Quantum phase transition in the dioptase magnetic lattice

The study of quantum phase transitions (see Sachdev S., Quantum Phase Transitions (Cambridge University Press, Cambridge) 1999), which are zero-temperature phase transitions between distinct states of matter, is of current interest in research since it allows for a description of low-temperature properties based on universal relations. Here we show that the crystal green dioptase Cu₆Si₆O₁₈ · 6H₂O, known to the ancient Romans as the gem of Venus, has a magnetic crystal structure, formed by the Cu(II) ions, which allows for a quantum phase transition between an antiferromagnetically ordered state and a quantum spin liquid.