Supergene Vermiculitization of A Magnesian Chlorite: Iron and Magnesium Removal Processes

Abstract--An X-ray diffraction study of vermiculitized micas in ultramafic and marie intrusive rocks from Cheongyang, Korea, shows the following weathering sequence: mica --* ordered mica/vermiculite interstratification --, vermiculite. Electron microprobe analyses show the general trends of K leaching and Ca enrichment with increased weathering. The vermiculitization of phlogopite from ultramafic rocks proceeds by means of a continuous decrease in Al-for-Si tetrahedral substitutions and a progressive increase in Al-for-(Fe 2+ + Mg) octahedral substitutions in the early stage of weathering. These substitutions occur to compensate for the excess of negative charge in the mica-like layer, in agreement with currently accepted vermiculitization mechanisms. They change to a slight increase of Al-for-Si tetrahedral substitutions in the late stage of the vermiculitization of phlogopite, owing to the oxidation of Fe despite its tow content. However, the behavior of Fe in the late stage of the transformation of biotite into vermiculite is significantly different; that is, Fe increases substantially. The reason for this Fe increase in the late stage remains unresolved. Recalculations of the structural formulas on the basis of several assumptions indicate that the oxidation of Fe is necessary for the vermiculite derived from biotite to form the reasonable structural formulas.

Section: Consideration Of Structural Formula For Vermiculitesupporting

confidence: 88%

Supergene Vermiculitization of Phlogopite and Biotite in Ultramafic and Mafic Rocks, Central Korea

Moon

1994

“…However, as pointed out earlier, neither corrensite nor Mg-rich chlorite has ever been described from modern evaporitic environments (Bodine and Madsen, 1987). Ordered regularly interstratiffed corrensite-like minerals have frequently been described from the weathering environment (Johnson, 1964;Proust et al, 1986); but it is almost certainly misleading to group these minerals with diagenetic corrensites. For instance, Wilson and Nadeau (1985) have argued that interstratified clays formed by transformation from coarse grained phyllosilicates during weathering are fundamentally different from the fine grained interstratified clays formed during diagenesis.…”

Section: Origin Of Chlorite Minerals In the 'Evaporite Carbonate" Assmentioning

confidence: 99%

Origin, Diagenesis, and Mineralogy of Chlorite Minerals in Devonian Lacustrine Mudrocks, Orcadian Basin, Scotland

Hillier

1993

114

Abstract--Chlorite and corrensite are common clay minerals in lacustrine mudrocks from the Devonian Orcadian Basin, Scotland. The relationship of their occurrence to vitrinite reflectance data demonstrate that they are authigenic minerals, formed during burial diagenesis/metamorphism at temperatures of -> 120"C. Whole rock mineralogical and chemical analyses show that chlorite authigenesis occurred by reactions between the detrital dioctahedral clay mineral assemblage and dolomite that was formed under early evaporitic conditions in the lacustrine environment.XRD and electron microprobe analyses indicate that phases intermediate between corrensite and chlorite are probably mixed-layer chlorite/corrensite with a tendency towards segregation of layer types. Chemically, the conversion of corrensite to chlorite involves an increase in A1 for Si substitution in tetrahedral sites, but there is no change in the Fe/Mg ratio of octahedral cations. There is also no relationship of mixed-layer proportions to paleotemperature; only a general paleotemperature interval of approximately 120 ~ to 260~ in which a range of phases between corrensite and chlorite occurs. Chlorite polytypes are exclusively IIb, indicating the formation of this polytype at diagenetic temperatures.The occurrence of corrensite and Mg-rich chlorite in evaporite and carbonate successions is probably a reliable indicator ofdiagenetic alteration at temperatures of >-100~ Burial diagenetic reactions between dioctahedral clay minerals and Mg-rich carbonates may possibly explain many occurrences of corrensite and Mg-rich chlorite in such rocks.

“…Chlorite can be transformed to vermiculite when the hydroxide sheet is disrupted by thermal dehydroxyl-ation or low-temperature acid leaching (Ross and Kodama, 1974;Bain, 1977;Proust et aL, 1986;Wilson, 1986). The removal of iron from the chlorite hydroxide sheet is an essential part of the process, and a high iron concentration in the hydroxide sheet appears to be a necessary prerequisite for chlorite vermiculitization.…”

Section: Introductionmentioning

confidence: 99%

“…The removal of iron from the chlorite hydroxide sheet is an essential part of the process, and a high iron concentration in the hydroxide sheet appears to be a necessary prerequisite for chlorite vermiculitization. The iron can be oxidized (Makumbi and Herbillon, 1972;Ross, 1975) or unoxidized (Proust et al, 1986) by the vermiculitization process, and sedimentary goethite is sometimes reported as a reaction product (Anand and Gilkes, 1984).…”

Section: Introductionmentioning

confidence: 99%

Chlorite Vermiculitization and Pyroxene Etching in an Aeolian Periglacial Sand Dune, Allen County, Indiana

Argast

1991

Abstract--Weathering has etched and deeply-dentieulated the constituent orthopyroxenes, and chlorite has been transformed to vermiculite in the upper 3 m of an aeolian, periglacial sand dune formed in northeastern Indiana about 13,000 b.p. Pyroxene weathering begins with the development of cleavageparallel etch pits on {010} and ( 100 } surfaces. These pits coalesce and eventually crop out on basal surfaces as denticulations. The mean denticulation size increases logarithmically toward the surface, and the denticulation size of the orthopyroxenes is a quantifiable feature of the weathering process. Ferruginous pendants, microboxworks of iron oxides, and other indications of iron redeposition within the orthopyroxene microenvironments were not observed.Chlorite in the dune has been weathered to vermiculite. The parent chlorite is a high-Fe variety, and the transformation to vermiculate does not involve the development of a chlorite/vermiculite intermediary phase. Fe 2 § is oxidized as part of the transformation process and this iron is retained in the sediment as discrete goethite and as crystalline and noncrystalline coatings on the dune grains. The vermiculite from depths shallower than 64 cm is only partly expandable and is completely collapsed by K-saturation or heat treatment. This is a hydroxy-A1 vermiculite and its formation is typical of intense weathering under the acid conditions prevalent at the dune surface.