Teresa Taboada

Smectite formation produced by weathering in a coarse granite saprolite in Galicia (NW Spain)

Abstract Weathering of a calcalkaline granite was studied in the south of Galicia (NW Spain) where the average annual precipitation is 1400 mm and the average annual temperature is 12°C. The original rock contains perthitic K-feldspars, plagioclases with inclusions of muscovite and opaque minerals, quartz and chloritized biotite, with apatite, zircon, sphene and opaques as accessories. In the saprolite the structure of the rock is preserved, the plagioclases show up to grade 4 weathering and the biotites, between 2 and 3. Weathering in quartz and potassium feldspar crystals is manifested only by fracturing. Biotite changes following the parallel linear model and its weathering products are interstratified biotite–vermiculite and iron oxyhydroxides. The plagioclases change to a microgranular material by pseudomorphic transformation. This material, which substitutes the plagioclase, includes small clearly delimited units which retain zones with the optic characteristics of muscovite. X-ray diffraction analyses of microsamples show that they are formed by 1:1 diocthaedral phyllosilicate, smectite and a small quantity of mica. From these facts we concluded that smectite is formed inside the plagioclase crystals, and probably originates from the inclusions of muscovite contained in these crystals, as it is suggested by the microscopic study which shows the increase in volume which occurs when the crystals of muscovite are transformed.

Pseudomorphic transformation of plagioclases during the weathering of granitic rocks in Galicia (NW Spain)

Abstract Microscopic study of twelve weathering profiles developed on granitic rocks in Galicia (NW Spain) showed that, in most cases, the transformation of the parent rock into saprolite occurs with pseudomorphic alteration of the plagioclases. It was seen that the transformation and the retention of plagioclases in the alteration profile are directly related to the conservation of the general structure of the rock and this, in its turn, depends on the mineralogical composition and texture. The most unstable constituents, in the Galician environment, are biotites and plagioclases, there being a clear difference in the rate of alteration between the plagioclases and the K-feldspars. This implies that a large part of the fabric of the rock remains unaltered when the plagioclases are completely weathered, so that the pores caused by the weathering of the plagioclases are filled by their own alteration products. With regard to the influence of the texture, it was observed that fine grained rocks are altered more easily than coarse grained ones, especially if they are rich in biotites. In this study we show various examples in which, even with high levels of alteration, the morphological features of the original plagioclase crystals are preserved: twins, original zoning, myrmekitic textures and perthites. The pseudomorphic transformation of plagioclase is independent of the products to which it gives rise, and here we show examples where gibbsite, 1:1 dioctahedral phyllosilicate, or a mixture of both, appear as secondary minerals. The mineralogical nature of a newly formed product depends mainly on the intensity of the leaching to which the profile is subjected, which is influenced by the rainfall in the area where they are found, as well as the size of existing voids in the material.

Multivariate statistical analysis of chemical properties of European volcanic soils

Within the framework of EU-COST action 622 twenty soils developed on volcanic materials from Italy (IT: EUR01 to EUR04), Azores Islands (AI: EUR05, EUR06), Iceland (IC: EUR07 to EUR09), Canary Islands (CI: EUR10 to EUR12), Greece (GR: EUR13 to EUR15), France (FR: EUR16 and EUR17) and Hungary (HG: EUR18 to EUR20) were described, sampled and analyzed for a large number of physico-chemical soil properties by different research groups. This database provides a good opportunity for the application of multivariate statistical methods, although the number of samples is moderate (94 horizons). In this chapter we applied two different approaches for the explanation of the variance structure of the chemical properties of the European volcanic soils: (1) an exploratory analysis using principal component analysis (PCA) aimed to identify the major trends and processes in these soils, and (2) a confirmatory analysis based on PCA separation and knowledge on andic properties using discriminant analysis (DA) to identify the minimum set of properties that enable a good separation of the different types of horizons (non-andic, vitric, andic).