Geology Reference
In-Depth Information
Plate 150 Microfacies, Weathering and Conservation of Limestones
The oolitic limestones, exploited near Savonnières-en-Pertois in eastern France, have been widely used as sculp-
tural stones in Central Europe (e.g. for the Gothic Cathedral in Cologne). These grainstones consists predomi-
nantly of ooids with a high porosity (30-35%) caused mainly by the dissolution of the nuclei. Porosity distribu-
tion however, assists in the workability of the stone, the combination with low permeabilities (about 30 mD
vertical and 100 mD parallel to bedding) and conspicuous differences of interparticle and intercrystalline pore
neck diameters facilitates on the other hand weathering and destruction (Lorenz and Ibach 1996). The plate
shows the microfacies of the not-weathered stone (-> 1), destruction types of sculptural stones -> (2-6) as well
as the effect of a conservation method (-> 7, 8). Figs. 2-6 are samples of building stones used for the outer front
of the Thomas Church in Leipzig, Germany.
The Ibach Total Impregnation method involves soaking well dried limestones and marbles with liquid me-
thyl-methacrylate, using vacuum and pressure.
1
Oolitic grainstone
. The nuclei of most ooids and mollusk shells are dissolved, causing high oomoldic and biomoldic
porosities (blue staining). Intercrystalline porosity (within ooid cortices and cement crystals) also occurs. Intercrystalline
pores may act as capillary systems that hold fluids and cause long-term rock destruction. The ooids have been cemented
by marine-phreatic isopachous cement crystals. Early Cretaceous (Berriasian): Savonnières-en-Pertois, eastern France.
2
Building stone
. Rain-exposed surfaces of building stones are prone to freezing damage. Water intruded via intercrystalline
ooid and cement pores, causing volumetric expansion and leading to cracking (arrow). The detachment of cortices is
caused by pronounced leaching along growth stages of the ooid. The ooid nucleus is completely leached. Savonnières
oolite used as external building stone for the Thomas Church in Leipzig, Germany.
3
Industrial pollution
contributes to the deposition of gasses and various particles in rain-protected niches, leading to the
formation of crusts (upper part). These 0.5 to 1 mm thick crusts consist of gypsum (white) and various soot particles
(black and brownish). Gypsum is formed by the reaction of SO
2
with water, producing sulphurous acid, which in turn
reacts with CaCO
3
(red, alizarine staining). Microbial activity has a great influence on crust formation. Gypsum also
occurs within oomolds several millimeters below the stone surface (-> 5). The crusts do not act as protecting cover.
Differences in gypsum and calcite with regard to thermic expansion and the reaction with fluids lead to a loosening of the
uppermost centimeters of the rock. Removal of the gypsum crusts accelerates rapid weathering. Open pores are blue.
Same locality as -> 2.
4
Surface of a building stone.
Porosity variations (bottom, with original porosity; top, with altered porosities) are respon-
sible for vertical and horizontal permeability differences. Red: calcite; blue: open pores; white: gypsum, dark: industrial
soot particles. The yellowish color (top and corner) is produced by the thin-section glass. Same locality as -> 2.
5
Oomoldic pore,
partly filled with idiomorphic gypsum crystals (formed during weathering; arrow) and brownish organic
matter (produced by hydrocarbons having migrated through the limestone). Same locality as -> 2.
6
Vertically cut stone surface.
The uppermost 'weakened zone' (-> 3) is affected subsequent to the loss of the outer gypsum
crust by frost weathering, causing splitting via microcracks (arrows; blue) parallel to the surface. Same locality as -> 2.
7
Conservation.
Various methods (surface sealing, acrylate total impregnation) are in use in order to protect stone sculp-
tures and building stones from further decay. The thin section shows the totally impregnated oolite. The normally color-
less acrylate (A) was stained in this sample to show that the impregnation has affected all the intra- and interparticles
pores. Isolated vacuoles (VA, white) within the acrylate are caused by volumes decreasing during polymerization. Be-
cause these bubbles are enveloped by acrylate, they are not accessible for gases or fluids. Note the complete compound of
the acrylate with the surface of the calcite cement crystals and within the leached ooids. This compound hinders further
intrusion of moisture and gases. Dark patches are organic matter (OM; see -> 5). Broadening and rounding of cement
crystal tips indicate meteoric overprint of marine-phreatic cements. Same locality as -> 2.
8
Conservation.
Inhomogenities within the oolite are caused by shell layers (SL) that were more strongly affected and
leached by rain and weathering, because of their primary aragonitic mineralogy. However, total impregnation also affects
these microfacially different parts of the rock. Note the complete impregnation (here white) of all molds and interparticle
pores with acrylate. The total adhesion of acrylate to the pore walls prevents the carbonate from taking up moisture and
pollutant and results in good consolidation of the whole rock. Vacuoles within the acrylate have been stained subsequent
to the impregnation and appear blue. Same locality as -> 2.
-> 1-8: Courtesy of H. Lorenz (Erlangen)
