MINERALS UNSTABLE IN PRESENT WEATHERING 95 



Fe, and, of course, anions, H and the halogens; S and so on. 



Sihcates containing only alkaUs and Ca combined with aluminium 

 and silica, mostly form light-coloured felspars. Dark minerals, like 

 biotite, augite and hornblende, also contain Mg, Fe and other metals. 

 The three groups of quartz, felspars and dark minerals together make 

 up by far the bulk of all crustal rocks. In addition ore minerals 

 occur, mainly in plutonic rocks and related veins. These contain 

 sulphides, like pyrites, FeS, and metal oxides like magnetite, Fe.'}04. 

 The sulphides in general are low-temperature minerals, the oxides are 

 formed at much higher temperature. 



Returning now to the process of weathering of rocks exposed at 

 the surface of the earth, we find that weathering attacks the crustal 

 rocks along two main lines: physical and chemical. Pure physical 

 weathering is rare. It is found, for instance, both in extremely cold 

 and in extremely hot and dry climates; in the tundras and the deserts. 

 In a cold climate rocks disintregate through frost splitting, in the 

 desert through sun blasting. Everywhere else on earth chemical 

 weathering is present, whilst it normally forms the main line of 

 attack on rocks of the crust. 



MINERALS UNSTABLE IN PRESENT WEATHERING 



Chemical weathering, under the circumstances of our present oxi- 

 dizing atmosphere, will attack all minerals but the oxides. Both the 

 felspars and the common dark minerals of the normal rocks, as well 

 as the sulphides of the ore veins, will be oxidized and form soluble 

 compounds. So the only things left are the oxides: the common 

 quartz, and the much rarer ore oxides such as magnetite. The solu- 

 tions carrying the material from the felspars, the dark minerals and 

 the ore sulphides are transported by rivers into lowlands and oceans. 

 There, mostly below the watertable, and often at the exclusion of 

 free oxygen, new compounds will form. The new combinations pre- 

 dominantly belong to the group of the clay minerals. 



This is, in an extremely schematized version, the normal sequence 

 of weathering — transportation — sedimentation in our present oxi- 

 dative atmosphere. Although extremely simplified, it illustrates the 

 points important for our study: that all minerals except oxides are 

 unstable in regard to chemical weathering, and, moreover, that the 

 ions derived from that chemical weathering of silicates and sulphides, 



