Pietraforte Sandstone

Steps in Pietraforte Sandstone (Piazza Santissima Annunziata).

Lithotype

Pietraforte Sandstone

Common names

Pietraforte

Rock classification

Sedimentary rock

Firenze (mainly in the southern area) and Province: Giardino di Boboli, Viale Galileo, Bellosguardo Hill, Ema Valley.

Turbidite sandstone belonging to the Supergruppo della Calvana, in the external Ligurian Domain, dated to the Late Cretaceous. Feldspathic and carbonatic quartz sandstones; they outcrop in thin layers (20-100 cm) alternated with silty argillites. The sandstone is classified as a lithic arenite.

It was the most used building material in the medieval Firenze, in ashlars of various sizes. The use continued in Renaissance, also for road paving.

It is mainly constituted by quartz granules, feldspars, mica, dolomite, and low amounts of calcite and clay minerals.

Macroscopic description

Fine-grained sandstone, gray-bluish in fresh cut, ocher for alteration. It is characterized by the presence of convoluted laminations and calcite veins. It is a hard, compact, not-polishable rock with low-medium porosity.

Macroscopic aspect of a Pietraforte Sandstone.

Microscopic description

The granules are constituted by quartz, feldspars, fragments of dolomite rocks, low-grade metamorphic rocks and silica-rich igneous effusive rocks. The matrix consists of prevalent micritic calcite and clay minerals. Calcitic cement is also present.

Thin section micrograph of a Pietraforte Sandstone.