Overview
Caminite is a rare magnesium-hydroxide-sulfate-hydrate mineral with an unusual origin: it forms on the deep-sea floor inside black-smoker chimneys at hydrothermal vents. It was described in 1986 from the East Pacific Rise at 21°N, where it was found intergrown with anhydrite in chimney walls precipitated around hot vent fluids. Its name comes from the Latin caminus, meaning chimney, a direct nod to the structures in which it grows. Caminite is of scientific rather than ornamental interest and is essentially absent from collector and lapidary markets.
Composition & structure
Caminite is a hydrated magnesium sulfate that also incorporates hydroxyl groups, with an approximate formula of Mg7(SO4)5(OH)4·H2O; the composition is variable and the structure accommodates a range of magnesium-sulfate-hydroxide ratios. It crystallises in the tetragonal system. Microscopically it forms small grains rather than the large, well-shaped crystals familiar from collectible minerals, consistent with rapid precipitation from cooling and mixing fluids.
| Formula | Mg7(SO4)5(OH)4·H2O (variable) |
| Crystal system | Tetragonal |
| Mohs hardness | About 2.5 |
| Lustre | Vitreous (commonly as fine grains) |
| Colour | Colourless to white |
| Type locality | East Pacific Rise, 21°N, submarine hydrothermal vent field |
Formation & occurrence
Caminite precipitates when cold, magnesium- and sulfate-rich seawater is drawn into the seafloor and heated within an active hydrothermal system. Laboratory experiments had predicted that such a magnesium-hydroxide-sulfate-hydrate phase should form from heated seawater, and caminite confirmed this in nature. It crystallises alongside anhydrite in chimney walls, where steep temperature gradients between near-boiling vent fluid and surrounding seawater drive rapid mineral deposition. Because it is metastable, it may be more widespread in hot seawater-recharge zones than the few reported occurrences suggest.
Identification & similar species
Caminite is difficult to identify by eye and is normally recognised through laboratory methods such as X-ray diffraction and electron microprobe analysis. Intergrown anhydrite is the most common associate, and caminite can be distinguished from it optically by its lower birefringence as well as by composition. Its softness, around 2.5 on the Mohs scale, and colourless-to-white appearance are consistent with a hydrated magnesium sulfate but are not on their own diagnostic.
Notable localities & collecting
The defining and best-known occurrence remains the East Pacific Rise type locality, with the mineral tied to active and recently active submarine vent systems rather than to land deposits. As a deep-sea phase recovered only through oceanographic expeditions, caminite is not available to ordinary collectors and is essentially a research mineral. Its interest lies in what it reveals about chemical exchange between seawater and the oceanic crust at hydrothermal vents.