Potassium Fixation by Clay Minerals during Hydrothermal Treatment

Abstract--The compositions, fabrics and structures of authigenic minerals that formed recently from silicic volcanic ash layers from a 1300-meter sediment column obtained at ODP Site 808 of the Nankai Trough were studied using XRD, STEM, AEM and SEM. Smectite and zeolites were first detected as alteration products of volcanic glass with increasing depth, as follows: smectite at 200 m below seafloor (mbsf) (20 ~ clinoptilolite at 640 mbsf (60 ~ and analcime at 810 mbsf (75 ~ A primitive clay precursor to smectite was observed as a direct alteration product of glass at 366 mbsf (approximately 30 ~ High defect smectite with lattice fringe spacings of 12 to 17 A and having a cellular texture filling pore space between altering glass shards occurs at 630 mbsf. Packets of smectite become larger and less disordered with increasing depth and temperature. The smectite that forms as a direct alteration product of volcanic glass has K as the dominant interlayer cation.With increasing depth, smectite becomes depleted in K as the proportion of clinoptilolite increases, and then becomes depleted in Na as the proportion of analcime increases. The composition of the exchangeable interlayer of smectite appears to be controlled by the formation first of K-rich clinoptilolite and then Na-rich analcime, via the pore fluid, giving rise at depth to Ca-rich smectite. Smectite reacted to form illite in the interbedded shales but not in the bentonites. Paucity of K in smectite and pore fluids, due to formation of clinoptilolite under closed system conditions, is believed to have inhibited the reaction relative to shales.

Section: Relation Of Interlayer Cation Composition To the Smectite-tomentioning

confidence: 99%

Relation Between Interlayer Composition of Authigenic Smectite, Mineral Assemblages, I/S Reaction Rate and Fluid Composition in Silicic Ash of the Nankai Trough

Masuda

1996

“…A more detailed description of the analytical techniques used can be found in Inoue et al (1978) and Inoue (1983); complete data are reported in Inoue et al (1978) and Inoue (1980).…”

Section: Ka-]mentioning

confidence: 99%

Further Investigations of a Conversion Series of Dioctahedral Mica/Smectites in the Shinzan Hydrothermal Alteration Area, Northeast Japan

Inoue

Utada²

1983

Self Cite

Abstract--A complete conversion series for mica/smectites was found in a hydrothermal alteration envelope around Kuroko-type ore deposits at the Shinzan area, Akita Prefecture, Northeast Japan. The minerals are an alteration product of volcanic glass of Miocene age and are commonly associated with zeolites and silica minerals. Degrees of ordering of interstratification of the minerals change discontinuously from Reichweite g = 0 (100-55% expandable layers) to g = 1 (45-20% expandable layers), and from g = 1 to g = 2 (<20% expandable layers). This pattern of conversion differs from the behavior of mica/smectites during burial diagenesis which undergo a continuous change in ordering type, and from the behavior of rectorite which displays a constant expandability and ordering (45-55%) over a wide range of conditions. Differences between these minerals were also found in the relationships between expandability and total layer charge, and between expandability and number of non-exchangeable interlayer cations. In mica/smectites from the Shinzan area, chemical changes in the interlayers and tetrahedral and octahedral sites are consistent with a reaction in which K-enrichment and K-fixation in the interlayers are controlled by an increase in negative layer charge. This conversion occurred in response to a steep geothermal gradient and migrating hydrothermal solutions.

“…The kinetics of the smectite to illite reaction during shale diagenesis have been studied extensively using experimental techniques (Eberl and Hower, 1976;Roberson and Lahann, 1981;Inoue, 1983;Howard and Roy, 1983;Whitney and Northrop, 1988;Whitney, 1990). Huang et al (1991) have shown that illite/smectite composition versus depth profiles can be calculated using an experimentally derived kiCopyright 9 1993, The Clay Minerals Society netic model assuming knowledge of pore fluid K concentration.…”

Section: Introductionmentioning

confidence: 99%

Experimental Determination of the Rates of Precipitation of Authigenic Illite and Kaolinite in the Presence of Aqueous Oxalate and Comparison to the K/Ar Ages of Authigenic Illite in Reservoir Sandstones

Small

1993

Abstract--The importance of precipitation rate as an effective control on illite and kaolanite formation during diagenesis has been examined by measuring precipitation rates, from A1 fluid concentration, in a Dickson fluid-sampling vessel at 160~ ~ and 500 bars (50 MPa). These experiments are considered to be analogues of the precipitation of clays in sandstones from porewaters containing dissolved carboxylic acids, which have a transient stability and may influence aluminosilicate solubility. Precipitated illite had a lath-shaped morphology and its composition was consistent with authigenic iUite in sandstones. Kaolinite formed tabular rather than vermicular shaped crystals. Kaolinite precipitation rate was two orders of magnitude faster than iUite precipitation and was rate-limited by the decomposition of oxalate; kaolinite formation should be equilibrium-controlled at virtually all stages of burial. Extrapolation of illite precipitation rate to burial temperatures indicates that the first appearance of illite in a burial sequence may be kinetically controlled. A model of illite precipitation based on these experimental results has been used to predict the time required to precipitate illite during burial of a sandstone, taking into account temperature changes during burial. For northern North Sea examples, a predicted illitization threshold of -60~ occurring at 60-80 Ma corresponds to the observed initiation of authigenic illite precipitation. Times of around 2-5 Ma would be required to reach a 98% approach to equilibrium at this threshold. The main phase of illite precipitation in the northern North Sea basin is a later, hydrologically controlled event (30-50 Ma). Equilibrium would be approached in around 0.1 Ma during this phase, which is consistent with the narrow illite K/Ar age range (1-5 Ma) recorded for some sequences.