Clays and Clay Minerals

Transcription

1 Clays and Clay Minerals Fields of interest for clays Various definitions Acients: Earths in the earth-air-fire-water system Definition of clay depends on discipline: Geologist grain size <2 μm Engineer: property of plasticity Ceramicist: hardening on firing No chemical definition, but: Most clays are high in aluminum Basic building block of silica minerals is the SiO -4 4 silica tetrahedron Linked together at apical oxygen to form sheets Tetrahedral sheet does not exist by itself General: Minerals which normally dominate the fine (<2 μm ) fraction of rocks and soils. of clay minerals In clay minerals the tetrahedral sheet is always combined with an octahedral sheet A cation (Al +3, Mg +2, Fe +2 etc. but not Ca +2, Na +1, K +1 ) is surrounded by 6 neighbors (oxygens or hydroxyls) Minerals composed of octahedral sheets. Brucit Mg 3 (OH) 6 Gibbsite Al 2 (OH) 6 Formula of the tetrahedral sheet: Si 4 O 10-4 due to negative charge: only exists in combination with cations and additional oxygens Formula of the octahedral sheet: Brucite: Mg 3 (OH) 6 all octahedrals occupied trioctahedral Gibbsite: Al 2 (OH) 6 2/3 of octahedrals occupied dioctahedral 1

2 1 tetrahedral sheet and 1 octahedral sheet linked together The unshared oxygen becomes part of the octahedral sheet Serpentine-Kaolin Group = 1:1 group no layer charge hydrogen bonds Simplest clay mineral: Kaolinite Serpentines Mg trioctaedric (generally not clay m.) Kaolins Al dioctaedric Kaolinite 2 tetrahedral sheets and 1 octahedral sheet linked together to form a sandwich structure Large interlayer cations Mica, Illite 2:1 layer minerals Variation of Bentonite (ca. 90% montmorillonite) weathering product of volcanic glass 2

3 2:1 layer minerals Illite-Smectite Mixed-layer clay minerals Definition: Two or more layer types (i.e. mica, smectite, chlorite, etc.) are intermixed in vertical stacking sequence within a single crystal. Bonding is strong within the layers but week between the layers Different types of layers have nearly identical configuration of tetrahedral oxygens bounding the outer surfaces Mixed-layer clay are widespread in nature Criteria for characterization: Types of layer involved (e.g. illite, smectite, chlorite) Amount of each layer type (%) Stacking arrangement, or vertical sequence, of the component layers (e.g. regular, random) Example: randomly mixed-layered smectite/illite with 10% illite layers Identification of clay minerals Identification using X-ray diffraction (advent of X-ray studies in the 1920 s; before generally regarded as amorphous or mixture of crystalline and amorphous components) Sample preparation: Cation saturation: with single, known cation prior to any treatment (K +, Na +, Mg 2+, Ca 2+ ), excess salt removed 1. Air dry specimen 2. Ethylene glycol or glycerol saturation 3. Heating to 350 C or 550 C 4. Dimethylsulfoxide intercalation (not routinely used) Identification by X-ray diffraction Kaolinite: 7-Å peak disappears upon heating / is shifted after DMSO treatment Identification by X-ray diffraction Identification by X-ray diffraction Illite: Smectite: 10-Å peak not affected by treatment Shifting of 001-peak affected by: type of interlayer cation single/double layer of H 2 O-molecules ethylene glycol solvation (swelling) heating (no H 2 O-layer) 3

4 Identification by X-ray diffraction Chlorite: 14-Å peak not affected by treatment In situ tests Clay = soil from a engineering point of view Special testing methods for in situ testing of fine grained soils SPT: Standard penetration test hammering of a sampler tube into the ground under defined conditions and counting the strokes CPT: elaborate testing method with a cone shaped probe; measurement of cone resistance, sleeve friction and pore water pressure Dilatometer / Flat Plate Dilatometer for measurement of the deformation and elastic properties of the soil Packer testing for in-situ determination of hydraulic conductivity Sisi Nubi project Sisi Nubi project Construction of new platforms Foundation and anchoring on the seabed site investigation CPT Cone Penetration Test Probe Seabed: mainly clay, silt, sand 4

5 CPT CPT Cone Penetration Test Cone Penetration Test Umbilical cable with data lines and hydraulic line CPT Results Continuous CPT profile Laboratory tests Clay = soil from a engineering point of view Various tests to determine the geotechnical of soils Direct shear test shear strength Triaxial shear test shear strength of drained or undrained sample Attenberg limits plasticity of clay, shrinkage Oedometer test consolidation and swelling Water content Hydraulic conductivity Laboratory tests Laboratory tests Sampling Extrusion of sample 5

