Decay patterns of weathered quartz sandstones : evidence for gypsum induced structural changes

Black gypsum crusts and thin black layers are considered to be typical weathering phenomena located on the surface of quartz sandstones. Black crusts are generelly found in sheltered areas not directly exposed to water runoff, but frequently soaked by rainwater. In contrast, thin black layers occur in zones which were subject to direct wetting by rain. Usually, enrichment of gypsum takes place not only on superficial crusts but also within the intergranular pore spaces ofthe stone substrate underneath crusts and thin black layers. Gypsum crystallization within the pore spaces may change the microstructural features of sandstones as well as dry-wet cycles leading to shrinking-expansion phenomena of gypsum, thus, causing decohension of silicate cemented detrital grains within the natural stone. The use of microscopic methods is helpful to characterize erosion features associated with crusts, thin black layers and inward gypsum migration. The surface samples were taken from Leineschloss in Hanover, St. Marienkirche in Zwickau und Erfurt Cathedral (Germany). The damage observed in the quartz sandstones under investigation is the result of a stone-gypsum interaction. Stone material incorporated within gypsum crusts has lost its primary grain bond. Beyond that, cyclic crystallization pressure and shrinkage-extensinal stresses of gypsum are the potential causes for fracturing surface near quartz grains. These cracks are supposed to be generated during the initial stage of damaging processes. Simultaneously, the development of secondary intergranular porosity is involved in these processes. Thus, the microstructural damage is not visible on the stone surface. If conventional cleaning is planned, the superficial decohension of mineral content in silicate cemented sandstones should be borne in mind.