Transcript Slide 1
Montmorillonite By Dominic Papineau General properties of montmorillonite “Hydrated Sodium Calcium Aluminum Magnesium Silicate Hydroxide” Montmorillonite is part of the smectite/montmorillonite group of clay minerals (dioctahedral smectites). The dioctahedral smectites also include beidellite and nontronite as other important end-members. Trioctahedral smectites include saponite and hectorite as the most important end-members. The water content is variable: when water is absorbed by the crystals, they tend increase several times their original volume. It typically forms microscopic or at least very small platy micaceous crystals. The physical properties of montmorillonite Color: usually white, gray or pink with tints of yellow or green. Luster: dull Cleavage: perfect in one direction (not seen in massive specimens) Hardness: 1 - 2 Specific gravity: 2.3 – 3 average Crystal habit: never in large individual crystals, usually found in compact or lamellar masses. Also seen as inclusions in quartz as fibers and powder-like masses. The optical properties of montmorillonite Montmorillonite is biaxial negative 2VZ = 5 - 30° = 1.485 – 1.535 = 1.504 – 1.550 = 1.505 – 1.550 = 0.015 – 0.020 (usually first order yellow or lower interference colors) Not pleochroic The chemical properties of montmorillonite Montmorillonite’s chemical formula: (My+ nH2O) (Al2-yMgy) Si4O10(OH)2 Common chemical substitution: Al3+ and Fe3+ can substitute for Mg2+ or Fe2+ Al3+ can substitute for Si4+ Octahedral site Tetrathedral site These can result in layers of negative charges which need to be balanced by interlayer cations (K+, Na+, Ca2+, Mg2+, etc.) and also organic ions. These exchangeable cations can be hydrated by nH20. In turn, this causes swelling of the crystals due to the interlayer water molecules. The crystallographic properties of montmorillonite Crystal system: monoclinic Point Group: 2/m Unit cell parameters: a = 5.17 b = 8.94 c = 9.95 = 99.9° Z=1 V = 453.04 Calculated density = 1.61 Space group: C2/m The crystal structure of montmorillonite Silicon tetrahedra Aluminum octahedron Magnesium octahedron Sodium, potassium or calcium ion In brief: T – O – T sheets The use of montmorillonite The water absorption property of montmorillonite makes it an important to farmland soils where extended periods of drought occur. It is the main constituent in bentonite (a volcanic ash) which is used in drilling muds. The bentonite gives the water greater viscosity and thus keeps the drill head cool during drilling. Montmorillonite may have promoted the catalysis of terrestrial polymeric biomolecules and could do the same on other planetary bodies. Origin of terrestrial life The RNA world has been proposed by many scientists to have been the precursor for terrestrial life because RNA molecules provide both the capabilities to store genetic information and to catalyze chemical reactions. RNA Nucleic acids O O O Mechanism of polymerization catalysis - - - - - - - - - Montmorillonite particle Negative charges due to chemical substitutions + - - - - - + - - + + + + + - + - - - - + - - - + - + Montmorillonite particle + + + + - + + - + - The cations include a wide range of possible cations among which organic ions (such as nucleotides and amino acids) can be attracted and oriented. The catalytic properties of montmorillonite It has been shown that montmorillonite-catalyzed reaction are up to 19 times more efficient than non-catalyzed biochemical polymerization reactions (Joshi, Pitsch, and Ferris, (2000) Origin of Life). Hence, montmorillonite is known to catalyze phosphodiester bonds and also to influence the regiochemistry of the phosphodiester bond produced. RNA molecule In fact, montmorillonite favors 3’,5’phosphodiester bonds O But not O 2’, 5’ phosphodiester bonds O Geological evidence of the origin of life: an hypothesis to test Ammonium (NH4+) can result from the biogeochemical degradation of biomolecules such as nucleic acids and proteins. NH4+ has the same charge and a very similar ionic radius to K+ and can thus substitute in K+ containing minerals (e.g. biotite, muscovite, K feldspar). NH4+ is very stable in high temperature processes and survives through crustal processes. If the RNA of the RNA world was indeed polymerized on clay minerals such as montmorillonite, then Hadean metapelites may contain high concentrations of NH4+ as evidence of the earliest terrestrial life. Such an hypothesis could also be tested on Martian clay-rich sediments from a Mars Sample Return. Occurrences of smectite/montmorillonite Locations on Earth: Montmorillonite deposits are located all over the globe and important sites include: 1. Himalaya - China, Tibet 2. Ural - Pakistan 3. Caucasus - Georgia (Russia) 4. Andes - Peru, Ecuador 5. Wasatch - Utah (USA) Present on Mars: Orbital infrared detection: Esposito et al., (2000) JGR Christensen et al., (1998) Science Detection in Martian meteorites: Bridges and Grady, (2000) EPS-letters Treiman and Lindstrom, (1997) JGR