When starting offering analytical service I got in contact with some passionate mineral collectors.
This allowed me too examine some minerals, I did not hold in my hands before.
As you can see in my current auction, I can offer paxite, mgriite and engelhauptite. The last one is a relatively new mineral (IMA-2013-09) with some novel physics at low temperatures.
For many materials magnetic order is expected at moderately low temperatures, when spin fluctuation becomes weak. In engelhaupte magnetic order is expected to be absent (frustration) or insetting at very deep temperatures (below a few K). The reason for this behaviour is thought to be found in the geometric structure, which prevents spins from aligning.
Synthesis in the lab for engelhauptite failed until now, so natural crystals remain the only available source for studying.
As only very few engelhauptite was found, it is a moral issue to use this for experiments.
The structure model below shows the frustration of the spins (arrows) for the atacamite mineral haydeeite, when frustration sets in below 6K.