R15- Marble

Equigranular, fine-grained rock that has a weakly developed foliation, with sub-parallel pale and dark grey layers entirely made of calcium carbonate grains (reacted with acid). The sample is a marble that comes from the contact aureole outcropping in the Campiglia Marittima area (Tuscany). The thermometamorphic aureole (characterized by marbles and skarn) was created at the contact of the shallow Botro ai Marmi intrusion (see R7 syenogranite) with host carbonate rocks, the latter mainly represented by the Calcare Massiccio Formation. The outcropping thermo-metamorphic aureole is approximately 1,5 km wide and 300 m thick (Geol.F.Trips, Vol.7 No.1.2 (2015), DOI 10.3301/GFT.2015.02).

Thin section description:

The field of view is entirely occupied by calcite crystals. In PPL view, the individual crystal grains are colourless but we can see a change in the relief of each of the grains relative to its neighbouring crystals. The changing relief is a characteristic of carbonate minerals and it is known as “twinkling”. This optic effect is due to the large difference in the refractive indices in different crystallographic orientations of the mineral; it can be observed in the PPL video by looking at two adjacent crystals of calcite and watching as their relative relief switches when the microscope stage is turned. In the XPL view, the crystals show a very high birefringence typical of carbonates; some of the crystals also display lamellar twinning (twinning in calcite crystals can supply information on the deformation mechanisms of the crystal; Ferrill et al., 2004, J Struct Geol. Acta, v. 26).

thin section PPL	thin section XPL