How grey limestones become white marbles

Abstract

The conditions required for generating white marbles are investigated in the contact aureole of the Adamello Pluton in northern Italy, where grey limestones pass into white marbles with increasing metamorphic grade. The grey-level or whiteness of the carbonate rocks is quantitatively measured using an integrating sphere, and values are linked to the mineralogical components and textural features. Nearly pure calcite rocks (> 99 wt % CaCO 3 ) undergo a transition from dark and light-grey very-low-grade metamorphic limestones to white and dull white marbles close to the intrusive contact. The grey colour of the limestones is caused by as little as 0.05 wt % of finely dispersed organic carbon. Approaching the pluton, the organic carbon content decreases with increasing metamorphic grade, producing a pure white to dull white marble with less than 0.02 wt % of organic carbon. Grain size and grain-boundary width have a secondary control on the appearance of high-grade white marbles by controlling light absorption, and thus brightness as well. The photon path is longer leading to higher absorption in coarse-grained marbles, thus explaining their dull appearance. Otherwise, changes in refractive index caused by wide grain boundaries are believed to enhance the light reflection in statically recrystallised marbles compared with very-low-grade metamorphic limestones which appear darker. Secondary phases such as silica or ore minerals, or significant fluid flow, do not induce colour changes.