What is the finest sediment found in caves that is invisible to the human eye?

Clay is the finest sediment found in caves and is characterized by its extremely small particle size, which can be invisible to the naked eye. Clay particles are typically less than 0.002 millimeters in diameter, making them significantly smaller than silt and sand particles. This fine texture allows clay to retain moisture and nutrients, making it an important component of cave sediments that can influence the cave ecosystem.

In contrast, silt consists of slightly larger particles, ranging from 0.002 to 0.05 millimeters, which can often still be seen. Sand is even coarser, with particle sizes between 0.05 and 2 millimeters, and is typically visible to the naked eye. Granite, on the other hand, is a type of rock and does not fit into the categories of fine sediment like clay, sand, or silt. Thus, clay stands out as the appropriate answer as it aligns with the criteria of being the finest and often invisible sediment in cave environments.