## 4 Discussion
### Origin of the Major Mineral Phases  
The origin and evolution of the major mineral constituents of the red mud, which hold the remarkable portion of the REEs, are significantly different from each other. The most Ti-minerals, but anatase are stable at high temperature and pressure and so are common phases of igneous and metamorphic rocks. As there are no sedimentary environments in which rutile, ilmenite or titanomagneticite would form, these phases must represent the original source rock of the precursor bauxite. Anatase is the stable low-temperature polymorphous variety of TiO\({}_{2}\) and probably represents the bauxite similar to goethite and hematite, what develop during lateritic weathering. Cancrinite nevertheless is not a common mineral in the most igneous rocks or bauxites and most probably developed during the industrial treatment [19] or following it, inside the pitfall due to diagenetic processes.  
Scanning microscopic observations show that cancrinite together with goethite and hematite appear in aggregates of intensively cemented clusters of very fine grains. This special microtexture is rather strange in bauxite, but is very similar to that develops from kaolinite under hyperalkaline conditions in some well-documented experiments (Figure 4, [20]). Hungarian bauxites, similar to most karst bauxites have relatively high Si/Al ratio. As a mineralogical consequence, in addition to the common Al-hydroxide phases (boehmite, gibbsite, etc.) they also contain a significant amount of clay minerals, usually kaolinite [16]. As the red mud samples studied contain no kaolinite at all, one can assume that it reacted with the elevated Na-content of the mud and formed the cancrinite pseudomorphs after kaolinite due either to the Bayer process or under diagenetic conditions.  
In the PC2-3 component (process) of the above calculation, Al and Ca appear with opposite signs (Al, -Ca) suggesting that the main Al- and Ca-phases do not form simultaneously in the pitfalls. So, development of the major cement minerals, cancrinite (Al) and calcite (Ca) seem excluding each other. A possible geochemical explanation for this phenomenon is the local difference in Ca/Al ratio of the mud. At places where Al concentration is below a certain threshold, cancrinite cannot crystallize. A more viable reason is the difference in CO\({}_{2}\) fugacity inside diverse regimes of the pitfalls. Results of experimental petrology suggest that at high fCO\({}_{2}\) calcite and another Na-phase (e.g., repheline) are stable, but cancrinite [21]. Although the current sampling strategy in the Almasfuzito area did not allow localization the zone, where carbonization is the most characteristic process, previous data suggest that the red muds in question contain a significant amount of calcite in the topmost horizon of the pitfalls. Formation of calcite replacing carcinrite in the shallowest zones due to the reaction with atmospheric CO\({}_{2}\) may explain the negative correlation between Ca and Al [22]. Nevertheless, it also suggests that as soon as carcinrite disappears, the shallow horizon is not capable to fix REEs any more, shown by the negative correlation between Ca and the REEs. Such a reduced concentration for each REE is suggested in the upper segment of all studied wells of the Almasfuzito area (Table 1).