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| Titel |
Multidisciplinary characterisation of raw materials used for “ghiara” mortars from the historical city centre of Catania (Eastern Sicily, Southern Italy): insights into their genesis |
| VerfasserIn |
Donatella Barca, Cristina M. Belfiore, Mauro F. La Russa, Teresa Pelle, Antonio Pezzino, Fabio Scarciglia, Marco Viccaro |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250037270
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| Zusammenfassung |
| This work deals with new data on the raw materials used for the production of typical ghiara
mortars, widely employed in historical buildings of Catania downtown (Eastern Sicily,
Southern Italy). Ghiara aggregates are interpreted as the product of thermal transformation of
soils rich in organic matter as a consequence of heating induced by the flowing lavas of Mt.
Etna volcano on them. This process changes the original brown colour of the soil to reddish
nuances.
Aim of this study is a multidisciplinary investigation of such raw materials by comparing
various ghiara samples from different ancient quarries located in areas covered by historic
lava flows (both in the historic centre and the periphery of Catania) with a still-preserved,
unburied and unburnt, volcanic soil profile presumably correlated with those affected by
transformation.
Relevant information about the compositional, physical, petrographic and mineralogical
features is provided, mainly on the weathering degree, the patterns that affect the primary
components and the newly-formed phases. This allows the acquisition of new elements on the
ghiara genesis and its physical-chemical formation conditions. Several analytical
techniques were used to characterise both the ghiara and soil samples from the same
profile.
Major and trace element patterns of ghiara and soil samples are very similar, supporting
the hypothesis that the studied soil really represents the parent material of ghiara unaffected
by the lava flow-induced transformation. Plagioclase, pyroxene and quartz grains of ghiara
samples exhibit important corrosion features, often coupled with Si-Al and Si amorphous
phases as well as Fe-oxides/hydroxides. Conversely, primary minerals are fairly fresh
within the soil. The clay mineralogy of soil horizons consists of illite and kaolinite
phyllosilicates (along with halloysite-7Å in one sample), whereas ghiara includes illite and
hematite and/or goethite. Organic matter occurs in significant amounts within the soil,
ranging between 1.6 and 5 wt%, whereas it is absent in ghiara samples. All these data
support the idea that ghiara materials derive from the burning of soils at temperatures
estimated in the range of about 550-900 Ë C, since illite is preserved whereas kaolinite
collapses from soil to ghiara specimens. Such temperatures are also able to promote a
fast and complete loss of soil organic matter, severe corrosion of all the primary
minerals (quartz included) and re-precipitation of amorphous phases, widespread
oxidation of iron forms. These features suggest a paramount role of circulating fluids,
presumably enriched in CO2 released from organic matter heating and degradation. |
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