another. For instance, zeolite 13X with the chemical formula Na2O · Al2O3· 2.45SiO2 · 6.0H2O have a pore size of 10 Å in diameter.
Correlation between the Warepan/Otapirian and the Norian/Rhaetian stage boundary: implications of a global negative δ13Corg perturbation
Published in New Zealand Journal of Geology and Geophysics, 2022
The Arawi Shellbeds and Ngutunui Formation are a succession of volcaniclastic sedimentary rocks dominated by thin sandstones, siltstones and shales, with minor but conspicuous conglomerates, tuffs and shellbeds (Figure 3). Limestones are not present, although the shellbeds approach coquina limestone composition in places within the Arawi Shellbeds (Grant-Mackie 1985), and there are no radiolarian cherts. Compositionally, the volcanic lithologies are broadly andesitic but range from basaltic andesite to dacite. The tuffs vary in vitric, crystal and lithic composition. In every respect, the sedimentary rocks in the Kiritehere section are typical of Murihiku Supergroup. They have been weakly metamorphosed to zeolite facies grade with conspicuous zeolite veining (laumontite, stilbite) and zeolite ‘cements’ (laumontite, heulandite, analcime). There are minor faults in places and also some ‘slumps’ (Grant-Mackie and Lowry 1964) but in general the stratigraphy is more or less ‘layer cake’, easy to recognise in the field, and the named formations and groups can be traced for many tens of kilometres. However, the sedimentary sequence as a whole has been folded, and a number of anticlines and synclines are recognised and named within a broader Kawhia Regional Syncline (e.g. Kear 1960; Edbrooke 2005). In the Kiritehere section, the sequence is dipping and younging to the east, and is part of a large homoclinal structure that may be interpreted as the west limb of a syncline (e.g. Kear 1960; Kear and Schofield 1964; Edbrooke 2005).
Metamorphism in the New England Orogen, eastern Australia: a review
Published in Australian Journal of Earth Sciences, 2020
K. Jessop, N. R. Daczko, S. Piazolo
In the SNEO, studies of metamorphism in the forearc basin of the passive-compressive Currabubula-Connors Arc phase show that metamorphism is very low grade. Illite crystallinity studies of K-white mica (Offler & Hand, 1988) revealed diagenetic (zeolite) to low sub-greenschist (prehnite–pumpellyite) facies. Offler, Roberts, Lennox, and Gibson (1997) studied rocks from the northern to the southeastern end of the Tamworth Belt (Figure 1) and recognised four zones increasing with stratigraphic depth from zeolite (heulandite–clinoptilolite ± stilbite) facies to sub-greenschist (prehnite ± pumpellyite ± epidote) facies with a calculated geothermal gradients of 9−12 °C/km. Offler et al. (1997) concluded that metamorphism resulted from burial and that relatively cold lithosphere was subducted at the time of basin sedimentation.
Murihiku rocks as potential petroleum reservoirs in Zealandia
Published in New Zealand Journal of Geology and Geophysics, 2018
Karen E. Higgs, Greg H. Browne, Angus D. Howden
Other authigenic minerals that are commonly associated with Murihiku sandstones are zeolites, with some of the more common ones including laumontite, heulandite, analcime, clinoptilolite, stilbite (e.g. Ballance et al. 1980; Boles 1974; Briggs et al. 2004; Challis & Geotech in STOS 1991). These observations are consistent with our results, which show a range of zeolite minerals occluding vugs and pores. Coombs (1954) and Black et al. (1993) noted a broad correlation between zeolite species and stratigraphic position, and Black et al. (1993) suggested that the type of metamorphic crystallisation is also related to open folding, hydraulic fracturing and veining. This could not be confirmed with our data. However, zeolites appear to be more abundant in samples with little or no degraded matrix (Table 2), which may imply that zeolite mineralogy and abundance are affected by variable sample degradation/alteration.