The crystallisation of large halite cubes is a rather common process, which can be studied in detail
in many salt mines worldwide. In fact, it is often a surprisingly fast process, where 10 cm or even larger crystals can form in some years or even only months time. However, the formation of giant halite crystals in the 1 m range is by far rarer. Apart from the well documented and readily accessible Merkers potash mine in Thuringia and some rumours about other giant halite cubes in northern Germany the "golden" halite cubes of the PCA mine near Carlsbad seems to be the only other well known example.
The potash deposits of the Carlsbad area are of permian age, similiar to most european potash deposits. Mineable potash seamd occur intercalated with halite and other evaporites and are generally little affected by any tectonics. They are however subject to highly pressurized natural gas in the overlying beds. To prevent extensive. roof falls caused by the pressurized gas, the gas is vented by regular drills into the ceiling.
In April 1962, a drill hole encountered a large cavity full of brine, which immediately rushed down into the adits and stopped hours later only. Mr. Perrini, the mine foreman at that time, decided to investigate the brine inflow and ordered to drive a raise, which finally hit a large, though very flat cave with the giant halite cubes shown here. Some months later a second similiar cave was discovered with even larger halite crystals as well as some exceptional large sylvite crystals.
Little is known about the further fate of this crystals, though some smaller ones were recovered by the miners. As potash mines are often subject to (catastrophic or systematic) flooding, it appears that either the cystals have been dissolved since many years or they are in danger to do so in the near future...
P.S. : The remarkable "golden" colour of the crystals originates from finely dispersed iron oxides, which are quite common in these deposits.
Update April 2006 : The mine was later bought by Eddy Potash Inc and was known as the Eddy mine until its final closure due to depletion of high grade potash ore in December 1997. In the moment there are plans under way to flood the whole mine and convert it into a salt brine production facility. This will definitely mean the loss of any remaining crystals !