  The objective of the present work was to analyze the possibility of
realization of quantum spin liquid in three volcanic minerals - averievite
(Cu5O2(VO4)2(CuCl)), ilinskite (NaCu5O2(SeO3)2Cl3), and avdononite
(K2Cu5Cl8(OH)4 2H2O - from the crystal chemistry point of view. Based on the
structural data, the sign and strength of magnetic interactions have been
calculated and the geometric frustrations serving as the main reason of the
existence of spin liquids have been investigated. According to our
calculations, the magnetic structures of averievite and ilinskite are composed
of antiferromagnetic (AFM) spin-frustrated layers of corner-sharing Cu4
tetrahedra on the kagome lattice. However, the direction of nonshared corners
of tetrahedra is different in them. The oxygen ions centering the OCu4
tetrahedra in averievite and ilinskite provide the main contribution to the
formation of AFM interactions along the tetrahedra edges. The local electric
polarization in averievite and the possibility of spin configuration
fluctuations due to vibrations of tetrahedra-centering oxygen ions have been
discussed. The existence of structural phase transitions accompanied with
magnetic transitions was assumed in ilinskite because of the effect of a lone
electron pair by Se4+ ions. As was demonstrated through comparison of
averievite and avdoninite, at the removal of centering oxygen ions from
tetrahedra, the magnetic structure of the pyrochlore layer present in
averievite transformed into an openwork curled net with large cells woven from
corner-sharing open AFM spin-frustrated tetrahedra ('butterflies'} in
avdoninite.
