Carbonate rocks - Marbles and Calcareous rocks

The main carbonate rocks cover calcareous rocks and dolomites, marbles being their metamorphic correspondents. The calcareous rocks are sedimentary rocks mainly composed of calcite (CaCO3), while dolomites are also sedimentary stones formed especially by dolomite (CaCO3.MgCO3).

Some other carbonate minerals, specifically the siderite (FeCO3), ankerite (Ca, MgFe(CO3)4) and magnesite MgCO3, are frequently associated to calcareous stones and dolomites, but generally in small proportions. The marbles are characterized by the presence of carbonate minerals with varied degrees of metamorphic re-crystallization.

Clay minerals (kaolinite, illite, chlorite, smectite, etc.) and its metamorphic products (sericite, muscovite, phlogopite, biotite, tremolite, actinolite, diopside, etc.), constitute common impurities as well disseminated as plated within carbonate rocks.

Quartz and sulfates are frequent accessories, whether as isolated crystals or disseminated in the matrix. Organic matter can also be finely spread, conferring black and dark brown colors to the carrying rocks.

Most carbonate stones come from biological or more properly biodetrital origin, forming in marine environment by deposition of shells and skeletons from other organisms (choral, bryozoans etc.). These shells and skeletons are preserved as more and less fragmented fossils, perfectly recognizable in the little or not metamorphosed rocks.
Depositional processes led by direct chemical and biochemical precipitation of carbonates in fresh water environments, determine the formation of non fossiliferous and rather heterogeneous stones such as the travertine and marl types.

Carbonate rocks represent thus sedimentary and metasedimentary materials, constituted of 50% or more of calcite and dolomite minerals. Calcareous, epicalcareous and calcic marbles contain predominating calcite, while dolomites, metadolomite and dolomite marbles are similar rocks with dolomite predominance. Common impurities include argils, quartz, micas, amphiboles, organic/ graphitic material and sulfates, characterizing a wide variety of color, texture, drawing, crystallinity and fossil content.

In the sector of ornamental and covering stones, the term ‘marble’ is used to indicate all carbonate rocks, metamorphic or not, capable of receiving polishing and shining. The recent growth of participation of granites in the use was, at least partially determined by its larger durable condition and resistance relating to marbles, besides the non traditional aesthetic standards and possibilities of paging over floors and façades.