Characterization and origin of the Mn-rich patinas formed on Lunéville château sandstones

Abstract

Abstract. The formation of iron- and/or manganese-rich dark patinas on sandstones is a common natural phenomenon that occurs also on building stones. Lunéville château, in eastern France, presents such patinas that developed either under natural conditions (rain and time) or after an accidental fire and exposure to significant amounts of water as part of attempts to extinguish the fire. The present study aimed at characterizing both types of patinas in an effort to determine their formation mechanisms and Mn sources. In both cases, Mn required for patina formation likely derives from the reductive dissolution of Mn-rich minerals present in pristine sandstones, as suggested by the contrasting mineralogy and chemistry of Mn-rich phases present in the bulk and in the patina of a given building block. Reduced Mn species then migrate to the exposed surface of building blocks where they are re-oxidized via undetermined processes. Patinas developing “naturally” over time result from the alternation of wetting-reducing and drying-oxidizing cycles and appear to be composed of birnessite. Patinas formed after the 2003 fire result from this single accidental event and form a much thinner, heterogeneous, and discontinuous layer of poorly crystalline lithiophorite at the sandstone surface (∼ 0–150 µm compared to ∼ 300–600 µm for “natural” patinas). The lack of Mn-rich patinas on areas of Lunéville château is likely related to the lower Mn content of pristine sandstone blocks.

Non-Destructive Analysis to Investigate the Stone Alterations at a UNESCO World Heritage Site

“… An increase of macro-roughness following the differential leaching of minerals from the rock;  An increase of surface permeability linked to cracks formation and the opening of grain boundaries [20] induced by dissolution.…”

Rock varnish revisited

“…This definition, however, does exclude rock coatings like heavy metal skins and case hardening that can be rich in Mn and Fe (Dorn, 1998, 2022). A variety of heavy metal skins form rock coatings (Dorn, 1998, 2022) that do not include an significant clay component, such as thin Mn-skins (Carpenter and Hayes, 1978; Eren et al, 2014; Gatuingt et al, 2021; Sharps et al, 2020; Tani et al, 2003) and thicker ferromanganese accumulations (Dorn, 1998; Spinks and Uvarova, 2019). Case hardening involves the reprecipitation of the remobilized rock coatings, including rock varnish constituents, inside the porous weathering rind of a rock (Conca and Rossman, 1982; Dorn et al, 2012a, 2017).…”

Thermodynamics and crystal structures of krautite, Mn[AsO3(OH)]⋅H2O, koritnigite, Zn[AsO3(OH)]⋅H2O and cobaltkoritnigite, Co[AsO3(OH)]⋅H2O

“…Zinc and cobalt for such minerals are released by weathering of primary ore minerals. Manganese, on the other hand, could be released from minerals of ore deposits (Holtstam et al, 2022;Cámara et al, 2022) but could have other sources in the environment (e.g., Gatuingt et al, 2021). This work focuses on thermodynamic properties and crystal structures on selected synthetic equivalents of the minerals in the krautite group.…”

“…Manganese, on the other hand, could be released from minerals of ore deposits (Holtstam et al , 2022; Cámara et al , 2023) but could have other sources in the environment (e.g. Gatuingt et al , 2021).…”