CONCRETIONS

Limestone cave carbonates are usually calcite and aragonite while gypsum caves can develop gypsum or calcite formations (Gewelt and Ek 1988; Calaforra et al. 2008). Calaforra et al. (2008) noted, “calcite speleothem evolution is mainly controlled by CO2 diffusion, while gypsum deposits develop mostly due to evaporation.” Freezing can also produce rare gypsum speleothems such as yozh (‘hedgehog’) formations, which can be similar to gypsum roses (Korshunov and Shavrina 1998). While the exact role they might play in speleothem formation is still debated, it is understood that some microbes do precipitate calcium carbonate in caves. It is possible they play a greater role in underwater cave environments (Tredici et al. 2018).

Formations include dripstones (stalagmites and stalactites), flowstones, moonmilk, frostwork, boxwork, coralloids, gours, helictites, cave pearls, and more (Gewelt and Ek 1988; Self 2004). Individual mineral crystals that precipitate are not themselves speleothems (except for certain cases like selenite needles), but build up and/or aggregate into speleothems. Crystal can split as they grow, and with enough splitting can form spherulites (Self 2004); splitting at a molecular level produces spherocrystals and botryoidal surfaces (as with cave formations of malachite).

Cave pearls are pisolites, mostly small spherical concretions formed by precipitation of thin, concentric carbonate layers around a tiny nucleus (though Roberge and Caron (1983) reported cuboid cave pearls in Canada). Concretionary grains smaller than 2mm are called oolites. Cave pearls often form in ‘nests’ in small basins where actively dripping water agitates the growing pearls just enough to keep them from cementing to the bottom (Melim and Spilde 2018), though some grow in shallow pools without active dripping, likely with microbial help (Jones 2009). Lacustrine pisolites are formed by algae (Jones and Wilkinson 1978).

(James St. John, CC-BY 2.0)