The mineralogical composition of the studied ceramics and clayey raw materials (Table 1) shows a predominance of silicate detrital minerals, such as quartz, feldspars and phyllosilicates, accompanied by, as accessories, goethite and hematite, calcite and dolomite. The clay fractions of the raw materials show illite as the main mineral, accompanied by kaolinite, with smectite (and illite-smectite) on smaller amounts.
Table 1 – Mineralogical composition (clay fraction composition in italic).
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Aveiro Cretaceous clays
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Barreiro Quaternary sandy clays
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Barreiro Pliocene
clayey sands
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Aveiro ceramics
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Barreiro ceramics
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Quartz
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30-35
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40-45
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45-55
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35-40
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45-50
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K-Feldspars
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5-8
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5-7
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6-8
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5-10
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5-10
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Plagioclases
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3-6
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3-5
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3-6
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3-5
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4-8
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Phyllosilicates
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45-50
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35-40
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25-30
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35-40
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30-40
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Calcite
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3-5
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<2
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<2
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3-5
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<2
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Dolomite
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5-8
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-
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-
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6-8
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-
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Hematite
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3-5
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2-3
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3-4
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6-10
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5-8
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Goethite
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4-7
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4-6
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3-5
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-
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-
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Mullite
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-
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-
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-
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<2
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-
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Illite
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50-55
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60-70
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55-65
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Kaolinite
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30-40
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20-25
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25-35
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Smectite
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10-15
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5-10
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<5
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Comparing with Barreiro raw materials, Aveiro Cretaceous clays are richer in phyllosilicates and slightly poorer in quartz; other distinctive features are the relative higher content in carbonates (with exclusive presence of dolomite), and slightly higher presence of goethite and hematite.
Concerning clay minerals content, all clayey raw materials are illite rich but Aveiro Cretaceous clays show the highest content in kaolinite (along some smectite), whereas Barreiro Quaternary sandy clays are those richer in illite, Barreiro Pliocene clayey sands showing an intermediate clay composition.
All Aveiro studied ceramics show a mineralogical composition very similar to traditional bricks produced in Aveiro, from Cretaceous clays: quartz, phyllosilicates (mainly micas), feldspars (mainly K-feldspars), Fe-oxides (mainly hematite), carbonates (mainly dolomite). They show some distinctive features, such as: relative lower content on phyllosilicates (and almost exclusively micas) and a discrete presence of mullite in some samples.
Studied Barreiro ceramics show significant differences towards Aveiro ones, such as: more quartz, less phyllosilicates, less Fe-oxides, no carbonates and no mullite.
Table 2 shows chemical results (major elements) of the studied ceramics and clayey raw materials.
Table 2
– Chemical composition of the studied ceramics and clayey raw materials.
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Aveiro Cretaceous clays
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Barreiro Quaternary sandy clays
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Barreiro Pliocene
clayey sands
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Aveiro ceramics
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Barreiro ceramics
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SiO2
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50–65
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60–75
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60–65
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60–70
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65–75
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Al2O3
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19–21
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15–18
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17–20
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19–21
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16–18
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K2O
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3–6
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4–7
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4-6.5
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2–5
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2-6.5
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Na2O
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0.3-1
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0.5–1.2
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0.4-1
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0.2-1
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0.5–1.2
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CaO
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0.4–1.1
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0.2–0.7
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0.2–0.5
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0.3-1
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0.2–0.6
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Fe2O3
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4–9
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3–5
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3–6
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5–10
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4–6
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MgO
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2–3
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0.3–0.9
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0.3–0.7
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1.5–2.5
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0.6–0.8
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TiO2
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0.5-1
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0.3-07
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0.3–0.6
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0.5-1
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0.3–0.7
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LOI
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8–10
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4–8
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4–7
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< 2
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< 3
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Raw materials show chemical compositions in accordance with their mineralogical compositions and reflecting their main differences. Aveiro clays, being richer in phyllosilicates and with less quartz, show naturally less amounts in SiO2 and slightly higher in Al2O3 and LOI; they show also relative higher contents in MgO (due to dolomite presence) and Fe2O3 (due to hematite and goethite). On the other hand, Barreiro raw materials are richer in SiO2 and slightly in alkalis.
All Aveiro ceramic samples show a chemical composition close to traditional bricks produced in Aveiro (Coroado et al. 1998), but with some relevant differences, such as: relative higher content in SiO2 and Al2O3 and a discrete trend to show higher Fe2O3 content. Barreiro ceramics show SiO2 higher content, Al2O3 and Fe2O3 lower contents and CaO (and MgO) very lower contents. Actually, mineralogical composition of Aveiro sugar jars are quite similar to local red ceramics typical raw materials, historically exploited on Upper Cretaceous marly (dolomitic) clays, in which the common clay minerals association is kaolinite plus illite, in some more red layers followed by illite-smectite, and usually also rich in goethite and hematite.
The persistence of dolomite is coherent with this maximum firing temperature (≈ 800ºC) which is not sufficient to achieve the total decomposition of this mineral that currently occurs in the local clays (Trindade et al. 2010 b); dolomite is absent or very discrete in heavy clays of any other Portuguese clay deposit (Coroado et al. 1998, Marques et al. 2011, Trindade et al. 2013).
On the other hand, the relative lower content on phyllosilicates as well as the discrete presence of mullite in some samples, points to a firing temperature slightly higher (more kaolinite, later production).
Red clays (commonly more kaolinitic) should have been increasingly used in advantage over green clays. Red clays are also richer in Fe-oxides, whereas green clays are richer in dolomite. Relative higher content on Si and Al, as well as the discrete trend to show higher Fe contents, shown by the same samples referred previously, reinforces this consideration (Amaral et al. 2020). It is well known that high iron contents favors the vitrification of the ceramic bodies at lower temperatures, which able the ceramic bodies to acquire good values of mechanical resistance at lower firing temperatures (Amaral et al. 2020, Moutinho et al. 2019); compressive strength analysis of Aveiro ceramics sugar jars pieces shows higher values (mean 9.5 MPa) (Moutinho et al. 2019).
Mineralogical and chemical composition of Lisbon (Barreiro) sugar jars are also quite similar to local red ceramics typical raw materials, historically exploited on Barreiro (portuguese word for clay quarry) Pliocene sandy clays, in which the common clay minerals association is illite + kaolinite + smectite. On Lisbon (Barreiro) sugar jars, the absence of high temperature phases, as well as the poor contents on Fe oxides, explains the lower quality of the firing products.