## 5 Weathering Processes
### Natural Weathering Processes
#### 5.1.1 Chemical Weathering Processes  
Petrological and geochemical analyses of rock samples from the weathered surface of the walls of the shelters indicate the existence of chemical alteration processes which include dissolution and/or transformation of minerals and the redistribution of ionic substances. The main effect observed was the formation of hardened superficial layers. These layers are enriched in Fe and/or Mn oxides, clays, and silica. This process has been widely referred in the literature as case hardening (e.g., [32; 33; 34]). The driving mechanisms for case hardening on sandstones are the hydrolysis and carbo-hydrolysis reactions which affect the aluminosilicate minerals. The carbo-hydrolysis phenomenon constitutes the mechanism whose action has the greatest effectiveness, in a coherent manner, with warm and humid climatic conditions. These conditions favour the adsorption of water and a chemical attack on the rock components through the action of meteoric water which is slightly acidic due to the dissolved CO\({}_{2}\). The CO\({}_{2}\) source may be atmospheric or organic (vegetable covering and soils rich in organic material in the upper area of the shelters). Next is detailed the geochemical expression that summarises feldspar alteration and clay authoringesis:  
\[6\text{KAlSi}_{3}\text{O}_{8}+4\text{CO}_{2}+4\text{H}_{2}\text{O}\to\text{K} _{2}\text{Al}_{4}(\text{Al}_{2}\text{Si}_{6}0_{20})(\text{OH})_{4}+4\text{K} ^{+}+12\text{SiO}_{2}+4\text{HCO}_{3}{}^{-}\]  
K-feldspar  
Illite  
\[2\text{KAlSi}_{3}\text{O}_{8}+2\text{H}^{+}+9\text{H}_{2}\text{O}\to\text{Al} _{2}\text{Si}_{2}\text{O}_{5}(\text{OH})_{4}+2\text{K}^{+}+4\text{SiO}_{2}+8 \text{H}_{2}\text{O}\]  
K-feldspar  
Koolinite  
\[2\text{CaAlSi}_{3}\text{O}_{8}+11\text{H}_{2}\text{O}+2\text{CO}_{2}\to\text{ Al}_{2}\text{Si}_{2}\text{O}_{5}(\text{OH})_{4}+2\text{Ca}^{2+}+2\text{HCO}_{3}{}^{-}+4\text{H}_{4} \text{SiO}_{4}\]  
Anorthite  
Koolinite  
Figure 4: SEM micrographs. (**A**) Detail of stromatolitic calcite layers enveloping grain surfaces (Shelter 11, Pehas de Cabrera). (**B**) Small lenticular gypsum crystals within the stromatolite crust (Cueva del Tajo de las Figuras). An example of this phenomenon can be observed in Figure 5A,B, with feldspar crystals altered by carbo-hydrolysis and kaolinite crystals neoformed as the result of the alteration. It is precisely the abundant presence of kaolinite (Figure 5C) in the Tajo de las Figuras samples which corresponds to the high humidity level in this location since this authigenic mineral has been commonly employed as an indicator of wet climates [35]. The availability of water is very important in the development of the chemical weathering processes [36], since the solubility of the feldspars is very low (e.g., 3 \(\times\) 10\({}^{-7}\) mol for the K-feldspar and 6 \(\times\) 10\({}^{-7}\) mol/L for the Na-feldspar) [37] its saturation is reached rapidly and the dissolution of feldspars needs a continuous renewal of under-saturated water. Figure 5: SEM micrographs. (**A**) Altered Na-feldspar crystal; on right, authigenic kaolinite cement. (**B**) Altered K-feldspar; note the development of intracrystalline fractures and etching features on crystal surfaces. (**C**) Detail of authigenic kaolinite aggregates showing booklet morphology. (**D**) Siliceous overgrowth on quartz crystal. (**E**) Organic biofilm coating quartz grain surfaces. (**F**) Brucite aggregates. The silica released (initially as H4 SiO4) migrates to the exterior of the rock according to the preferential directions of water flow and starts to form the superficial hardened rinds and fillings in the system of joints and fractures (Figure 5D). A similar chemical attack, even more rapid and efficient, is produced simultaneously on the micaceous ferromagnetism minerals, such as biotite (2K(Mg,Fe,Mn)\({}_{3}\)AlSi\({}_{3}\)O\({}_{10}\)(OH)\({}_{2}\))).