Cuprospinel from Aogashima Island, Tokyo, Japan

Cuprospinel, which is rarely been found in nature, was identified on the Yuhama coast in Aogashima Island, Tokyo, Japan. Cuprospinel occurred as one of the sublimated minerals in a rim of black spherical aggregates with hematite as the core. In one sample, the cuprospinel was found to have a composition of (Cu0.86Fe2+0.08Mg0.04Mn0.02Ca<0.01)(Fe3+1.98Al0.01Ti<0.01)O4, and exhibited a crystal structure with tetragonal symmetry (I41/amd) and unit cell parameters of a = 5.826(2) Å, c = 8.682(5) Å, and V = 294.7(3) Å3. Another sample had a composition of (Cu0.60Mg0.21Fe2+0.10Mn0.05Ca0.03)(Fe3+1.99Al0.01Ti<0.01)O4, and its crystal structure exhibited cubic symmetry (Fd3m) with unit cell parameters of a = 8.358(3) Å and V = 583.7(6) Å3. Both samples may have undergone growth at high temperature and subsequent annealing at low temperature, so that the difference in crystal structure may be mainly due to the abundance of Cu2+ Jahn-Teller cations. This is the first demonstration of the occurrence of tetragonal cuprospinel in a natural environment.