Chlorite weathering

Weathering of the silicate minerals Is primarily a process of hydrolysis. Much of the silica that Is released by weathering forms silicic acid but, when liberated in large quantities, some of it may form colloidal or amorphous silica. Mafic silicates usually decay more rapidly than felsic silicates and, in the process, they release magnesium, iron and lesser amounts of calcium and alkalies. Olivine is particularly unstable, decomposing to form serpentine, which forms talc and carbonates on further weathering. Chlorite is the commonest alteration product of augite (the principal pyroxene) and of hornblende (the principal amphibole). 

Chlorite A magnesium-rich clay mineral produced by terrestrial weathering under polar and subpolar conditions. 

Clay mineral A layered aluminosilicate, such as kaolinite, dUte, chlorite, and montmordlonite. Most are formed by chemical weathering of rocks on land. 

Anand, R.R. Gdkes, R.J. (1984) Weathering of hornblende, plagiodase and chlorite in meta-dolerite, Australia. Geoderma 34 261—280 Anand, R.R. Gdkes, R.J. (1984a) Weathering... 

Murakami, T., Isobe, H., Nagano, T. Nakashima, S. 1991. Uranium redistribution and fixation during chlorite weathering at Koongarra, Australia. Proc. Scientific Basis for Nuclear Waste Management XV, held 4-7 November, Strasbourg, France, published by the Materials Research Society, USA, 473-480. 

Si, Fe and Fe is variable. Illite also appears to be the early product of weathering in cycles of intense alteration or one of the stable products under intermediate conditions (Jackson, 1959). It is apparently stable, or unaffected by transport in rivers for relatively short periods of time (Hurley, et al., 1961) but does change somewhat in the laboratory when in contact with sea water (Carroll and Starkey, 1960) it has been reported to be converted to chlorite or expandable minerals upon marine sedimentation (Powers, 1959). However, Weaver (1959) claims that much sedimentary illite is "reconstituted" mica which was degraded to montmorillonite by weathering processes. It is evident that a certain and usually minor portion of illite found in sedimentary rocks is of detrital origin (Velde and Hower, 1963) whether reconstituted or not. 

In sum, one can say that 14 8 trioctahedral brucitic chlorite is largely unstable in most weathering environments, but aluminous soil chlorites are common under acid conditions. The bulk of chlorite found in sediments is certainly detrital in origin. 7 and 14 8 chlorites can be formed from 50°C upward in temperature until above 100°C where 14 8 chlorite becomes one of the most common minerals in sedimentary rocks. 

The initial increase in hydrostatic pressure in a sedimentary basin appears not to change mineral stabilities from those of the weathering environment. The formation of potassic, iron-rich micas such as ferric illite and glauconite both in lacustrine and shallow ocean basins demonstrates their stability at low pressures and temperatures. The same is true of the 7 8 chlorite chamosite or berthierine. Most likely the chemical variables of pH, Eh and the activity of the various ions in solution are predominant in silicate phase equilibria in sedimentary environments. 

JOHNSON (L.J.), 1964. Occurrence of regularly interstratifled chlorite-vermiculite as a weathering product of chlorite in a soil. Amer. 

QUIGLEY (R.M.) and MARTIN (R.T.), 1963. Chloritized weathering products of a New England glacial till. Clays and Clay Min. 10, 107-16. 

VEN1ALE (F.) and VAN DER MAREL (H.U.), 1963. An interstratified saponite-swelling chlorite mineral as a weathering product of Lizardite rock from St. Margherita Staffora (Pavia Province), Italy. Beitr dge Min. Petr. 9, 198-245. 

Some of the high Fe values may be real. During weathering under neutral to acid conditions, the Mg-rich brucite sheet tends to be stripped out and removed from the immediate environment. If new material is precipitated between the talc layers, it is more apt to be Fe and Al than Mg. As chlorites go through the sedimentary cycle, perhaps several times, their average Fe and Al content will tend to increase. 

Trioctahedral clay chlorite is an abundant constituent of soils formed by the weathering of basic volcanic pumice and tuffs in North Wales (Ball,1966). The adjusted chemical analysis (29.35% Si02, 16.82% A1203, 4.42% Fe203, 15.08% FeO, 0.25% MnO, 21.54% MgO, 12.00% H20+, 0.54% H20 ) produces the following structural formula ... 

The X-ray method affords a reasonable estimate of the structural formula based on chemical data. Using X-ray data, Ball calculated the structural formulas for twenty-six weathered soil and vein clay chlorites from North Wales. Tetrahedral Al ranged from 1.0 to 1.7 which is similar to the values for chlorites in shales. Octahedral Fe ranged from 0.8 to 2.4, with all but two values being less than 1.6 these values are much lower than those calculated (X-ray) for shale samples but almost identical to the shale values based on chemical determination of the Fe content. 

Most of the chlorite-like material formed in soils is dioctahedral rather than trioctahedral. In the process of weathering, illite and muscovite are stripped of their potassium and water enters between the layers. In these minerals and in montmoril-lonites and vermiculites, hydroxides are precipitated in the interlayer positions to form a chlorite-like mineral (Rich and Obenshain, 1955 Klages and White, 1957 Brydon et al., 1961 Jackson, 1963 Quigley and Martin, 1963 Rich, 1968). Al(OH)3 and Fe(OH)3 are likely to be precipitated in an acid to mildly basic environments and Mg(OH)2 in a basic environment. The gibbsite sheets in the soil chlorites are seldom complete and the material resembles a mixed-layer chlorite-vermiculite. The gibbsite may occur between some layers and not between others or may occur as islands separated by water molecules. 

Vermiculite and vermiculite layers interstratified with mica and chlorite layers are quite common in soils where weathering is not overly aggressive. (A few references are Walker, 1949 Brown, 1953 Van der Marel, 1954 Hathaway, 1955 Droste, 1956 Rich, 1958 Weaver, 1958 Gjems, 1963 Millot and Camez, 1963 Barshad and Kishk, 1969.) Most of these clays are formed by the removal of K from the biotite, muscovite and illite and the brucite sheet from chlorite. This is accompanied by the oxidation of much of the iron in the 2 1 layer. Walker (1949) has described a trioctahedral soil vermiculite from Scotland formed from biotite however, most of the described samples are dioctahedral. Biotite and chlorite with a relatively high iron content weather more easily than the related iron-poor dioctahedral 2 1 clays and under similar weathering conditions are more apt to alter to a 1 1 clay or possibly assume a dioctahedral structure. 

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