Phase changes in aged sesquioxide gels and some analogies with katamorphic processes

Abstract

The phase changes detected in gel mixtures aged under different conditions offer certain analogies with natural katamorphic phenomena, particularly bauxitisation. pH is by far the most important single factor while composition and order of mixing come a close second. Certain factors also influence the distribution of aluminium in 4- and 6-fold co-ordination, while early stabilization of the latter in a boehmitic phase is an essential precursor to significant yields of kaolinite (including halloysite and hydrated and curled kaolinitic phases) but not for montmorillonite. Boehmite is the earliest phase in most gels and can be retained and crystallised under increased temperatures, pCO2, or reduced pH conditions. Generally however, it is replaced by bayerite, gibbsite, and/or nordstrandite. In the light of these experimental and relevant geological data three main paragenetic sequences are examined: a) Montmorillonite-halloysite-(boehmite) associated in stagnant or waterlogged conditions; b) Kaolinite with subordinate boehmite (and gibbsite) under moderately drained conditions; c) Gibbsite from alkali-rich silica rocks with optimum drainage. As phase changes are in the large part determined by pH conditions attention is drawn to the importance of parent rock type in determining such levels. However, CO2 is also shown to play a significant role. Finally a brief correlation is made between the experimental observations made here and various other paragenetic phenomena- including bauxitic deposits associated with marine activity.