@article{fdi:010037919,
  title = {{W}eathering and allophane neoformation in soils developed on volcanic ash in the {A}zores},
  author = {{G}{\'e}rard, {M}. and {C}aquineau, {S}andrine and {P}inheiro, {J}. and {S}toops, {G}.},
  editor = {},
  language = {{ENG}},
  abstract = {{O}n {F}aial and {P}ico islands ({A}zores), we studied two profiles on basaltic pyroclasts that contain buried horizons, and we focussed on petrography, micropedology and mineralogy. {E}mphasis was given to weathering of the lapilli and ashes, and the neoformation of allophane. {A} combination of optical studies, in situ chemical analyses, {X}-ray diffraction and infrared spectroscopy of clay fractions revealed that allophane is present both in the micromass of the groundmass, in alteromorphs after lapilli or pumice, and in clay coatings. {W}hereas most studies describe allophane as a colloidal fraction formed by the congruent and total dissolution of the ashes, this study shows evidence for the formation of allophane alteromorphs, due to leaching of {S}i and cations, with preservation of the original shapes of the tephra. {T}he allophane alteromorphs often display optical characteristics that resemble those of palagonite. {I}ncreasing alteration is observed through three steps: (i) hydration of the glass associated with strong cation and {S}i leaching, (ii) allophane hypocoatings, and (iii) allophane alteromorphs with development of intragrain bridges. {T}he chemical signature of the alteromorphs varies from a pure alumino-silicate at one extreme to an {F}e({T}i) enriched alumino-silicate at the other. {B}etween those two extremes, the colour grades from yellow to dark orange, with microzonations. {A}n {A}l-rich allophane composition is associated with gibbsite in the {EUR}6-{P}ico profile, whereas at the base of the {EUR}5-{F}aial profile, {S}i-rich allophane is associated with halloysite. {S}ome variations of the palaeo-environment are also suggested by strong iron segregation observed with various secondary phases (ferrihydrite, haematite, iddingsite).},
  keywords = {},
  booktitle = {},
  journal = {{E}uropean {J}ournal of {S}oil {S}cience},
  volume = {58},
  numero = {2},
  pages = {496--515},
  ISSN = {1351-0754},
  year = {2007},
  DOI = {10.1111/j.1365-2389.2007.00910.x},
  URL = {https://www.documentation.ird.fr/hor/fdi:010037919},
}