Environmental Engineering Reference
In-Depth Information
Within this layer, calcite has been replaced by gypsum through a process called
“epigeny”. This process is very particular because it implies only minor volume
changes, even if the new mineral does not have the same molar volume as the
original one. The color of epigenic layers is mainly due to impregnation by water-
soluble organic products mentioned in the preceding section, and also in certain
cases by iron oxides originating from the stone itself. The epigenic replacement goes
inward, i.e. from the stone surface toward its core, but is always very shallow. The
epigenic layer thus develops immediately under the stone's surface, and underlines
all its details (tool marks, among others) in an extremely precise way [VER 93]. Its
thickness depends on the porosity of the stone, not on that of the upper-lying black
crust. It is never higher than 0.5 mm. Epigenic layers may even develop without any
black crusts; in such cases, the stone surface becomes darker, yellower and more
compact.
Epigenic gypsum layers should not be mistaken for the French concept of
“calcin”, which was introduced by stone masons and sculptors at the beginning of
the 20th century. Viollet-le-Duc, in his dictionary of architecture published in 1854
[VIO 54], does not use this term. Calcin is a superficial transformation of the stone
that may sometimes reach a depth of 5 mm, and is recognizable while carving
because it is harder than the stone core. This characteristic was verified by Mamillan
in 1964 [MAM 64]. This author has also shown through chemical analyses that the
calcin corresponds to the stone's enrichment with gypsum. This in turn corresponds
with the sulfation zone in the stone. Since the Mamillan publication, the term calcin
has sometimes been replaced with “sulfo-calcin”. Petrographic investigations have
allowed us to show that calcin consists of several sulfation zones (see Figure 8.24),
the upper one being the epigenic layer itself [BRO 96].
Figure 8.24.
Different locations of gypsum on and under the surface of a limestone: thin
section, transmitted light, nicols // (A);
⊥
(B); and explanatory sketch (C)
−
CN = black crust;
CGE = epigenic gypsum layer; MC = carbonated matrix; F = crack filled with gypsum;
fp = microfossil disrupted by gypsum crystallization [BRO 96]
