Il marmo di Rocca Bianca (Val Germanasca, Alpi Occidentali).

The Rocca Bianca marble (Germanasca Valley, Cottian Alps), is a white statuary marble employed in the building of many historical monuments located in Turin and its province (e.g. Palazzo Madama, 1675 and Palazzo Carignano, 1681). This marble, was extensively exploited from the 17 century until 2003, in two different quarries at an altitude of ca. 2000 m, and forms several transposed layers embedded in the polymetamorphic garnet-chloritoid micaschists belonging to the pre-Mesozoic basement of the Dora Maira Unit, one the continental crust units of the Penninic Domain of the Western Alps. This marble is characterized by a cm-thick layered bands of tremolite rich- green levels in dolomite-rich marble (known as "calcefiro zonato verde"), alternating with a mylonitic marble with grey levels due to grain sized reduction of the carbonate. The heterogeneous geomechanical behaviour, related to the variable calcite/dolomite ratio in the different levels, produced some difficulties in the exploitation of the marble, because the more dolomitic volumes were generally intensely fractured. The distribution of the Rocca Bianca marble is mainly controlled by plurikilometric folds, related to the second deformation phase of alpine age, developed in HP-LT metamorphic conditions and complicated by post-metamorphic regional fault systems. The exploitation was interrupted in 2003 when the fracture density in the quarry fronts raised excessively.