Fjölrit RALA - 15.06.2004, Page 76

3° Crystallinity and texture. Crystallinity expresses the degree to which a pyroclast (or
rock fragment) is composed of crystalline material in relation to the glass phase. Vitric
particles are characteristically glassy, whereas holocrystalline fragments consists wholly of
mineral grains. In pyroclasts composed of a mixture of glass and crystals, the resultant texture
is called hypocrystalline (or merocrystalline). Pyroclasts consisting exclusively of crystals
too small to be discemible with the naked eye or using a hand lens are said to be aphanitic-
textured, whereas the term porphyritic refers to fragments carrying relatively large, usually
well-shaped crystals (phenocrysts) set in a fine-grained or glassy groundmass. When there is
a parallel alignment of tabular crystals (especially feldspars), suggestive of flow, the texture is
called trachytic.
4° Composition. The IUGS modal classification bases the names of volcanic rocks on the
relative proportions of five mineral groups: Q (quartz, tridymite, cristobalite), A (alkali
feldspars), P (plagioclases Ans to Anioo), F (feldspathoids) and M (mafic and related minerals)
for which volume modal data must be determined. The texture and the nature of the minerals,
and some physical properties of volcanic glass provide valuable information on the chemistry
and the cooling history of the source magma. Colour in transmitted light and relief (or
refractive indices) of volcanic glasses show a close correlation with chemical composition,
basaltic glasses (sideromelane) being coloured and having a positive relief, whereas rhyolitic
glasses tend to be colourless, with negative relief. Palagonite is the primary alteration product
of sideromelane. It has a wax-like or resinous appearance and in thin section it is yellow to
orange, isotropic or weakly birefringent. During palagonitisation some chemical elements are
leached out, while others show gains. This is reflected by either a decrease, or an increase of
the refractive index.
Volcanic glasses are inherently unstable and decompose more readily than mineral phases
they are associated with. Alteration of basaltic glasses may change drastically the chemistry
of interstitial pore waters and frequently results in deposition of secondary mineral phases
(zeolites, carbonate minerals, opal, etc.) in cements and pore spaces. In contrast, alteration of
silicic glasses promotes lithification, cementation by redistribution of silica and formation of
bentonitic clays.
It is the main purpose of our poster presentation to illustrate the granulometric and textural
features related above and their impact on weathering, using field photographs and
micrographs of soil and rock thin sections.
References and further reading
Fisher, R.V. and H.-U. Schmincke. 1984. Pyroclastic rocks. Springer-Verlag, Berlin.
Le Maitre, R.W. (ed). 1989. A classification of igneous rocks and glossary of terms.
Recommendations of the Intemational Union of Geological Sciences Subcommission on the
Systematics of Igneous Rocks. Blackwell Scientific Publications, Oxford.
MacKenzie, W.S., Donaldson, C.H. and C.Guilford. 1987. Atlas of igneous rocks and their
textures. Longman, Harlow, UK.
Schmid, R., 1981. Descriptive nomenclature and classification of pyroclastic deposits and
fragments: Recommendations of the IUGS Subcommission on the Systematics of Igneous
Rocks. Geology 9:41-43.
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