Silica extraction from sea water
- 1. SILICA EXTRACTION FROM SEA WATER, BIOGENIC CHERT , DIAGENESIS CONTENTS Chemical Extraction Biogenic Extraction BIOGENIC CHERT o Introduction o Diagenesis of Opal o Factors affecting rate of Diagenesis By : RAJESH KUMAR 419AG2010
- 2. CHEMICAL EXTRACTION Experimentally, Quartz from seawater containing dissolved silica (4.4 ppm) precipitated onto nucleation surfaces of clean Quartz. Quartz not readily crystallizes under normal pH & Temp conditions in ocean ( Even In solutions whereby Silica conc. Exceeding Quartz solubility) Thus Formation of Chert containing Microcrystalline quartz through INORGANIC PROCESS is Unlikely. Chert precipitates where Volcanism occurs and water highly saturated with silica. Cherts found to occur with Dolomite of Hydrothermal origin in Lower Missisipian age Some silica precipitates on clay/sillicate minerals and get removed from sea water to ocean.
- 3. BIOGENIC EXTRACTION Silica secreting organisms (Radiolarians, Diatoms, Silicoflagellates, Sponges) extracts dissolved silica from oceanic surface water and produces OPAL A bearing skeletal structures. Diatoms responsible for bulk of silica extraction from modern ocean. Residence time of Sillicates in ocean is 10-15ka Glacial/Interglacial Perturbations Proxy for Climate Change Diatoms forming opalescent films
- 4. BIOGENIC CHERT INTRODUCTION Opaline Tests (Opal A) of Siliceous organisms start dissolving after their death and accumulates on ocean floor. Siliceous Oozes (>60% siliceous skeletons) forms where ; Opaline tests productivity exceeds Dissolution Continuous Burial of Siliceous Oozes/Clayey sediments Continuous dissolution of Opaline skeletons and accumulation in the pore spaces of sediments Pore fluid enriched in Silica and ultimately precipitates Chert (Microcrystalline quartz).
- 5. DIAGENESIS OF OPAL Biogenic OPAL -A to OPAL CT to MICROQUARTZ to MEGAQUARTZ Opal CT Interm. & Metastable Phase Low temp Cristobalite with interlayered tridymite lattices Occurs as LEPISPHERES (Bladed Microcrystalline aggregates), Nonspherulitic, Rim cements, Overgrowth, Massive cement Solution -Reprecipitation Opal A Opal CT then Opal CT Microquartz Solid-Solid Transformation Metamorphism Microquartz to Megaquartz
- 6. FACTORS AFFECTING RATE OF DIAGENESIS Temperature Increased temperature promotes increased rate of transformation Rate of Sedimentation/Burial High rate promotes High Geothermal gradient and so Rate of transformation. In the Deep ocean, OPAL-A to OPAL-CT occurs at 45属C while OPAL-CT to MICROQUARTZ at 80属C Presence of Mg(OH)2 Serve as nucleus for OPAL-CT crystallization Surface to Volume Ratio Increases with Decreasing Particle size, Increasing Opal solubility and the Rate of Transformation Extreme low level of Detrital minerals Opal A to Opal C transforms faster and forms Porcellanite (Lithified Opal CT) from sediments rich in OPAL A NOTE (Path B) Due to these factors, Transform can also take place at shallow depth & low T