Overview
Baylissite is a very rare hydrated potassium-magnesium carbonate. It was described from an Alpine tunnel in Switzerland and is prized mainly as a curiosity by collectors of rare carbonates rather than as a display mineral. Because it is water-soluble and breaks down in humid air, baylissite is fragile and difficult to preserve, which adds to its scarcity in collections. It is named after the Australian chemist Sir Noel Stanley Bayliss, who studied the synthetic equivalent of the compound before it was recognised in nature.
Composition & structure
Baylissite has the formula K2Mg(CO3)2·4H2O, a double carbonate of potassium and magnesium with four molecules of water of crystallisation. The water content is essential to the structure, and its loss is what makes the mineral unstable outside of damp, sheltered settings. It is monoclinic and tends to form colourless, transparent crystals. Chemically it sits among a small family of hydrated alkali-bearing carbonates, a group that also includes species such as gaylussite and eitelite, all of which depend on water-rich, alkali-rich conditions to form.
| Formula | K2Mg(CO3)2·4H2O |
| Crystal system | Monoclinic |
| Mohs hardness | 2 to 3 |
| Lustre | Vitreous |
| Colour | Colourless |
| Type locality | Gerstenegg-Sommerloch tunnel, Bern, Switzerland |
Formation & occurrence
Baylissite forms in low-temperature, water-rich environments where potassium-, magnesium- and carbonate-bearing solutions can crystallise without being washed away. At its Swiss type locality it was found in a hydroelectric tunnel through the Grimsel area of the Aar massif, where it formed as a delicate secondary mineral in fissures. The same dependence on persistently damp, alkaline conditions has led to reports from a few other settings, including cave and groundwater environments elsewhere in Europe and Russia. In all cases it is a fragile secondary phase rather than a primary rock-forming mineral.
Identification & similar species
Baylissite is recognised as soft, colourless, transparent crystals with a vitreous lustre and a low hardness of about 2 to 3. Its most diagnostic property is practical: it dissolves in water and deteriorates in humid air, behaviour that distinguishes it from many superficially similar colourless carbonates and sulphates. Without careful handling it can be confused with other soluble, low-hardness alkali carbonates, so secure identification usually relies on chemical or X-ray analysis combined with knowledge of the specimen's source.
Notable localities & collecting
The Gerstenegg-Sommerloch tunnel in the Bern canton of Switzerland is the type locality and the classic source. A small number of additional occurrences have been reported, but verified specimens are scarce everywhere. For collectors, baylissite is among the more challenging rare carbonates to own: it must be kept dry and sealed to survive, and it is best suited to specialists who can provide controlled storage. Genuine material is uncommon on the market and generally circulates through dedicated rare-mineral dealers.