Outstanding white crystal group of the very rare Ca-Mg chloride mineral tachyhydrite from the TL in Germany: Stassfurt, Saxony - Anhalt, Germany. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Tachyhydrite
A highly deliquescent calcium-magnesium chloride — it dissolves in humid air almost as you watch.
Intro adapted from Wikipedia (CC BY-SA).
Tachyhydrite is an IMA-recognized mineral species in the halide class, with the chemical formula CaMg₂Cl₆·12H₂O.
It crystallizes in the hexagonal crystal system (space group R-3).
Key properties: Mohs hardness 2; specific gravity 1.667; Vitreous luster; optically uniaxial.
IMA symbol Thy, grandfathered status — known before the IMA’s founding, named in 1856.
A rare mineral found in bedded salt deposits of oceanic origin.
It is found associated with Kainite, carnallite, sylvite, halite, kieserite, bischofite, anhydrite.
Its name comes from the Greek for quick and water, remarking on its deliquescence.