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tration (pmol g"1'

Fig. 2.3.1. A Spatial structure of a Si-tetrahedron (left) and an Al-octahedron (right). B The conversion of primary minerals into secondary clay minerals. C Composition of important rocks with respect to their base and Si content. (Scheffer-Schachtschabel 1998)

Primary clay minerals

Secondary clay minerals

Vermlcullte Smectite

Primary clay minerals

Mica

Chlorite (primary)

Feldspar Mica Pyroxenite Olivine Vulc. glasses

Secondary clay minerals

Vermlcullte Smectite

Secondary chlorite

Weathering products

Kaolinite

Smectite, Vermlculite lllite

Allophane, Imogollte '-♦•Hal loyslte *■ Kaol in ite Kaolinite, Goethite, Hematite, Glbbsite

- Mg and Fe Increasing, K decreasing -

Fig. 2.3.1. A Spatial structure of a Si-tetrahedron (left) and an Al-octahedron (right). B The conversion of primary minerals into secondary clay minerals. C Composition of important rocks with respect to their base and Si content. (Scheffer-Schachtschabel 1998)

- Mg and Fe Increasing, K decreasing -

Granite Granodiorlte

Diorlte Gabbro Peridotlte Andesite Basalt Pikrite

Quartzite

Olivine

Pyroxene

Other minerals

Si concentration (% Si02)

Si concentration (% Si02)

Granite Granodiorlte

Diorlte Gabbro Peridotlte Andesite Basalt Pikrite

Quartzite

Olivine

Pyroxene

Other minerals availability of alkaline compounds. Basalt, for example, is much more alkaline than granite, i.e. contains more potassium, calcium and magnesium.

Secondary Minerals

From the primary intrusive and igneous rocks as well as from metamorphic rocks (e.g. slate), secondary sediments are formed during the course of weathering and erosion, and are deposited and solidified. Depending on the original rock and its "weathering history", sedimentary rocks have different cation contents. Clay-rich soils possess a greater capacity for exchange of cations than sandy soils, not only be cause of their much larger surface area for ion exchange, but also because of the greater number of covalent bonds. Secondary minerals include those in sedimentary rocks; for example, in limestone, there are aluminium oxide, iron oxide, manganese (hydr)oxide. Clay minerals are particularly important components of soils which are formed as new products of the Si-Al minerals during the course of weathering from silicates. According to the climatic conditions when the clay minerals were formed, these could be layer lattices in which the cations are imbedded more or less tightly between the layers of Si-tetrahedra and Al-octahedra. These cations can be reversibly exchanged against H+ without

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