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laterite, aluminum, silicates and plants

     Thanks to Pat Jellison, Roger Miller and others who have helped 
     clarify my understanding of what laterite is. Let me cover a few 
     points as I understand them and correct me where I go wrong.
     The term laterite is not precise. There is not a consistent scientific 
     definition nor should we expect commercial aquarium additives to 
     necessarily be labelled or identified precisely. A loose definition of 
     laterite would be soil primarily composed of iron oxide and hydroxides 
     which have little or no calcium, magnesium or carbonate compounds and
     which are low in aluminum and silicates.
     For the purposes for use in planted aquariums, it _may_ not be that 
     important that we use a laterite which has had every last bit of 
     aluminum or silicate leached out. The laterites commonly sold probably 
     contain some amounts of kaolinite (a low solubility aluminum silicate: 
     Al2Si2O5(OH)4). My suspicion is that one of the reasons for using 
     laterite is that it won't greatly affect the water hardness and pH.
       1. Will the presence of silicates make any difference to aquatic 
       2. Will aluminum silicates be sufficiently soluble to pose aluminum 
     toxicity problems for plants if the substrate pH goes below 6?
     I could only see a substrate with low pH if there were significant 
     amounts of organic material such as peat or humus. My questions on 
     aluminum toxicity are more to do with why laterite might be preferred 
     to various clays or soils and how we might evaluate the potential for 
     problems with cheaper commercial clays such as pottery clays (see my 
     next posting on Terrastone clay analysis)
       3. Would these organic materials tend to form aluminum complexes, 
     mitigating any problems with aluminum plant toxicity?
       4. Suppose there were aluminum ions in the substrate which diffused 
     into the water which had a pH of 6.5 or higher; would it be 
     precipitated or oxidized? Would it pose a problem for fish or 
       5. What is sesquioxidic Fe or Al? Fe2O3? Al2O3?
       6. What are halloysite, Oxisols, Ultisols and Spodosols?
     BTW, I did check my reference on silicon dioxide/aluminum silicate = 
     glass and my excuse is that it was late at night. The chemistry book 
     did mention some special types of glass with trace aluminum or other