Secondary Minerals in Volcanic Caves: Data from Hawai‘i

Lava tube caves contain a variety of secondary minerals formed by seepage waters extracting components from the overlying soils and basalt and depositing them as crusts, crystals, and small speleothems in the underlying tubes. Fifty one specimens were analyzed in the course of a reconnaissance of a selection of caves on Hawai'i. Many of these were from caves at low elevation where the cave environment is wet and at ambient temperature. Some were from a cave at 2900 meters on Mauna Loa where conditions were much drier. Minerals in hot fumarole caves in the Kilauea Caldera were observed but not sampled. Mineral identifications were made by X-ray diffraction with some assistance from a scanning electron microscope with energy dispersive X-ray detector for rough chemical analysis. Calcite is surprisingly common, appearing in the form of small coralloids and other crusts and coatings. Gypsum is common as crusts and as puffballs. Two other sulfate salts, thenardite and mirabilite, were identified. Two unusual mineral deposits contained the transition metal ions vanadium and copper. Bright olive-green patches in Lama Lua Cave were characterized as a hydrated copper-vanadium silicate with composition Mg 2 Cu 4 (VO 4 ) 2 Si 5 O 13 (H 2 O) 5.6 . Although the mineral produced a high quality X-ray diffraction pattern, the pattern did not match any minerals in the database. The blue-green stalactites in the Kapuka Kanohina system are a non-crystalline hydrated copper silicate coating lava stalactites.