Final answer:
Speleothems, or cave formations, are primarily made out of calcite (CaCO3) option (b). Calcite forms from the precipitation of calcium carbonate in CO2-rich water solutions as they come into conditions where CO2 can be released, resulting in the growth of formations like stalactites and stalagmites within caves.
Step-by-step explanation:
Speleothems, which are cave formations, are mostly made out of calcite. Calcite, or calcium carbonate (CaCO3), is the primary component of limestone, the rock that is most commonly associated with the creation of caves.
When limestone becomes saturated with CaCO3 in a CO2-rich water solution and this solution comes in contact with a place where the CO2 can be released, such as when the water rises toward Earth's surface or warms, CaCO3 precipitates from the solution and forms speleothems, including both stalactites and stalagmites within the cave environment.
Understanding cave formation involves knowledge of the chemistry of cave formation as described in Figure 16.4.1. Limestone caves form via a process involving the dissolution of limestone by acidic water that contains dissolved CO2.
In the right conditions, as the acidic water percolates through the limestone, it dissolves the rock and ultimately deposits calcite to form the various cave decorations such as the iconic stalactites hanging from the cave ceiling and stalagmites rising from the floor.
Therefore, the correct answer to which substance speleothems are primarily composed of is B. calcite.