Marble

Marble forms when limestone or dolomite, sedimentary rocks rich in calcium carbonate, undergo regional metamorphism during orogenic processes. Under pressures of 2–5 kbar and temperatures between 200 and 800 °C, calcite crystals recrystallize into larger and more homogeneous aggregates, eliminating original impurities and imparting marble with its characteristic granular structure. Recrystallization obliterates original sedimentary structures, transforming the material into a massive and compact rock.

Marble varieties vary enormously in color and composition depending on impurities present in the original limestone. Pure white marble, such as Carrara marble, contains nearly pure calcite; gray, pink, or red marbles derive from inclusions of graphite, iron oxides, or silicate minerals such as quartz, feldspar, and mica. Common mineral associations include dolomite, talc, tremolite, and forsterite, which impart distinctive mechanical and chromatic characteristics. From an economic perspective, marble represents one of the most valuable ornamental stones, with historic deposits in Italy (Carrara, Tuscany), Greece (Thassos), Turkey (Afyon), and China, where quality and color determine commercial value.

Marble is a medium to fine-grained metamorphic rock composed primarily of trigonal calcite (CaCO₃) with rhombohedral crystal structure. Mohs hardness 3–4; density 2.7–2.8 g/cm³. Ordinary refractive index (no) approximately 1.658; extraordinary refractive index (ne) approximately 1.486; positive birefringence Δn ≈ 0.172. Solubility in dilute HCl at room temperature with pronounced effervescence, characteristic of calcite.

Crystal structure: trigonal, space group R3c, lattice parameters a ≈ 4.99 Å, c ≈ 17.06 Å. Metamorphic recrystallization produces xenomorphic aggregates with irregular contacts (granoblastic texture); the degree of metamorphism determines mean crystal size (0.1–10 mm). Raman spectroscopy shows characteristic CaCO₃ peaks at 1086 cm⁻¹ (symmetric CO₃²⁻ stretching), 1435 cm⁻¹ (asymmetric stretching), 713 cm⁻¹ (in-plane bending), and 879 cm⁻¹ (out-of-plane bending).

Varieties containing dolomite CaMg(CO₃)₂ exhibit intermediate compositions and slightly different physical properties. Silicate marbles contain accessory minerals such as tremolite Ca₂Mg₅Si₈O₂₂(OH)₂, quartz SiO₂, and feldspar, which increase hardness and resistance to alteration. Coloration derives from trace elements: Fe²⁺/Fe³⁺ for gray and red tones, Mn²⁺ for pink, graphite for black. UV-Vis spectroscopic absorption reveals characteristic bands dependent on impurity composition.