6 Laboratory tests Laboratory tests Sample preparation Shear test Mark of penetrometer test More likely problems for Swiss geologists More likely problems for Swiss geologists (from Keusen, 2000) (from Keusen, presentation ETH IGT ) More likely problems for Swiss geologists The clay cycle Formation of clays and clay minerals What type of soil? Restricted to upper crust - cohesive/non-cohesive Pore water pressure? -low/high.. 6

7 Occurrence of clay minerals Formed primarily by reaction of feldspars (and other unstable minerals) with the hydrosphere Generally absent in rocks formed at temperature above 50 C (igneous/metamorphic rocks) Abundant in soils, unconsolidated sediments, sedimentary rocks and in many hydrothermal alteration zones Sedimentary rocks are the most widespread rock type on the earth s surface shale is the most abundant rock type clay minerals are major constituents of shales clay minerals are the most abundant minerals at the earths surface (possible exception: quartz) Important sheet silicates Kaolinite Al 4 [(OH) 8 /Si 4 O 10 ] Muscovite KAl 2 [(OH) 2 /AlSi 3 O 10 ] Montmorillonite (Al,Mg) 2 [(OH) 2 /Si 4 O 10 ] (Na,Ca) x (H 2 O) n Chlorite (Mg,Fe,Al) 3 [(OH) 2 /(Si,Al) 4 O 10 ] (Mg,Fe) 2 (HO) 6 Illite: muscovite with partial substitution of K by H 2 O Clay origins Most clays in modern sediments are detrital. Example: K-Ar dating of illite reveals Paleozoic ages older than the ocean basin where samples were taken Clays are eroded from continents and transported to a basin controlled by 3 factors: mineralogy of the bedrock mineralogy of the soil developed on the bedrock Clay origins Bedrock sources Unmetamorphed igneous rocks Small amount of mica and chlorite Low- and medium grade metamorphic rocks (slate, phyllite, schist) Rich in mica and chlorite Hydrothermally altered volcanic/volcanoclastic rocks Smectite, chlorite and ML smectite/chlorite Mafic rocks Mg-, Fe-rich trioctahedral clay minerals transport (river, wind, ocean current) Ultramafic rocks Chlorite, talc, serpentine Clays origin Clays origin Sisi Nubi Sisi Nubi Samarinda Samarinda Mahakam river Mahakam river 7

8 Clay origins Soil sources Most detritus does not derive from bedrock directly but from soil formed on the bedrock. Mineralogy of soil is influenced by: parent material (bedrock) climate topography drainage time Generally: tropical/subtropical climates kaolin-rich soils (in extreme: bauxite-gibbsite-rich soils) warm dry climates smectite rich soils; or illite/smectite ML temperate climates vermiculite cold climate (extreme N and S) minimum of chemical weathering chlorite Soil sources Kaolinite Soil sources Soil sources Illite Chlorite Clay origins Transport Small grain size Transport over great distances by rivers but also by wind and gentle currents Transport can be changed abruptly when off-shore Flocculation of clay mineral near the river mouth due to change in salinity Glacial transport in the extreme N and S Minimum chemical weathering chlorite and illite Clays origin Sisi Nubi Samarinda Mahakam river 8

9 Clays origin Bentonite Distribution pattern in a basin in direction of sediment transport Definition: Bentonites are clay rocks But: illite is the most abundant clay mineral in the oceans Mainly formed from alteration of pyroclastic/ volcanoclastic rocks Bentonites consist predominantly of smectite Bentonite deposit formed from pyroclastic flow (Milos) Newspaper Headline in April 2010 Volcanic activity Ordovician K- Bentonites K-Bentonites represent episodes of explosive volcanism o/video html K-Bentonite begin as smectite rich layers; then convert to illitsmectite rich beds over time Reduces value for industrial applications Ordovician K- Bentonites but have values as stratigraphic marker horizons Remnants of large caldera-forming events Ordovician K- Bentonites Prominent ash beds in the USA and Northern Europe Deicke and Millbrig K-Bentonits were correlated with geophysical logs and chemical fingerprints throughout the eastern midcontinent of the USA the Millbrig bed covers an area of at least 2.2 x 10 6 km 2 Kinnekulle bed covers an area of at least 6.9 x 10 5 km 2 Calcultated volumes of silicic magma to produce these beds: Millbrig: 1509 km 3 Kinnekulle: 972 km 3 9

10 Ordovician K- Bentonites Results of supervolcano eruption The Deicke-, Millbrig- and Kinnekulle-beds are among the largest ash falls recorded They are the result of supervolcano events A supervolcano is a volcano that eject more than 10 9 tons of ash Explosion of a supervolcano is about 30 times stronger than the eruption of Krakatau, Indonesia (1883) Last supervolcano eruption: Toba (Sumatra) ca years ago 10