The microfacies of Pleistocene calcareous crust in Tunisia revealed the presence of diverse aspects of acicular crystallizations in a porous system. These crystallizations are in the shape of branched filaments, mono or polycrystalline rods which are either smooth or serrated (calcite), and serrated edge needle (calcium oxalate). The genesis of these acicular structures seems to be directly or indirectly linked to the organic activity. In fact, the facies are tied to the biological activity or biochemical interactions, which occur between living organisms and the mineral (calcite), triggering a succession of precipitation and dissolution reactions. These reactions, which are characteristic of vadose environments, have actively contributed to the endurance of calcareous crusts due to premature or delayed diagenesis. The acicular calcite, found in the Tunisian Pleistocene calcretes, reflects regional climates that influence the form and mineralogy of needle fiber calcite.

$\bf{Highlights}$

$\bullet$ Diverse aspects of acicular crystallizations in the microfacies of Pleistocene calcareous crusts investigated.

$\bullet$ The needle fiber calcite has different origins, which may be biogenic, physicochemical or organic.

$\bullet$ The form and mineralogy of the needle fiber calcite are affected by the environment and climate change.

$\bullet$ The lithogenesis of the crusts is influenced by the redistributions and the organizations of the needle fiber calcite.