Black Crusts: "Fossil" and Modern. A Comparison from two Case studies in Coimbra Monuments (Central Portugal) / Schwarze Krusten: "Fossil" und von heute. Ein Vergleich von zwei Fallstudien durchgeführt an Baudenkmalen in Coimbra, Zentral-Portugal

On two limestone monuments in Coimbra, which are located near each other, two different kinds of "black crusts" were observed: "fossil" and modern. The crusts were analyzed and the data compared. The very thin "fossil" black crusts of Santa Clara-a-Velha Monastery are formed by pellicules of gypsum, anhydrite and syngenite rich in Fe and Mn and depleted of Pb,Ti, Zn, Cu and Ba. The anthropogenic, modern crusts of the Santa Cruz Church are thicker, richer in gypsum and trace elements such as Ti, Pb, Zn, Cu and Ba. Calcium oxalates, weddellite and whewellite, appear in areas where lichens are growing. While the "fossil" black crusts have resulted from burial for at least 300 years, representing a geogenic weathering product in a rural area, the black crusts of the Santa Cruz Church represent an anthropogenic product generated from limestone attack in an urban modern environment