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