New mineralogical record of guano-associated phosphates in Colombo cave (Toirano, Liguria, Italy)

Colombo Cave is part of Toirano karst system and opens at 247 m a.s.l. The wide entrance passage was used during prehistoric times, and a 4.5 m deep archaeological excavation pit is located 10 m from the entrance. The main room is dominated by a large central rock pillar and the floor is completely covered by important ancient bat guano deposits. Samples were taken in separate containers and were representative of old guano deposits and a hard-yellow crust covering the guano heap. All samples were identified by XRD and SEM-EDS analysis. The results indicate that old guano samples were composed mainly of calcite, quartz, spheniscidite and minor amounts of xenotime (Y), monazite, zircon, and rutile. In particular, spheniscidite (NH4,K)(Fe3+,Al)2(PO4)2(OH)·2H2O forms from the reaction of ammonium-rich fresh guano leachates with probably clay sediments containing Fe and muscovite (source of K and Al) (Sauro et al., 2014). The mineral associations present in soft yellow and whitish crusts that cover guano were represented by gypsum CaSO4·2H2O, brushite Ca(HPO4)·2H2O, ardealite Ca2(HPO4)(SO4)·4H2O and newberyite Mg(HPO4)·3H2O. Brushite and ardealite occur as cryptocrystalline aggregates varying in color from white-ivory to yellow-ivory and form by the reaction of sulfuric and phosphoric acid with limestone rock (Hill & Forti, 1997; Puşcaş et al., 2014). Newberyite was less common and likely comes from the interaction of phosphates with Mg provided by the disaggregation of the dolomitic host rock. The identification of these minerals is important because they were never reported from Liguria. Further investigations will be conducted in order to understand the minerogenetic processes involved in these guano-associated minerals, and their evolution through time in the damp cave environment (Audra et al., 2019).