Über glimmer- und feldspatverwitterung sowie entstehung und umwandlung von tonmineralen in rezenten und fossilen Lössböden

Abstract

In all eight types of loess-soils, two recent chernozems and braunerden (brown forest soils), one syrosem of young Würm age, one chernozem of older Würm age, one braunerde of the last interglacial period, and one earthy braunlehm of the Middle Pleistocene Period, were investigated by polarization and phase-contrast microscopy as well as by the X-ray method. The problem was to determine the contents of primary minerals and clay minerals in order to answer the question as to what extent layer silicates and feldspars > 2 μm play a role in the formation of clay minerals. The results are as follows. 1. (1) In none of the eight soils investigated could a variation of the particle size of minerals in the silt fractions caused by frost cracks by discovered; in particular, no coarse clay formation (2-0.6 μm) explainable by cryoclastic fragmentation could be proved, not even in the syrosem of synsedimentary origin and of young Würm age. 2. (2) In the mechanism of pedochemical formation and transformation of clay minerals, the weathering and transformation of layer silicates > 2 μm plays a dominant role. The percentage of layer silicates declines by 1.2 to 1.8 times as much as the feldspars, independent from the type of soil and the intensity of weathering, respectively. The decline of feldspars amounts to 30% in the braunerde of last-interglacial age and is greater than in recent braunerden; in the rubefied earthy braunlehm of Middle Pleistocene age, the decline of feldspars amounts to 40% as compared to the decline of layer silicates of 45% in the braunerde of last-interglacial age and 75% in the earthy braunlehm, respectively. The weathering of silicates and the pedochemical formation of clay is important in the recent braunerde-chernozem and even in the recent chernozem with primary carbonates, as well as in the chernozem of old Würm age. 3. (3) The pedochemical formation and transformation of clay minerals is characterized to a very large extent by the formation of illites (25–40%) and particularly of montmorillonites (50–60%). Part of the montmorillonites is to be considered as a new formation of the weathering products of the feldspars. It is to be concluded that another part of the montmorillonites is the weathering products of the micas resulting from a transformation through the illite-stage. 4. (4) Compared with the importance of the montmorillonites and illites, the other minerals play an unimportant part in the pedochemical formation and transformation of clay minerals. This holds good also for the vermiculites which are merely present in the coarse and medium (0.6-0.2 μm) clay fraction, and not, however, in the generally dominant fine clay (< 0.2 μm) fraction. It is to be concluded that illites (mostly < 0.2 μm) and vermiculites always > 0.2 μm) are not always formed one after another, but beside each other in the same soil. 5. (5) In none of the loess soils investigated could a further transformation resulting in “Bodenchlorit” be found, not even in the intensively weathered and rubefied earthy braunlehm of Middle Pleistocene age.