A textural and chemical study of Icelandic palagonite of varied composition and its bearing on the mechanism of the glass-palagonite transformation

Journal Article · Fri Mar 01 04:00:00 EST 1991 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(91)90337-5· OSTI ID:5890743

Thorseth, I H; Furnes, H; Tumyr, O ^[1]

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1. Univ. i Bergen (Norway)

Palagonite of basalt and basaltic andesite parentages from hyaloclastite deposits in Iceland has been investigated. SEM studies indicate a sharp to diffuse alteration front which may propagate along microfractures in the glass, resulting in a progressive partial dissolution yielding palagonite of variable, but generally increasing porosity towards grain surfaces. The palagonite has a granular texture. Incipient alteration is indicated by the development of individual globules, whereas at an advanced stage chains of globules, defining a sponge-like texture, characterize the palagonite. In the basaltic andesite, the precursor to brown Ti- and FE-rich palagonite is a white variety, for which an evolutionary model is presented. EDS line-scans across the fresh glass-palagonite boundary show the existence of a 2-4 {mu}m thick zone of glass-like material in which all elements have been depleted, except Si, and in some cases Al, which have been relatively enriched. This gives evidence for selective element mobility prior to a variable degree of congruent network dissolution of the Si-rich residue, yielding zoned palagonite with a different porosity. In order for Fe, Ti, and Al to dissolve, a pH < 3 is necessary. In an oxidizing environment and at pH-values > 3, Fe, Ti, and Al will precipitate in the pores of the white palagonite as oxides/hydroxides, thus creating the brown variety, which is characterized by highly variable contents of the above-mentioned elements. The applicability of this model to palagonite derived from basalt parentages at different pH conditions is discussed.

• OSTI ID: 5890743
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Journal Name: Geochimica et Cosmochimica Acta; (United States) Vol. 55:3; ISSN GCACA; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
AMORPHOUS STATE
ANDESITES
BASALT
CHEMICAL ANALYSIS
CHEMICAL COMPOSITION
CHEMISTRY
CLAYS
CRYSTAL STRUCTURE
CRYSTALLOGRAPHY
DEVELOPING COUNTRIES
DISSOLUTION
ELECTRON MICROSCOPY
EUROPE
GEOCHEMISTRY
GEOLOGIC MODELS
ICELAND
IGNEOUS ROCKS
ISLANDS
MICROSCOPY
MINERALOGY
PETROCHEMISTRY
PH VALUE
QUANTITATIVE CHEMICAL ANALYSIS
ROCK-FLUID INTERACTIONS
ROCKS
SCANNING ELECTRON MICROSCOPY
SOLUBILITY
VOLCANIC ROCKS
WEATHERING