Vugs

A vug is a small to medium-sized cavity or opening within a rock that forms after the rock has already solidified. Vugs are typically irregular in shape and larger than ordinary pore spaces, and they are often lined with mineral crystals that precipitated from fluids moving through the rock after its formation.

Formation and characteristics: Vugs arise when soluble minerals are dissolved or when surrounding material is removed, creating voids that can later be mineralized by hydrothermal or meteoric fluids. The crystals lining a vug may form a drusy coating, yielding a dense layer of tiny crystals on the walls. Common minerals found in vugs include calcite, quartz, dolomite, and, less commonly, zeolites, fluorite, or celestite. Vugs are frequently associated with carbonate rocks such as limestone and dolostone, but can also occur in other settings where dissolution and mineral precipitation occur.

Relation to other features: In volcanic rocks, primary gas vesicles can be enlarged or partially filled by post-eruption mineralization, but vesicles are typically the original voids formed during solidification. A geode, by contrast, is a hollow rock cavity that generally has a more complete outer shell and is often lined with crystals; vugs are usually open to the surrounding rock and irregular in shape.

Significance: Vugs are notable for their mineral-rich crystalizations and can influence rock porosity and permeability. They are of interest to mineral collectors and to studies of diagenesis and hydrothermal processes.