hydrophanous

Hydrophanous refers to a property of certain minerals that become translucent or transparent when wet and opaque when dry. This characteristic is due to the way water interacts with the mineral's structure. When a hydrophanous mineral is exposed to water, the water fills microscopic pores or fissures within the mineral. This filling process causes light to scatter differently, making the mineral appear more transparent or translucent. As the mineral dries, the water evaporates, and the air within the pores scatters light again, returning the mineral to its opaque state.

This phenomenon is most commonly observed in certain varieties of opal, particularly common opal. For example, some opals might appear milky white and opaque when dry, but when immersed in water, they can become noticeably clearer, allowing light to pass through more readily. The degree of hydrophanousness can vary significantly between different specimens and types of minerals. The term itself is derived from Greek roots: "hydro" meaning water, and "phaino" meaning to appear or show. Therefore, "hydrophanous" literally translates to "appearing with water." While less common in other mineral groups, the property highlights the dynamic relationship between a mineral's internal structure and its interaction with external substances like water.